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Re: [Amps] SB220 step start blows fuses

To: David Gow <>
Subject: Re: [Amps] SB220 step start blows fuses
From: Warren Volz <>
Date: Wed, 5 Jul 2017 00:06:44 -0600
List-post: <>
What about any large inrush of current through the on/off switch?


> On Jul 4, 2017, at 9:49 PM, David Gow <> wrote:
> There is little to be gained by adding a soft start to an SB-220.  I don't 
> have the original articles handy any more but it was common knowledge back 
> during the SB-220 production life and confirmed by Heathkit that the design 
> of the plate transformer actually does create a soft start effect and that it 
> was purposely designed that way.  The 3-500Z tubes were designed as instant 
> heating tubes and although a filament soft start my have some slight thermal 
> shock advantage it would be minimal and not worth the trouble and added 
> complexity and one more thing to fail.
> Dave W7VM 
> Past owner of at least (5)  SB-220s over the decades but now I love my solid 
> state 1200 watt amp instead.
> -----Original Message-----
> From: Amps [] On Behalf Of Ron Youvan
> Sent: Tuesday, July 4, 2017 2:51 PM
> To: amps
> Subject: Re: [Amps] SB220 step start blows fuses
>    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 winding.
>> 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:
>>> 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.
> -- 
>   Ron  KA4INM - Youvan's corollary:
>                 Every action results in unwanted side effects.
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