Bassman 100 eventually blows fuse

[Kenzo1979]

Thanks, Sluckey!

Coming along nicely.  Found a couple questionable things while going through the rebuild

[Kenzo1979]

Update: amp is repaired, ultimately I believe the issue was one of the output coupling caps.

I’m glad I did everything I did even though in the end most of it may have been unnecessary.

Replaced all the boards, components, tube sockets and power tubes.    Got the boards and sockets through the site which shipped immediately and arrived in a matter of days.  Can’t say enough good things about that.  Also used some new wire in places.

Originally after rewiring and replacing the boards and sockets, I fired it up with the existing tubes and existing output coupling caps which were Orange drops dated from early 2000’s.  Amp blew the fuse again.   I had done quick cap checks on these back when I suspected that area of the circuitry, but the readings were within spec.  So I pulled power tubes and did some tube swaps thinking maybe one or more of the power tubes had gotten trashed from constantly putting them through the troubleshooting process;  but the issue continued regardless.  So back to the output coupling caps virtually the only components I hadn’t swapped out that could be affecting things this way.   Wish I had the ability to do more thorough leak-down tests, but in the future if I suspect something, I will just change it and see if there’s change.  I never would have imagined one of those OD’s was bad, but here we are. So swapped out both caps and the voltage stabilized, back in business.

Put two power tubes in at a time, one on each side and measured the current through the output transformer.  At about -48VDC bias, two of the tubes were pulling 33mA, one was pulling 36mA and the other was 26ma.  Ewww, not going to be very balanced with that set up, so I put in new JJ tubes, couldn’t get a matched quad so I got the closest to matched pairs and split them up between each side of the amp to balance it as closely as possible.  Biased them to about 30mA each, bias voltage for that was about -49 VDC.  I also took the Rob Robinett website advice of running my heater wires all in phase except for between the two pairs of power tubes. 

All I can say is the amplifier sounds amazing now. Crystal clear with no audible idle noise at all.  I’m very happy, and now I have some good spare parts. 

So thanks everybody who participated in giving me advice and trying to point me in the right direction. Greatly appreciate. Cheers!

[trobbins]

A worthwhile test is to pull the tubes, and use a variac to bring up B+ to max allowed by voltage rating on filter and coupling caps, and go around every grid leak resistor and measure the voltage across it to confirm there is no leakage through any coupling cap (its a bit more complicated for a fixed bias output stage).  That's probably easier to set up unless you have an insulation resistance tester and test each coupling cap for leakage at some voltage that may not be the cap voltage rating.

Easy to say in hindsight.  Not easy to do safely unless you have a variac.  Perhaps more commonly done by those restoring vintage amps than those relying on 'new' parts.