Tone stack question.... sorta

GlennLewis · February 16th, 2006, 04:57 PM

Okay, I'm reworking an old tube amp, from what I can remember sitting here at work without the amp in front of me is: I've got (2)12au7's in the preamp, a 6au6 as the phase inverter, and dual 6v6gt's in the power section. 3 controls, volume, treble, and bass.

Now the "problem/question" part. The treble control is too bright and controls too much of the output. The treble control seems more like a volume control on the treble part of the frequency response, and goes from bright to make-your-ears-bleed. And the bass pot seems to do very little, and the amp doesn't have much bottom end at all. Both the treble and bass controls are 2.2MOhm pots, and I cannot recall the cap values for each control.... I'll post that in a couple of hours when I'm looking at it again.

What I'd like is to get some additional low end out of it, and so I am wondering about the following.

Replace the 2.2MOhm pots with different value and taper?
Change the cap values to alter the bandpass?
Remove the coupling cap between the ground lug and the ground on the bass pot?

Any thoughts and suggestions are much appreciated.

Tremo · February 16th, 2006, 05:04 PM

It's unlikely that the 6AU6 is the PI.

Without a schemo, any advice would be an absolute shot in the dark. Specifically, what amp is it, and we need a schemo. You need to help us to help you.

tkdrew · February 17th, 2006, 01:51 PM

It is likely that the coupling caps between stages could be part of your problem too, too small cap will cut a lot of low end. Also could change the cathode bypass caps(if it has any) to larger values to get more low end. If all else fails you could add a low value cap to the circuit going from one side of the output transformer to ground to help roll off the high end, just be sure to use a high enough voltage rating. Hope this helps, you might try posting at the ax84.com site-- lots of help and good info there.

Quote:

Originally Posted by GlennLewis

Okay, I'm reworking an old tube amp, from what I can remember sitting here at work without the amp in front of me is: I've got (2)12au7's in the preamp, a 6au6 as the phase inverter, and dual 6v6gt's in the power section. 3 controls, volume, treble, and bass.

Now the "problem/question" part. The treble control is too bright and controls too much of the output. The treble control seems more like a volume control on the treble part of the frequency response, and goes from bright to make-your-ears-bleed. And the bass pot seems to do very little, and the amp doesn't have much bottom end at all. Both the treble and bass controls are 2.2MOhm pots, and I cannot recall the cap values for each control.... I'll post that in a couple of hours when I'm looking at it again.

What I'd like is to get some additional low end out of it, and so I am wondering about the following.

Replace the 2.2MOhm pots with different value and taper?
Change the cap values to alter the bandpass?
Remove the coupling cap between the ground lug and the ground on the bass pot?

Any thoughts and suggestions are much appreciated.

David Barnett · February 17th, 2006, 09:01 PM

Quote:

Originally Posted by Tremo

It's unlikely that the 6AU6 is the PI.

A single 7-pin pentode can't do the job? :)

I suppose they could have wired it up as a cathodyne splitter...

Tremo · February 18th, 2006, 04:37 AM

Quote:

Originally Posted by David Barnett

Quote:

Originally Posted by Tremo

It's unlikely that the 6AU6 is the PI.

A single 7-pin pentode can't do the job? :)

I suppose they could have wired it up as a cathodyne splitter...

I stand by my comment.

GlennLewis · February 21st, 2006, 02:03 PM

I should have spent more time diagramming this amp before posting the question. The 6au6 is being used as part of the pre-amp section straight off the volume control, then the treble and bass controls are seperately amplified, each with their own "half" of a 12au7. The second 12au7 is used to drive the screen grids on both 6v6's, and the second half of the second 12au7 is being used to drive the control grid of both 6v6's (feedback loop?).

Kind of a wierd amp, not done diagramming it yet, and I surely do appreciate all the help and suggestions.

Sounds pretty good, but it's an oldy with some hum, so my next question is going to be about replacing the cap can.

Thanks again!

Tremo · February 21st, 2006, 04:34 PM

Say what?? The screens and control grids are BOTH being driven by the audio signal?? HUH?? Very, VERY strange. Schemo please.

February 16th, 2006, 04:57 PM	#1 (permalink)
GlennLewis Tele-Holic Join Date: Mar 2003 Location: Chicago Posts: 542 4 Photos	Tone stack question.... sorta Okay, I'm reworking an old tube amp, from what I can remember sitting here at work without the amp in front of me is: I've got (2)12au7's in the preamp, a 6au6 as the phase inverter, and dual 6v6gt's in the power section. 3 controls, volume, treble, and bass. Now the "problem/question" part. The treble control is too bright and controls too much of the output. The treble control seems more like a volume control on the treble part of the frequency response, and goes from bright to make-your-ears-bleed. And the bass pot seems to do very little, and the amp doesn't have much bottom end at all. Both the treble and bass controls are 2.2MOhm pots, and I cannot recall the cap values for each control.... I'll post that in a couple of hours when I'm looking at it again. What I'd like is to get some additional low end out of it, and so I am wondering about the following. Replace the 2.2MOhm pots with different value and taper? Change the cap values to alter the bandpass? Remove the coupling cap between the ground lug and the ground on the bass pot? Any thoughts and suggestions are much appreciated.

February 21st, 2006, 02:03 PM	#6 (permalink)
GlennLewis Tele-Holic Join Date: Mar 2003 Location: Chicago Posts: 542 4 Photos	Tremo, you're right I should have spent more time diagramming this amp before posting the question. The 6au6 is being used as part of the pre-amp section straight off the volume control, then the treble and bass controls are seperately amplified, each with their own "half" of a 12au7. The second 12au7 is used to drive the screen grids on both 6v6's, and the second half of the second 12au7 is being used to drive the control grid of both 6v6's (feedback loop?). Kind of a wierd amp, not done diagramming it yet, and I surely do appreciate all the help and suggestions. Sounds pretty good, but it's an oldy with some hum, so my next question is going to be about replacing the cap can. Thanks again!

February 16th, 2006, 05:04 PM	#2 (permalink)
Tremo Banned Friend of Leo's Join Date: Mar 2003 Location: Silicon Valley, CA, USA Posts: 3,803	It's unlikely that the 6AU6 is the PI. Without a schemo, any advice would be an absolute shot in the dark. Specifically, what amp is it, and we need a schemo. You need to help us to help you.

February 21st, 2006, 04:34 PM	#7 (permalink)
Tremo Banned Friend of Leo's Join Date: Mar 2003 Location: Silicon Valley, CA, USA Posts: 3,803	Say what?? The screens and control grids are BOTH being driven by the audio signal?? HUH?? Very, VERY strange. Schemo please.