6550s in '65 or '66 blackface bassman?

[lowell]

Hi all,
I know 6550 are much harder on an amp. They have 1600ma heater current and have max dissipation at 44watts. So they might fry a PT heater winding and/or OT windings. Anyone know the VA specs for said bassman and or if the OT can handle it? Obviously I will need to rebias too.

[GibsonLover]

I wouldn't mod a Blackface amp, but if it's already been tampered with, why not. Maybe just mount an auxillary heater transformer to the inside of the cabinet and run your 6550's off of that. Your stock PT should handle the extra B+ load, and the change isn't too obtrusive. It should have more headroom, but not a lot more noticeable power. If you like it, you can always cut metal and install a larger PT for a stock look. I put a Bassman PT in my Silverface Princeton to run 6L6's. It barely fit, and it sure is cool.

[boyer]

I read somewhere on a forum someone doing this recently, without mention of using an auxiliary heater trafo, but I doubt if that's a good idea. BF Bassman's IIRC have a max heater supply of 3.15 amps. This leaves you with only 150ma to spare, and those 6550 suck more that that EACH.

I've been interested in seeing if this would work, but haven't wasted the money on the tubes yet.

If you decide to try it, measure the heater voltage across one of the tubes, and make sure that it's with in +/- .2 volts. I wouldn't trust my heaters running at 6v or below. You'll damage your cathodes operating at any voltage outside of the tubes specs (ie 6.3v).

Please post your results here as I'm interested to see how they'd work out. The extra clean headroom would be nice. Some like to take the heads off of audience members with chainsawesque distortion, while I like to blow them off their seats with clean

[lowell]

I decided it was not worth trying. The potential meltdown of the PT was not a risk I was willing to take.