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Kofi Annan in: "Tube Amp for Multi-Way Speakers"
Kofi Annan in: "Tube Amp for Multi-Way Speakers"
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Old 18th October 2019, 01:41 AM   #111
Kofi Annan is offline Kofi Annan  United States
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Quote:
Originally Posted by kevinkr View Post
Are you sure those 20W resistors are actually 1.2K? At 70VDC the resistor would be dissipating just 4W, consequently it seems unlikely that that is the resistor value.

But I wonder why you are doing this at all?

I would normally apply power to the entire amplifier and measure all voltages and currents starting at a small fraction of the rated line voltage. You can also use a ballast lamp in series with the primary to minimize mischief in the event of a mistake.
Thanks for the reply.

So, I Did bring up the voltage slowly and found that the voltage dropping resistors started smoking about 70VDC in. Thatís when I tested the supply across the 1.2k and it burned out at around the same voltage. Iíve probably got a little fatigue and Iím not thinking clearly, but Iím not understanding why the resistors would fry in these circumstances.

Iíll try again tomorrow. Any clear, non-exhausted insights would be welcome.

Kofi
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Old 18th October 2019, 02:37 PM   #112
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Did you measure the 70VDC directly across the 1.2k resistor?
Now, 200VDC across a 1.2k-20W, that would get real hot.
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Old 18th October 2019, 02:56 PM   #113
Kofi Annan is offline Kofi Annan  United States
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I did. Just to prove that I hadn't discovered a loophole in Ohm's Law, I tried again with a 400R 10W job and was able to get about 60VDC across it without letting the smoke. The resistr was labeled 12W, but I pulled it from an old amp and it may have seen better days.

The fact that I smoked the two 75R resistors in series with the circuit implies a short somewhere. I also noticed that one of the screen grid windings has about half the resistance to ground as the others. I'm a little concerned that it implies a shorted turn somewhere.

More testing to come.

Kofi
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Old 18th October 2019, 03:26 PM   #114
scott17 is offline scott17
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So the latest test with the 400R-10W is with the amplifier circuit connected?

Do you have a power supply schematic handy that you could post?
Have you tried powering just the supply with 120VAC and see what the output voltage is? If there is problem, with the supply you will blow the main fuse.
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Old 18th October 2019, 03:41 PM   #115
Kofi Annan is offline Kofi Annan  United States
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Quote:
Originally Posted by scott17 View Post
So the latest test with the 400R-10W is with the amplifier circuit connected?
Nope. Just across the resistor. Wanted to pull the circuit and test separately, which, admittedly, is not conclusive. I think I just wanted to make sure I was getting rectified DC.

Quote:
Do you have a power supply schematic handy that you could post?
Have you tried powering just the supply with 120VAC and see what the output voltage is? If there is problem, with the supply you will blow the main fuse.
I have attached the PSU design. Note the 80R resistor was in series with the circuit but without the final cap. And it smoked at around 50VDC.

Thanks so much for all the help!

Kofi
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Old 18th October 2019, 04:25 PM   #116
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With 60VDC across a 400R resistor, it is dissipating 9W, so yes, it will get very hot and I believe de-rate quickly from there. 65VDC and you are over the limit of 10W.

I modeled your same power supply in PSUD2 and got the same exact results, of course. I'm kind of at a loss.

Did you measure the AC transformer output voltage with the secondaries in parallel?
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Old 18th October 2019, 04:42 PM   #117
Kofi Annan is offline Kofi Annan  United States
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Quote:
Did you measure the AC transformer output voltage with the secondaries in parallel?
Do you mean pull the AC from the circuit and measure the full output voltage from the toroid? Nope. Haven't done that. I'll give that a go and post results.

My paranoia is kicking in here regarding a resistance measurement. One of the left channel screen taps is measuring about 60R to ground while the other three are measuring about 140R to ground. Wondering if there's a shorted turn in the transformer that may be causing this?

Kofi
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Old 18th October 2019, 05:23 PM   #118
scott17 is offline scott17
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I don't think you should read resistance that low from any of the output xfmr primary taps to ground including the screen taps. You would get a high charging reading through the power supply caps. But if your power supply is disconnected from the amp circuit, you should not read any resistance from the taps to ground.
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Old 18th October 2019, 06:05 PM   #119
Kofi Annan is offline Kofi Annan  United States
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Hmmm... so with the PSU *disconnected* from the circuit, I read a couple of megaohms from the B+ supply to ground. With the supply *connected* to the circuit, I get about 100 ohms from B+ to ground.

Gotta be a fault somewhere, right?

Kofi
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Old 18th October 2019, 06:34 PM   #120
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Yes, somewhere. That means if you were to measure resistance from where your B+ connects to the OT, and ground, (with the PS disconnected), you would read about 100 ohms. That's a problem.

You said you were getting strange resistance readings on the OTs, why not try disconnecting the OT primary completely from the amp circuit and see what you get. Also disconnect one end of R24 feeding B+ to your preamp/splitter section just to remove that from the equation as well.

Nothing in your final schematic indicates that you should read 100 ohms to ground.

No wonder your dropping resistors initially failed. If you indeed have 100 ohm load on the power supply, then even at 70VDC, it's trying to pull 700mA.

Last edited by scott17; 18th October 2019 at 06:59 PM. Reason: additional comment
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