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Re: [Amps] SB220 step start blows fuses

To: amps <>
Subject: Re: [Amps] SB220 step start blows fuses
From: Ron Youvan <>
Date: Tue, 4 Jul 2017 17:51:02 -0400
List-post: <>
   Warren Volz wrote:

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

  Tube pins 2, 3 & 4 should all measure zero Ohms in any direction.

        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?

  Zero Ohms is a good reading across a 5 Volt 30 Ampere transformer

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!

  You do not say if the filaments light or start to light when you turn
on the power.  You do not say if you have the unit in the high power or
low power mode.

  I do not know anything about the step start circuit, but it is suspect.

On Jun 25, 2017, at 11:54 PM, Warren Volz <> wrote:


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.

  Ron  KA4INM - Youvan's corollary:
                Every action results in unwanted side effects.
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