Reverb Problem with Blackface Twin

kbdrost · Post by **kbdrost** » 3 Apr 2004 1:51 pm

I have a '66 Twin Reverb with JBL speakers. I love the tone, but periodically, the reverb will lose its "depth" (the chorus-like aspect of the effect) It still has a reverb, but it is a very clean tone. Nothing seems to cause it; it just kicks out. Just as mysteriously, it will kick back in again. Has anybody experienced a similar problem, or is my amp just possessed? If you have had a similiar experience, do you have any idea how to get rid of it?

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Ken Drost
steelcrazy after all these years

Tyler Macy · Post by **Tyler Macy** » 3 Apr 2004 4:03 pm

Check the connections to and from the reverb tank. I have had problems like you describe from intermittant rca cable connections.

Bill Terry · Post by **Bill Terry** » 3 Apr 2004 4:22 pm

I agree with Tyler, the cables are always suspect with intermittent reverb issues.

I have several older Fender amps that have the original braided shield reverb cables (as will your BF Twin if it's original). The area where the braid is soldered to the outside of the RCA connector at the chassis and then bent down inside the rear panel is particularly susceptible to weirdness, i.e. intermittent shorts.
<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Bill Terry on 03 April 2004 at 04:23 PM.]</p></FONT>

Bill Hatcher · Post by **Bill Hatcher** » 3 Apr 2004 4:46 pm

Most likely the wire as others have said, but you need to open the reverb tank and check those little wires from the plug to the transformers on each end. Also check the third tube from the right as you are facing the back of the amp. If I remember right, that drives the verb circuit.

Ken Fox · Post by **Ken Fox** » 3 Apr 2004 7:58 pm

If all else fails, change the cathode bypass cap on tube 4 (reverb recovery). If it opens you will have reverb but very thin sounding. This cap allows the signal to bypass the 1500 ohm cathode resistor. When it goes open, the signal has to go thru the 1500 ohm resistor and is degraded considerably. The cap is 25 microfarads at 25WVDC. Cheap and easy to change out.
This cap is also prone to cause an awful feedback in the reverb circuit, as it can go microphonic. Seen and fixed that three times in the last two years in Twin Reverb amps.
Electrolytic caps tend to be a problem as they get older. I just got a 1967 Twin with a shorted one in the power supply, it was causing the amp to blow fuses. All the other filter caps have drifted very high in value (common as they get older). The older caps tend to start acting like a resistor and then tend to load the amp's power transformer down, often they finally short out. This amp will get all new filter caps and be ready for another 40 years or so of music!