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Re: [Amps] SB220 step start blows fuses

To: amps <>
Subject: Re: [Amps] SB220 step start blows fuses
From: Warren Volz <>
Date: Wed, 12 Jul 2017 00:40:52 -0600
List-post: <>
Well I have a major update!

The amp shipped with 2 3-500zs that are “not full output” according to the 
seller. Since the amp was in shambles I didn’t have a way to test them until 

One of the 3-500zs is unmarked and missing the phenolic spacer below the glass 
envelope. The other is clearly labeled as a Eimac. I tested each tonight with 
the HV plugged in. The unmarked worked without issue but the Eimac arced right 
after turning the amp on!

So, I guess I’m in the market for a new 3-500z. Based on what I see online, 
there isn’t really a way to fix this gassy tube but I’m going to let it sit 
with no HV and filament voltage for ~30-60 min to see if that has any impact.

Tonight I also removed the fan and oiled the bearings + the surrounding fiber. 
The fan is a lot louder than I expected, but with the oil it’s slightly quieter.

Thank you all for your help!


> On Jul 11, 2017, at 6:35 PM, Warren Volz <> wrote:
> Again thank you all for the advice and guidance. 
> Replacement fuses showed up today so I'm being a little more
> adventurous. As multiple people pointed out the dim bulb tester I have
> as built won't work unless I can find at least one 300W bulb. 
> So, I remove the dim bulb tester and tried each tube in the amp. Good
> news is that both filaments light up just fine! To me that rules out the
> filament supply and paints a target on the HV supply as many of you have
> suggested. 
> The amp as I received it was a pile of parts, and so I have no reference
> in terms of working/not working prior to my mods. Both the QSK and the
> RM220 were used pulls off of an auction site. The RM220 is squarely in
> my sight now since it had a burned out metering resistor when I got it.
> When I checked previously the diodes were all ok. Also, all components
> (caps + resistors) on the filtering board are new. 
> I'll likely try reconnecting the HV primary without the plates connected
> to see what that does next. 
> -Warren 

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