Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 67polara on April 07, 2011, 10:14:37 pm

Title: 72 twin reverb master volume pot
Post by: 67polara on April 07, 2011, 10:14:37 pm

I have a 72 Twin Reverb with master volume that has a dpst pull switch and 70k tap on it, is this pot available?  If not what happens if I don't use one with a tap and leave the cap out?  Hey and what does the pull switch do exactly?  When I pull it out I don't notice any change.

Tony

Title: Re: 72 twin reverb master volume pot
Post by: navdave on April 07, 2011, 11:35:22 pm

Looks like the pull option is a dark switch if the amp is clean you probably wont notice much change.
If the pot aint bad dont worry it should out last you if it croaks a regular 1M pot with a DPDT switch should work with a
a little modding any tech could pull it off.

Title: Re: 72 twin reverb master volume pot
Post by: 67polara on April 08, 2011, 09:10:50 am

It is bad, that is why I asked the question.  This man has owned this amp since new.  He knows what it sounds like and I need to get it as close as possible to the same sound.  What pot do I use and how do I mod it so it sounds the same or very close.

Thanks

Tony

Title: Re: 72 twin reverb master volume pot
Post by: LooseChange on April 08, 2011, 07:53:22 pm

I remove the pot and all the circuitry that goes with it. I replace the pot with a 1m and wire just the pre pi master circuit.

Title: Re: 72 twin reverb master volume pot
Post by: 67polara on April 09, 2011, 12:55:21 pm

Thank you I will do that.

Tony

Title: Re: 72 twin reverb master volume pot
Post by: topbrent on April 11, 2011, 01:22:43 pm

Another alternative is to convert it to post-phase inverter master volume.

The best one I have used is the updated Type II, aka LarMar Master Volume.

It uses a dual 250K pot, and two 2.2M safety resistors. 

Easy to implement, and it works like a charm.  When you dial it all the way up it is effectively out of the circuit.

Title: Re: 72 twin reverb master volume pot
Post by: 67polara on April 12, 2011, 09:41:53 am

Thank you for the info.  I found a 1 meg pot with switch in my stuff and just replaced the pot leaving out the cap.  Only problem I had is that the switch end of the pot was just a little large due to the angle of the face plate.  A few moments with a file fixed that though.  Works great and sounds good.  When he brought this amp to me it didn't work at all.  He said he lent it to someone and when he got it back no sound at all. 

Here is what I found:

12at7 failure in PI
Master volume switch only worked at 7 or above
V1 microphonic
bad solder joint at reverb jack
1 blown speaker
Recap and remove death cap

Tony