SB220 Question - no plate current

[Mark Graalman WB8JKR]

  Do you have DC continuity between the 3-500 grids
and the chassis?

Mark  WB8JKR

On Sun, 11 Apr 2004 16:46:32 -0400 Jim Mullen <jmullen at ROCKYS.COM>
writes:
> 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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Products bought, sold or traded here is the responsibility of the
parties involved.  This list and the City of Tempe are not responsible
for losses or misrepresentations of any kind.  Buyer beware!

-----------------------------------------------------------
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