Thursday, April 26, 2012
Retrofitting an Ultralinear Twin Reverb
It seems the scientists at the school decided it had a bad power transformer, removed it, and then decided as it was too expensive to buy one that they'd dump the amp in the trash. So he got it sans power transformer and it sat for a few years until the work picked up and he could afford to get it repaired.
The ultralinear Twin Reverb was an uprated amp capable of producing a rated 135w, and they did it by using an ultralinear output transformer. This uses screen taps at about 70 per cent and the idea is that you can run the plates really hard and still keep the tubes alive. It yields clean, glassy tone that seems a bit sterile. But did I say they're loud?
And thereby hangs a tale. Power transformers for these things are a little unusual because the rectifier setup is different than the older Twin Reverb amps. It delivers more voltage because it is a full wave bridge, with a non center tapped secondary.
According to the good folks at Hammond the previous way of doing things yielded an average DC voltage of about .45 of the AC voltage and the ultralinear amp setup yielded more like .90 of AC voltage. So you can't just drop any old transformer in and hope to have a working combination.
The quest began, and the price for a replacement Mercury Magnetics power transformer was nearly three hundred dollars. Ouch!
I did locate a used one but the fellow first wanted a new Classic Tone (Magnetic Components) Twin Reverb international voltage power transformer and a new Classic Tone output transformer to go along with it in exchange for a used, single ultralinear power transformer.
I thought it was too much, highway robbery really, and he dropped the demand for the output transformer-no doubt figuring he could use the one he had.
I still don't like the idea of being hosed for parts by strangers so I nixed the deal entirely and decided to find something else more suitable, as I'd seen used ultralinear power transformers on ebay for around forty to fifty bucks.
I then obtained a standard Twin Reverb power transformer used for a Grant and a new choke from Mojo and built a bias-rectifier eyelet board, redid the power supply capacitors and the necessary rewiring, converted the bias to a level set and revised the phase inverter circuit to standard and then......nothing. No output to speak of.
Some minor tweaks to the wiring, relocating the bias control to a safe place, and other minor touches
Volume, lots of volume. Tons of tone and plate voltages right at a steady 450v DC.