Bad idea to run indicator lamp this way?

[Gregg]

Considering that most of those switches are rated at 10 to 15A at 250VAC I don't see the problem. This one for example (actually the only one I've sen so far) is rated even at 10A/400VAC with peak current of 30A:

http://www.farnell.com/datasheets/1697794.pdf

[Gaz]

Considering that most of those switches are rated at 10 to 15A at 250VAC I don't see the problem. This one for example is rated even at 10A/400VAC with peak current of 30A:

http://www.farnell.com/datasheets/1697794.pdf

I agree, that's why I'm hoping jmaf can give some clarification on why he avoids AC switching. I just don't see how it could really be more stress on the switch than 650vdc...

[jmaf]

I agree, that's why I'm hoping jmaf can give some clarification on why he avoids AC switching. I just don't see how it could really be more stress on the switch than 650vdc...

I would not move the standby switch to the AC side. We get away with switching higher DC voltages than the switches are rated for because of the low current in tube amps high voltage circuit. But if you move the switch to behind the rectifiers, the switch is going to get hit with the capacitor inrush current + 2 times it's peak rated AC voltage.

Anywhere before the first reservoir cap, the switch is hit with maximum voltage and maximum current when turned on, AC or DC.

On the AC side you have both the AC insulation problem and you have the inrush current problem. On the DC side before the reservoir you have the DC current switching problem (pulsed DC) and capacitor inrush current. So in both these cases before the reservoir cap you have 2 issues together.

So the best deal IMO is to place the switch after the reservoir cap.

[Gaz]

Anywhere before the first reservoir cap, the switch is hit with maximum voltage and maximum current when turned on, AC or DC.

On the AC side you have both the AC insulation problem and you have the inrush current problem. On the DC side before the reservoir you have the DC current switching problem (pulsed DC) and capacitor inrush current. So in both these cases before the reservoir cap you have 2 issues together.

So the best deal IMO is to place the switch after the reservoir cap.

Thanks for the clarification! Appreciate it.

With that said, is there any place I could add a standby switch to that Hiwatt power supply using only a SPST switch, like in series with the fuse, for example? Or is that a horrible idea?

[Gregg]

Anywhere before the first reservoir cap, the switch is hit with maximum voltage and maximum current when turned on, AC or DC.

On the AC side you have both the AC insulation problem and you have the inrush current problem.

If we assume a "regular" 100W amp with a secondary voltage of say 350VAC:

1/ How much is the "maximum voltage" ?
2/ How much is the "maximum current" ?

[jmaf]

Thanks for the clarification! Appreciate it.

With that said, is there any place I could add a standby switch to that Hiwatt power supply using only a SPST switch, like in series with the fuse, for example? Or is that a horrible idea?

Gut instinct tells me that removing the ground reference by opening the fuse circuit might not be a good idea.

The 280 VAC secondary would be free to float, but the 170 VAC secondary still has a path to ground via the OT and tubes, also the 220k resistors set a reference to the chassis....

I'm not sure what would happen but this is a 650 V amplifier. There's a reason why Hiwatt switches off those two high voltages at once and not the ground reference. I'd play conservative with 650 VDC

[Gaz]

Gut instinct tells me that removing the ground reference by opening the fuse circuit might not be a good idea.

The 280 VAC secondary would be free to float, but the 170 VAC secondary still has a path to ground via the OT and tubes, also the 220k resistors set a reference to the chassis....

I'm not sure what would happen but this is a 650 V amplifier. There's a reason why Hiwatt switches off those two high voltages at once and not the ground reference. I'd play conservative with 650 VDC

Sure, I'm just wondering why then this is an acceptable location for the fuse then, since opening the SB switch would be just the same as the fuse blowing.

[jmaf]

Sure, I'm just wondering why then this is an acceptable location for the fuse then, since opening the SB switch would be just the same as the fuse blowing.

The reasoning makes sense but I've never tried it. The fuse is supposed to fail under exceptional circumstances, I'm not sure that's where I'd place a standby switch for normal operation. Why didn't Hiwatt do it?

[Gaz]

The reasoning makes sense but I've never tried it. The fuse is supposed to fail under exceptional circumstances, I'm not sure that's where I'd place a standby switch for normal operation. Why didn't Hiwatt do it?

True.

[pdf64]

OK, I'll be the one to say it; how about just leaving standby off the amp?
Being as there's no technical benefit, but rather drawbacks to both its implementation and use.
If a mute function is a customer spec, it could be much better implemented in the signal path.

[Enzo]

If you are designing or building one, sure, why not?

Companies have found that the consumer expects a standby switch, and sales increase when one is added. Peavey found that when they revamped their popular Classic 30 into the Classic 30-2. Added a standby switch after 20 some years. The market speaks.

As to the old Hiwatt, I see no reason in the world to change it from the original. There may have been "better" ways to make it originally, but they have worked this way the last 40 years, and have no reputation for problems involving the power supply or switch. And to remove it at this point would just leave a hole. Literally or figuratively.

[pdf64]

I think Gaz indicated on post #3 that he wanted to use a progressive ie combined 'off-standby-operate' type switch.
So either he didn't have the extra panel space for a dedicated standby switch, or he had another use for that panel space.

Marshall puts them on the tube cathodes on the JCM 900s, I like to place HT fuse right before the OT center tap.

I think it would be a bad idea to cut the HT to the plates whilst leaving the screen grids energised, as that may cause the screen grids to conduct heavily, likely to their detriment.

A fuse in the cathode return seems a neat way to catch both, but it misses out a short that sends HT into the heater circuit.

It's difficult to protect against every common failure mode!

[Gaz]

OK, I'll be the one to say it; how about just leaving standby off the amp?
Being as there's no technical benefit, but rather drawbacks to both its implementation and use.
If a mute function is a customer spec, it could be much better implemented in the signal path.

I did consider it, but I thought that 650vdc+ DC may be a bit high for the no-standby-necessary category.

If you are designing or building one, sure, why not?

Companies have found that the consumer expects a standby switch, and sales increase when one is added. Peavey found that when they revamped their popular Classic 30 into the Classic 30-2. Added a standby switch after 20 some years. The market speaks.

As to the old Hiwatt, I see no reason in the world to change it from the original. There may have been "better" ways to make it originally, but they have worked this way the last 40 years, and have no reputation for problems involving the power supply or switch. And to remove it at this point would just leave a hole. Literally or figuratively.

I try not to muddy-up the technical questions with my selfish motives for doing things, but here's why I asked the the initial question: I have this amp built, and it's essentially a DR201 clone, but I really want to add another useful control to the front panel, but have absolutely no room. I thought it would grand if I could use one of these switches I linked to earlier for both power and standby functions.

Problem is that the original Hiwatt standby requires a double-pole switch, and so I am in search of a safe place where I could instead use a SPST for the standby. My initial question about the indicator was spawned by a now-forgotten hair-brained idea, and I am still looking for an alternate location in the power supply for the standby. I actually don't really care if it's just a mute, traditional standby, or something inbetween (soft-start?), so I'm open to any suggestions.


Prior to posting I did just try this below, by moving the standby where the red "X" is marked, and leaving the plate supply connected all the time. It worked like a charm, but I don't know if there's any danger in doing it that way.



With this arrangement, in Standby, the plate voltage is reduced from 680vdc to about 300, and the screen supply is reduced to about about 50vdc. Why? My limited knowledge tells me think that without the screen supply's rectifier 'active' and only the top rectifier working, the only path to ground is now through the 220K balancing resistor, which is floating the plate supply 50vdc above ground. That 50vdc is consequently the only voltage which is being fed to the screens and preamp nodes downstream.

This does work as effective complete mute, and the 50vdc supply is low enough to put the tubes into full cut off. I'm just not sure if there's any reason not to do this. It actually seems like a pretty good way of doing it on first glance because the HT only rises initially to 300vdc instead of just being slammed with the full 680vdc plate supply as if there was not standby at all.

And I'm not try to look Daves Reeve's in the mouth, but I just need the panel space and would like a mute function in a single switch if possible. I suspect it was done originally the way it was to provide full isolation for servicing/testing, but who knows...

[pdf64]

I was going to suggest applying standby to the screen grids somehow; your X method seems good.
But they're still receiving tubes and are being used within their limits, so I don't see the technical need for a HT standby.

[g1]

I put HT fuses on the DC side, after the big caps. Marshall puts them on the tube cathodes on the JCM 900s, I like to place HT fuse right before the OT center tap.

I think it would be a bad idea to cut the HT to the plates whilst leaving the screen grids energised, as that may cause the screen grids to conduct heavily, likely to their detriment.

A fuse in the cathode return seems a neat way to catch both, but it misses out a short that sends HT into the heater circuit.

Probably not relevant, but just to clarify a bit, the Marshall cathode fuses are for output tube failure only, they also have a HT fuse right after the OT feed. But I guess that would still miss a plate to heater short.