Effect of raising and lowering B+ - diyAudio
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Old 29th October 2015, 08:00 PM   #1
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Default Effect of raising and lowering B+

I recently started poking around and modding an EL34 PP amp that I've been using for the past 7 years or so. I measured B+ from the supply at 500V which was surprising given that the last stage PSU cap that the B+ is taken from is only rated at 450V.

After poking around some more it looks like someone (previous owner perhaps) modded the PSU to draw from the 400V xfmer output tap instead of the 360V tap, thereby raising the B+ and pretty much over-volting all of the caps in the amp by 20-60 volts.

My question is, has anyone experimented with increasing or decreasing supply voltage and its affect on the sound quality? This amp sounds pretty good so I'm not sure I want to mess with it. Is it still safe to continue using the amp in this configuration, considering all the caps are being run at or slightly above their ratings? On the other hand it's been run like this for nearly a decade and nothing has blown up so...
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Old 29th October 2015, 09:09 PM   #2
stajo is offline stajo  Sweden
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I dont think anyone has tried that yet
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Old 29th October 2015, 09:18 PM   #3
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I tried...I like more tube current with lower +U, than bigger +U with low tube current.....according max. plate power.
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Old 29th October 2015, 10:04 PM   #4
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It is NOT SAFE to continue to operate caps over their rated voltages.
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Old 5th November 2015, 07:54 PM   #5
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Wouldn't this also increase the gain of the amp?
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Old 5th November 2015, 07:58 PM   #6
rayma is offline rayma  United States
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Originally Posted by simplefi View Post
Wouldn't this also increase the gain of the amp?
If used, a global nfb loop will keep the overall gain nominal.
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Old 5th November 2015, 08:52 PM   #7
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Originally Posted by simplefi View Post
Wouldn't this also increase the gain of the amp?
i think it would cause distortion because too much or too little plate voltage changes the design parameters of the amp by creating more or less current flow causing the load line to shift away from the most linear portion of the tube characteristic curve. you would have to change the bias point and the total grid voltage swing to correct this problem but then you would end up with less amplification.

increasing plate voltage would increase volume, i think, but also cause a decrease in fidelity.

im just a noob.

Last edited by chopchip; 5th November 2015 at 08:55 PM.
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Old 6th November 2015, 09:18 AM   #8
DF96 is offline DF96  England
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Increase in voltage can cause a reduction in life for both caps and valves. It will probably allow more power output. Distortion may or may not change, and it could go up or down, depending on the details of the amp design.

Generally a bad idea.
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Old 6th November 2015, 08:06 PM   #9
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General speaking here. With my guitar amps on the pre-amp tubes; reducing the plate voltage & helps to generate more harmonics & raising the plate voltages typically cleans up the amplified signal. All the time with the same cathode resistor/capacitor combo.

With the final amplifier section; I do the same thing to vary the sound except that it's important to know where you place the tube in the positive & negative sine & what the compression level will be. I raise the cathode resistor when I raise the voltage (well over Max spec) to move into a cleaner tone placing the tube in a more positive sine. The more the tube is away from the negative sine, typically the more clean sound it will pass. The higher the cathode resistor, the less current the tube will conduct, however then lowering the plate resistor & then you thus raise the plate voltage (as you question) will cause more gain as already stated. Exceeding the optimum gain will again enter into the various types of distortion, however slightly different then operating in the negative sine as mentioned above.

Now this is with my specific design operating down in the "brown sound" region (low pre-amp B+) with class A / SE configuration. With the class AB (Push-Pull) one can work with current caused B+ "Sag" and go from low drive (high B+) clean to heavy driven brown (low B+) sound with a high enough plate/bias resistor.

The best way is to do all the reading you can get on the subject as you are doing now, and then acquire a decade box full of resistors, variac (of which you may already posses) and start switching up resistors. I have 5 decade boxes, 3 resistor & 2 capacitor & still end up soldering in other swappable parts dialing in tone.

So you see in my world no component is safe! Double up those 450 Vdc caps in series for a 900 volt rating. Then measure the AC component on the DC line to watch how bad it gets. Add 2 more in parallel if you need to. Caps can be bombs, ask my plastic army men (when I was 10) what a 5Vdc, 2000mfd cap does when over' volted. I refuse to pay x 4 for any cap over 450Vdc rating.


Last edited by MaicoDoug; 6th November 2015 at 08:11 PM.
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Old 6th November 2015, 11:27 PM   #10
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I've tried running a circuit well below B+ volt spec'd (400-0-400) once due to a problem with a transformer, one leg of the high voltage pair I wanted to use was more than 25% over spec'd (like 460+) and didn't match other so, I wired it to run the lower voltage of the 2 HT pairs, which was balanced, but a bit more than 15 or 20% spec'd for the circuit/tubes - like 350-0-350, maybe less, have to look it up... anyhoo -

It ran/made music, but had something missing (headroom) and it ran hot, meaning the iron/transformers/chokes. The Transformer provider (ed-something ;^) fixed it and now this thing is running at spec'd voltages and the iron runs much cooler. This was a 300b (and still is, although being experimented still). I bet your caps are good quality, were under rated but still, running anything way out of spec, higher or lower, is a bad idea, esp. caps and NOS tubes. Have fun and be safe.
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