1966 Fender Super Reverb

[xtian]

Sweet, mostly unmolested SR on bench. Check out the bronze colored caps in the doghouse!

Previous repairman installed 3-conductor power cord, but left the death cap in place. Shouldn't he have disconnected the death cap from ground?

Pots sound dirty, and amp occasionally says "SNAP SNAP SNAP SNAP" when running. Otherwise, sounds great; good power, and very quiet at idle. Going to clean the pins and pots and see if it's better.

[Cygnus X1]

Yes.
But also why did they only replace two of the filter caps?

[jelle]

Great amp. The snap sounds are probably the carbon comp plate resistors.

Remove Death Cap.

The noisy pots are most likely due to some DC leaking through the black phenolic paper eyelet board. Check the decoupled side of the treble cap for DC. Also check the other caps's decoupled sides. If you see a few mV, then you know what it is.

I am always amazed by the sonic improvement I get from replacing the E-caps (also cathode and bias caps) on these old amps. Jelle

[jelle]

Yes.
But also why did they only replace two of the filter caps?

It probably stopped humming after that fix.

[Blackburn]

Beautiful guts! Such a clean board!

[xtian]

The pots are all perfect now, after applying DeOxIt FaderLube. No scratchiness.

The popping I've heard sounds like a short or intermittent connection. The tube in V2 is microphonic, and made a couple similar SNAPs when I tapped it. I'll replace and try again.

Has all old preamp tubes: National (Yugoslavia), GE, Mullard. And Groove Tubes for 6L6's, you know, to round out the set!

Can't believe how clean and quiet this thing is!

[xtian]

I note that the cabinet is solid pine, but the baffle is pressboard. Is that expected?

[jelle]

Yes, my 64 is like that too. Jelle

[xtian]

New tube in V2 seems to have solved the snapping. All good.

Next observation: the Groove Tubes power tubes are marked GT-6L6C.

Are these equivalent to 6L6GC?

They are only drawing ~18mA at 463 plate voltage. Very cold.

[jelle]

Great!

What do the GT-6L6C look like? Some versions I would bias cold. But not that cold.

Jelle

[xtian]

Photo of power tubes attached. I'm using Aiken's method:

The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side.

http://www.aikenamps.com/Biasing.html

[jelle]

Chinese coke bottles. I like 25-35 mA on those. just listen to the tone you get. Lower bias current has more headroom.

[xtian]

All good. Thanks for the help, jelle!

-a

[jelle]

Great, you are welcome!

[beasleybodyshop]

What do you mean by "death cap" ?