Bugera 6262 master volume mod

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kingkorg

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Apr 15, 2017
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Hello kind people of GroupDiy.

So i've got this awesome amp which is almost 1:1 replica of Peavey 5151 II.

I was wondering what would be the best spot/value for another master volume pot after the FX loop.

I am using it as a power amp with some other pres, and it's often too noisy when i send stuff to it, or as FX return. It would be great to be able to crank external FX, but lower the master on the power amp.

Schemo:
https://groupdiy.com/index.php?action=dlattach;topic=73585.0;attach=70319
 
Just got done with the mod, i wanted to report it works like a charm. I get very different tones out of the amp now with stock volume cranked and attenuated with this post inverter volume control.

I checked, the added components don't affect the sound on their own, the amp sounds exactly the same when pot is attenuating and at full. It seems the phase inverter tubes sound different when pushed harder. More saturation and midrange.
 
Note that PPIMV breaks the presence control because the feedback is lost. So the amp will not sound "tight" like it should with the presence up. You're also not pushing the power tubes which can affect the sound of course. Dummy loads and power scaling is the proper way to limit level but there's really no substitute for just cranking it.
 
Wait, is it killing presence function even when PPIMV pot is at maximum?

It could be what i am hearing is absence of presence then, therefore more mid range.
 
kingkorg said:
Wait, is it killing presence function even when PPIMV pot is at maximum?
No. If MV is at max, it should function as if the MV was not present.

Understand that the "presence" control is actually applying some feedback from the output into the inverted input of the PI circuit which significantly linearizes the signal and flattens the frequency response. It's a very important part of the circuit. So if you put a level control in that loop, you're not just adjusting the overall output level, you're also adjusting the feedback. Less feedback yields less linearity and less flattening of the frequency response.

OTOH, if you clip the output tubes, that also breaks feedback (because the clipped part is lost) and can make the amp sound harsh. So the presence control is supposed to be (circuit originates from bassman / plexi amps) used when you're not driving the power section super hard. It's common for guys to just crank all knobs and play, but if the power amp is clipping hard, it can be better to turn down / off the presence.

One plus of the PPIMV is that if you use a largish pot like 1M, as you back off the MV from near maximum, you still get a high level signal into the power tubes but with significant series resistance. That series resistance can affect blocking distortion which occurs when the grid starts to conduct, charging up the 100n. So that's something to try if you (and your neighbors) can tolerate it.
 
Hi guys. I'm not a newb but it's been years since I've done any mods or reading of schematics. I have one of these amps I'm interested in doing this mod to. I have a question about the schematic. Im assuming these are 0.1 uf capacitors. Where I have lines drawn on the schematic I re-uploaded, I understand that's the middle lugs on the pots. The one cap leg is soldered to the middle lug, where does the other leg go?
 

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Microfarad, but these aren't really needed since the high voltage had already been removed by C45 & C56. The attached pic illustrates how I usually do mine.
 
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Microfarad, but these aren't really needed since the high voltage had already been removed by C45 & C56. The attached pic illustrates how I usually do mine.
Hmmmmmm....am I missing something? Without caps on each wiper, the bias voltage will vary with the position of the pot, dropping to zero volts when the pot is all the way "off".

Bri
 
these aren't really needed since the high voltage had already been removed by C45 & C56. The attached pic illustrates how I usually do mine.
When the MV is cranked down, the grid bias is shorted to ground, which results in blowing the output tubes (and maybe frying the output xfmr).
That may work on a cathode-bias output, but I wouldn't do it on a fixed-bias.
 
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Folks - thanks for the correction and potentially saving someone from doing something stupid (I edited my post). Sorry, I got in a hurry and posted an incorrected schematic that I had cut and pasted together some time ago and forgotten about. The power tubes in my amps were actually cathode biased and don't seem to have an issue, but probably should play it safe and use the 0.1uF coupling caps there too.
 
I know you guys have lives outside of this forum and I'm sure you're just as busy as me with work. If anybody has the time and could do a proper edited schematic of exactly what I need to do I would so greatly appreciate it and would pay $10 as well for your time. If anybody is interested talk amongst the group and figure out who is going to do it.

I mean absolutely zero disrespect to the OP of the 1st edited schematic. I sat down with an electrical engineer from work and we both were able to make sense of what we saw but still had some questions. I don't want to have to keep coming back asking a million questions and I REALLY don't want to blow anything. Guitar is my lifeline and my amps, well if you're a musician like me you know... Thanks in advance
 
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