6550s in Univox 1221?

[glebert]

I've got this amp that is currently working OK, so of course I want to mess with it. Measuring 750V on the plates, 375V on the screens. I think the tubes in it are tired, and I don't have a set of robust 6L6s sitting around, but I have three old pairs of 6550s (Tung Sol, RCA, Svetlana). I know the 6550s take much more heater current, so that is something to watch for, and I was reading that 6550s want lower grid feed resistors (less than 50k?) What other problems could I run into putting 6550s into this amp? It was "supposed" to be 100W out of two 6L6, which no one believes, but would I have to worry about stressing the OT with 6550s?

TIA,
Greg

[loudthud]

6550s should work if they will fit in the sockets. Check that pin 1 of the sockets is not hooked to anything. On some 6550s it's connected to the metal shield around the base. Data sheet says -31V bias with 300V screens 600V plates. Be prepared to adjust the bias.

RCA_6550.pdf

[glebert]

6550s should work if they will fit in the sockets. Check that pin 1 of the sockets is not hooked to anything. On some 6550s it's connected to the metal shield around the base. Data sheet says -31V bias with 300V screens 600V plates. Be prepared to adjust the bias.

[ATTACH]n981678[/ATTACH]

I had forgotten about the pin 1 thing, thanks for the reminder!

[glebert]

Put the RCA 6550 tubes in, no problems with filament voltage dropping. There was nothing attached to pin 1 of the socket. Had to remove the bear trap tube retainers so the tubes could seat. I added 1k screen resistors since that was something else that seems necessary. Haven't done anything with the bias feed resistors, seems like 100k is not uncommon in some classic amps. I'm getting about 50mA per tube with the bias voltage at -35V, so probably need to bias a little cooler than that since that is 90% of max per Rob Robinette's calculator. Amp definitely more lively than before, not surprising. Can hear more hum now than I could before, so will probably have to replace a few more caps too. I don't know if the hum is worse because the amp is now amplifying more effectively or because there is higher current being drawn from the HV supply.

[g1]

I'm getting about 50mA per tube with the bias voltage at -35V, so probably need to bias a little cooler than that since that is 90% of max

For rough comparison, SVT runs the 6550's at 24mA each with approx. 700V plate.