[Amps] SB220 step start blows fuses

Warren Volz warren at warrenvolz.com
Tue Jul 4 17:30:58 EDT 2017

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 <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.
> Any advice would be very much appreciated!
> Thanks!
> -Warren

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