Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Geo on May 15, 2011, 01:21:57 pm
-
I just got a Blackface Bandmaster head and cab for $600 (!!!). One power tube was dead so I popped in a quick replacement, which probably didn't bias well with the other original 6L6. My bias voltage is -57v where the schematic calls for -48. Checking voltages, I took a reading on pin 6 of the phase inverter tube. It was close to spec, but with the test lead in place on that pin, the amp began to hum. I spotted the other original 6L6 redplating so I shut the thing down immediately. It doesn't redplate now when brought up, but I'm leery.
I've also noticed that the PT gets EXTREMELY hot--so much so that it was still hot maybe half an hour after shutting the amp down.
This amp is all original I think. I should probably start with replacing the filter caps... but does anyone have any thoughts about the above symptoms? :dontknow:
Thanks. ;)
-
I'm surprised no comments yet? I'd start by adjusting the bias down first to get it up and running so atleast it doesn't red plate. With a high/hot bias you're running not just the tubes hot but the tranny too w/ all of the extra current being sucked. Check the B+ and the other power nodes' voltages while you're in there. Listen for other obvious signs of cap, speaker or whatever issues. Then you could go through the rest of it and see what's up w/ everything and if you want to put any money and work into it. It would likely be a good idea to replace all your e-lytics & check all tubes. They get old & used up like your guitar strings if it hasn't been serviced for years or decades.
-
Yes replace those old PS caps. Thats one of the first things to check to see if there blown out or showing signs of saturation with an anolog meter
-
Thanks guys. I think I will start with a matched set of power tubes and new PS caps.
Good to know that the high bias may be the reason for the redplating and hot PT.