Thursday, December 29, 2016
Reviving A Sick Twin Reverb
I got a call from a new customer about a sickish Twin Reverb that he had. It had died on the job and another local tech had retubed it but for whatever reasons decided he was done with it. I talked to him over the phone and it appeared as if the amp had been inartfully modified by person or persons unknown and the preamp tubes flashed on powering up. There was no output to speak of.
So I hauled it home and performed an autopsy. Sure enough as you can see the center power supply capacitor was missing, and the bias setup was still the balance control with insufficient voltage at all the relevant grids. This requires using the layout (easiest way) and duplicating the original component values.
The first task was to do enough revival to assess the rest of the amp's condition. With no tubes in it the B+ voltage was well over 525v which is very high even for no tubes. A rebuild of the capacitor board was in order and a careful tracing to determine that there was no kooky stuff going on with the wiring.
After that, a look at the bias board convinced me to make another one with new diodes and such and convert the bias balance to the preferable level set system of the previous Twin Reverbs. Here's what it looks like.
With all that the voltage was still way too high, and then I noticed that the center tap (red-yellow) from the power transformer had broken off at the ground lug. Once that was taken care of, the voltage came right down to normal, and reinstalling the power tubes and setting the bias around 36 ma was a cinch.
And that flash from the preamp tubes? They were Groove Tubes, but they must have been old stock EIs from Serbia. They're well known for doing this on startup, along with Bugle Boys, Telefunkens and some others.
A few tweaks got the vibrato (broken 100k resistor on the control) and reverb working, and I'll finish the cathode capacitors in the morning and send it back home.