SB220 Question - no plate current

[Jim Mullen]

OK Gang, any help with this would be appreciated.

I picked up a pretty beat SB220 at an auction last year. Had some Harbach
mods done, front panel was hacked up and lots of stuff had been removed.
Missing was the meter switch, relative power circuit and the ALC circuit.
Some weird wiring done to make it switchable between 120/240 but never
completed. It was wired for 240. It was from a silent key, his widow said it
did work up to the time he bought a new Ameritron.

I didn't have 240 available in the shack and it was not possible to return
it to 120 without extensive rewiring.

So, I rebuilt the whole thing. Just finished it up, basically returned it to
stock Heath with the exception of the Harbach mods. Found a lot of good
parts on ebay to replace the missing ones. It had the rectifier/metering
board, capacitor bank and parasitic mods when I got it. I added the soft
start and soft key when I rebuilt it.

Started testing today. HV is OK, but no resting plate current. I've traced
all the bias circuits and they seem to be both wired correctly and working
correctly. With my meter I have about 115VDC applied to the filament CT,
changing to 4.8V when the amp is keyed. There is never any indication of
plate current flowing. I traced the HV from the plate caps back to the HV
cap bank with an ohmmeter (no HV probe in the shack) and there is
continuity. I have no reason to believe there isn't HV on the plates when
the amp is on. The TR relay is switching correctly and the bias voltages
noted above are there also.

Just in case I was looking at a dead plate current meter I put some drive to
the amp. 100W in, zero output to a dummy load, pretty much as I expected.

Before I spend a crapload more time tracing circuits I guess my first
question is: If the tubes are bad will there be zero IP? I've never had an
amp with 3-500's, but it's hard to believe there wouldn't be a hint of
current. Yes, the filaments light up just fine, filament voltage is 4.75 on
my meter.

Second question (assuming the tubes are not the problem). Any suggestions
where else to look? It's not that complex a circuit, I'm puzzled I don't see
the problem.

Thanks in advance for any suggestions.

Jim, KK1W

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