Hammond has reverse-engineered every useful guitar amp OT and PT and have a line of 1700-series output transformers and 290-series power transformers. These are all drop-in replacements, so if you have an amp with a dead OT or PT you can use these parts without having to drill any holes.
because the transformers are modelled after named amp models, hobbyists and builders can be assured of the tonal range possible assuming similar or identical circuits and tubes are used, too. In any case, if you want an amp with the power of the original Bassman, say, you can buy the matching OT and PT set. More adventurous builders may wish to mix and match these offerings with those of other brands. By this I mean, say, using the repro Bassman OT from Hammond but use a non-Hammond PT.
There is a critical point of interest regarding the PT specifications in the Hammond 290-series inasmuch as the plate winding uses an AC voltage but a DC current. This is the same as they did in the "Classic" transformer lines, and it will confuse most hobbyists and builders born in the sixties or later.
For example, the Bassman PT for a 50W amp has a 660Vct plate winding rated at 230mA. We can take this as 330Vac at 230mAdc given the half-bridge rectification.This results in about 470Vdc and 108W of power. The Bassman OT is 4kaa, representing a 1ka load. At 100Wpk audio power, there needs to be 316Vpk at 316mApk. Then we need about 60Vsat across the tube at full load and have a loaded supply of about 380Vdc The sag is 90V, or almost 20%. If we look at the 316mA at 380V we have 120W, which is not too far off the 108W calculated previously, so we will say it is "close enough".
Most of Fender's amps evolved to use about the same supply voltages and a variation of OT impedances. Hammond has added the devices Fender should have switched to when tube rectifiers were dropped. That is, non-CT plate windings. This allows the use of a full bridge rectifier modules and requires a separate winding for bias. The latter helps to improve grounding, as TUT3 shows.
Power supply design defaults should always be:
full-wave rectification is always used
half-wave rectification is NEVER used especially for bias supplies
every supply has its own winding
dual primaries for world-wide use
electrostatic screen and/or core band for noise reduction
Hammond has reverse-engineered every useful guitar amp OT and PT and have a line of 1700-series output transformers and 290-series power transformers. These are all drop-in replacements, so if you have an amp with a dead OT or PT you can use these parts without having to drill any holes.
because the transformers are modelled after named amp models, hobbyists and builders can be assured of the tonal range possible assuming similar or identical circuits and tubes are used, too. In any case, if you want an amp with the power of the original Bassman, say, you can buy the matching OT and PT set. More adventurous builders may wish to mix and match these offerings with those of other brands. By this I mean, say, using the repro Bassman OT from Hammond but use a non-Hammond PT.
There is a critical point of interest regarding the PT specifications in the Hammond 290-series inasmuch as the plate winding uses an AC voltage but a DC current. This is the same as they did in the "Classic" transformer lines, and it will confuse most hobbyists and builders born in the sixties or later.
For example, the Bassman PT for a 50W amp has a 660Vct plate winding rated at 230mA. We can take this as 330Vac at 230mAdc given the half-bridge rectification.This results in about 470Vdc and 108W of power. The Bassman OT is 4kaa, representing a 1ka load. At 100Wpk audio power, there needs to be 316Vpk at 316mApk. Then we need about 60Vsat across the tube at full load and have a loaded supply of about 380Vdc The sag is 90V, or almost 20%. If we look at the 316mA at 380V we have 120W, which is not too far off the 108W calculated previously, so we will say it is "close enough".
Most of Fender's amps evolved to use about the same supply voltages and a variation of OT impedances. Hammond has added the devices Fender should have switched to when tube rectifiers were dropped. That is, non-CT plate windings. This allows the use of a full bridge rectifier modules and requires a separate winding for bias. The latter helps to improve grounding, as TUT3 shows.
Power supply design defaults should always be:
full-wave rectification is always used
half-wave rectification is NEVER used especially for bias supplies
every supply has its own winding
dual primaries for world-wide use
electrostatic screen and/or core band for noise reduction
Have fun
Very helpful! I thought I'd add a little of what I know about how the 290 line was developed from my conversations with the engineer at Hammond and from CE Distribution employees. After Classic Tone and Marvel Electric which were sister companies under the Magnetic Components Company announced they were going out of business in 2020 CE bought and sent Hammond many of their transformers to Hammond for reverse engineering. At that time Hammond had some direct replacements but not nearly as many as they do today. Many of the new models are based off of those transformers even to the point where many of the OT are made with paper bobbins (not that it matters).
Another interesting thing is that Heyboer which makes transformers for lots of US suppliers bought all of Classic Tone's winding equipment, so their transformers are in some ways related to Classic Tones old transformer lines too.
I've used many of these newer Hammonds in builds and for repairs and they are quality transformers.
Hammond has reverse-engineered every useful guitar amp OT and PT and have a line of 1700-series output transformers and 290-series power transformers. These are all drop-in replacements, so if you have an amp with a dead OT or PT you can use these parts without having to drill any holes.
because the transformers are modelled after named amp models, hobbyists and builders can be assured of the tonal range possible assuming similar or identical circuits and tubes are used, too. In any case, if you want an amp with the power of the original Bassman, say, you can buy the matching OT and PT set. More adventurous builders may wish to mix and match these offerings with those of other brands. By this I mean, say, using the repro Bassman OT from Hammond but use a non-Hammond PT.
There is a critical point of interest regarding the PT specifications in the Hammond 290-series inasmuch as the plate winding uses an AC voltage but a DC current. This is the same as they did in the "Classic" transformer lines, and it will confuse most hobbyists and builders born in the sixties or later.
For example, the Bassman PT for a 50W amp has a 660Vct plate winding rated at 230mA. We can take this as 330Vac at 230mAdc given the half-bridge rectification.This results in about 470Vdc and 108W of power. The Bassman OT is 4kaa, representing a 1ka load. At 100Wpk audio power, there needs to be 316Vpk at 316mApk. Then we need about 60Vsat across the tube at full load and have a loaded supply of about 380Vdc The sag is 90V, or almost 20%. If we look at the 316mA at 380V we have 120W, which is not too far off the 108W calculated previously, so we will say it is "close enough".
Most of Fender's amps evolved to use about the same supply voltages and a variation of OT impedances. Hammond has added the devices Fender should have switched to when tube rectifiers were dropped. That is, non-CT plate windings. This allows the use of a full bridge rectifier modules and requires a separate winding for bias. The latter helps to improve grounding, as TUT3 shows.
Power supply design defaults should always be:
full-wave rectification is always used
half-wave rectification is NEVER used especially for bias supplies
every supply has its own winding
dual primaries for world-wide use
electrostatic screen and/or core band for noise reduction
Have fun
Kevin, a question for you on output transformers. I bought a London pre kit from you a while back. Thank you very much, it is wonderful, and I appreciated the guidance received during assembly. If you don't recall (and I wouldn't expect you to) I used your kit in place of a DSP board that had failed in a Fender Champ X2. So the preamp controls are all stuffed into the X2 chassis, along with a Celestion neodymium 10" speaker.
It's a neat rig, but I was considering changing the output transformer. As a consequence of my former business (used to provide voice/music audio for a lot or large temporary events like airshows, car races, World cuo ski events, etc) I have a tone of left overs of autoformers and isolation transformers.
My question is what sort of ratio do I need to match the output of the tube driven Fender X2 to the speaker I am using? For example, I have a bunch of EV AT100 autoformers. Would these be a good match, and what info would you need to make such a determination?
Or am I overthinking this? Should I just find a good transformer in my stockpile that is large enough to handle the output voltage and yet presents the tube side with a somewhat higher load than 8 ohms? if so, 50 ohms? 100 ohms?
That would be a Super Champ X2, rated at 15W using 2x 6V6.
The amp has a full and proper tube power amplifier with a proper output transformer that matches the push-pull tube stage to an 8R speaker. Why do you need to change it? There are lots of mods you can make to the PA, but changing the OT is not the first one that comes to mind - if at all. And as always, do not waste your money on Mercury Magnetics. Total hype and not as good as "generic" brands.
"Autoformers" are auto transformers )single winding with taps and a shared end for input and output)designed for audio, generally for low-quality public address systems. Note that such systems should NEVER have full bass response, let alone extended bass response, as this low-frequency content becomes more of a background "rumble" than sounding anyways musical. Background music systems are explicitly that - BACKGROUND and should therefore be unobtrusive.
In a PA system, the power amp may have a 70V line transformer built-in, or simply be rated at such high 8R power that it can produce 70Vrms output directly. The latter would have to be rated for close to 800W into 8R - not too common. In typical systems, the driving OT has a step-up voltage ration that matches the amp's nominal rated voltage output (what corresponds to its 8R output) to the 70V line. Then at each speaker location is a smaller autoformer to tap a portion of the power.
None of the 70V system transformers are applicable to your guitar unless you wish to distribute the sound through such a system, with notably reduced fidelity /altered tone compared to the combo speaker. Using an over-sized line AT may reduce the fidelity loss.
That would be a Super Champ X2, rated at 15W using 2x 6V6.
The amp has a full and proper tube power amplifier with a proper output transformer that matches the push-pull tube stage to an 8R speaker. Why do you need to change it? There are lots of mods you can make to the PA, but changing the OT is not the first one that comes to mind - if at all. And as always, do not waste your money on Mercury Magnetics. Total hype and not as good as "generic" brands.
"Autoformers" are auto transformers )single winding with taps and a shared end for input and output)designed for audio, generally for low-quality public address systems. Note that such systems should NEVER have full bass response, let alone extended bass response, as this low-frequency content becomes more of a background "rumble" than sounding anyways musical. Background music systems are explicitly that - BACKGROUND and should therefore be unobtrusive.
In a PA system, the power amp may have a 70V line transformer built-in, or simply be rated at such high 8R power that it can produce 70Vrms output directly. The latter would have to be rated for close to 800W into 8R - not too common. In typical systems, the driving OT has a step-up voltage ration that matches the amp's nominal rated voltage output (what corresponds to its 8R output) to the 70V line. Then at each speaker location is a smaller autoformer to tap a portion of the power.
None of the 70V system transformers are applicable to your guitar unless you wish to distribute the sound through such a system, with notably reduced fidelity /altered tone compared to the combo speaker. Using an over-sized line AT may reduce the fidelity loss.
Have fun
Hey, Kevin, Thanks so much for the reply. The following may not be useful given your statement that changing the OT may not be worthwhile, but a couple of comments first.
The transformers I am speaking are not the simple voice only you are familiar with but instead designed for full range response. The Electrovoice AT100 for example is a 6lb monster of Iron and copper, I have over 100 of them which were taken out of the Indianapolis Hoosier Dome for their speech/music system, and the performance level is very high. (Spec attached) From that same installation I have numerous 300-watt Crown isolated transformers (not autoformers) so for me it would be a no-cost improvement, if in fact it is an improvement. The Crown ones have only a fixed 3:1 ratio, so while they are huge and hifi, this may noy be enough of a change in the reflected impedance. For example, attaching one of these Crown 300 watt Xformer to an 8 ohm speaker would change the load the amp sees to only 24 ohms. I expect the impedance load the amp side would want to be higher?
I also have a lot of 32 watt transformers, again designed for high performance audio. I'll Have to go take a look at one to get the make and model number, but I would suspect they are similar in performance to the Hammond stuff. They certainly weigh similar amounts. These 32 Watt models give me a few more taps than the 300 watt Crown, so perhaps I can arrange loads close to 50 or 100 ohm impedance.
Just as aside, my Crown Slide rule (and Crown's engineering documents) tell me that 625 watts @ 8 ohms = 70 Volts. Not that this info would be useful to you, but you never know.
Hammond has full-range autoformers up to 250W, so I am familiar wioth these, plus Plitron made custom units for Bryston's PA-oriented installation amplifiers.
None of these devices are appropriate replacements for the OT in a tube amplifier.
The amp you have has an OT with a primary impedance around 8k-aa, that is anode to anode. The load each tube sees is one-quarter of that, so 2k. The impedance ratio of primary to secondary is the square of the voltage ratio, and as you can see, all the numbers are quite high when matching tube plates to speakers.
Fender tended to use slightly under-sized OTs, as do most MI amps. This means that the bandwidth is wider at low output than at full output, which is only of concern if it bothers you. There are benefits to having rolled off bass with guitar inasmuch as the overdriven amplifier will be less "farty sounding". To optimise that operating mode further requires changing some capacitor values within the circuit. TUTs explain all of this, along with transformer specification methods.
For a line transformer (regardless of single or separate windings), the ratio is quite small.Say you have a 100W line match for 70V. The 70V impedance is 142R, just from using the power equations and Ohm's law. The taps on the device are usually given power ratings assuming 8R for the speaker. There may be an 8R tap rated at 100W that is used to match the PA output; the 70V tap (full winding)connects to the other distribution transformers; the low-wattage taps go to specific local speakers.
Hammond has full-range autoformers up to 250W, so I am familiar wioth these, plus Plitron made custom units for Bryston's PA-oriented installation amplifiers.
None of these devices are appropriate replacements for the OT in a tube amplifier.
The amp you have has an OT with a primary impedance around 8k-aa, that is anode to anode. The load each tube sees is one-quarter of that, so 2k. The impedance ratio of primary to secondary is the square of the voltage ratio, and as you can see, all the numbers are quite high when matching tube plates to speakers.
Fender tended to use slightly under-sized OTs, as do most MI amps. This means that the bandwidth is wider at low output than at full output, which is only of concern if it bothers you. There are benefits to having rolled off bass with guitar inasmuch as the overdriven amplifier will be less "farty sounding". To optimise that operating mode further requires changing some capacitor values within the circuit. TUTs explain all of this, along with transformer specification methods.
For a line transformer (regardless of single or separate windings), the ratio is quite small.Say you have a 100W line match for 70V. The 70V impedance is 142R, just from using the power equations and Ohm's law. The taps on the device are usually given power ratings assuming 8R for the speaker. There may be an 8R tap rated at 100W that is used to match the PA output; the 70V tap (full winding)connects to the other distribution transformers; the low-wattage taps go to specific local speakers.
Have fun
OK, got it, I think. Even if I were able to find taps on my other "hi-fi" transformers that would get me a 2K reflected impedance, it still wouldn't be great because in this case too much bass would get me a muddy signal that probably would not be responsive to where the tone controls are centered? Did I get that right?
So I guess I'll abandon that idea, which was planted by reading about OT upgrades from your favorite vender
But just curious since you raised the issue. If you were to perform the mods you mentioned above before worrying about the OT, what mods would those be?
This line is the most important regarding the autoformers:
None of these devices are appropriate replacements for the OT in a tube amplifier.
Regarding the amp you have and what mods you might make, you have to think about what tone you want to have compared to the tone you actually have. Then try to close the gap appropriately. An easy change to make to the PA is to increase the feedback loop values by a factor of ten. Gain remains the same but the tone will be more dynamic.
This line is the most important regarding the autoformers:
None of these devices are appropriate replacements for the OT in a tube amplifier.
Regarding the amp you have and what mods you might make, you have to think about what tone you want to have compared to the tone you actually have. Then try to close the gap appropriately. An easy change to make to the PA is to increase the feedback loop values by a factor of ten. Gain remains the same but the tone will be more dynamic.
Have fun
OK, perhaps I didn't get it. As humbly and sincerely as possible, why would that be the case? They are iron and copper windings. If I can provide a reflected impedance load that the supply side wants to "See", why would a higher quality piece of copper and Iron give you less performance?
But thank you for the second part, and that is exactly the type change I would hope to make. I am guessing you have some information on the site that can walk me through this? If not, would you let me know how this is done?
What you wrote here says you did not understand the description of the output transformer:
" If I can provide a reflected impedance load that the supply side wants to "See", why would a higher quality piece of copper and Iron give you less performance?"
The first part of the assumption is incorrect inasmuch as the OT has a centre-tap to feed power to the tube plates and your line ATs do not. You cannot use what is essentially a single-ended transformer designed for zero standing-DC as a replacement for a real push-pull transformer.
The second part of your statement has nothing to do with anything I stated. Guitar amps are not about fidelity per se; rather, they are about tone creation. For some players fidelity is improved tone, where others find such a change contrary to what they want. There is no right and wrong there.
Computers and guitar amps both have resistors and capacitors but they are wildly different devices. The same goes for line transformers and tube output transformers - apples and oranges. TUT volumes 2,4,5,6 have explanations about how output transformers function and how to specify them.
In a push-pull circuit, each tube only amplifies half the signal, working on its own end of the OT primary with CT tied to B+. The signal is "added together" within the OT. While one tube works the other rests but both ends of the transformer are magnetically in parallel so thew working tube does not see half the end-to-end impedance but one-quarter. The secondary is designed to match the speaker. OTs come in all power ranges and with narrow or wide frequency response. B+ can be hundreds of volts and the end-to-end voltage approaches twice the B+ value. OTs are designed for this environment.
On the other hand, line distribution transformers like you have are designed to match an amplifier speaker output to a 70V line, or to match the 70V line to a speaker load. Neither side can tolerate DC current or voltage. To reflect an impedance of k-ohms would require a very nonstandard speaker and everything else about the transformer is truly inadequate for the goal of replacing a real tube amp output transformer. As far as that application goes, your ATs are the wrong device. Why not use them to distribute sound through your house? The point of the high-voltage distribution is to allow the use of thin and inexpensive wire.
What you wrote here says you did not understand the description of the output transformer:
" If I can provide a reflected impedance load that the supply side wants to "See", why would a higher quality piece of copper and Iron give you less performance?"
The first part of the assumption is incorrect inasmuch as the OT has a centre-tap to feed power to the tube plates and your line ATs do not. You cannot use what is essentially a single-ended transformer designed for zero standing-DC as a replacement for a real push-pull transformer.
The second part of your statement has nothing to do with anything I stated. Guitar amps are not about fidelity per se; rather, they are about tone creation. For some players fidelity is improved tone, where others find such a change contrary to what they want. There is no right and wrong there.
Computers and guitar amps both have resistors and capacitors but they are wildly different devices. The same goes for line transformers and tube output transformers - apples and oranges. TUT volumes 2,4,5,6 have explanations about how output transformers function and how to specify them.
In a push-pull circuit, each tube only amplifies half the signal, working on its own end of the OT primary with CT tied to B+. The signal is "added together" within the OT. While one tube works the other rests but both ends of the transformer are magnetically in parallel so thew working tube does not see half the end-to-end impedance but one-quarter. The secondary is designed to match the speaker. OTs come in all power ranges and with narrow or wide frequency response. B+ can be hundreds of volts and the end-to-end voltage approaches twice the B+ value. OTs are designed for this environment.
On the other hand, line distribution transformers like you have are designed to match an amplifier speaker output to a 70V line, or to match the 70V line to a speaker load. Neither side can tolerate DC current or voltage. To reflect an impedance of k-ohms would require a very nonstandard speaker and everything else about the transformer is truly inadequate for the goal of replacing a real tube amp output transformer. As far as that application goes, your ATs are the wrong device. Why not use them to distribute sound through your house? The point of the high-voltage distribution is to allow the use of thin and inexpensive wire.
Have fun
Thanks for your reply, Kevin. Would you mind terribly providing me with the info on where I can read the info on "An easy change to make to the PA is to increase the feedback loop values by a factor of ten. Gain remains the same but the tone will be more dynamic." I believe this is the sort of tonal changes I was looking for.
And again, understanding the mission of this forum is to be as respectful as possible, I'll be brief and if you are curious to hear more I'd be happy to explain.
My business used to be in providing large scale speech and music systems in large acreage events. I doubt very much there are many more people who have designed and successfully implemented 140V distributed systems of this type than I, using 11 gauge wire, and 2 x Altec 2200 series amps in a push/pull arrangement for 1150 watts @8 ohms, (times x number of racks of this arrangement) and large format EV devices at the receiving end. I've hooked up numerous transformers using a real impedance bridge and other tools available to provide foreground music over miles, and actually used some of these components for the delay rings at Kenny Loggings Closing Ceremonies at Mile High Stadium in Denver, CO for the 1995 US OLympic Fest.
I am not writing any of this as braggadocio, I am explaining why I profoundly disagree with the statement these systems are designed to get away with thin wire and only as background music. My real life experience states this just is not correct.
I'll leave my comments about this issue at that, unless you are interested in hearing some of the very neat and creative things I've done in past. Once again, offered with respect.
And if you could point me in the right direction to increase the feedback loop by a factor of 10, I would be appreciative. Thanks very much.
The mod to the feedback loop of the PA is as simple as I've described. Can you read a schematic? This is different than wiring diagrams for PA systems, or for wiring guitar pickups, as those are closer to layout drawings - pickup wiring are in fact just layouts.
In most Fender amps, the feedback resistors are 820R in series from the speaker output working against 100R in shunt to ground at the bottom of the splitter. These values give a gain of 9.2 - colloquially referred to as" a gain of 10". Were the amplifier inverting the gain would be 820/100=8.2, but then the input impedance facing the preamp would be way too low. Most PAs are non-inverting, and gain is 1+(820/100)=9.2. With tubes, things are never quite as it should be, so even with this strangle hold, and an open-loop gain of just 100, the real gain will be a little bit off.
Increasing the values by 10x says use 8k2 and 1k instead. This does nothing to the gain as the ratio is still the same. It does nothing to the bias of the splitter. The effect is to loosen up the sound and make the amplifier even more stable than it already is. Note that this works for tube circuits and not for solid-state, for reasons too involved to get into here.
---
Regarding the public address systems, the bulk of installations are small-scale and intended for low-key SPLs. The 70V North American standard and 100V Euro standard is in fact intended to allow the use of small wire over long distances, the same as using higher voltages for electric railway power distribution allows thinner catenary, or simple mains power distribution using extremely high-voltage for long runs.. Lots of power can be distributed this way These PAs come in all sizes, and you have been lucky to work on some impressive ones .
It is simple Ohm's Law. 100W at 100V requires 1A and thus can be transmitted via a thin wire. 100W at 10V requires 10A and thus a thicker wire is needed. Obviously, the length of wire between the source and load introduces its own resistance and a power loss to the load. In your big system, you may need very thick wire close to the source, but outlying branches can be handled with smaller gauge wire. The compromise of loss versus cost of the copper comes into play here. Whether the "allowable" thinness is taken advantage of or is seen as a detriment in reality, is up to whoever oversees the installation.
This a factor in wiring guitar amps, particularly for heaters, where high currents are needed. The PT fly-wires must handle the total current of the load, and these leads are often quite thick. The wiring can be fanned out from one point, allowing thin wire to each socket, or if the sockets are daisy-chained, graduated gauges of wire can be used as you move away from the PT to the end of the chain. TUT3 shows both methods.
Fidelity is a moving target with public address systems, and this includes stage PAs.The required fidelity also varies with each installation. A train station only needs intelligible voice. A hockey rink in the 1950s might have only needed the same, where one wired today wants more of a hifi sound.
I was just looking at the Fender Super Champ XD schematic to cut and paste the PA section and this one is different from the usual inasmuch as it uses a tweed circuit: a concertina splitter preceded by a gain stage. That is actually quite a nice sounding circuit with the right values.
The feedback loop is R23=2k7 (series) and R17=47 (shunt). The ratio here is 57.5, so a gain of 58.5 which is pretty high. This could make the tone a bit woofy or dark. It is odd that so much gain is being demanded from the PA considering the number of opamps preceding it.
The output tubes share a cathode current-sense resistor and there is only one bias pot. A useful mod would be to split the cathodes and add a second resistor, then split the bias feed and add a second bias pot in parallel. This allows hum-balancing of the output stage, which in itself is good with an added benefit of improving the bass response through the OT (not that you might hear that).
The mod to the feedback loop of the PA is as simple as I've described. Can you read a schematic? This is different than wiring diagrams for PA systems, or for wiring guitar pickups, as those are closer to layout drawings - pickup wiring are in fact just layouts.
In most Fender amps, the feedback resistors are 820R in series from the speaker output working against 100R in shunt to ground at the bottom of the splitter. These values give a gain of 9.2 - colloquially referred to as" a gain of 10". Were the amplifier inverting the gain would be 820/100=8.2, but then the input impedance facing the preamp would be way too low. Most PAs are non-inverting, and gain is 1+(820/100)=9.2. With tubes, things are never quite as it should be, so even with this strangle hold, and an open-loop gain of just 100, the real gain will be a little bit off.
Increasing the values by 10x says use 8k2 and 1k instead. This does nothing to the gain as the ratio is still the same. It does nothing to the bias of the splitter. The effect is to loosen up the sound and make the amplifier even more stable than it already is. Note that this works for tube circuits and not for solid-state, for reasons too involved to get into here.
---
Regarding the public address systems, the bulk of installations are small-scale and intended for low-key SPLs. The 70V North American standard and 100V Euro standard is in fact intended to allow the use of small wire over long distances, the same as using higher voltages for electric railway power distribution allows thinner catenary, or simple mains power distribution using extremely high-voltage for long runs.. Lots of power can be distributed this way These PAs come in all sizes, and you have been lucky to work on some impressive ones .
It is simple Ohm's Law. 100W at 100V requires 1A and thus can be transmitted via a thin wire. 100W at 10V requires 10A and thus a thicker wire is needed. Obviously, the length of wire between the source and load introduces its own resistance and a power loss to the load. In your big system, you may need very thick wire close to the source, but outlying branches can be handled with smaller gauge wire. The compromise of loss versus cost of the copper comes into play here. Whether the "allowable" thinness is taken advantage of or is seen as a detriment in reality, is up to whoever oversees the installation.
This a factor in wiring guitar amps, particularly for heaters, where high currents are needed. The PT fly-wires must handle the total current of the load, and these leads are often quite thick. The wiring can be fanned out from one point, allowing thin wire to each socket, or if the sockets are daisy-chained, graduated gauges of wire can be used as you move away from the PT to the end of the chain. TUT3 shows both methods.
Fidelity is a moving target with public address systems, and this includes stage PAs.The required fidelity also varies with each installation. A train station only needs intelligible voice. A hockey rink in the 1950s might have only needed the same, where one wired today wants more of a hifi sound.
Hey Keven;
"Can you read a schematic?" No, not very well. By staring at it long enough I can likely identify basic signal flow, or recognize resister values when I see them, but no, I would be at a grade 1 level in this.
"This is different than wiring diagrams for PA systems, or for wiring guitar pickups," Absolutely correct.
Having agreed to all that, would I recognize "Increasing the values by 10x says use 8k2 and 1k instead" when I see them? If I understand this correctly, I would simply replace a few of the resistors near the amplifier output transformer. I would recognize these resistors by the color of the bands around the resistors?
The documents below indicate they are from a Super Champ X2 service manual. Is there anyway you can draw a big circle around the parts I need to swap, and, in a perfect world, sell me the parts and bill me for your help? Not a novice to soldering, but I just don't work on componenet level stuff.
The mod to the feedback loop of the PA is as simple as I've described. Can you read a schematic? This is different than wiring diagrams for PA systems, or for wiring guitar pickups, as those are closer to layout drawings - pickup wiring are in fact just layouts.
In most Fender amps, the feedback resistors are 820R in series from the speaker output working against 100R in shunt to ground at the bottom of the splitter. These values give a gain of 9.2 - colloquially referred to as" a gain of 10". Were the amplifier inverting the gain would be 820/100=8.2, but then the input impedance facing the preamp would be way too low. Most PAs are non-inverting, and gain is 1+(820/100)=9.2. With tubes, things are never quite as it should be, so even with this strangle hold, and an open-loop gain of just 100, the real gain will be a little bit off.
Increasing the values by 10x says use 8k2 and 1k instead. This does nothing to the gain as the ratio is still the same. It does nothing to the bias of the splitter. The effect is to loosen up the sound and make the amplifier even more stable than it already is. Note that this works for tube circuits and not for solid-state, for reasons too involved to get into here.
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Regarding the public address systems, the bulk of installations are small-scale and intended for low-key SPLs. The 70V North American standard and 100V Euro standard is in fact intended to allow the use of small wire over long distances, the same as using higher voltages for electric railway power distribution allows thinner catenary, or simple mains power distribution using extremely high-voltage for long runs.. Lots of power can be distributed this way These PAs come in all sizes, and you have been lucky to work on some impressive ones .
It is simple Ohm's Law. 100W at 100V requires 1A and thus can be transmitted via a thin wire. 100W at 10V requires 10A and thus a thicker wire is needed. Obviously, the length of wire between the source and load introduces its own resistance and a power loss to the load. In your big system, you may need very thick wire close to the source, but outlying branches can be handled with smaller gauge wire. The compromise of loss versus cost of the copper comes into play here. Whether the "allowable" thinness is taken advantage of or is seen as a detriment in reality, is up to whoever oversees the installation.
This a factor in wiring guitar amps, particularly for heaters, where high currents are needed. The PT fly-wires must handle the total current of the load, and these leads are often quite thick. The wiring can be fanned out from one point, allowing thin wire to each socket, or if the sockets are daisy-chained, graduated gauges of wire can be used as you move away from the PT to the end of the chain. TUT3 shows both methods.
Fidelity is a moving target with public address systems, and this includes stage PAs.The required fidelity also varies with each installation. A train station only needs intelligible voice. A hockey rink in the 1950s might have only needed the same, where one wired today wants more of a hifi sound.
Hey Keven;
"Can you read a schematic?" No, not very well. By staring at it long enough I can likely identify basic signal flow, or recognize resister values when I see them, but no, I would be at a grade 1 level in this.
"This is different than wiring diagrams for PA systems, or for wiring guitar pickups," Absolutely correct.
Having agreed to all that, would I recognize "Increasing the values by 10x says use 8k2 and 1k instead" when I see them? If I understand this correctly, I would simply replace a few of the resistors near the amplifier output transformer. I would recognize these resistors by the color of the bands around the resistors?
The documents below indicate they are from a Super Champ X2 service manual. Is there anyway you can draw a big circle around the parts I need to swap, and, in a perfect world, sell me the parts and bill me for your help? Not a novice to soldering, but I just don't work on componenet level stuff.
Thanks for your help in all this.
Those files didn't come out quite the way I thought they would. Feel free to remove if they are causing problems
I believe you did not see post-13? I think I posted it while you were making up your post. If you review that post, I suggested that changing the feedback values may have little benefit in this specific amp, but did suggest a different mod. Really, you need to say how the sound seems to you now and what you would like it to actually sound like, if it needs to change.
Yes, the images of the manual look wonky but at least you can read the values and part names. The PDFs I have for your amp are practically illegible.
The schematic is akin to reading music on a staff as both are abstract. The layout drawing is like reading tablature, which shows how to position your fingers on a fret board. The best way to learn how to read a schematic is to first have a resource of schematic symbols, then to look at a physical circuit while looking at the schematic. This is easier to do on a simple hand-wired circuit than with a PCB amp as it is easier to see what connects to what.
Like most things, you do not have to know everything about the subject to be successful or to make a living from it.
I believe you did not see post-13? I think I posted it while you were making up your post. If you review that post, I suggested that changing the feedback values may have little benefit in this specific amp, but did suggest a different mod. Really, you need to say how the sound seems to you now and what you would like it to actually sound like, if it needs to change.
Yes, the images of the manual look wonky but at least you can read the values and part names. The PDFs I have for your amp are practically illegible.
The schematic is akin to reading music on a staff as both are abstract. The layout drawing is like reading tablature, which shows how to position your fingers on a fret board. The best way to learn how to read a schematic is to first have a resource of schematic symbols, then to look at a physical circuit while looking at the schematic. This is easier to do on a simple hand-wired circuit than with a PCB amp as it is easier to see what connects to what.
Like most things, you do not have to know everything about the subject to be successful or to make a living from it.
Have fun
Thanks for that. I have let the amp sit for close to a month, as I was expecting I might want to work on this amp, and wanted to make certain the caps had time to discharge.
I think want I am going to do at this point is fire it up and play it for a few days, and that way identify exactly what the mods are that I want.
You are correct, I did not see post 13, and now that I have, I'll give the amp a more thorough listen. If that's a tweed circuit there should be no reason I can't get a great sound out of it when using your preamp.
A long time to reply... but I was reading this thread and realised that the feedback loop value change could be beneficial after all.
The gain is set by 2k7 / 47R = 57.4. Even though the closed-loop gain is high compared to the nominal open-loop gain of 100, the resistor values are low, as is typical for Fender. Going to 27k and 470R could open up the sound.
In general, when the loop is "opened up" to allow more actual voltage gain, say fro 9.2x to 20x, the sound will be a bit looser, maybe with less-damped bass. The value change without gain change gives a different dynamic feel, going from a "dry" or "choked" tone to one that is more "lively".
A warm welcome to tube amp modding fans and those interested in hi-fi audio! Readers of Kevin O'Connor'sThe Ultimate Tone (TUT) book series form a part of our population. Kevin O'Connor is the creator of the popular Power Scaling methodology for amplifiers.
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