This is a repair on an old Xeron Mini-Max Sweep Tube Amp. This was built back in the early 80's, as best as I can tell. It originally used 12GB3 sweep tubes, this one had been converted to 12DQ6B tubes, and used 4 of them. This is a grid driven amp, with about +700vdc on the plates, +150vdc on the screens, and about -35 on the control grids. The input is swamped with a 47 ohm load, and is tuned. The output is a conventional Pi-network for one band, with the plate tune adjustable external & the plate load internal and set for an output load of about 50 ohms.
These are neat little amps, probably mostly designed for AM operation. I've used it with a HTX-10 10m mobile QRP rig, and found it to be decent on SSB if the bias was adjusted accordingly. AM with 100% modulation yielded about 500w pep output, and about the same on SSB with the bias adjusted for that mode. Operation on FM, due to duty cycle, had to be held to about 120 watts output. CW operation in the SSB mode provided adequate break-in delay.
This one had a toasted power transformer. These amps are kinda prone to that, as it's cramped quarters inside for a decent transformer and adequate cooling. A larger transformer on an external chassis solved that problem. Here's a few pics of the repairs & mods ...
Ripped all the "guts" (seen below) from the chassis with the "new" larger transformer, and rewired a cable for remote connection to the mini-max...
Comparing old transformer (left) to new transformer (right)
Cable and quick disconnect installed and ready to go...
Testing things out, adjusting bias, checking voltages, tuning input coil, etc...
External bias adjustment installed by someone in the past...
Tuned input (now with pink wax installed to "lock it down")...
Internal load control in output Pi-network, I drilled hole in cover to allow for external adjustment...
All finished up and running great. Pretty strong for a little sweep tube amp!
