Parasitic Oscillation in an otherwise fine amplifier...

[MarkB]

Mark and Ron, thanks so much. So, being as my amp mostly just squeals with certain preamp tubes, and only when the preamp gain is MAXED, then I don't have a NFB issue, correct? Plus, my amp is basically an AC30 circuit, and that doesn't use NFB, right?

Then, I most likely have an issue with some wires in the preamp being a bit too close or running together in the wrong way, right? That I can check, by poking and prodding with a chopstick and seeing when the squeal goes away. The only trick figuring out how to get constant signal to my amp while I poke, so I can hear it if/when the squealing stops. Maybe hook up a CD player?

THANK YOU!!!

No NFB, no positive feedback problem that can be solved by swapping output tube wires - correct. Does it only happen when you play guitar? Many amps will squeal without any guitar signal - the noise in the circuit is enough to set it off. If it only happens when you play, it may be microphonics in the tubes. A classic car repair problem is replacing a bad part with a new, but bad out of the box, replacement. Then the problem still occurs, and you're lead to think that the part you replaced wasn't the problem. You'd need a known good non-microphonic tube to rule out that possibility. There can also be microphonics generated around the socket. EF86 tubes are more subject to that problem - a wire between the board and the socket can vibrate enough to set off microphonics in the tube.

[dehughes]

Mark and Ron, thanks so much. So, being as my amp mostly just squeals with certain preamp tubes, and only when the preamp gain is MAXED, then I don't have a NFB issue, correct? Plus, my amp is basically an AC30 circuit, and that doesn't use NFB, right?

Then, I most likely have an issue with some wires in the preamp being a bit too close or running together in the wrong way, right? That I can check, by poking and prodding with a chopstick and seeing when the squeal goes away. The only trick figuring out how to get constant signal to my amp while I poke, so I can hear it if/when the squealing stops. Maybe hook up a CD player?

THANK YOU!!!

No NFB, no positive feedback problem that can be solved by swapping output tube wires - correct. Does it only happen when you play guitar? Many amps will squeal without any guitar signal - the noise in the circuit is enough to set it off. If it only happens when you play, it may be microphonics in the tubes. A classic car repair problem is replacing a bad part with a new, but bad out of the box, replacement. Then the problem still occurs, and you're lead to think that the part you replaced wasn't the problem. You'd need a known good non-microphonic tube to rule out that possibility. There can also be microphonics generated around the socket. EF86 tubes are more subject to that problem - a wire between the board and the socket can vibrate enough to set off microphonics in the tube.

Okay. Good to know.

The squeal ONLY makes itself present when I have the preamp gain up almost all the way. At any other setting, the amp is really, really nice sounding. I'm very pleased with it, and would be DONE with it, if I could just get it to behave at high preamp settings...grr....

I've tried other preamp tubes, and while the EH tubes seem to not be as prone to oscillate (they only do so VERY slightly when the gain is maxed...), the problem is still there, just less so with different tubes, and the issue SEEMS to be connected to the cathode follower tube/circuit. I can put just about any tube I want in the amp, but if I put a Tung Sol in the cathode follower position, the squeal is at its most noticeable. So, I'm inclined to think that it is most likely a layout issue...don't know what, obviously...

[Ron]

Correct, the AC30 does not have feedback.

You can choose to select tubes that don't squeal, or you can try to find out whether you are getting some feedback in your wiring layout. Have you determined that you don't have microphonic tubes? If you tap the Tung Sols with no signal input to the amp, do they start to ring?

Since the amp squeals only with the preamp gain almost all the way up, I would suspect microphonic tubes and simply find ones that don't squeal. On the other hand, slightly microphonic tubes can give you an edgy sound with detail that other tubes cannot. I have to say that I don't think there's anything wrong with your amp that tube selection cannot fix, based on what you've said.

[dehughes]

Correct, the AC30 does not have feedback.

You can choose to select tubes that don't squeal, or you can try to find out whether you are getting some feedback in your wiring layout. Have you determined that you don't have microphonic tubes? If you tap the Tung Sols with no signal input to the amp, do they start to ring?

Since the amp squeals only with the preamp gain almost all the way up, I would suspect microphonic tubes and simply find ones that don't squeal. On the other hand, slightly microphonic tubes can give you an edgy sound with detail that other tubes cannot. I have to say that I don't think there's anything wrong with your amp that tube selection cannot fix, based on what you've said.

I'm pretty sure the tubes are not microphonic. I've tried them in other amps, and have had no problems. Also, I have tapped on them and there are no signs of microphonics.... My instinct says that the Tung Sol 12AX7s should be just fine (Top Hat has been using them in their King Royale amp with great success, and my amp is very similar to the King Royale...and the tubes came from a reputable dealer who tests them thoroughly), so I'm inclined to consider the layout to be a more likely suspect.

In poking around the wires in my amp, nothing seemed to make a difference when moved. However, I have found that the Master Volume pot ground wire, and the wire that goes from the Master Volume to the PI coupling cap (left side, on the picture of the preamp...), are both sensitive and easily transmit noise (whump whump...bump...noises as I poke with the chopstick...). Now, this is kind of what I suspected all along, as the run from the MV pot to the PI coupling cap was going to use shielded wire, but I ran out. I moved the MV pot ground wire and the MV to PI input wire around, and this seemed to lessen the squeal/phase shift, but not quite remove it. Very interesting...

Also, I noticed that the MV pot ground solder point is touching the ground solder joint for the shield of the wire that runs from the preamp volume pot to the input grid of the cathode follower. They are situated to the left of the buss bar by the left inputs, on the picture I posted of the preamp. Check it out.

Now, that's kinda interesting...makes me wonder if I should 1) run shielded wire from the MV pot to the PI input, and 2) move the ground point of the MV pot and the ground of the grid input wire of the cathode follower away from each other.

Whaddya think?

[Ron]

I know that Fender used shielded cables between the MV pot and the parts board.

[dehughes]

Well, the noise is still there, except now it is worse. I ran some shielded wire from the master volume pot to the input cap on the PI, moved/cleaned up some grounds in the tone stack section, re-ran some eq wires, and straighted up some of the shielded input cables. Now, the noise is even more present and obnoxious. So, perhaps I'm on the right track but headed in the wrong direction. TOTALLY frustrating...

So, here is what I've found to be the case now:

1) The EF86 channel sounds absolutely KILLER. Perfect. Volume and tone knobs, and that's it. No troubles, no nothing, just great sounding tone for days.

2) The 12AX7 Master Volume side sounds great, too, until you crank the 500k volume pot (gain) up past 3/4 open, then it gets phase-shifty and thin, and the squeal-scratch-oscillation starts happening on the top end. Gain down, no issues...

3) Turning the Cut and Treble knob down helps tame the squeal a bit, but the pot that has the most affect on reducing the squeal is the MIDRANGE pot. Interesting... Turning the mids all the way down almost takes care of the problem, though a little bit is still present. Don't know how to interpret that...

4) When the amp is in high power and high filtering modes (32uf instead of 16uf in various places...and higher voltages throughout), the squeal is more noticeable. Mostly, it is with higher voltages and with higher preamp filtering (1st gain stage...V1) that the squeal is more prominent, as raising the filtering in the power amp section doesn't affect the squeal at all.

So, it seems like the higher voltages, higher preamp filtering, and higher midrange settings are the straws that break this oscillating camel's back. Personally, I'm considering shooting this camel in the head and burying its body in the back yard, but then I can tell that I'm THIS close to having a truly fine amplifier on my hands....so I'll hold off on that.

Any suggestions, given the above? I've not tried switching the grids on the PI or the primaries on the OT yet, as my instincts say that the problem lies upstream somewhere.... Your help and input is, as always, greatly appreciated!

THANKS.

[MarkB]

Try the new Ampage forum - there are some pro techs who post there.


http://music-electronics-forum.com//for ... y.php?f=10

[dehughes]

Try the new Ampage forum - there are some pro techs who post there.


http://music-electronics-forum.com//for ... y.php?f=10

Will do. Thanks!

[dehughes]

FIXED IT!!!!!

It was the fact that I had a turret very close to the input jack for the second channel - the plate cap from the cathode follower was connected to this turret, and the the wire from that went off to the treble pot. Moving this wire even 1mm closer to the tip of my guitar cable (when in the jack...) made an EXPONENTIAL difference (for the worse...), so I knew that was it. I've temporarily re-wired that cap to another spare lug on the strip (that was way farther away...), and jumpered the wire to the treble pot, and BINGO, no squeal!!!

However, the gain is a bit edgy still, so I'll have to tweak some more.

THANKS THANKS THANKS!!!!

[MarkB]

Good to hear you worked it out.