Debugging with a scope?

[d2camero]

Icetech, a little confused - the layout diagram shows V4 as reverb driver tube - is this correct?

Also, the layout does not show the OT, so I cannot see where the NFB is to connect back into the circtuit. Any chance for a complete schematic?

Any chance for a detailed pic of the amp wiring, so we can see lead dress?


thanks d1

[Icetech]

Sorry dark, v4 is the driver tube, i just thought 1 2 3 4, i didnt see he labeled them funky on the layout:)

The nfb on this comes from one of the lugs on the speaker jack to the 4.7k resistor and to where it shows on the layout..

I can try the OT thing after work tonight.. and take some pics..

[d2camero]

A couple of "observations"

- what tap are you using on the OT for NFB? This will affect the total amount of NFB and you will have to adjust the 4.7k resistor accordingly.


- the layout is missing a connection from the 0.1uF cap to the 1M grid bias resistor on the PI. Although, I doubt you left this unconnected


- can you post a pic using a sine wave? Cannot quite see of the issue is a parasitic oscillation.

- Aiken has a good article on NFB, well worth reading. http://www.aikenamps.com/NegativeFeedback.htm


d1

[Icetech]

Donor was a bassman head.. only has 1 tap.. basically the nfb wire goes straight from the speaker output to the 4.7k resistor..

Will read that article and try other things when i get home tonight.. thanks guys.

[skyboltone]

I would just try to isolate the secondary of the OT from ground. Generally, the speaker jack has the ring grounded with the pin ungrounded. What I'm wondering is if you've got an insulation failure in the secondary (8 ohm) winding that allows the feedback portion of the signal to get dirty at higher levels.

A speaker output jack with no ground connection would be a good test with everything wired as normal.

Dan

Continued clarification after sleeping on it: I'm thinking a winding is arcing to ground internally in the OT. You are then injecting this arc interference directly back into the amp at the PI. The arcing could be on the primary or secondary side. So here's what you do. Unbolt the OT from the chassis and slide an insulator (garolite, dry cardboard etc) under the feet so that the case is not grounded. Isolate the speaker jack from ground. Now we have no ground on the OT at all. CAREFUL!! A LEAKAGE RESISTANCE EVEN OF A COUPLE OF MEG COULD PUT A NASTY FATAL CHARGE ON THE CASE OF THE OT. DON'T TOUCH THAT SUCKER!! This is for experiment/diagnosis only!!! Put some earplugs in. Leave the speaker hooked up so that you got a load. We used to salvage big power transformers this way in old AM transmitters. The case would be hotter than a pistol but it's inside the transmitter where everything is dangerous anyway.

Tell us what happens

[Darkbluemurder]

I would just try to isolate the secondary of the OT from ground. Generally, the speaker jack has the ring grounded with the pin ungrounded. What I'm wondering is if you've got an insulation failure in the secondary (8 ohm) winding that allows the feedback portion of the signal to get dirty at higher levels.

A speaker output jack with no ground connection would be a good test with everything wired as normal.

Dan

Continued clarification after sleeping on it: I'm thinking a winding is arcing to ground internally in the OT. You are then injecting this arc interference directly back into the amp at the PI. The arcing could be on the primary or secondary side. So here's what you do. Unbolt the OT from the chassis and slide an insulator (garolite, dry cardboard etc) under the feet so that the case is not grounded. Isolate the speaker jack from ground. Now we have no ground on the OT at all. CAREFUL!! A LEAKAGE RESISTANCE EVEN OF A COUPLE OF MEG COULD PUT A NASTY FATAL CHARGE ON THE CASE OF THE OT. DON'T TOUCH THAT SUCKER!! This is for experiment/diagnosis only!!! Put some earplugs in. Leave the speaker hooked up so that you got a load. We used to salvage big power transformers this way in old AM transmitters. The case would be hotter than a pistol but it's inside the transmitter where everything is dangerous anyway.

Tell us what happens

This does not sound like a test for the inexperienced. In addition I would not know what to look for. What would happen if the OT were OK? What would happen if not? Checking another OT seems to be less dangerous to me although certainly more expensive.

[skyboltone]

This does not sound like a test for the inexperienced. In addition I would not know what to look for. What would happen if the OT were OK? What would happen if not? Checking another OT seems to be less dangerous to me although certainly more expensive.

I agree. About the danger. If the ungrounded OT works OK with the normal feedback connected as drawn, rip the OT out and throw it away. Replace it with a new suitable transformer. The test is a last ditch diagnosis tool only. NEVER OPERATE AN AMP WITH AN UNGROUNDED TRANNY CASE!!!!! The only reason I suggested it is because the usual suspects are not solving the problem. This test may not either.

Dan

[Darkbluemurder]

Another tip would be to check RG Keen's site (I believe it is www.geofex.com). There's a debugging page with good info. There are some tests to identify a bad OT. Maybe you can detect whether your OT is bad through these suggested measurements. I would not set hopes too high though. My last bad OT passed all the measurement tests yet it was bad (only put out a fraction of normal volume).

[Icetech]

Ahh!! Sky, thanks for the clarification... the way i read it before was like you wanted me to remove the nfb and ot tap from the jack and solder them together.. thats why i was like WTF....

I understand what you're saying now.. and not that dangerous.. just common sense not to eat a few hundred volts (again)

Once again, thanks for all the help guys... if worst comes to worst, i have a old princeton amp with 50 or 100 watt (huge) trannies in it, i could steal the OT to see if thats the prob:)

[skyboltone]

My last bad OT passed all the measurement tests yet it was bad (only put out a fraction of normal volume).

This is an excellent point. All we are testing with my suggestion is the primary to ground and secondary to ground insulation integrity. We could still have leakage primary to secondary or either winding to another portion of itself. If you have a known good OT it might be just as well to wire it in there and see what comes of it.

Dan

[Icetech]

Sorry for the slow response, had company over tonight, tried it with the speaker jacks and OT isolated.. and no sound output at all (which would make sense to me since there is no ground for the OT..) with just the speaker jacks grounded, i got sound and same prob. im gonna steal a OT outa something this weekend and see if the prob goes away.. then try to find a cheap replacement..

Have a reverborocket reissue here thats a complete turd.. might see if i can get that to work in there for testing...

[skyboltone]

Sorry for the slow response, had company over tonight, tried it with the speaker jacks and OT isolated.. and no sound output at all (which would make sense to me since there is no ground for the OT..) with just the speaker jacks grounded, i got sound and same prob. im gonna steal a OT outa something this weekend and see if the prob goes away.. then try to find a cheap replacement..

Have a reverborocket reissue here thats a complete turd.. might see if i can get that to work in there for testing...

An output transformer is an impedance matching device. Like any transformer it changes one voltage and current into another voltage and current based on turns ratio. There is high voltage DC on the primary side with an AC voltage component mixed with it. The transformer (which does not pass DC) simply separates the DC voltage from the AC and passes the AC at a lower voltage/higher current so that it can drive the voice coil of the speaker. The grounded negative terminal is for safety only. Should the high Voltage DC short to the secondary windings the grounded secondary winding will provide a path back to the power supply and blow the fuse. This keeping your B+ out of the speaker loop. The ground has nothing at all to do with whether or not the circuit produces a sound output. Check your wiring on the test. If it is wired like we discussed and still behaves the same way, I believe you have an open secondary winding. But why it makes any noise with the secondary grounded at the jack and the transformer case isolated falls under the category of FM. Or in the vernacular, freaking magic. Maybe the secondary winding is burnt in two with both sides touching the case. Intensely weird mister. I dunno. You got me on that one. In any case if it won't make noise with all grounds lifted you can safely assume the OT is toast. I'm sure of that. Take the end bell off and look around in there. Bob I was successful repairing a shorted OT recently. While you're at it with everything isolated and the circuit energized check the transformer case to ground with your volt meter. I'd like to know if it's hot.

If anybody sees something leaky in my analysis here pipe up now.



Dan

[Icetech]

I got it figured out... and i feel like an idiot.. When i built the amp, i didnt have a 2k pot for the presence pot.. so i stuck in a 5k and forgot bout it, heck.. whats 3k among friends anyway...

Anyway.. i usually had the presence down around 2-3.. i happened to bump it when checking things tonight. and the signal cleared right up..

I never bothered to look at the pi/driver section much, always seemed to think bout preamp and such.. didnt realize what the presence was doing til i stared at it for a minute..

Anyway, sorry for all the posts... now i gotta put the damn OT back in:)

[skyboltone]

Hehehehehehehehehehehehehehe. Ain't life grand. Boy we had fun talking about it though. So it was a capacitor leaking down like you said it sounded originally. Hehehehehe. It was leaking down through the 5K pot.

Wow.

Dan

[Icetech]

Yeah, i am not sure why being a 5k instead of 2 has it so crazy, one of my other amps i had a pro build uses a 5, of course thats a much simpler design, and all different. so apples and oranges:) Once again, thanks:) and now i can finally get around to doing all of dogears upgrades to that design:)