Bad idea to run indicator lamp this way?

[mhuss]

Why not just go inline with the HT fuse? At that point you still technically have AC (albeit 'folded' AC), and that would completely cut off the HT current.

[jmaf]

Hey OP, speaking of cathodes.



If you place a resistor like 47K to 100K x 5 Watt across the standby switch and you interrupt the green circled ground connection using it, when you open the switch it'll lift the KT88's cathodes and will send the tubes into cutoff. Minimal current and low voltage. Could be a solution to your 3-way switch.

[Gaz]

Why not just go inline with the HT fuse? At that point you still technically have AC (albeit 'folded' AC), and that would completely cut off the HT current.

Yes, that was my initial thought. I figured: "If it's a good enough place for a fuse..." But Jmaf intuited it was not a wise idea. What do you mean by "'folded' AC"? Is floating like that any different than in an amp that uses a Full-Wave rectifier and lifts the CT?

Do you think that's better than my idea to cut off the screen supply on the AC side of the 'bottom' rectifier?

Hey OP, speaking of cathodes.

[ATTACH=CONFIG]39476[/ATTACH]

If you place a resistor like 47K to 100K x 5 Watt across the standby switch and you interrupt the green circled ground connection using it, when you open the switch it'll lift the KT88's cathodes and will send the tubes into cutoff. Minimal current and low voltage. Could be a solution to your 3-way switch.

I tried this and it didn't completely mute the amp (which defeats the purpose if I'm not even cutting off the the HT). The cathodes float at about 50vdc with a 47K resistor (only dissipates >1W at full signal, btw), which is fine, but it raises the B+ across the filter caps to about 720vdc. Seems like it's worse on all accounts than the method I tried, except for that it's theoretically easier on the switch as you explained. But the switch is rated at 10A @ 250VAC with a 1000vdc dialectric rating, so I'm not sure that's a practical concern.

[mhuss]

The AC at that point is rectified, but not filtered. I guess you could consider it "pulsing DC." I called it 'folded' because the waveform looks like one half of the sine wave was 'folded' across the 0 volt line, so the 'bumps' are all on side of zero.

However, with the switch (or fuse) open, there's going to be a pretty large potential across the open circuit (which is probably why DR did it the way he did as opposed to in series with the fuse). Perhaps a heavy duty relay might be the best bet.

[jmaf]

Other than these simple mods that use the switch directly, you could build a separate switch board where you can control 2 switches with one SPST.

These are 1000 V MOSFETS with really low Rdson (<= 2 ohms).

Using 2 optoisolator IC's to isolate the gates you can switch both FETs at once using just one SPST switch.

[Gaz]

The AC at that point is rectified, but not filtered. I guess you could consider it "pulsing DC." I called it 'folded' because the waveform looks like one half of the sine wave was 'folded' across the 0 volt line, so the 'bumps' are all on side of zero.

However, with the switch (or fuse) open, there's going to be a pretty large potential across the open circuit (which is probably why DR did it the way he did as opposed to in series with the fuse). Perhaps a heavy duty relay might be the best bet.

Right, and perhaps the placement of the fuse there in the initial design was somewhat of a compromise ensure both supplies would be cut off (opposed to two fuses). Do you see any glaring problems with the way I suggested in post #28? It doesn't fully cut off HT, but I'm okay with that (I always put a resistor a 220K/2W resistor across Standby switches anyway per Merlin's recommendation). I'm more concerned with the mute function. I just wanna make sure I'm not missing some reason not to do it that could stress any components needlessly. Thanks.

[pdf64]

Do you see any glaring problems with the way I suggested in post #28? It doesn't fully cut off HT, but I'm okay with that (I always put a resistor a 220K/2W resistor across Standby switches anyway per Merlin's recommendation). I'm more concerned with the mute function. I just wanna make sure I'm not missing some reason not to do it that could stress any components needlessly

I suggest to check that the caps for the screen grid supply don't get reverse biased somehow, when in standby mode.

[mhuss]

With your solution, you're basically opening up the bottom coil. The way the bridge diodes are arranged, the top coil still has a path to ground, hence the lower voltage you're measuring. As the previous poster commented, I'd just make sure that all the caps are biased correctly, and that the screen voltage is less than the plate on the output tubes. I'd expect the plates to see 300~whatever volts and the screens to be close to zero. This is a safe scenario.

[Gaz]

I suggest to check that the caps for the screen grid supply don't get reverse biased somehow, when in standby mode.

With your solution, you're basically opening up the bottom coil. The way the bridge diodes are arranged, the top coil still has a path to ground, hence the lower voltage you're measuring. As the previous poster commented, I'd just make sure that all the caps are biased correctly, and that the screen voltage is less than the plate on the output tubes. I'd expect the plates to see 300~whatever volts and the screens to be close to zero. This is a safe scenario.

Dumb questions, but how do I check for this? Do you mean a negative voltage gets applied to the positive side of the caps somehow?

@mhuss: The screen voltage is at zero when the bottom coil is opened and the the plate voltage is also around 300vdc as you guessed. Initially I said that the screen were resting at 50vdc, but I forgot to mention I had bleed resistor across the switch terminals per Merlin's suggestions. Perhaps that would be a way to avoid the reverse bias issue if that's the case. Again, just not really sure how to check for the 'reverse biased' caps....

Thanks!

[mhuss]

Yes, just make sure the '+' side of the five filter caps are all the same as or more positive than the '-' end.

[Gaz]

Ah, okay, I think I understand. The reverse biasing of the caps would only happen if the screen supply (feeding the junction of the stacked caps) was to be higher than the plate supply, correct?

[mhuss]

yep

[Gaz]

Thanks for the clarification. I think I'm in the clear. This wouldn't be possible without your schematics, so thank you!

[jmaf]

If he's cutting the circuit where the red X is on post #28 then it is impossible for the caps to get reverse biased under any circumstance. At worst possible scenario the difference would be 2 diode drops correctly biased.

[Steve A.]

Gaz: As I posted a few nights ago (#6) you could use a 3PDT center-off toggle switch without changing any of the stock secondary wiring (assuming that you just need a single pole to switch off the main power- otherwise you would need a 4PDT switch.)

When they first came out with the progressive toggle switches 15 years ago I was really excited and picked up several from Allen Amps as I recall. But as Bruce Collins pointed out you could just as effectively use a regular center-off DPDT toggle switch although without the progressive feature. Just flip the switch one way for Standby and the other way for On. (During the short time while going from Standby to On the tube filaments should stay hot.)

Here is one eBay auction for a 3PDT switch which may or may not fit in the intended space for it. If not a rotary switch might work. IMO it is better to not change the original standby design unless absolutely necessary. (As someone else suggested you might want to incorporate some sort of mute function instead.)

Steve Ahola

Carling Technologies HM254 73 Toggle Switch 3PDT on Off On | eBay