Saturday, December 29, 2012

Marvel Amp Update

I dropped by Bruce Collin’s Mission Amps shop yesterday for a little work on the newly acquired Marvel vintage tube amp. This was the first time I’d had the chassis all the way out of the amp. The circuit is sparse and primitive, reminding me of a 1947 Gibson amp I once owned. It has a half wave rectifier, just a tiny diode. Weird.

Mostly I wanted Bruce to install a grounded power cord and test the caps, because I really like the tone of the ratty little amp as it is. Here is what we ended up doing:

-Install grounded power cord
-Remove the “Death Cap”
-Install cathode bypass cap on the power tube. This gave the amp a bit more punch.
-Test all tone caps for leakage. 
-Test all circuit voltages.
-De-oxidize the tube sockets
-Install speaker jack

We then tried five different preamp tubes: 

-The vintage RCA gray plate 12AX7 I had in the amp
-a 5751 from JJ Tesla
-a 12AY7 from EH
-an NOS 12AZ7
-an NOS RCA 5965

Of those tubes I liked the 12AZ7 best, so I have it in the amp for now. We did not change the power tube. The original 7591 tube is a gem; a Sylvania rebranded as Multivox.

We found some date codes on the chassis and filter cap. It looks like it was made late in 1963.

The amp tested at 7 watts just as it began to clip. It still has the same old-school raspy sound but slightly browner. Sounds good! It’s a keeper.

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