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1977 Deluxe Reverb. Schematic calls for a power transformer input of 120VAC, and a secondary HT winding producing 330VAC (x2, center-tapped). This gets rectified by a 5U4GB (which consumes 50VDC in the process), and is filtered to produce approximately 420VDC B+, and 415V on the power tube plates.

My original power transformer appears to have been wound 'hot'. At approximately 120VAC wall voltage, my HT winding is at 377VAC, not 330. The downstream effect is that my B+ is closer to 465VDC. At the same time, the heater winding is right where it should be, 6.3-6.4vac, at 120-121vac input. So it seems isolated to the HT winding.

The amp works fine, and has for years, but in the interest of learning and experimentation, I'd like to bring the B+ down to spec levels. I'd like to drop 35-40VDC from the B+.

I've googled and come up with shunt regulator, bucking transformer, etc. I don't know what these mean in practical terms, in the context of a vintage tube amp. I'm hoping there's some component, or small analog circuit, I can place after the rectifier and filtering, and before the standby, to reduce the voltage.

I'm aware there are other ways to reduce the B+. I don't want to apply a variac to reduce the input voltage, because it affects the heaters, too. And for this discussion, I don't want to swap the 5U4GB rectifier for a 5R4GB. While feasible, it would only drop a further 10-15V. Also, it's my understanding that this solution would introduce more 'sag' in the power supply. Finally, the capacitor input voltage of the 5R4 is only 20uf, and my (spec) primary filter caps are (total) 32uf. Probably not an issue, but still. In short, I'm trying to get the amp closer to spec, not farther.

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    \$\begingroup\$ It isn't at all likely that the transformer was wound 'hot'. Are you using a 5U4GB? Most probably you're using another rectifier, with less voltage drop. Another possibility is a shorted primary turn, which would upset the turns ratio. \$\endgroup\$
    – user207421
    Feb 17, 2017 at 0:49
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    \$\begingroup\$ You could put a 36V Zener in series. But I strongly recommend to leave well enough alone. If it hasn't blown up yet it probably won't. \$\endgroup\$
    – user207421
    Feb 17, 2017 at 1:15
  • \$\begingroup\$ Likely or not, I'm measuring the vac right at the secondary. No question it's putting out 377 on each side of the center tap (actually the sides differ by a volt). This is before the rectifier, but yes, as I say above, I am using a 5U4GB. If my turns have a short, intuitively it would act like fewer winds, not more, right? Regardless, I have no desire to replace the PT unless I'm forced to at some point. The amp works fine. I'd just like to lower the B+. \$\endgroup\$
    – joe
    Feb 17, 2017 at 5:23
  • \$\begingroup\$ Is it putting out the correct heater voltage? Or is that also 10% too high? \$\endgroup\$
    – user207421
    Feb 17, 2017 at 5:27
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    \$\begingroup\$ What about another 5U4GB in series to drop another 50V? \$\endgroup\$
    – KalleMP
    Nov 5, 2018 at 16:18

7 Answers 7

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There are many ways of reducing the plate voltage on valve equipment .I have done this on radios but not on a guitar amp .The most common way I have done this is to place experimentaly 2 100 ohm 5 watt wirewound resistors in the HT transformer secondary .One in series with each leg before the rectifier .You will notice the HT drop .Now you can ballpark calculate your final resistor value which in general wont be 100 ohms .The resistors get pulsating DC currents with a high peak to average ratio which is why the correct value is lower than most people think.The total power that the resistors waste is the same as a resistor in the DC side or a series Zener or a linear reg .If your total filter cap is generous which is the expectation of high Quality Audio and you are doing a lot of class A then your concerns about droop need not be concerns .This AC HT resistor approach does have the advantage of providing additional protection .Although I have not seen this I have heard about rectifier tubes flashing over and ruining HT transformers .The AC resistors limit fault current and hence Arcing and they will fail open circuit .The AC resistors make the rectifier current pulses slightly broader making the transformer copper slightly cooler .

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  • \$\begingroup\$ This is interesting. Not only does it address the root problem, instead of adjusting the B+ later, I'm guessing this won't introduce the noise that the Zener will, since this is pre-rectification, and pre-filtering. Are you saying it's a smaller resistor than one might need for the DC side because we're talking RMS, not peak DC? Anyway, are there other things to consider in this approach? What about heat? Placement re noise? Anything to avoid? Thanks! \$\endgroup\$
    – joe
    Feb 17, 2017 at 19:04
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    \$\begingroup\$ @joe .The resistors I have used have been wirewound .This is so they dont blow on surges .This was 30 years ago when most resistor manufacturers did not say anything about spike power ratings .Wirewound resistors have good surge ratings.Because you are using a vacuum tube rectifier the surge issue is not so bad but you have a guitar amp which is more grunty than a radio so use wirewound resistors to be safe .Put the resistors near the rectifier tube socket which will be away from the preamp and you will not increase hum .Your HT ripple will be slightly less with the resistors than barefoot . \$\endgroup\$
    – Autistic
    Feb 17, 2017 at 20:23
  • \$\begingroup\$ Solutions using resistors will introduce 'sag', as you (@joe) put it. \$\endgroup\$
    – gbarry
    Oct 24, 2017 at 21:31
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there probably isn't a whole log of current going through those things. If so, put some bulbs, or even LEDs in there.

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  • \$\begingroup\$ Hmm, 10 white LEDs does sound rather tempting as a case mod :-) \$\endgroup\$
    – KalleMP
    Apr 30, 2017 at 14:29
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You can lower the voltage by adding a huge 40V Z-Diode into your B+ supply.

Obviously these diodes will disipate a lot of heat and need a heat-sink.

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    \$\begingroup\$ A zener will introduce some noise on this supply, might filtering be required afterwards? \$\endgroup\$
    – Colin
    Feb 17, 2017 at 9:15
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    \$\begingroup\$ @Colin__s I would expect a healthy amount of RC filters in any tube amplifier B+ supply, so noise is a non-issue here. Good call in general though. Zeners are noisy. \$\endgroup\$
    – Nils Pipenbrinck
    Feb 18, 2017 at 4:55
  • \$\begingroup\$ @Colin__s It's a push-pull amplifier. Whatever noise it introduces should be cancelled. \$\endgroup\$
    – user207421
    Feb 18, 2017 at 9:04
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Honestly, I would leave things just as they are.

One benefit of higher supply voltage is more output power before clipping occurs.

FWIW - I used to run my old tube-type amplifiers Really hot, with excellent results.

So long as the filter caps are holding up, just leave it alone.

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    \$\begingroup\$ Respectfully, I'm not asking for opinions on what sounds best. I'm asking how one would lower the B+ without resorting to the usual tactic of swapping rectifiers, which has side effects. It's an experiment. I'd like to see the amp operate in it's "designed state". \$\endgroup\$
    – joe
    Feb 17, 2017 at 5:23
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Putting resistors in series with each leg of the secondary windings of the PT will drop voltage but don't forget the power rating of the resistors - when dropping 20Vac on each each leg with a resistor, 5w may be insufficient. The power dissipation of a 100Ohm resistor with 20V dropped across it is: P= Vsquared/R = 20Vx20V/100R = 4W. If you require a smaller resistor to drop the 20V the power dissipation will increase, so calculate the power dissipation and select a resistor accordingly.

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Put a couple of zener diodes in series with the red/yellow power transformer center tap to ground. These need to be reversed polarity in order to pull the voltage down.

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  • \$\begingroup\$ Bruce Collins of Mission Amps originally suggested this proven technique. \$\endgroup\$
    – Dave Adams
    Feb 29, 2020 at 5:12
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Adding resistors to the B+ or on the leads of your PT secondary is a poor option. Remember, a voltage drop occurs in a resistor in relation to the current that is flowing through the resistor. So your voltage drop will be all over the place, not to mention the extra sag you introduce to your power supply. Very inaccurate. Large Zener diodes in the 40v / 40 watt range are very expensive, so you might as well spend a little more for a new PT of the right specs of you were to go that route. No. Your best option by a mile is to build an "amplified Zener", mosfet-based circuit and place it between your high voltage tap and ground. It costs all of about $3 in parts.

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