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Re: [Amps] SB220 step start blows fuses

Subject: Re: [Amps] SB220 step start blows fuses
From: Warren Volz <>
Date: Tue, 4 Jul 2017 15:30:58 -0600
List-post: <>
Thank you for the replies. I had some time to circle back to this today and 
checked a few things.

- With the tubes removed, it looks like they have no shorts but the grids 
measure ~0 ohms (maybe it’s too low for my meter?). Is that expected?
- Checking the filament transformer I see (All are while in circuit with the 
power switch off)
        BLK-BLK/YEL = 3.2 ohms
        BLK/GRN-BLK/RED = 4.1 ohms
        RED-RED = 10 ohms
        GRN-GRN = 0 ohms
        GRN/YEL-GRN = 0 ohms
        Am I correct in assuming that 0 ohms across the GRN/GRN winding is not 
a good sign?
- Checking the anode to chassis resistance without tubes and the HV interlock 
defeated I see ~750 Kohm, is this normal?

When I rebuilt this, I did the following modifications:
        - W7RY board with fuse option installed
        - ES4SB220 filter capacitor board installed with new electrolytics 
        - RM220 replacement metering board installed
        - Grids grounded
        - Molex connector on the HV transformer primary windings

As I mentioned previously I think the fan isn’t working, but I haven’t had a 
chance to verify that yet. I only have one more replacement set of fuses so I’m 
trying to be smart about my testing until I can get replacements (likely later 
this week). I can easily disconnect the HV transformer. Am I correct in 
assuming that if the fuses blow after disconnecting the HV primary I can assume 
the filament transformer is bad?

Thanks again for your help!


> On Jun 25, 2017, at 11:54 PM, Warren Volz <> wrote:
> All,
> I’ve been restoring a SB220 and finally got to the testing stage this 
> weekend. When I turned it on, the HV went up to ~1kV and then started to 
> drop. Successive cycles of the power switch did nothing. Oddly, the meter 
> lights and fan never started running. I looked at the bottom of the amp 
> tonight and the 3A fuses I added to the W7RY step start board blew. What 
> areas should I look at now to determine what is going wrong? I have both 
> tubes in, and I believe I previously checked them for filament shorts. I’m 
> worried the HV supply has an issue.
> Any advice would be very much appreciated!
> Thanks!
> -Warren

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