Hi Guys

Power Scaling has been around for decades and has undergone continuous improvement and change as far as the kits go, but the goal and the performance overall has always been the same. We have been forced by parts availability issues to make wholesale circuit changes, such as the switch from the SB Super Budget series that used an expensive mil-spec pot and a very simple circuit, to the SV Super Versatile kit series, when the special pot became exceedingly expensive. So, we designed it out and the kit became a little more complex to accommodate its absence. We tried to use the budget of the old pot as a guide for the new circuitry cost.

Along the way, many amp builders, techs and hobbyists have tried to copy what we do. Most have since disappeared. One tech, Dana Hall, saw the original Classic-PS circuit in an amp he was to design a PCB for and decided to market his own kit. He called it "vvr" for variable voltage regulator. It is a Human foible that when we see how something is done, we inevitably say, "Oh... I knew that". One might have been aware of the basic circuit yet never did they apply it how we did.

Dana made his circuit simpler and changed the active current clamp to one that is hit and miss, depending on an unpredictable mosfet specification. It would have saved a component to leave this feature out. He also made a design choice that we described in TUT4 (The Ultimate Tone volume 4) as being "not preferred", because in some cases indirect control is preferred over direct control. For Dana this allowed another simplification and he sold his kit for a very low price benefiting a lot of players who wanted to make their amps quieter. He also sold his kits to a few amp builders who incorporated it into their products.

One of the problems with Dana's interpretation of Power Scaling, was that in most cases the whole amp is controlled. There are a few problems with doing this. One is that the amp tone changes with the power setting. This is distinctly not Power Scaling. The other problem is that every Volume pot that connects to a tube grid becomes "scratchy" when rotated. This is due to the changed DC current through the pot. The fix is to add a coupling cap to isolate the pot from DC, but this further requires that a grid-leak resistor be added for the tube grid as most guitar amps use the pot for that function.

The scratchy pot problem extends all the way back to the input where the guitar is plugged in. Most tube guitar amps do not have a coupling cap at the input; rather, they have a direct DC connection. The guitar pot is suddenly behaving quite rudely!

With Classic-PS as offered originally, none of those issues existed. Only the output stage was Power Scaled. Classic-PS had different issues that mostly made installation a little trickier, but once that was done correctly the tone stayed the same as one dialed the controls down. For the player, the main imposition was that there were two panel controls, Power Scale and Drive Compensation. Both had to be set about the same to retain the amp tone. They could be used independently to achieve three alternate performance ranges. Dana did not include the Drive Compensation control either out of further simplification or of simply not knowing why it was needed?

Other things, such as Power Dampening, were a copy of Mesa-Boggie's Limit control from a specific bass amp model. This varied the bias to the Schmitt splitter and thus limited drive to the output stage and subsequently of output power. This approach has the scratchy pot problem which we fixed in our SL-MV Splitter Limit Master Volume kit. We added three components to the existing one, quadrupling complexity, but making the approach actually useful for anyone that might need to change the control setting more than once per performance.

Marshall introduced a two-thirds form of Power Scaling on its Slash and Yngwie models. They followed the concepts presented in SSH Secrets & Secret Holders, but made an interpretive error in the execution. Their error is pretty common for techs and engineers not used to dealing with mosfets in power control positions in tube amplifiers for musical instruments. Yorkville Sound made the same error, although not in a variable power circuit, rather, in an active hum filter. In the Marshall amps, techs reported that if they disconnected the "Electronic Power Attenuator" that the amp sounded as it should, but once reconnected the amp sounded "stifled". Marshall combined the power control and drive compensation on a single control and that part worked inasmuch as the stifled sound was consistent over the loudness sweep.

In electronics there are countless ways to achieve the same goal and every tech, engineer or hobbyist will try to re-invent everything to put their own mark on whatever they are attempting. Sometimes, the best and/or easiest ways have been found. It was said once with respect to our Power Scaling kits, "Kevin O'Connor likes complicated circuits". It is not that I like them so much as I believe in Einstein's wisdom: "A thing must be made simple enough to achieve the goal, but no simpler" So, for me I do not want to sacrifice performance niceties, such as "smoothness of control" or player ergonomics just to save pennies, or to have an aesthetically simpler circuit.

All of the above is explained in much greater detail in TUT4 and TUT6.

Hi Guys

The bias regulator used in our Power Scaling kits requires a higher raw bias supply voltage than many stock bias supplies provide AND a lower impedance. Bias supplies derived from the plate supply through high-value resistors DO allow an outrageous voltage to be attained, up to the absolute value of the B+. However this is at high-impedance. To make the bias regulator happy, the dropping resistor has to be made much lower in value, usually by paralleling many more resistors. This leads to an excess of heat in the chassis. The output of this supply needs a zener clamp as a minimum to protect the pass element of the bias regulator.

Other situations allow the stock bias winding to be separated from ground and a voltage doubler circuit to be implemented. This works reasonably well in Hiwatts, some Fenders and some Marshall amps.

Using an auxiliary transformer wired backwards and powered by the heater supply eliminates all of the concerns with the other methods, while providing a high-voltage at medium- to low-impedance. Our RBX Raw Bias Auxiliary Supply kit provides -84V with a typical bias-set network attached to the bias regulator output. With no load its output can pop up to -110V, but this is okay for the BJT in the regulator.

Readers of the TUTs (The Ultimate Tone series books), this forum and our FAQ will know that it is always a good idea to have excess sweep of the bias controls to allow complete turn-off of every tube sample. Usually, having 15% of the absolute screen voltage is sufficient. For example, if Vs=500V then -Vb should extend to -75V; for 400Vs, -60Vb, and so on. The target bias voltage for large-bottle tubes and 6V6 is around 10%, but we do not want to make the mistake of some amp companies of ONLY providing that amount of bias voltage as some tubes will red-plate and some will be stone cold.

From the numbers above we can see that there is a limit of compatible B+ for RBX. The bias regulator BJT can go to having zero volts across itself with -84V output. This would correspond to a screen voltage of 560Vs. For screen voltages higher than 560V RBX is inadequate for the task and the tubes will be overbiased and likely red-plate. The solution is to use a larger PT in the RBX format, i.e. go from 6VA to 12VA, then the output voltage will rise to almost double. An amp with 750Vs needs -113Vb with standard tubes to assure a proper control range of the tubes. Even at 600Vs, as in a Marshall Major, we need -90V for adequate controls, so RBX is not quite good enough there.

Note that the RBX PCB is sized only for 6VA transformers, which is suitable for the majority of amplifiers. For Vs>560V you would need a separate PT and RBX-LT with higher-voltage caps than usually provided.

Going to the larger auxiliary PT allows us to reconsider the decision of wiring the new PT forwards or backwards.The forward wiring has no loss per se and the output voltage will be as one would expect, allowing the use of the same 6VA PT provided this will support the bias-set network et al. We would need a 115Vac or so secondary. The primary can be a single or duals - with the 229-series from Hammond every PT has dual primaries and secondaries out of necessity of the design. The primaries are wired in series or parallel, as required for your mains, and the secondaries are wired in parallel. We now have enough bias voltage for Vs=800V. For higher Vs we wire the secondaries in series and regulate it down to a reasonable range less than 200V so as to protect the bias regulator BJT

Have fun

Hi Guys

In the quest towards a Power Scale circuit with "ideal" control, I developed some alternative circuits, some similar to the current kits and others that are quite different.

As discussed in the "Power Scale pot sweep" thread, the nominal issues are to avoid having dead spots in the sweep at the ends of the pot rotation, and to have reasonable resolution of control at the quiet end. One would suppose that the ideal solution would be to use an opamp with a tailored response to potentially allow the use of a common llinear pot but achieve a modified log control shape? Opamps are wonderful devices and there are lots of low-power types available. However, they introduce their own problems and would still require interfacing to the high-voltage circuitry. Feedback would definitely be incorporated, and would have to be fancy to achieve the response mentioned, as well as be stable over the entire range of input voltages. The kit afterall has to accommodate the whole range of supply voltages of all the guitar and bass amps on the market.

Of course, it is simple enough to build a discrete high-voltage opamp with flexible input voltage capability, and that is one of the paths investigated. Over a supply voltage range of 200V up to 880V it worked perfectly after some refinements were added. In this design, both NPN and PNP transistors are used and even from the same manufacturer they are not available with the same voltage capabilities. Placing devices in series is a necessity - just like the current kits have - but three devices in cascode for the NPNs and two devices for the PNPs. This is only a minor inconvenience. For precision, all of the resistors are metal-film 1% 600mW types wired in parallel or series to achieve the required net values and to accommodate the heat dissipation at the highest input voltage. All of this adds up to a lot of components, so a "sandwich" board assembly is required, that is two PCBs that plug together and are parallel to each other.

Overall, the high-voltage opamp approach can be made quite small. I laid out the first version using cordwood style resistor installation. The solder pads were too small and too tightly spaced and I could not see myself soldering it, nor the average hobbyist. Someone with fantastic soldering skills and/or a magnifier, and/or maybe someone who does some surface-mount soldering would have no problem. Anyway, I laid out a second then third version to make it more "buildable", which has all the Rs lying down.

One of the amendments added to the circuit allowed "programmable" output voltage at the quiet end of the pot sweep. Four DIP switches allow this selection. There was a4-way rotary switch alternative that turned out to be very low-quality. Overall, the control board began as slightly narrower than the present SV1 board width of 2.3" (59mm) and ended up slightly larger at 2.6" (66mm). The power board changes for SV1 and SV2 applications. The two boards plug together with 0.1" header pins and receptacles, then two bolts with lock nuts to secure them safely.

I found that the heart of the programmable amendment could be added to the present kit circuit and that this greatly enhanced the sweep of the Power Scale pot. The minimum voltage is set to a fixed value and the pot sweep is very good. The board size for the SV1 increased by 0.15" (4mm) width and the SV2 remained at its present width. This keeps everything familiar for techs who have installed a lot of the current kits. These new versions are still called SV1 and SV2 as common designators, but have a smaller designation of SV1-M36 and SV2-M36. The 'M' designates a change to 1M pots for Power Scale and Drive Compensation, and the '36' refers to the 36k resistor in parallel with the PS pot. These boards arrive before the Holidays.

One other amendment to the new kits is the use of 3W resistors in some positions. These are marked on the schematic and the PCB as there are still two positions using 1W of the same value. As always, it is best to sort all the parts before assembly.

Have fun

Hi everyone!  I'm interested in hear what opinions about what speakers work best for using Power Scaling and playing at low volumes.  I've had mixed success selecting speakers for this task.  I find that many speakers seem to have "minimum volume" needed to get them to start sound like themselves. It isn't too loud for stage but too much for practicing alone.   Some speakers especially the Celestion speakers I've tried sound sluggish at low volumes.  I've had more success with mid powered American voiced speaker like vintage C12Ns but of course that speaker doesn't work for all styles or amps.  What speakers do you guys have success with when using power scaled amps at lower volumes?  Are there certain specs I should be selecting for?  Any insight would be helpful!  Thanks!

Hi all,

I used to have circuit cards that were rather easy to cut. Just score it a little and it would break failry clean.
But that was a long time ago.

The cards I have now will break rather ugly.
It will follow not the cut but the structure inside.

Does anyone have tips?

Warm regards,

Strlok

Hi everyone.  I'm looking for some input.  I had a custom-built amp come in the shop that essentially is one channel a 60's Fender Bandmaster with a three-tube spring reverb unit in front.  The transformers are a Super Reverb clone for the power and Bassman clone for the output.   The thing I'm concerned about is current draw on the high voltage secondary since the amp is now powering a 6K6 power tube in the reverb portion as well as the 6L6GCs.   

The power transformer has rating of 300 ma for the 325-0-325 secondary.  In a Super Reverb with a 5U4GB about 248 ma would be drawn leaving a healthy safely margin.  My best calculation is that this circuit would draw about 270 ma.  Is that enough of a safety margin? The 6k6 is drawing about 30 ma.  Should I be concerned?

Hello, TuTians,

I wasn't exactly sure which category I should post this question, but anyway, this is a question about a phenomenon that I have rarely seen addressed in guitar amplifier forums. And that is:

Which components or stages of a guitar amplifier should I direct my attention to, for modification or parameter tweaking, that would have the most success in enhancing the phenomenon of having that entirely desirable sort of spontaneous, sustaining feedback, that can happen while you are playing, that seems to be a natural by product of SOME certain rare amplifiers? What is it that I can change or modify about a circuit in a tube amplifier to build this kind of interactivity with the guitar into an amplifier? Is such a thing even known?

I KNOW that this kind of playability in an amplifier is possible, within reach, and NOT built with some special kind of "un-obtanium," because I had the opportunity to play through such an amplifier, ONE time. This was when I was in a studio during some recording sessions, in which I was tasked with dubbing in a little solo part into a section of a song. I only had my guitar with me that day so, the engineer just grabbed one of his friend's heads that happened to be at the studio that day. When I plugged into that amp, the tone completely floored me. I wasn't even ready for it! 

No. seriously. When I plugged my guitar into that amplifier, hit a chord to check for tune and then played a couple of notes, just merely bending into vibrato, the thing CAME ALIVE!  How easily it would just sing... and sustain perfectly into these notes that were some portion of the harmonic series, that seemed to work with every note I played. I was so astonished at the incredibility of the tone from that amplifier, that I actually DID NOT get to play on the solo in that song that day. 

I know this sounds silly, but every time I tried to do a take, I got completely distracted by this amazing electric tone that I had never heard coming from my guitar before. So much so, that those emotions totally devolved my playing into nothing but fits of laughter and Tourette's Syndrome like cursing, just stuck in postures of complete exasperation, high-fiving nothing but air; like some over-excited Patriots fan watching some epic touchdown. Embarrassing. I simply couldn't keep it together. And because time is money in a studio, after about 3 or 4 tries, I got quickly switched out to give someone else a try on the solo. (Which he nailed, by the way.) 

I'm sorry for such a long dramatic anecdote of my first time with a really great amp, but up until that day, I had NO IDEA that kind of tone was possible in a guitar amplifier. Of course, you guys will relate, because that kind of feeling is, after all, why we are all here. So, I guess what I am asking is, what do I need to do to get, build, modify or acquire an amplifier like that one? And I'm talking about getting that singing spontaneous feedback, that would just miraculously appear at the end of a run.

The gear I used couldn't have been more straight forward. That is why I KNOW that tone was ALL the amp. I had my own guitar with me that day, which was an all stock 1996 Japanese Fujigen made Fender, '68 reissue Strat in naturally finished swamp ash, and a maple neck. This guy's mythical amp that I plugged into was a non-master volume, non-plexi (i think), '68-72 or so, 100w Marshall Super Lead head, plugged straight into an older, slanted Marshall 4x12 cabinet. The engineer told me that the amp WAS modified in some way, but he didn't know how. And curiously, I WAS NOT playing at any deafening kind of level or even really loud at all.  

He had grabbed that amp for me to use because it was very similar to my OWN, which was a stock '69 100w Marshall Super Bass that I plugged straight into a vintage 30 loaded slanted Marshall 4x12 cab. But it didn't sound like that!

So, can that kind of feedback come from any type of amp? If so, what kinds of things should I focus on in an amp build, or WHICH KIND of amp build should I target in building to get that kind of feedback? Does it have to be a Marshall style circuit? I do love that Marshall kind of tone, by the way. Could I get something like that from a modified or tweaked 18w clone or a JCM800 micro kind of kit, or something else? 
(Am I playing 30 years and still an amp noob?   )

Thanks for ALL of your kind help, 

Scott

Hi I'm interested in knowing a bit more about the Z Pre-amp design.  From the description it says it's Soldano inspired?  I'm assuming that it does away with the cathode follower to lower the tube count and make the EQ more effective?   

I'm looking to be build something with the gain level of SLO-100 but smoother and fuller and sweeter sounding---a little less fizzy than the SLOs I've heard.   Will the Z-Pre amp work for that sort of thing?  Are there any suggestions about places in the circuit to tweak?  I'm interested to hear people's experience with it.  Thanks everyone!

Hiya!

I have access to a CNC router table at the local makerspace, and realized it can probably cut down on the time for me to fabricate some cabinets. The one thing I'm unsure about is the finger-joints. Assuming I'm only able to work with the bit perpendicular to the sheet stock, I'll have to cut "dogbones" for the internal corners of the finger joints (picture from here https://cutlasercut.com/getting-started/cnc-machining/):

 Now I've seen that there are ways to minimize their visibility (https://fablab.ruc.dk/more-elegant-cnc-dogbones/), but I was wondering if anyone has any strong opinions about dogbones in general for finger joints. Effects on strength, durability, air-tightness of cabinet, extra work to fix these issues, etc.

Thanks!

Hi Guys

Most modern solid-state power amps use symmetric supply voltages, commonly referred to as "split rails". This just means there is, say, +/-50Vdc supporting the circuit. In absolute terms each rail is the same voltage, only the polarity is different.

The supply voltage has to be sufficient to accommodate the peak signal amplitude into the speaker load at full load. So, the +/-50V will accommodate a 40Vpk signal into 8-ohms, corresponding to 200Wpk or 100Wrms. There is 10V extra, which accommodates losses over the transistors and current-sense resistors (emitter resistors). If the supply stays solid at 50V at full load, there should a few more volts possible into the load as we do not want to rate the power of the amp right at its limit lest it may not reach that value under low-mains or other conditions. On any case, the supply rails need to be in proportion to the power output desired.

Most PA circuits live within the same supply voltage as the output stage requires. However, there are some designs where the front-end circuitry requires a higher voltage, and these higher rails can be created in various ways. The two broad methods are supply stacking and having independent sources.

Supply stacking is exactly as it sounds: a second supply is stacked on top of the main supply. Where a typical PSU has a center-tapped winding, a full bridge over the whole winding and filter caps and bleeder resistors from each of the bridge outputs to the CT, the stacked supply adds another small supply on top of each rail. This smaller supply is often a small OPT with dual secondaries, where each secondary has a full bridge across it, followed by a cap and bleeder resistor. These small supplies are identical and one has its negative end tied to the positive main rail, while the second small supply has its positive end tied to the negative main rail. We end up with V+HI, V+, CT (0), V-, V-HI. Note that the V+/-HI are relatively low-current and would not be suitable in a class-G or class-H amplifier.

To size the small PT, we look at the current draw of the front-end circuitry. The typical differential input stage draws constant current. The transimpedance stage may have a constant current source for one side, but the other side may draw more current under load. Predrivers or driver stages draw modest currents but can sink and source much higher currents, so these should be assessed in simulation or in a real circuit. How much current the small supply must support depends on the circuit load, and we most often have drivers and predrivers supported by the main supply, alleviating some of the variable loading on the small supply.

We also consider the nominal voltage that the front-end circuit operates from. Because it sits atop the main supply, which will sag under load, we have to account for that sag plus any sag that may occur due to low-mains to both PTs. This generally means that the small supply voltage might be twice the difference we need between the idling main rail and the idling front-end rail. For example, a mosfet amp might have +/-65V main rails with +/-75V for the front-end. We intuitively view this as needing 10V boost supplies. If that is all we add, then as the 65V sags, so too will the 75V, and this may not be good for the stability of the quiescent point for the output stage, or possibly for the overall circuit stability.

If we double the boosted supply to 20V and add a ground-referenced regulator to provide 75V, then we accommodate up to 10V of sag for the front-end. The reality is that we need a bit more sacrificial voltage here.

As a reference,the inherent regulation of typical toroidal PTs is quite good, varying with the VA rating:
80VA   10%
120VA  9%
160VA  7.7%
225VA  7.2%
300VA  7.1%
500VA  5.1%
625VA  4.1%

Some front-end circuits incorporate active current sources while others use resistive ones. For the latter, a regulated supply maintains bias stability, but means the amp might be unstable while powering up. As mentioned, the regulator must be ground-referenced and it is simplest to use a discreet circuit for this. For active CS circuits, we may still wish to regulate the voltage, or at least have an active hum filter for each boosted rail.

The PT for the boosted supplies may be quite small depending on the circuit, maybe 10-15VA, but could be up to 50VA for a large amp or when significant drive currents must be supported by the boosted rail. Each boost supply is derived from one secondary, so whatever the current and voltage product is for one boost supply, we must double it for the PT VA rating.

The small PT VA goes up proportionately if we use it to create fully independent high rails that are ground-referenced. The circuit is identical to the main supply, just a smaller PT with higher voltages.

The boost supply PT can be a small toroid, semi-toroid or EI type. EMI from this PT should be low as the load is balanced over the secondaries and fairly constant.

Have fun