High Frequency Ringing

[EVMode]

Hello,

I have posted about this previously in another channel but I have more information and hopefully will get more views here. I have a custom built AB763-ish circuit. This one is only one channel and does not have the vibrato circuit.
v1: 12ax7 - pre-amp
v2: 12at7 - reverb pre
v3:12ax7 - reverb recovery
v4: 12at7 - PI
p1: 6V6
p2: 6V6
rectifier: GZ34

The amp has this high-frequency ring which tails the transients of certain notes. I have tried the following:
- Used a different cabinet/speaker
- Used different guitars
- Used 2 reverb tanks (neither mounted inside of amp)
- Again Swapped v1-v3 many times with a handful of tubes
- Poked around with chopstick: basically tried to dampen every component inside of the amp
- Dampened each transformer and tube
- I even played the exact loop through another tube amp and verified this amp did not ring

Here is a great demo video from today. You can hear the ring with the amp reverb turned to 0. Increasing the reverb exaggerates the ring.

https://drive.google.com/file/d/1BqMFNi ... sp=sharing


After all of this I am still not able to locate the source of the problem. Any and all help is much appreciated.

[EVMode]

Here is the original video I posted on this forum a couple weeks back

https://drive.google.com/file/d/19UQnrT ... sp=sharing

[EVMode]

Here are some pictures of the amp circuit:

https://drive.google.com/file/d/1C16ah4 ... sp=sharing
https://drive.google.com/file/d/1BvZfqs ... sp=sharing
https://drive.google.com/file/d/1BxCiHF ... sp=sharing
https://drive.google.com/file/d/1C2xHLq ... sp=sharing

[xtian]

Here is a great demo video from today. You can hear the ring with the amp reverb turned to 0. Increasing the reverb exaggerates the ring.

Sorry, I cannot hear any "ring" or odd sounds in this video. When you turn on the reverb briefly, I hear...reverb. Otherwise, I can't hear anything odd. (I'm listening at my mixing desk on good quality monitors.)

[EVMode]

Here is a great demo video from today. You can hear the ring with the amp reverb turned to 0. Increasing the reverb exaggerates the ring.

Sorry, I cannot hear any "ring" or odd sounds in this video. When you turn on the reverb briefly, I hear...reverb. Otherwise, I can't hear anything odd. (I'm listening at my mixing desk on good quality monitors.)

I'd suggest listening to the second video link in my reply post. It is more noticeable/exaggerated in that video. I'm not sure what your hearing range is but it may just be too high pitched for you to perceive, just a thought.

[pompeiisneaks]

I also only hear reverb on the video from today.

In the second video the one from 'earlier' I can definitely hear some ring, but it almost seems to me like it's frequency dependent, and may just be that you've got the amp a bit too 'bright' and loud and it's feedback at that frequency because the reverb is accentuating that higher frequency. Does dialing the treble back a bit help? Do you have a bright switch that you can toggle and see if it helps as well?

~Phil

[sluckey]

I also don't hear any ringing, but I do have a substantial high freq hearing loss. Disconnect the NFB wire. Any better?

[EVMode]

I also only hear reverb on the video from today.

In the second video the one from 'earlier' I can definitely hear some ring, but it almost seems to me like it's frequency dependent, and may just be that you've got the amp a bit too 'bright' and loud and it's feedback at that frequency because the reverb is accentuating that higher frequency. Does dialing the treble back a bit help? Do you have a bright switch that you can toggle and see if it helps as well?

~Phil

This does seem to be somewhat frequency dependent. The ring does not sustain on all notes. But once I started noticing, it has been driving me crazy whenever I play the amp. I guess I could always take it to a gig and see how it does in the mix.

This amp does not have a bright switch. The treble does make the ring more prominent but I've not tried turning to 0 to see if it goes away.

[martin manning]

I hear trailing oscillation around 4.5 kHz in the second recording. Most likely a parasitic, and lead dress related. Maybe try moving the power tube grid leads, maybe even try twisting them together.

[EVMode]

I hear trailing oscillation around 4.5 kHz in the second recording. Most likely a parasitic, and lead dress related. Maybe try moving the power tube grid leads, maybe even try twisting them together.

Would manually dampening with a chopstick work to locate the problem leads? I was poking around inside of the amp today with a loop running and I couldn't find anything that I was able to manually dampen.

I'm just not positive if a chopstick would work for oscillations that high.

[xtian]

Would manually dampening with a chopstick work to locate the problem leads? I was poking around inside of the amp today with a loop running and I couldn't find anything that I was able to manually dampen.

I'm just not positive if a chopstick would work for oscillations that high.

You're not trying to solve mechanical vibration. The parasitic oscillation that Martin proposes is caused by signal-bearing wires being too close to each other, or especially, running closely in parallel. A chopstick can be helpful here, because you use it to push the wires around, in an attempt to improve the situation.

[pdf64]

Crikey, it’s hard to do amp tech stuff in the absence of test kit
Does the issue manifest if the amp is operated with the reverb driver tube in V2 removed (and reverb control turned to minimum)?

[martin manning]

I took a shot in the dark on the power tube grids since the AB763 layout sometimes has problems there. If pushing those leads around with a chopstick has any effect, you may be on to something. Your power tube grid leads are long, elevated, and close to the PI grid leads. Look at Fender's layout to see how they are routed, and run them down on the chassis surface as much as possible.

[Stevem]

Read up on how to install your coupling caps the right way, as this must be right first before doing any further trouble shooting if getting this right does not cure your issue on its own.

In general in terms of good signal flow wire layout, grid wires should be very short, as short as possible since they act like antennas and you can even make plate wires longer without harm many times to get to this end!

Moving tone stack caps and resistors off the board and onto the pots is many times a great way to achieve shorter grid wires.

Another thing you can do to locate this oscillating problem is to one at a time solder a long enough lenght of wire to each preamp tubes plate that can then be rapped around that gain stages grid wire a 2 or 3 turns.

If this makes the issue worse, or makes a new issue then that stage is not the culprit, if it makes it better then you have atleast narrowed it down to wiring / parts layout issues in and around that gain stage.

Be sure to insulate then bare end of that wire you solder to the plate(s) for this test.

[martin manning]

Read up on how to install your coupling caps the right way, as this must be right first before doing any further trouble shooting if getting this right does not cure your issue on its own.

Might help some, but I wouldn't say it's necessary before doing anything else.