Bugera 6262 master volume mod
« on: October 23, 2019, 04:24:29 PM »
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
« Last Edit: October 23, 2019, 04:40:13 PM by kingkorg »


CJ

Re: Bugera 6262 master volume mod
« Reply #1 on: October 23, 2019, 08:28:20 PM »
like this>
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
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Re: Bugera 6262 master volume mod
« Reply #2 on: October 23, 2019, 08:31:08 PM »
AWESOME! Thank you! What pot value do you recommend?

iampoor1

Re: Bugera 6262 master volume mod
« Reply #3 on: October 24, 2019, 02:09:21 AM »
500k or 1m is a pretty standard PPIMV value.

Re: Bugera 6262 master volume mod
« Reply #4 on: November 11, 2019, 09:47:30 AM »
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.

squarewave

Re: Bugera 6262 master volume mod
« Reply #5 on: November 11, 2019, 11:09:50 AM »
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.

Re: Bugera 6262 master volume mod
« Reply #6 on: November 11, 2019, 12:01:57 PM »
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.
« Last Edit: November 11, 2019, 12:42:39 PM by kingkorg »

squarewave

Re: Bugera 6262 master volume mod
« Reply #7 on: November 11, 2019, 01:37:15 PM »
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.


 

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