Master Volume Placement on a Bogen CHB35A

[Freddy]

I started working on a Bogen CHB35A with the intention of turning it into a guitar amp. I semi gutted the working Bogen CHB35A and removed all the extraneous signal path wiring (jacks, pots, wiring for aux input, mic 2 input, etc). I cascaded the preamp tubes..1st half 6eu7--2nd side 6eu7--1st side 12ax7-2nd side 12ax7 and then into the 6c4 phase inverter. This amp now has a single input. I have the volume control between the 1st and 2nd half of the 6eu7 tube. The tone stack is inbetween the 1st side and the 2nd side of the 12ax7. At this point the amp was working fine. It sounds a little dull but I wanted to leave the tone tweaking for later.
I wanted to be able to bring up a little preamp distortion at times so I thought I would place a master volume pot right before the phase inverter. Once I did this the amp lost about 70% of it's volume and I started to get some squealing (high frequency oscillation) when the master volume was turned up. When I take the pot out of the circuit it goes back to normal operation. I thought about placing the master volume pot after the 6c4 phase inverter but I do not have a dual gang pot handy and I do not think the little 6c4 is amplifying the signal anyways.
I might try grounding the master volume pot in a different spot or moving the pot to a spot right after the tone stack. I thought I would ask for suggestions or guidance. Any suggestion would be appreciated.

[MrMike]

If it is squealing, you are getting parasitic oscillation. Perhaps one of the MV wires is close to an output wire from the PI, preamp or power tube? Perhaps too close to the OT wires? try a chopstick to see if the oscillation will go away.

If you loose 70% of the volume, use a higher value pot for the MV - 1meg?

mm

[Freddy]

Thank you for the suggestion.
The pot I tried for the master volume was one meg..I placed it right after the decoupling capacitor and right before the PI tube grid. I am going to try moving some of the wires around and checking to see if everything has a proper ground. This amp is point to point and it is a rat's nest. I can't figure out why there is a power loss though. I am gong to take the master volume pot back out and see if that solves the power loss problem for the time being.
Before I started my rewiring the amp had a little bit of high pitched squeal when it was turned up and it is much worse after the rewiring. I thought I had a bad tube so I swapped them out with some I knew were good and the squeal was still there. Could this be solved with a few 68K resistors between the gain stages?

[Aharon]

The best thing to do is use all the parts you can and build a brand new amp.
The point to point wiring in those amps was never meant for the gain of guitar amplification.
I rebuilt 4 of those.
Aharon

[Freddy]

I am not quite ready to do a build from scratch. I wanted to get my feet a little wet by modifying something that was in working condition before diving in. That is why I started with a working amp. I will eventually get to a build from scratch and this Bogen may become a parts donor. I have repaired a few Gibson amps that were pretty much a point to point rat's nest but these were not what I would consider high gain amps.
I thought some of these Bogens were a popular item to modify although the transformers look kind of wimpy to me. I am not looking to make a metal distortion monster...just something I can plug into and play a little jazz and blues with.

[Freddy]

I think I have a handle on my problem. One bad ground, a piece of unshielded wire too close to the output tube socket, and way too much gain going on after adding another gain stage and removing two of the one megohm pots out of the circuit.

[tubetwang]

few Bogen links...

http://www.rru.com/~meo/Guitar/Amps/PA2Guitar/

http://richardstevens.home.mindspring.c ... index.html

Aharon has a good point.

I used one of those. Kept the transformers only.

[Freddy]

Thanks for the links. I looked over the PA2Guitar link a while ago and that is what inspired me to mess around with this Bogen amp. The other link I had not seen before. The Bogen amp was working well, has good tubes, and has been recapped recently so I really did not have the heart to gut it. Up until this point, I have only done repairs on amps like recapping, replacing tubes, replacing ac cords, etc. If it wasn't broke I left it alone. I thought I would get my feet wet by messing around with the Bogen before I started one from scratch. I have a couple of Filmosound amps (with 6V6 tubes)that I plan to hack up for a build from scratch after I do a little more lurking and reading in this forum.
I straightened out my problems with the Bogen amp this evening and placed a master volume right after the tone stack and it works pretty well. Moved some of the wires around and relaced most of the signal path with shielded wire and now it is quiet as a mouse. I can get some of the bluesy distortion I wanted now. There is still a little too much gain in the preamp section I am going to try to tame along with a rewiring of the tone controls.