[Amps] SB-220 Problem has me stumped, looking for some guidance on what to look at next

[Ron Youvan]

    Colin K8MDX Cooper wrote:

> I bought a SB-220 as a project at Hamvention in Ohio, I think I paid about 400 dollars for it, It was pretty dirty but complete. I wanted to restore one of these to compliment my SB200 that I have already restored.
> The problem:
> When the amp is plugged in and turned on resting at idle, it seems to be fine. However when you key the amplifier the Plate and the Grid meters are both PEGGED. The meter lights also dim as to indicate that the amp is using quite a bit of current.

> Things I have changed/updated:
> I Installed the Harbach electronics boards, so it has a new cap string as well as a new hV and metering board.
> I also installed a new TX/RX relay and the “Soft Key” so that my new radios could key the amp without damage.
> I Directly grounded the grids on the tube sockets, removing the mica capacitors.

> Things I have checked:
> hV: hV is good, the cw/tune / ssb switch works I have 2400 volts in cw and 3000 in ssb
> Tubes: I also own two other TL-922 Kenwood amps that use the same tubes, so I swapped a set of proven good Eimac 3-500z’s from one of those amps into this one - no change
> 120-vs-220 Voltage: it does this regardless of how the input power is configured
> The Bias Diode circuit on the Harbach board, I triple checked my assembly/installation and see no problems
> The 120 V. Bias/relay Power supply is working.
> The Bias relay: I initially thought this was a bias issue and so I triple checked the configuration of the bias circuit. I have also reconfigured the bias circuit to resistive bias and there was no change. The grid and plate meters still peg when keying the amp.
> The center tap of the filament transformer: It does not appear to be shorted to ground.
> The amp acts like something somewhere is shorting to ground but only when the amp is Keyed up.

> I would really really appreciate any help or guidance that the group could provide on what to look at next.
   I would run a wire from the cathode (through a choke ? 2.5 mH) out to
a multi-meter and see of the bias Voltage changes with drive.
I would lower the RF drive to five or ten watts and slowly increase it
and see what drive causes the current rises over the idling value,
looking at the output power.
-- 
   Ron  KA4INM - Youvan's corollary:
                 Every action results in unwanted side effects.