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#1 2013-03-10 15:26:10

New member
From: Cambridge MA
Registered: 2013-03-10
Posts: 1

Advice on recapping and rebuilding a V4B ?

Hello All:
Im just beginning my journey into the weird and wonderful world of Ampeg and was hoping for a little advice from the community. A client came in with a V4B that was blowing fuses after it was played into a 4 ohm cab while set on 8 ohms. So far it seems to have taken out one of the diodes in the bridge (blew to short), and there may be more as well, but certainly the power caps are old, possibly original and really should be changed. I could just do a swap with the same components, but I have seen some complaints about the the CE reissue stuff and  this approach seems to be about 3x the price of using commodity components.
Heres what I plan on doing:

Replacing the first cap can (70/40 uF) with a 500V 50/50 uF (in parallel to make 100uF) and the second cap can (70/40/40 uF) with a second 500V 50/50 uF (in parallel to make 100uF) plus an external 47uF 500 V cap attached in series to the 40 uF normally attached to the third 40 uf section of the cap can. I will probably swap out the existing 40 uF with a second  47uF 500 V. Probably going to pursue a similar strategy for the third cap can, a 500V 50/50 uF plus an external 47uF 500 V.

So a couple of questions:
Math seems to work, am I more or less in the correct ball park here ?

As these values are close to tolerance ranges, Im not expecting any major changes in sound (although the caps are old enough that who knows what their value currently is, so obviously some change is expected.) Should I be ?

Are there any other common faults that I need to deal with while Im inside ?

Many Thanks



#2 2013-03-10 17:35:22

From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: Advice on recapping and rebuilding a V4B ?

What you wrote looks good to me,  although I would suggest considering using  one 100x100 500v can to replace the second 70x40x40,  and then using a 100uf 450V axial cap in place of that 47uf 500v cap in series.
(I am talking about the screen supply here).  Instead of two 40uf caps in series,  you could use two 100uf caps in series.   This makes the filter for the screen supply 50uf total instead of the roughly 20uf it is stock.

The increase in capacitance in this case doesn't make a huge difference in the sound,  20uf for the screens always seemed a little low to me anyway.

Are you sure the blown diodes are the rectifiers and not the flyback diodes?

Replace the caps in the bias supply including the .047uf cap.



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