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Re: [Amps] SB220 step start blows fuses

To: "'Carl'" <>, <>, "'amps'" <>
Subject: Re: [Amps] SB220 step start blows fuses
From: "David Gow" <>
Date: Wed, 5 Jul 2017 09:49:08 -0700
List-post: <>
I pretty much agree with Carl about the cement resistors. When I rebuilt my 
Henry 3KA I stuck with good old Ohmite vitreous resistors and was amazed that 
they are still in production, including the mounting hardware and not at all 
expensive.  I got a full set for the Henry from Mouser and only because someone 
had dropped something on the disassembled chassis and shattered all of the HV 
bleeders.  It helped that the Henry was free for the pickup (in a state of 
disassembly ).  The hardest part was figuring out the wiring harness and Ted 
Henry helped me with that about a year ago. If the switch is the big worry 240 
volt power strips are plentiful and cheap at your friendly industrial supplier 
or on the web.  Otherwise I still think the soft start is a waste of money but 
that is just my opinion and I respect yours.

73 and SK Dave W7VM
-----Original Message-----
From: Carl [] 
Sent: Wednesday, July 5, 2017 8:21 AM
To: David Gow;; 'amps'
Subject: Re: [Amps] SB220 step start blows fuses

Both the filament and power transformer characteristics are well known BUT the 
On Off switch is unobtainium and the HV transformer will still thump at times 
when turned on depending on where in the AC cycle it is at.
To further add to switch injury is the tendency to increase filter cap values 
beyond reason.

I build my own soft starts since the kits with cement style resistors were 
known failure points, dont know if that has been changed....but nothing beats 
an old style vitreous enameled wire wound resistor.


----- Original Message -----
From: "David Gow" <>
To: <>; "'amps'" <>
Sent: Tuesday, July 04, 2017 11:49 PM
Subject: Re: [Amps] SB220 step start blows fuses

> 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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