Using the 6AV5

[Tiny_Daddy]

Someone brought a Sovtek Mig 100 and a box of 6AV5 and asked for cathode bias. I used 390 ohm / 47uF cathode resistors on each tube and connected the plate voltage to the 270V with 120V zeners following a 1.2K 10W resistor and 22uF filter cap dropping the screen voltage to 150V. After getting a bad oscillation I added 100 ohm resistors in series with each plate. Tubes are various and run 10-14 watts with no input. Measures 20 watts out before clipping.
Fortunately the power supply with bridge and center-tap allows this mod, and the double-pole standby switch disconnects both AC leads to the bridge.

[darryl]

I've used 12AV5's, and had no problems with unwanted oscillation. I did use grid stoppers and screen stoppers as a routine precaution, but did not require plate stoppers for stability. The power supply was a voltage doubler, supplying 310VDC B+ and 155VDC for the screens, and the amp had fixed bias.



The construction details are here: http://www.guitargear.net.au/discussion/index.php?topic=29864.0

[Tiny_Daddy]

The amp works fine with 2 tubes. The oscillation was with 4 output tubes.
NICE LOOKING AMP.

[DummyLoad]

The amp works fine with 2 tubes. The oscillation was with 4 output tubes.
NICE LOOKING AMP.

air gap inductor: some 20AWG enamel wire wound on a pencil form; about 20-25 turns then connected from plate to plate would help tame oscillation as well.

agreed - very nice looking amp.

--DL

[darryl]

The oscillation was with 4 output tubes.

Sorry, I should have taken more notice of "Mig 100".