Help with Peavey Tube Sweetener

[imloggedin]

This is getting ridiculous. Sorry, but I had to edit my post.
regards
Bernd

Oh my bad you said VR1A. I just saw VR1

 

[imloggedin]

Hi,
in response to your pm. There is not much left then.
regards
Bernd

I bypassed everything like was suggested by Bernbrue. Then I put a 100k pot in the mix. One way I shunted it to ground to bleed off the signal (it worked as volume).. the other way I used it as a resistor in series with the signal, and it did nothing when turning it. I'm pretty sure as John said that bleeding signal to ground causes the LPF. When I turn down any of the volume knobs (thus bleeding more to ground) the LPF starts happening. When I turn them up, the LPF goes away, but then the signal distorts (tubes driven hard). Same applies to VR1A and VR3. I don't know of any other way to attenuate signal without using ground. John, I believe you suggested using R1 as a pad, but I don't know if that can be variable on a pot cause its 275v? UGHHH

 

[emrr]

Did you bypass V1A like I mentioned?  That'll definitely turn down the gain.  Any time you turn down a pot in a high impedance circuit, even more with high mu tubes, you start losing high frequencies.  And R17?

 

[JohnRoberts]

Hi,
in response to your pm. There is not much left then.
regards
Bernd

I bypassed everything like was suggested by Bernbrue. Then I put a 100k pot in the mix. One way I shunted it to ground to bleed off the signal (it worked as volume).. the other way I used it as a resistor in series with the signal, and it did nothing when turning it. I'm pretty sure as John said that bleeding signal to ground causes the LPF.

No HPF...  loses bass from pole with C2

When I turn down any of the volume knobs (thus bleeding more to ground) the LPF starts happening. When I turn them up, the LPF goes away, but then the signal distorts (tubes driven hard). Same applies to VR1A and VR3. I don't know of any other way to attenuate signal without using ground. John, I believe you suggested using R1 as a pad, but I don't know if that can be variable on a pot cause its 275v? UGHHH

I made many suggestions, I don't feel like repeating them. They shouldn't be hard to find.

JR

 

[imloggedin]

Did you bypass V1A like I mentioned?  That'll definitely turn down the gain.  Any time you turn down a pot in a high impedance circuit, even more with high mu tubes, you start losing high frequencies.  And R17?

I did not bypass that. I am not sure what you mean by bypass. I can remove it from R13 and set the pot to 0 ohm. Is that what you meant by bypass? Or do you mean jumper from before R5 to before C6? I am not sure what purpose that would serve though (not being arrogant, just dont understand)? I can jumper over R17. Its quite difficult to completely remove a pot from the board though without just ripping it apart.

 

[emrr]

Ok, you've misunderstood twice now.  V1A, the first 12AX7 stage, not VR1A the pot.  Lose it.  VR1A/3 to C7.  Disconnect the stuff in between those two points from those two points.  Gain will go down. 

That after trying the R17 strap-out first, it may help the high freq situation, may not. 

 

[imloggedin]

Ok, you've misunderstood twice now.  V1A, the first 12AX7 stage, not VR1A the pot.  Lose it.  VR1A/3 to C7.  Disconnect the stuff in between those two points from those two points.  Gain will go down. 

That after trying the R17 strap-out first, it may help the high freq situation, may not.

OK. I did what you said. Removed C6 and R17. I jumpered between them. Removed R1 so I'm not providing voltage to the tube still. This left me with 1 working knob for level (VR1). It seems to be pretty flat although I can still hear roll off above 16k, and its pretty gritty sounding for the most part. I am not sure if it will work on my 2bus or not but I am not going to mess around with it anymore. I'll set the other channel up the same and play around with it. This first attachment is the knob at 100%

Thanks for your suggestion EMRR, youre not so bad after all LOL  8)

 

[imloggedin]

Knob at 75%

 

[imloggedin]

Knob at 0. Wish i knew howto attach multiple pictures to a post 

 

[emrr]

Yep. 

Bypass R5.  No reason to have it.  Might make treble even better. 

Experiment with small cap (0.00Xish) between R23/C11 and ground.  Will boost high frequency, you might find a value that doesn't increase too much at full pot, and helps at lower settings. 

I'd leave R1 in.  It still has filament cooking away, so lack of B+ means there's no plate to suck stray electrons out of the vacuum.  At any rate you aren't saving that half of the tube, probably making it worse. 

Then gut the whole thing and put something better sounding in it like 12AU7, with a design meant for it.  12AU7 in a similar correct circuit replacing V1B/V3A should be better in every way.