Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: DrRogersA on April 16, 2024, 09:42:03 am
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Hello, all. I am making plans for rebuilding my 18W SuperLite into something more like a Hoffman Stout. In particular, I have been looking at the Blues Bandit variant described here: https://el34world.com/Forum/index.php?topic=6895.0 (https://el34world.com/Forum/index.php?topic=6895.0)
The layout in that thread has a DPDT On-Off-On switch that switches between tube rectification, standby, and solid state rectification.
I really like the idea of being able to switch rectification types. But that's high voltage from two sources, with a lot of AC ripple to boot (since the switch is before the first filter capacitor). And when I'm working on amps, my first, second, and third priorities are safety, safety, and safety.
So what sort of switch should I be looking for? Any particular rating? I see Carling switches, for instance, rated at so many amps at so many volts, but this is different: low current but high voltage. Or are these ratings really about power - current times voltage?
And is using a DPDT in this situation safer than a SPDT?
Has anyone else implemented a similar switch, and if so, can you tell me what you used?
Thanks much.
Regards,
Austin Rogers, PhD
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You need an On-Off-On switch to get all three benefits. SPDT will do.
Personally, I have never seen a switch specifically rated for DC, but that doesn't mean they don't exist.
A nice heavy Carling switch rated for 250VAC, I think, is probably what most people would say they use for such things.
Maybe others will chime in with more complete information.
Dave, Meritorious Service Medal (LOL)
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Switches typically have a DC rating that is much lower than the AC rating. The rating varies with current. For example, 30 VDC at 2A, 60VDC at 1A.
The switch ratings even for small switches usually don’t go down to the small currents (milliamp range) we’re talking about.
Even small toggle switches should work fine. If you want to really be on the safe side, use something the same size as the mains switch.
If you want to reduce stress on the Standby or Tube/SS rectifier switch, put a resistor across it.
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These are the switches I use. In the distant past I tended to use the big bad boys at the bottom of the page -- one is rated for 20A (!) at line voltage. As I've looked at the specs of these switches, it's hard to compare the mini switches with the big guys as they do not quote the same variables. But, the mini switches appear to be very robust, and I suspect they are at least the equal of the biguns in the high voltage (eg, tube amp) realm.
If you're concerned about arcing between the tabs on the switch, it takes several kV per mm separation -- so not likely to be a factor, though I do use heat shrink tubing and keep my connections clean.
For high current, the big guys are the way to go, of course.
The manufacturers aren't really going to help us out with more detailed specs . . .
https://www.tubesandmore.com/products/switches?filters=4083a4089c242a4083b4103c4103a4108 (https://www.tubesandmore.com/products/switches?filters=4083a4089c242a4083b4103c4103a4108)
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Thank you to all who replied!
After reading your replies, I started poking around in datasheets and came cross the concept of dielectric strength. I think that's what I was ultimately worried about, and it looks like I'm just being overcautious here. As long as I practice good lead dress (and I'll also use heatshrink) everything should be fine at these voltages and currents.
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Provided that the amp is properly fused and earthed, it's safe.
For high V DC, quick make / break switches are theoretically better.
Slow make / break have a tendency for arcing with DC.
Regular Carling switches are an example of quick make break, note their 250V AC DC rating.
Note that standby is a generally bad idea, so it'd be preferable to switch rectifier modes with the amp mains power switched off.