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

[John Simmons]

Colin,

While I'm certainly no tube amp guru, I have a couple thoughts. Have you 
tried running the amp without tubes and keying it? How about unplugging 
the amp and manually operating the relays and doing resistance measurements?

73,
-John NI0K

> Colin Cooper via Amps <mailto:amps at contesting.com>
> Monday, January 14, 2019 2:21 PM
> Hello everyone,
> 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 Harbaugh 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 120v 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.
>
> Thank you and 73
> Colin Cooper
> K8MDX, Grass lake Michigan
>
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