2205 Repair Questions

[stephenl]

I replaced the power tube sockets, power supply caps, bias supply caps, and added 10ohm 1% resistors to measure bias current at the cathode. I put in JJ KT77's - matched and burned in.

Measured:
Va = 475vdc
Vs = 472vdc
Vb = -40vdc - 37mA/tube(cathode + screen screen current)

@ 70% dissipation:
.7 * 25watt + 17.5watt
17.5watt/475v = 37mA

As previously mentioned, it sounds like the typical target Vb is around -37vdc. Is it worth it to go there

Normal channel sounds like a Marshall
Boost Channel (splitch model)...not a fan. It uses a diode rectifier + an extra diode for clipping.

[xtian]

I don't recommend targeting 70% bias by default. 50-60% is sufficient for good tone, and the tubes will last longer.

[stephenl]

So I played it for a couple of hours before the owner picked it up and all was good.

He had it at practice tonight, and sent me a few texts that it sounded great...after 30 mins of playing, he said the volume dropped off substantially on both the normal and boost channels.

Any ideas on this?

[TUBEDUDE]

Did he see any redplating? Both channels likely means power tubes or supply.
I'd bench it till it fails and see if the bias makes it to the tubes. Then power supply rails, etc. Gotta make it fail on the bench to see what's up.