[Amps] SB220 step start blows fuses

David Gow d.gow at frontier.com
Tue Jul 4 23:49:18 EDT 2017

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 [mailto:amps-bounces at contesting.com] 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 <warren at warrenvolz.com> 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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