Hello,
I'm building the third of this design, a Coyote (by me), and have run into something Weird. It was making a horrible noise from the first time I tried it. A little poking around revealed that there is a 60 cycle AC voltage in the signal path that varies from 45 to 51 volts! I made some pics of the layout to explain this weird thing. There are arrows showing where the voltage shows up. It's on both plates of the PI though not equally; one only has about 30 volts (still way weird).
So, to try to find the source I lifted one leg from the power source - the pic shows it with a little gap in the wire. When I checked to see where the voltage was coming from it had disappeared! I checked everywhere in the amp - no stray AC voltages! None! I used an alligator clip to jump the gap and the voltage came back in all the places I found it before. I put that wire back and lifted the other plate power wire - voltage gone!
So, well, wtf?!
I cannot imagine where it could be developing. If anybody has a any clue Please let me know. I need to finish this amp.
Can't figure source of 50v AC
Moderators: pompeiisneaks, Colossal
Can't figure source of 50v AC
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Activax Amplification
Re: Can't figure source of 50v AC
The two lowest hanging fruits I can think of would be trying a known good tube, and disconnecting the nfb. Do either of those make it go away or get worse?
Re: Can't figure source of 50v AC
Yes, after checking those, as it's on the LTP outputs, are the signals differential, common mode, or unrelated?
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Re: Can't figure source of 50v AC
@nworbetan: I tried a known good tube - the voltage was very slightly lower but still there. This amp design has no NFB.
@ pdf64: I'm ashamed to admit it but it's been awhile since I was doing serious studying of LTPs. Been more about power supplies of late and mostly just putting together designs I know are good. So here's the schematic. Is it helpful?
Just so you know, this is the third one of these I've built. The others sound Great, and no phantom voltages. I have one here and I can't find any difference between them.
@ pdf64: I'm ashamed to admit it but it's been awhile since I was doing serious studying of LTPs. Been more about power supplies of late and mostly just putting together designs I know are good. So here's the schematic. Is it helpful?
Just so you know, this is the third one of these I've built. The others sound Great, and no phantom voltages. I have one here and I can't find any difference between them.
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Activax Amplification
Re: Can't figure source of 50v AC
where is the coupling cap for pin 2 on the schematic? I see your layout has one.
Re: Can't figure source of 50v AC
How many ac volts measuring from plate to plate of the PI? How many from the Rp10/Rp12 junction to ground?
If it says "Vintage" on it, -it isn't.
Re: Can't figure source of 50v AC
Mr Fowler: here's a wider view of the schematic -
@jjman: it measures about 70v AC across the plates of the PI. Again, this vanishes if either of the plate feeds is lifted. There is no voltage to ground at the junction of Rp10 & Rp12. In fact, there is no input voltage at all either to the LTP or anywhere else in the amp besides those places I have described.
This amp is built rather 'on the cheap' compared to its predecessor. So was the first one I built, a prototype for proof of concept. But it sounded so good it was bought up almost immediately.
Here are guts shots of the amp in question and it's working predecessor.
] Thank you guys for looking at this problem. So far I'm pretty stumped. How on Earth does voltage develop on its own? There must be something I'm missing. I made the board; it's exactly like the one in the prototype so no di-electric problems. I have check beneath the board and found nothing out of order there. So I'm hoping someone can have a flash of insight and say that it's this or that. I don't want to tear it apart and start over.
So thanks again for sharing your wisdom and tech skills. Now I'm going to read up on differential, common mode and unrelated signals and LTPs in general.
@jjman: it measures about 70v AC across the plates of the PI. Again, this vanishes if either of the plate feeds is lifted. There is no voltage to ground at the junction of Rp10 & Rp12. In fact, there is no input voltage at all either to the LTP or anywhere else in the amp besides those places I have described.
This amp is built rather 'on the cheap' compared to its predecessor. So was the first one I built, a prototype for proof of concept. But it sounded so good it was bought up almost immediately.
Here are guts shots of the amp in question and it's working predecessor.
] Thank you guys for looking at this problem. So far I'm pretty stumped. How on Earth does voltage develop on its own? There must be something I'm missing. I made the board; it's exactly like the one in the prototype so no di-electric problems. I have check beneath the board and found nothing out of order there. So I'm hoping someone can have a flash of insight and say that it's this or that. I don't want to tear it apart and start over.
So thanks again for sharing your wisdom and tech skills. Now I'm going to read up on differential, common mode and unrelated signals and LTPs in general.
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Activax Amplification
Re: Can't figure source of 50v AC
Shooting from the hip as usual, I might actually hit something sooner or later...
Those blue and brown wires from the plate of the output tubes to the output transformer are smack dab underneath the phase inverter input. Anything interesting change if you move those high power wires away from the sensitive high impedance part of the circuit?
If anyone can explain whether capacitive or inductive coupling is more likely in that scenario I'd like to read all about that.
Those blue and brown wires from the plate of the output tubes to the output transformer are smack dab underneath the phase inverter input. Anything interesting change if you move those high power wires away from the sensitive high impedance part of the circuit?
If anyone can explain whether capacitive or inductive coupling is more likely in that scenario I'd like to read all about that.
Re: Can't figure source of 50v AC
Good point! I checked to make sure no wires had compromised insulation, no shorts, but didn't think of a field effect. I'll check it out. Thanks!
Activax Amplification
Re: Can't figure source of 50v AC
Nope. Not the problem. I tried chopsticking everything to no avail, but I didn't expect much. The plate wire to the OT carry a lot of voltage but it's DC, not AC.
However, while I was messing around in there I discovered some more odd behavior. I mean really weird. I'm going to scope it all out and maybe I can tell more but I'm getting AC up 86v in various places. It seems to be originating from the 1st stage, not the LTP or PI.
Curiouser and curiouser.
However, while I was messing around in there I discovered some more odd behavior. I mean really weird. I'm going to scope it all out and maybe I can tell more but I'm getting AC up 86v in various places. It seems to be originating from the 1st stage, not the LTP or PI.
Curiouser and curiouser.
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Re: Can't figure source of 50v AC
Why are you tying pin 6 (plate) to the tail of the PI in the schematic? I see it is not that way in the pictures...
"Education is what you're left with after you have forgotten what you have learned" - Enzo
Re: Can't figure source of 50v AC
Sometimes "messing around in there" and "shooting from the hip" will get the job done, but it looks like it's time to get serious.
There's a simple troubleshooting theory that goes something like "Between the good and the bad is the ugly." If you have a circuit that's working correctly at point A, but things aren't working correctly any more at points C, D, E, F, etc, then point B is the ugly part that needs work.
So first things first, let's establish what your amp is doing correctly and call that point A.
Step one: pull all the tubes and check every single point on the power supply starting with the reservoir cap and moving back toward the preamp. The DC voltages are going to be higher than with the tubes, but that should be fine. The filter caps and bleeder resistors should have been chosen to be able to tolerate the unloaded voltage. The important part is that there should be practically zero AC at all with no load. Then check the power tube bias supply. Is your p/s good? Okay, that's point A.
Step two: plug the output tubes in and assess. Good? Point B.
Step three: plug in the PI and repeat. Good? Point C.
etc
There's a simple troubleshooting theory that goes something like "Between the good and the bad is the ugly." If you have a circuit that's working correctly at point A, but things aren't working correctly any more at points C, D, E, F, etc, then point B is the ugly part that needs work.
So first things first, let's establish what your amp is doing correctly and call that point A.
Step one: pull all the tubes and check every single point on the power supply starting with the reservoir cap and moving back toward the preamp. The DC voltages are going to be higher than with the tubes, but that should be fine. The filter caps and bleeder resistors should have been chosen to be able to tolerate the unloaded voltage. The important part is that there should be practically zero AC at all with no load. Then check the power tube bias supply. Is your p/s good? Okay, that's point A.
Step two: plug the output tubes in and assess. Good? Point B.
Step three: plug in the PI and repeat. Good? Point C.
etc
Re: Can't figure source of 50v AC
Okay, finally got it sussed. There were two ground connections that were crappy, one not connected at all. But moreover, this is cathode biased (EL84s). The resistors have Very tiny print and I accidentally put a 150K instead of a 150R. Boy, what a Duh! But now it sounds Great. Cranked all the way with nothing plugged in there's only a faint hiss. And, if I do say so myself, it has a unique look. This one is industrial - smells like cutting oil. Sits on a bench with dirty rags and edge grinders, right next to the welder. (okay, it doesn't really smell)
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Last edited by sepulchre on Sun Apr 29, 2018 11:09 am, edited 1 time in total.
Activax Amplification