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#1 2007-12-05 15:02:03

Diablo
Member
Registered: 2007-12-01
Posts: 139

V4 - Fading Reverb

My V4 reverb is acting up. It was very thin, so I replaced the 6cg7 tube (driver). That fixed it up just fine, or so I thought. If I first turn on the amp from cold, the reverb works great with plenty of depth. However, after it warms up for a few minutes, the reverb gets very thin, almost like the depth knob is turned very low. I tried swapping in a new 12AX7 in the other part of the reverb circuit, but it didn't improve things. Anyone have a clue?

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#2 2007-12-05 19:07:23

paully
Member
From: Northern New Jersey
Registered: 2007-11-02
Posts: 200

Re: V4 - Fading Reverb

Two questions, When you say 'thin', do you mean the level drops or the tone changes? Two different questions/solutions. Also do you plan on attempting repair yourself. You'll need a couple tools and a knowledge of how to work around, and respect the high voltages inside a V4. These things can fry you. At the very least you'll need a multimeter.

3 areas to look at:
1) dirty connections(pot's internal wiper, solder points, tank's RCA connections, tube sockets)
2) the specific supply voltage that feeds the reverb circuit
3) component breakdown

If steps 1, 2 are OK, I'd recomment that you point-up all component solder points in the reverb's input and recovery path. Then, by my count there are 7 caps in the rev circuit; 2 electrolytics and some non-polarized ones. That's next. These are always suspect. There is one in particular, the .47mF cap that connects tube 1 to the tank. That's the first one to replace. Any other cap could cause the level to drop. If you have a scope to signal trace, that's a plus.

You really have to test these problems with the amp open and running. Again, if you're attempting this yourself, be aware of the high voltages inside. Happy hunting.

Best, Paul


WADAYAKNOW.. For the first time in my life, I'm wrong again!

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#3 2007-12-08 02:14:12

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: V4 - Fading Reverb

Thanks Paul. Your info was very informative. By thin, I mean the amount of reverb is greatly reduced, not a change in the tone.

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#4 2008-02-24 03:46:01

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: V4 - Fading Reverb

I just spent the afternoon replacing all the electrolytic caps in the amp, all the caps in the reverb circuit, all the resistors in the reverb circuit, and the four 5W plate resistors on the power tubes. I took the shotgun approach because every componenet was original. Turned the amp on and played it for an hour and it's perfect - reverb sounds beautiful and no fading. I think it's good to go for another 38 years.

I bought the parts from tubesandmore.com
For those interested in replacing the power supply filter caps, here's the part#s you need:

C-EC40X3-525   40/40/40 microfarad 525V triple in one can.
C-EC80-40-30-20 80/40/30/20 microfarad 525V quad in one can.
You need two of the 80/40/30/20 quads.
C-SA40-500 Sprague 40 microfarad 500V.

The 80+30 caps are in parallel to make 110 microfarad.
The 20 micro farad caps don't get used.
One of the 40 micro farad cap goes in series with the other 40 micro farad cap. The other 40 in the quad doesn't get used.

These are twist lock caps that fit in the same location as stock.

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#5 2008-02-24 05:41:42

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: V4 - Fading Reverb

Diablo, 

I hope that you reused the covers from the old caps.  Since the two 110uf (80+30) are in series with one another,  one of them has about 270VDC on the negative side (The metal casing)
in normal opperation,  this is no issue,  but if you ever touched it by accident, you wouldn't be pleased.  especially if it was still on.

-steve

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#6 2008-02-24 12:45:28

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: V4 - Fading Reverb

Thanks for the warning on that. I was aware of it, but hadn't put the cardboard covers back on. Is there an easy method to remove the cardboard from the old caps? Heat gun perhaps? What holds them on? Wax or glue?

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#7 2008-02-24 19:02:20

hangman
Banned
From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: V4 - Fading Reverb

I figured you were probably aware... but i figured, better safe than sorry.
I believe it is a glue of some kind... I don't recall it being that difficult to remove though.
don't worry about ruining them... if you happen to mess them up, I've got so many I could just give you a few.

-steve

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#8 2008-02-24 23:07:33

Diablo
Member
Registered: 2007-12-01
Posts: 139

Re: V4 - Fading Reverb

Hey Steve, thanks for the offer on the cardboard covers. I removed the cardboard tubes with a heat gun. There is some kind of black waxy tar substance inside that loosens after about 5 minutes of heat.

For my next project, I'm going to make the power amp switchable. I want to use the V4 preamp combined with a Fender Hot Rod Deluxe power amp. That way I can get some power amp distortion without blowing out the windows in the house. After that, I might try building one of those 18 watt Marshall clone amps.

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