Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chuggy on September 29, 2011, 09:51:24 pm

Title: PPIMV in a Twin Reverb
Post by: chuggy on September 29, 2011, 09:51:24 pm

I'm contemplating installing a PPIMV in a master volume Twin Reverb in which the stock master volume has been removed.
Any reason to deviate from the standard circuit of .1 mfd cap to 1 meg pot to .1 mfd cap for each side?
Will the hookup wire run to the stock control position be excessive?
Is shielded cable needed?
Any lead dress tips?

Thanks-
Jim R

Title: Re: PPIMV in a Twin Reverb
Post by: phsyconoodler on September 30, 2011, 10:49:41 am

Use shielded cable for sure.
  The trainwreck type master works great.

Title: Re: PPIMV in a Twin Reverb
Post by: The_Gaz on September 30, 2011, 01:38:17 pm

I'd just try it with twisted wire first, myself, as the Twin circuit is pretty low gain.

Title: Re: PPIMV in a Twin Reverb
Post by: phsyconoodler on September 30, 2011, 01:41:37 pm

Just do yourself a favor and use shielded cable.Then you don't have to go back in and redo it.

Title: Re: PPIMV in a Twin Reverb
Post by: Fresh_Start on September 30, 2011, 02:10:24 pm

Most folks seem perfectly happy with a 0.1uf coupling cap before and after the Master Volume Pot(s).  However, if you have two 0.1uf caps in series, the net capacitance would be 0.05uf.  Things get more complicated because you have a variable voltage divider (i.e. the master volume pot) in between the two caps.  If you want to stay as close to the original Twin Reverb as possible, theory might say that the first coupling cap should be 10x the value of the second coupling cap (IOW 1.0uf).  That way, the big coupling cap keeps DC voltage off the MV pot(s) but the knee frequency it creates with the MV pot(s) is so low that it won't matter.  Then the "normal" 0.1uf coupling cap right before the 220K grid return resistors forms the stock frequency filter.  Remember, the bias voltage is AC ground too.

Don't worry if that doesn't make sense - just build it with the full set of 0.1uf caps.  If the tone changes too much with the MV on full or if lowering the MV affects the tone, then start fiddling with big in series with small.  Personally, I like smaller coupling caps on Blackface Fenders anyway so the net effect of the two 0.1uf caps might be just fine.

Cheers,

Chip

Title: Re: PPIMV in a Twin Reverb
Post by: chuggy on October 02, 2011, 08:17:38 pm

Installed a PPIMV in my Vibrosonic Reverb that had been converted to a BF Twin Reverb circuit.  Tried twisted unshielded wire and used the original MV pot location.  Works quite well and does not seem to suffer for lack of shielded cable.  Thanks for the held, guys.

Jim R