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In Reply to: RE: Which Diode ? posted by Tre' on October 06, 2023 at 11:41:55
OK, thanks. Specifically, the resistor was not shorted, it was the diode that was shorted. But the resistor was in parallel with the diode, so I could not confirm that the resistor was fine until I removed it from the circuit.Also, the wire was shorted to the leg of the diode, but the very first thing I did was remove the wire and wrap with electrical tape. I thought I had found the problem, and retook the measurements and still got the 3 volts. But the diode was damaged and was still shorted.
But since the Cap, resistor and diode were all in Parallel, they would all see the 'Short'. Correct?
Edits: 10/06/23Follow Ups:
the 330 ohm resistor is in parallel with the 330uf cap (one set for each tube).
The diode is in series with that paralleled set and so is the 10 ohm resistor.
the diode is shared between the two tubes.
If the resistors are good and the caps are good and there are no wires touching them but the one tube is still drawing too much current then something is wrong but it can't be the diode. If the diode is causing the one output tube to draw too much current, then it would be causing both output tubes to raw too much current. So that is not the cause of the one tube drawing too much current.
Have you checked the coupling cap that feeds the control grid of the tube that is drawing extra current? (the .22uf 600v cap) If it is leaking DC voltage to the grid of the output tube from the plate of the driver tube that would cause your problem.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
No the diode is in Parallel with the Cap and the resistor.... Maybe the schematic is wrong (I usually have a problem matching the 'supposed' schematic with the actual circuit), but here is the picture of a Tube in the circuit... Technicians are known for not updating the documentation with revisions to a circuit. I suspect that is what is going on here.There is a diode for each power tube, along with a 330 Ohm resistor, And a 330 uf Cap. Here is a Picture. The Cap, Resistor and Diode all attach to the two same terminals. Diode is indicated with an Arrow....
And remember my Bad Diode is a DEAD Short.... It is Bad ! --- BTW - This was my very first Amplifier Build. I would have done a much neater job today.
Edits: 10/06/23 10/06/23 10/06/23 10/06/23
Oh, well that changes everything.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Would it have been possible for a Power transformer wire to get hot enough to melt the coating into the leg of the diode wire, causing the diode to short out when it was a metal to metal contact.
Edits: 10/09/23
What voltage is normally on the brown transformer wire?
And did it weld to the cathode (stripe end) or anode of the diode?
Cheers
The transformers get hot. The wire is hooked to the transformer.
And it was the striped end of the diode.You tell me. Why do you think it got so hot?
But, I can tell you this... The problem that existed is now gone. The amp played great today for 10 straight hours. Football Games early in the day and Music the last 4 hours...
Edits: 10/08/23 10/08/23
Suggestion go research 'zener diode used as shunt regulator.'
Then find out what the transformer brown wire does & what AC voltage it supplies.
Cheers
What do you think caused my bad diode to fail and short out?
With the Kathode resistors?
Just saying & what's the diode part number?
Cheers
Build Instructions are very specific on where to attach diodes. Same terminals as the resistor and Cap. I was not smart enough or lucky enough to come up with an 'accidental' working design. Back then or Now! The 4 diodes are 1N5363B(Note: This diagram does not show all parts. There are quite a few of these in the build instructions.)
Here are the attach points for Diodes, Caps and Resistors. - All in Parallel.
Edits: 10/06/23 10/07/23
So I'm assuming they are a fail safe if for example Rk develops an o/cct or pin 8 on valve base looses continuity with valve pin. According to spec sheet the diode has a max regulated current of 158ma. I'm assuming if this is a EL34 PP amp the valves are each running 50-60ma individually. So across 330 ohm cathode resistor @ say 50ma current you should read around 15-18V across each cathode branch (resistor).But if a open cct in that branch occurred I'm assuming the voltage would go high and the zener would conduct to stabilize the cct in which you would hear distorted sound through that channel. Why the zener popped is unknown to me but I would replace all the parts and check in question the associated solder joints and make sure pin 8 on socket is tight. Best of luck, cheers.
It is basically an EL34 PP Amp, but you can run a variety of Power Tubes, as it is self biasing. You can use 5881s, KT-66s, E34Ls and KT-90s I have run KT-88s for over 20 years in it.The only parts I am replacing is the resistor and the diode and a new set of Power Tubes. I've only got two solder points for these parts. So, I have cleaned off all the solder and will re-solder when I get the diode in place.
If you're interested there was a review done on this amp over 20 years ago at the below link.
Here is a picture of my amp which had wood side panels, which came with the kit.
Edits: 10/07/23
Have not seen this application before, anyone can explain.
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Is fwd or reversed biased across the cathode in question(amp)?Surely that amp diode would be conducting as the voltage would be well above Vd if the diode in question is fwd biased.
Cheers
Edits: 10/06/23
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