The reasons why the power transformer failed were pretty simple. There is a small 22 ohm resistor in the bias supply and that failed open. With the loss of bias voltage, the power tubes red plated and died, bad smells came out and the power transformer primaries expired-no doubt it was the thermal fuse within, which is unrepairable.
However, I did have an old style Vibrolux Reverb power transformer that's a drop in fit and it was a pretty simple thing to fix up all the burned stuff, add a couple of spade connectors , plug everything together and power it up. It worked fine, too, but I had the germ of an idea.
Now that I had a power transformer that was rated at 180 ma instead of 120 ma, what if I installed a more substantial output transformer as well? Then, I'd have a 35 watt amp in a 22 watt package with a pair of 5881 power tubes.
It also presented the opportunity to do some tinkering to make it all work together, upgrade that ridiculous 22 ohm grain of wheat resistor, and gave me a chance to try out my prototype vibrato roach (optocoupler) which I am calling the Doodle Bug.
There was the Surf Bug and the Black Widow roaches but they're out of production now and the maker of the Surf Bug has also passed off this mortal coil, more's the pity.
But there's the Doodle Bug.
So Doodle Bug number one has been installed and it's a winner. Smoother than the original, it's more like a tube modulated vibrato than an optocoupler.
So the transformers went in in pretty quick order although the output transformer must be moved aft about half an inch to clear the speaker frame. It's tight but there's about 1/4 inch clearance which is plenty enough. You do have to drill a new hole for the mounting screw, and it helps to angle it a little bit because it comes up against the grommet where the wires go through.
I'd previously installed F&T power supply capacitors for tone and reliability. The bias supply resistor required some adjustment to increase the range to a usable level.
This amp has a 5A filament fuse which is OK for 6V6s but a little light for 5881s so I uprated it to an 8A fuse which should be fine given that most Deluxes didn't have a filament fuse to begin with and the Supersonic 22, which uses 6V6s has a 10A filament fuse. To be on the safe side I cleaned off the traces on the underside of the board where the fuse holder is attached and laid in a more thorough bead of solder. This was all because the 5A fuse failed after about three hours of service.
It's got a lot more guts than the DRRI, that's for sure.
The amp was donated by Matt Wellendorf, a local guitarist and also a great pal. I provided the brain power for this project.
I used Marvel Electric transformers which are plenty good and at a fair price. Currently the output transformer is a 4 ohm number but the amp can stand the mismatch as all Fenders do. For a high end deal I'd probably install a Classic Tone Multi tap output, but the Marvel iron is good enough for proof of concept and some reliability testing. I see where Marvel is offering an 8 ohm output of this configuration which would allow a closer impedance match.
If this is something you'd like to try you can get in touch with me at all the usual places. At this point with all the parts added in about $350 should do it. If you figure that some routine maintenance and improvement should cost you $150 or so, it could be a good time to upgrade your 97 pound weakling.
What you've created here is in essence, a 1x12 Vibrolux Reverb!
ReplyDeletePretty much. On a budget too.
ReplyDelete