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Old 16th January 2003, 12:40 PM   #11
dhaen is offline dhaen  Europe
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Paul,

Yes, you must test with the source disconnected, and the input properly terminated. Otherwise there are too many factors involved.

Cheers,
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Old 19th January 2003, 04:48 AM   #12
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Hi John,

If you are still there? I made a RCA shorting plug for the input on the amplifier. Wire from + to - .

The voltage on the DVM is from the B+ cap.

The probe from the scope was connected to the inside of the chassis on the binding post solder lugs. Ground from probe attached to chassis grounded binding post. Passive probe attached to positive binding post.

23 step GoldPoint attenuator all the way off.

Spikes are 120hz apart, every other spike ( all positive spikes on sine wave ) are 60hz.

How bad is it? I'm braced and sitting down.
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Old 19th January 2003, 12:21 PM   #13
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Paul,

To measure what's actually being fed to the speakers, move your scope ground probe to the -ve speaker terminal.
Now re-evaluate.

The spikes are probably rectifier switching. To prove this, disconnect the heater transformer from on side of the rectifier while the thing is running (careful!), then, in the second before the heaters actually go out, see if the spikes disappear.

There are ways to reduce these, but let's be sure of the cause first.

Paul, If you want to "wake me up", there's no problem with sending me an e-mail. I'll probably reply on the forum though, as others might be watching for hints.

Cheers,
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Old 19th January 2003, 03:47 PM   #14
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Hi John,

Quote:
the -ve speaker terminal
Could you explain what -ve is? Would this be the negative speaker binding post? That is where I connected the probe ground.

Quote:
The spikes are probably rectifier switching. To prove this, disconnect the heater transformer from on side of the rectifier while the thing is running
The heater transformer wire to the GZ37 tube? Could I temporarily solder in a SPST toggle switch? This would be the safest way, I think.

John, I also noticed on the Audio Asylum that you are an Audio Professional, just curious as to what exactly it is you do? You obviously know your electrons.

Thanks again.
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Old 19th January 2003, 03:57 PM   #15
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Paul,

Quote:
Could I temporarily solder in a SPST toggle switch? This would be the safest way, I think.
Yes, that sounds fine.

Sorry about the negative binding post confusion. I was thinking that you were connected to your star earth point. (Which is a valid point for all other measurements).

Yes, I'm an electronic engineer.
Most of my recent work has been in Broadcast. But I've been around for longer than I'd like to mention .

Cheers,
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Old 19th January 2003, 07:06 PM   #16
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John,

I did as instructed. There is no difference when switching off the filament to the GZ37. The spikes do not disappear, they decrease, as the B+ decreases.

Here is a pic of the setup:
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Old 19th January 2003, 07:11 PM   #17
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Here is a pic of the test gear output when B+ is decreasing. About 5 sec. after the filament switch turned off.

Hate to ask but, now what?
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Old 19th January 2003, 07:18 PM   #18
dhaen is offline dhaen  Europe
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Paul,

I'm sorry, another misunderstanding. I must read your posts more carefully. It's my fault.
I meant one if the wires to the heater rectifier for the 300B.
It's those Schottky diodes that I'm suspicious of.

Cheers,

John
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Old 19th January 2003, 07:29 PM   #19
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OK John,

Just to make sure I am going to do this right . I need to put the switch in series with one of the 300b filament wires?


And thanks again
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Old 19th January 2003, 09:19 PM   #20
dhaen is offline dhaen  Europe
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Hi Paul,

Not quite....
In series with just one of the wires that runs from the transformer to the Schottky rectifier bridge.

Cheers,
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