[AMPS] SB 220 meters and the 110 volt supply

[nospam4me@juno.com]

>:Be sure to remove the wire between the bias contacts on the relay and
>the 110VDC power supply.  If you don't do this, a shorted tube can
easily 
>:destroy the filament transformer.  

>In your opinion, would a series glitch resistor in the bias supply
>prevent xmfr damage?    

:yes.  10k ohm, 2w to the bias contacts ought to limit the short circuit 
:current to a safe value.  

Then it would be a toss-up about doing the conventional "rewire mod" (as
also described on Rich's web page) vs the install of one resistor. 
Ahhhh...  the lazy man has choices to make... 

>Since the meters are near hens teeth to replace,

:Harbach has them. 

Yes, but the last time I looked, they were not scaled/labeled for the SB
220 or an exact fit. Seemed a shame to cut into the front face plate
which I believe the Harbach meters might require. 

>I'd like to protect the movements if possible. 

A meter/shunt protection circuit is at Figure 2 on my Web site.  

It looks like a great circuit.
In the past, I had a problem with the meter calibration changing too
much. 
I hope to source that animal along with the failed Ip Meter in the next
round.  The diodes are a great idea I've used before. But something in
this circuit doesn't play properly. I've got some new shunt resistors on
the way.  New known small parts, a whip and chair after a pasta dinner
surely will help tame the beast. 

>I hope to work up a
>circuit to do that properly.   Notes in the original owners manual about
>the meter movement values to follow once I confirm them.
>
The movements are 200uA and approx 1k ohms.   

That works...
The notes in the owners manual, from the original builder say about 1250
ohms. I'll report back what I find installed in this amplifier. 

onward...
skipp

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