Hiss/oscillation on the OD channel of an ODS

[talbany]

Pictures Please!
Otherwise we're guessing

[pompeiisneaks]

The other random thing you may not have checked, is the OT may be out of phase and causing oscillations. Sometimes the problem is subtle and only shows up at pretty high levels of output. You could test quickly by either swapping the OT leads to the output tubes or by swapping the PI output wires.

Other ideas sound more likely but this is one that's bitten me more than once. It wasn't the usual "SCREAMING NOISE" you get if it's reversed in some cases.

~Phil

Good one!
Or disconnect the nfb wire

Very true, that's a very quick easy test, if that solves it, then you do need to swap the leads, if not, reconnect, only one lead to desolder

~Phil

[bepone]

Yes you're right. I also did some changings in the wirings. In fact I use this OD schematic for years, and now I use very few wires. I don't use turret boards anymore, and I solder the components directly to tubes/pots as much as I can (big work), Matchless like, but cleaner !... But this way or with boards, I still get this hiss/oscillation at some point.

i dont see any pic? try DC heaters for the first tube (you can use some lead 6V battery to test)
try to install MF resistors in apsolutely all section of the first input tube (grid leak, grid stopper, anode, cathode)
try to improve grounding
use the coax between some stages

[butwhatif]

I have found JJs to be the noisiest for hiss--

[alkuz1961]

I do not think the author is talking about the natural hissing of the tubes. Most likely, this is some kind of positive feedback between the stages. I once had a similar effect from the primary wires of the OT, which were laid under the board, next to the elements of the PI. I moved these wires to the outside of the chassis, and the problem was solved.

[CB Amp]

I do not think the author is talking about the natural hissing of the tubes. Most likely, this is some kind of positive feedback between the stages.

Yes, that's it ! Thanks.
It's a feedback, not from the OT, but I don't understand where that comes from.

I did all the things you mention : swapping tubes, MF resitors, with or without NFB on the OT, etc...
The only thing is that I use common coax wire, not a branded one. But I will buy some good one, even if I don't think it can come from a few pF in capacitance.

And I will post some pics ! Some of you will scream, just as the OD channel

[erwin_ve]

I do not think the author is talking about the natural hissing of the tubes. Most likely, this is some kind of positive feedback between the stages.

Yes, that's it ! Thanks.
It's a feedback, not from the OT, but I don't understand where that comes from.

I did all the things you mention : swapping tubes, MF resitors, with or without NFB on the OT, etc...
The only thing is that I use common coax wire, not a branded one. But I will buy some good one, even if I don't think it can come from a few pF in capacitance.

And I will post some pics ! Some of you will scream, just as the OD channel

CB,
before we start to see pictures and scream, a few things on your comments that raised my eyebrows:
1. If you dont know where the oscillation is coming from how would you identify if its coming from the OT or not?
2. Coax wire shields the signal from picking up all kinds of signals you dont want. If it's shielded it does the job. Using certain kind of brands or specs can have a impact on the tone of your amp BUT: the capacitance has nothing to do with the oscillations you have.
3. Pics!

[Reeltarded]

Based on new information I also believe this is the commonest problem ever to happen on any amp with some NFB.

Disco the NFB.

If it stops screaming..

Swap the darned OT wires and hook the NFB back up where it was.

[CB Amp]

If you dont know where the oscillation is coming from how would you identify if its coming from the OT or not?

Based on new information I also believe this is the commonest problem ever to happen on any amp with some NFB.
Disco the NFB.
Swap the darned OT wires and hook the NFB back up where it was

I already tried it, and the problem remains the same.
A feedback from the OT usually produces a huge low noise, at any volume and on both channels. It's difficult to miss.
The feedback I get is a high frequency one on the OD channel only, related to gain levels. It could come from the NFB indeed, but as I wrote, I tried many different NFB values from a few ohms to open circuit (also playing with the presence value), and the oscillation is still present.

[CB Amp]

Pics !
Here is an old design, from an amp that I built 15 years ago. The wiring is not that good, carbon comp resistors, no oscillation.

[CB Amp]

The one I'm working on.
I use terminal strips now, and my amps are dead quiet, no hum at all. MF resistors on OD, shielded cable, but it doesn't seem to suit the OD design. Oscillation on this one.

[pompeiisneaks]

If you dont know where the oscillation is coming from how would you identify if its coming from the OT or not?

Based on new information I also believe this is the commonest problem ever to happen on any amp with some NFB.
Disco the NFB.
Swap the darned OT wires and hook the NFB back up where it was

I already tried it, and the problem remains the same.
A feedback from the OT usually produces a huge low noise, at any volume and on both channels. It's difficult to miss.
The feedback I get is a high frequency one on the OD channel only, related to gain levels. It could come from the NFB indeed, but as I wrote, I tried many different NFB values from a few ohms to open circuit (also playing with the presence value), and the oscillation is still present.

Glad you tried it. BUT the statement that feedback from the OT usually produces a huge low noise is completely wrong. I've had that noise, A high pitched wail, I've had a really annoying oscillation hum at mid to high frequency that was barely audible, but the OT swap made it disappear. If you said that swapping OT had no impact then that's not right either because if you had it right and swapped there'd be a very clear impact. You'd then get the noise. If you swapped and noticed nothing, then your NFB isn't connected likely? Or maybe you just meant it didn't resolve the other problem AND you heard the requisite hum/noise/screech. I've had the great luck of having every amp I've built so far with NFB have the OT leads backwards, not too good of luck I guess . BUT every single one was different sounding. I think part of it is based upon the amp's overall frequency response/design etc.

As has been mentioned, the lead dress is probably the only other issue that could cause that kind of thing, I've had one do a pretty annoying buzz/hum and when I moved the OT leads with a chopstick it resolved it.

But maybe a sound sample will help confirm what it is?

~Phil

[alkuz1961]

...I already tried it, and the problem remains the same.
A feedback from the OT usually produces a huge low noise, at any volume and on both channels. It's difficult to miss.
The feedback I get is a high frequency one on the OD channel only, related to gain levels. It could come from the NFB indeed, but as I wrote, I tried many different NFB values from a few ohms to open circuit (also playing with the presence value), and the oscillation is still present.

I want to show you a small example of what can be the noise from the location of the wires
https://youtu.be/8kSkPprM-Rk
https://youtu.be/X6KH5o7aFYI

[CB Amp]

If you said that swapping OT had no impact then that's not right either because if you had it right and swapped there'd be a very clear impact

Of course swapping the leads has an impact, but it did not solved my problem.
And yes, I will try to make a sound sample.

I want to show you a small example of what can be the noise from the location of the wires
https://youtu.be/8kSkPprM-Rk
https://youtu.be/X6KH5o7aFYI

Thanks for the examples Alkuz. Unfortunately, neither of the two correspond to what I've got.
But thanks also for the Youtube idea, I won't have to record anything ! I found what I've got, here at 2'30 :
https://www.youtube.com/watch?v=Gd-WDIYRalo

I precise that I don't have the awfull hum/hiss, my amp is very quiet. But I've got a similar whistle, with a higher tone, and not so much tone amplitude.
The guy says that disconnecting the reverb tank solved the problem. That's something I also did (the amp has a reverb section), but the whistle was still here.
Too much gain ok, but it comes even with low settings.

[erwin_ve]

Swapping the OT primaries should result in a big squeal or no squeal.
So it's uncertain what you actually did.