I was reading TUT4 (I think) that details various transformer set combinations for 50W amps. With that section was statement about how some builders/players favor the sound of high impedance primaries over lower ones and visa versa. I've experimented with different primary impedances 5E3 designs but I'm not sure if the difference in impedance was great enough to experience a big difference and I didn't do it with two amp sitting side by side. To my ears the higher impedance transformer seemed to sound little softer and warmer but that could have been my imagination.
What has been other people's experiences? Also given the same supply voltage what effect should we expect between say a higher and lower impedance primary as far as tone?
Thanks!
Hey
Wouldnt the high impedance make less power if B+ is the same? maybe that gets you to clipping sooner too.
(12-07-2022, 02:27 AM)nauta Wrote: [ -> ]Hey
Wouldnt the high impedance make less power if B+ is the same? maybe that gets you to clipping sooner too.
Different impedances of primary have different suggested Va (B+) values but most are lower than the supply voltages that most guitar amps have. My question is that given that the supply voltages are adequate for full power for either transformer what is the possible tone difference?
Hi Guys
The differences between OTs are many. If you try to narrow them down by taking samples from a single product line by a single manufacturer, even then you may not be making a "scientific" comparison, even if the same person designed every unit in the line. For example, Hammond has many OTs in the 1650-series, but these were designed by different engineers over a period of time, and then modified later by different engineers. Different techniques were used on different models.
The narrowing to given product line DOES give you a comparison of what is available, which in the end is what matters.
Because the tube has a nominally-set transconductance, the amount of current it wants to push through the OT is set for a given grid-1 voltage change given Vs and Va are stable. Changing the OT impedance means the effective voltage gain of the tube is different with each OT sample. Whatever fixed PA circuit you use, if it has feedback or not, the impact on how hard the circuit is driven or how it tries to accommodate the changed load varies.
Each OT design has its own parasitic elements and these impart distortions to the signal trying to pass through it. Combine this with the circuit adaptations above and you have a very unpredictable situation tone-wise. This means you cannot make a general statement, such as "going to high-z adds harmonics", which would be the intuitive notion. Higher turns ratios DO introduce stronger parasitic impacts in general, but it cannot be assumed to be a guarantee of tonal shift.
I know from talking with many techs and builders that for a given build where they have tried a range of primary values for OTs, that they can hear a difference and choose something specific for the build. This may sacrifice power, but amps are designed for tone. You may find the impact is greater in simpler circuits, particularly where other distortion mechanisms have been dealt with, particularly grounding and wiring.
You really have to try it for yourself.
Thanks for the reply! It's helpful. I have some idea about how different sectionalization schemes and grades of laminations affect the sound from a book on transformer design I read. Most manufacturers don't publish info about either the lamination types used or how a transformer is sectionalized which goes to your point about the Hammond 1650 series. My guess would be that various "clone" transformers are even less consistent.
There are a lot of factors involved so I can see your point about how unpredictable it may be.
Hi Guys
This is an old thread but a very important point was left out with respect to other factors that make hearing the different OTs easier: bias method and tube idle dissipation, and tube type.
The OP said he was using a Fender 5E3 to try out different output transformers. The 5E3 is cathode-biased which makes the tubes run hot. This heat can stress the screens of the tube especially if the screen is physically a grid rather than beam-forming plates. Even with the latter, too much heat can cause thermal distortion of the signal going through the tube.
Being able to reduce the idle heat will reduce the smearing of the sound as the amp is driven harder. The easiest way to control the tube idle is with fixed-biasing that has adjustable bias (not a contradiction).
So, having a lower idle heat reduces thermal distortion and will allow the OT swaps to be more audible. Using fixed-bias allows this to be easier. Using more neutral tubes will keep the subtlety of the OT tones from being swamped by the tube character.
Of course, you may want to use cathode-bias and/or a "sonicly rich" tube type in which case assessing the OT differences is more difficult and may result in no audible difference. That could lead you to having an incorrect conclusion regarding the efficacy of the method if you do not consider the context of your test.
Personally, I find the "beam-forming plate" tubes to be the cleanest and most neutral sounding. KTs and other gridded screen structure tubes have a distinct tone of their own.
(04-06-2025, 12:03 PM)K O'Connor Wrote: [ -> ]Hi Guys
This is an old thread but a very important point was left out with respect to other factors that make hearing the different OTs easier: bias method and tube idle dissipation, and tube type.
The OP said he was using a Fender 5E3 to try out different output transformers. The 5E3 is cathode-biased which makes the tubes run hot. This heat can stress the screens of the tube especially if the screen is physically a grid rather than beam-forming plates. Even with the latter, too much heat can cause thermal distortion of the signal going through the tube.
Being able to reduce the idle heat will reduce the smearing of the sound as the amp is driven harder. The easiest way to control the tube idle is with fixed-biasing that has adjustable bias (not a contradiction).
So, having a lower idle heat reduces thermal distortion and will allow the OT swaps to be more audible. Using fixed-bias allows this to be easier. Using more neutral tubes will keep the subtlety of the OT tones from being swamped by the tube character.
Of course, you may want to use cathode-bias and/or a "sonicly rich" tube type in which case assessing the OT differences is more difficult and may result in no audible difference. That could lead you to having an incorrect conclusion regarding the efficacy of the method if you do not consider the context of your test.
Personally, I find the "beam-forming plate" tubes to be the cleanest and most neutral sounding. KTs and other gridded screen structure tubes have a distinct tone of their own.
You make an excellent point. In the past few years I have found that majority of 5E3 and similar hot cathode biased builds that came in my shop using the usual Rk values are actually dissipating well above 100% with most tube samples. There is muddiness to the sound. With the Tweed Deluxe I think this partially because Fender never amended their Rk values as plate voltages rose in their amps. Fenders first Deluxes ran about 350 VDC on the plates---by the end production the plate voltages were about 415 VDC. Guys building kits tend to copy this.
I've increased the value of the Rk in my own builds and they do sound much clearer. After doing so I heard more of difference when accessing other components.
My understanding is that there is an optimum impedance that will match the power tubes, whatever type they are, for maximum power. If you deviate up or down from there, it will give less power and more overall distortion with different mixtures of harmonic content. If you go too far out, then damage can result. It's possible to simulate the impedance mismatch with the primary winding by using a different speaker load to an extent, and I've read that going higher than the optimum match makes the tubes work harder and there is a possibility of arcing inside the transformer with high voltage and wide swings.
This is quoted from R.G. Keen's Geofex page;
'generally a mismatch by a factor of 2 either way is ok, with lower than expected load impedance being preferred.
Extended A: It's almost never low impedance that kills an OT, it's too high an impedance.
The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.
If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.
There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.
This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.'
And here's a quote from the '57 Custom Classic Owner's manual apparently;
'There are multiple things that can happen when you mismatch the impedance, and what is actually safe really depends on the particular design. If the impedance is too low, the anode dissipation of the tubes and the power lost in the output transformer (due to winding resistance) will increase, causing both to heat up more than during normal operation. If the impedance is too high, anode dissipation will decrease, but if you overdrive the amp the screen grid dissipation will increase. There is also the possibility of excessive flyback voltages, which can destroy the output transformer or the output tubes (or both).'
My brother and I used to run his Silvertone 1484 stock 4 ohm tap into an 8 ohm speaker and it had less power, more distortion, and kind of a squashed sound. It sounded fantastic for single note leads, but chords were a bit muddled sometimes. Surprisingly the output transformer is still working fine 25 years later with constant use, though now it is properly matched up to a 4 ohm speaker load, and is louder and cleaner with more definition than it used to be with the mismatch.
I remember way back on Ampage/Music Electronics Forum, 18 watt, and some other forums guys were talking about JTM45's and different impedances with those and the sound differences. You may search out those threads if you're curious.
Greg
Hi Guys
The original post was about the effect of the primary impedance not secondary loading or matching. Two different issues.
To begin with: for a given PSU voltage and current capability, there will be an optimum OT primary impedance that will allow maximum audio output power to be tapped from the PSU. You can increase the primary impedance and draw less power from the PSU.
The secondary load match is another issue, and is what your references are talking about exclusively. They leave out a few points of context.
As most people know, guitar and MI circuits began as hifi circuits meant to be "mobile PAs" that you could plug anything from guitar, bass, accordian, organ, radio, etc into and have them be amplified. It was expected that if the amp or speaker distorted that one would turn down the Volume control. As guitar amps became more focused on making guitars sound good, output transformers became smaller for a given power rating BUT the OEM expectation was still a clean output.
In the original context of design and use, "clean" was the emphasis, so clipping the output stage or mismatching the secondary load was not on anyone's mind. But...
Along came players who wanted to be louder in a given space, or found that the overdriven sound fit into their musical expression. Eventually guitar amps were voiced to sound good when overdriven, and often sounded quite thin when not, and new preamp arrangements allowed distorted sound to be created without overdriving the power amp. Even with this new design paradigm, players will push the envelope and on a modern stage, a Master Volume amp will still be driven to clipping.
An example of amp clipping is easily heard on the live album for the Heaven and Hell tour. The beginning of 'I' starts relatively quietly but distorted. Then for the chordal section the guitar volume is dialed up and you hear the sound get louder but there is a "sizzle" on top. That is PA clipping.
Guitar amp OTs are band-limiting inasmuch as they will not provide full power at low-frequencies but will at mid- and maybe high-frequencies. This reduced size makes the OT more vulnerable to damage by overload. The tubes are natural power limiters but you could easily assemble and output stage where the OT is in peril almost all the time. Most OTs are not designed with much margin for abuse - abuse that is considered normal operation in MI. So, one way to assist OT reliability is to use a full-power-bandwidth unit, which also makes the dynamic tone more consistent, at least as far as the OT's contribution goes.
To protect the OT from other mishaps, some amps have clamping diodes on the primary side, from the driven end(s) to ground. This is a brute force method that results in shorted diodes if they are sized to ever come on at a useful point. Do they save OTs? No one can say for certain.
I prefer a benign suppression approach of simply tethering the driven end(s) of the OT primary to ground using resistors. These never fail and they work continuously over the audio cycle to sap energy from the parasitic elements of the OT. Yes, they consume some audio power as well, but an insignificant amount. If the load becomes discontinuous, or the player has set an extreme secondary mismatch, the universe remains stable.
Hi Guys
Another context is the "hifi ideal" for tube PA design. In this ideal, grid-stops of all kinds are not needed. There is an assumption of ideal layout and ideal components, neither of which exist.
As TUT promoted since '95, the use of higher-value screen-stops will provide a great deal of protection to the tube and the OT. Fender and other builders who began during the Tweed era had either no screen stops or very low-value ones, which was okay in the context of avoiding clipping. When you go from a very low value to our standard 1k-5W (specified that way to encourage buying flame-proof wire-wound components), there will be a tone change. You can hear it in an amp like the SVT (which is riddled with poor design decisions). Players say the amp has its "loud tone all the time" and the biggest impact is the amp never "eats" tubes.
In a hot-running amp the 1k-5W protects the vulnerable screen which is constantly in peril due to the excess heat. Upside-down tubes do not help. The high-value screen-stops can make assessing OT tone easier, too.