Sunn Model T reissue squealing

[nsubulysses]

Well I am really having a bad time with this one. Sunn Model T reissue won't stop squealing. I have gone through the usual checklist and need a fresh insight! I have no more ideas.

I have:
- tried new tubes / swapping tubes
- cleaned and re-tensioned tube sockets, cleaned all pots with de-oxit
- tried a new input jack, and also cleaning and retensioning speaker jacks and FX loop jacks just in case
- made sure all pots and jacks are tight and making good contact with chassis
- tried bridging a capacitor over power supply and cathode bypass caps, even though they test ok for ESR and capacitance. Power supply voltages are about 10-15V low but I don't think this matters
- tried to find bad ground connections but have had no luck
- looked for bad solder joints at tube sockets and where the pots and input and FX loop jacks enter the PCB but no luck.

Basically the amp squeels at around 3.8K and it is not consistent. With something in the input jack both channels squeal. With no input only the gain channel squeals when you turn it up. When you pull the first preamp tube the squeal is gone. Coupling caps in V1 and V2 area seem ok via capacitance and short checks with a DMM.

From the looks of it this amp has never been worked on or even opened. The pre tubes are the original Fender branded "Groove Tubes" which seem to be a mishmash of Sovtek and JJs. The power tubes are semi-new Groove Tubes and are probably the only thing that has ever been replaced/worked on in the amp. From what I know it was bought off eBay, worked for a couple months and now squeals

Sorry this is a long post but hopefully it will save 20 questions and a long thread in general.

[Jazz P Bass]

First things first.
Verify the integrity of all of the power supply nodes.
If V1's supply, on the other side of the plate resistor, is not decoupling the signal properly, the signal will ride on that node.

[nsubulysses]

Jazz if I understand what you mean, all the power supply nodes are good.

supply // measured // expected
B+ // 477V // 489
Z (screen supply) // 477 // 487
Q+ // 382 // 389
R+ // 279 // 284
S+ // 259 // 266

Plate resistors for V1 both measure 100K and drop the supply voltage from 266V to about 170V or so.

Could the FX loop board assembly have anything to do with this? At TP34 I get the proper voltage on channel 1 but the voltage when switched to channel 2 is -.008V and it should be .127.

[52 Bill]

Jazz if I understand what you mean, all the power supply nodes are good.

I think JPB is thinking bad filter cap in the preamp power supply.

Have you checked the low voltage power supplies? How about the relays? It's been a while since I've seen the inside of one of these, but what about lead dress, are the wiring loops arranged so ins and outs are separated?

Do any of the controls change the squeal?

[g1]

Can you be more specific about the symptoms?
Will channel 1 squeal under normal playing conditions? Ch.2? All volume levels?
When you said "something in the input jack", what was at the other end? Guitar? If so, how does the guitars volume control affect the squeal?

[nsubulysses]

The +/-16V supply is there. I did not check the relays but I guess I didn't think to since channel switching works I thought they were alright. Could be wrong but I don't think it's a lead dress problem. This bitch of a design is on one single huge PCB. Other RI Model Ts I've had have the same lead dress and do not squeal.

Channel 1 and channel 2 will both squeal under normal playing conditions with a guitar plugged in. Basically when you get the volume up to about 4 or so for a live music volume it squeals. The more the treble, mid, presence or gain are turned up the earlier on in volume knob setting the amp will squeal. If you have the gain on 10 on the distortion channel it will squeal when the volume knob gets to about 2.

When I have it on the bench with no input channel 1 will not squeal, but channel 2 still does. When I have an input (sig gen) into the input both channels squeal when you start turning up the volume. YOu can play guitar or a sig gen and kind of overpower the squeal, but as soon as turn down the sig gen or stop playing guitar, there is the squeal.

I am unsure how the guitar volume control alone affects the squeal. I will have to check that and get back.

[nsubulysses]

I think JPB is thinking bad filter cap in the preamp power supply.

Don't think this is it because I clipped a paralelled filter cap over every electrolytic in the amp other than the radial ones which are for the +/-16V supply. BUT, I did pull these from the board and check them for capacitance and ESR and they check ok with my meters. Also the +/-16V is there. Could electrolytics possibly still do a poor job at decoupling if their supply voltages are on?

[g1]

Sounds exactly how a microphonic V1 would react. I realize you have tried a swap there, but am wondering if you tried several in that position?
Are your DC voltages at V1 correct?
Maybe try the AC voltage measurements in accordance with the schematic notes. I'm wondering about excess gain at TP2.

[g1]

Could electrolytics possibly still do a poor job at decoupling if their supply voltages are on?

Yes, especially further from the supply, as there will be less ripple anyway. So you can have correct DC levels even without the cap, and decoupling problems. But I would expect that the paralleling you tried should adequately decouple.

[52 Bill]

When I have it on the bench with no input channel 1 will not squeal, but channel 2 still does.

This is why I thought about the relay K1, with no input Ch1 is grounded through the relay and the input jack, while Ch2 is grounded only through the relay.

Or if the relay supply voltage is low, the relay may not kick in fully and cause a high resistance contact.

[nsubulysses]

Thanks all for your kind and helpful suggestions. Coincidentally I do have one of those relays in stock. I will have to check the supply voltage at the relay and see where to go from there.

My V1 cathode and plate voltages are there. Unfortunately the schema does not list plate voltages for the preamp tubes. I believe they are ok though because they both measure the same on the supply side of the plate load resistor and 165-170V on the plate side.

This really gives me some new things to think about thanks.

I may have to parallel the caps in the low voltage supply with an equivalent electrolytic just to make sure, since i didn't yet.

I have tried quite a few new tubes in V1, as well as taking tubes from the other V positions to see if there was any difference and no luck. I also placed all new tubes in the preamp positions and a used but good set of 6L6s in just to make sure, and they amp performed the same.

[nsubulysses]

Are your DC voltages at V1 correct?
Maybe try the AC voltage measurements in accordance with the schematic notes. I'm wondering about excess gain at TP2.

DC voltages at V1 are correct. However I do seem to have excessive gain after the relay, but not at TP2. Input level at TP1 was set as per ch 1 and ch 2 numbers on the schematic.

CH1
TP // expected // measured
TP2 // 1.09V // 1.05V
TP4 // 114mV // 234mV
TP5 // 7.43 // 13.6

CH2
TP2 // 372mV // 372mV
TP8 // 2.21mV // 12.4mV
TP9 // 115 mV // 238mV

[g1]

Variations of the volume and gain pots would account for that. I don't think you would have to move them much (from levels specified in notes) to get the right readings.

[nsubulysses]

Ok good to know thanks. I tried to set all knobs at the levels specified in the schematic for testing but it was really just a ballpark since I don't have the knobs on. Still determined to sort this out.

Pulling this board over and over again is not a good time I am almost ready to just shotgun the V1 socket, caps in the low voltage supply, and the relay and test my luck. I know this is not a good route but I'm up the creek. I will report back with my good or bad results

[nsubulysses]

Wow unfortunately still struggling with this one.

Swappying the K1 relay had no effect, as well as bridging the caps in the low voltage supply. Putting a 12AU7 in V1 lessens the squeal so I'm basically on the verge of shotgunning the coupling caps in V1 and possibly the tube socket. Coupling caps check normal for capacitance with a DMM but it seems like this might not be a definitive test for decoupling.

I know eveyone has already chimed in with some "out there" possibilities for the squeal since the usual suspects have been checked, but if anything else comes to mind please let me know.