squealing out of control when powered on

[dscottguitars]

Hi, I just built/re-built an old McMartin 6L6 push pull amp The inside has been completely re-made using other 6L6 schematic parts. I have a really bad high pitch out of control feedback squeal as soon as I turn on the standby switch. I can't leave it on-I'm afraid to. I have isolated the problem to the phase inverter by pulling every other tube and that one is the only one the stops the noise so I can check voltages. To check the PI voltages I pulled both power tubes. All the voltages on every preamp tube plate are a little higher my schematic by quite a bit, but the voltage before the plate resistors is fine, 350v. The rest are below.

The PI and power stage circuit I used is a long tail inverter and from the AA864 Fender Bassman reissue. Assuming the OT transformer is good, I cannot figure out what this is or how to remedy it. I used two 12A-7's in the preamp prior and a 12AT7 in the PI, which I tried different ones, AT7's and others. All connections look good, I used only allen bradley resistors (checked) and new good quality caps.

Voltages:
W/O PI tube- 12AX7 ~300v on both plates, 12AV7 (wired together in parallel) 180v; both tubes supplied by 350v before the resistors. 6L6's have ~413 on both the plates and screens(?) using a 470 resistor.

W/ PI tube and no 6L6's- ~325v and 188v respectively. Also, 226v on pin1-205v on pin6 of PI. There's 398v (368v with the above configuration) before the resistors and the increase in voltage appears in the power stage making the higher plate voltages given the two different scenarios.

I hope I've covered everything.



Any help would be greatly appreciated...

[Alex R]

Swap the plate leads over?

(could be the feedback goes back in phase instead of out of phase)

[dscottguitars]

on the 6L6's?

[Jazz P Bass]

It may be that the two leads that go from the output transformer to the 6L6 plates need to be reversed.(blue & brown).
If you have a negative feedback circuit & the leads are incorrect then what you get is positive feedback.

[Chuck H]

+1

[dscottguitars]

That helped the squeal but now it's a bad low pitch out of control hum...

Thank you for your help all...

That took care of the squealing from the phase inverter. Now I have a low building out of control hum, can't leave it on it's so bad. I've isolated the problem to the 12AV7 tube wired in parallel. There is high plate voltages throughout. I'm not sure if that's it but I cannot seem to get the voltages down. I've included a schematic of what I've done, with the voltages present when the 12AV7 is pulled out. The voltages go up about 15% when I pull the power tubes to check the 12AV7 plate voltage. Any advice would be appreciated.

I added another 10K/5W resistor in series with the present one and the voltages only came down about 5-10 volts.

[Chuck H]

There's an obvious problem. The 12av7 shouldn't ever draw enough current to drag down the B+ rail 15%. There's probably a wiring error.

[Alex R]

You might check resistance to ground on pins 3 and 8 of v1.

[loudthud]

You need a 220K resistor from pin 5 of each 6L6 to ground.

[dscottguitars]

Hi and thanks again...

Alex: I checked the resistances to ground, but I'm not sure if I did correctly. There's a resistor there and the resistance is that value. There is no resistance between grounds in the amp, I did check that.

Loud: Although I missed them on the schematic they are there, thanks for noticing.

What I did notice is no grid resistor on the first tube so I put one on the grids (they are wired together to split the signal-not sure if this is an okay thing to do) and the building hum went away!! I can now have all the tubes in and check voltages, which are responding better. I still have a loud hum, just not building, and isolated the noise to the 12AV7 on the drawing. It shows to have 50v on the grid of the next stage- other side of coupling cap- and I don't think there's supposed to be any. It's a new cap-haven't changed it yet-later today... I just wanted to respond to the posts and update the progress. It is progress.

Thank you all for your help.

[Chuck H]

The building hum was likely the effect of V1 heating up and going into meltdown due to lack of bias and therefore generating more gain. The 12av7 may just be a hummy tube. Do you have another you can try?

[dscottguitars]

Yes, I have a few different ones: AU, AY, AT & AX. I found a loose connection also on V2, the socket tab was loosening and broke as I was changing the coupling cap. I'll be changing that and try it again. Thanks...

[dscottguitars]

voltage question

I have voltage on pins 2 & 7 of the phase inverter. It is about 50v and there is 86v on the junction of the cathode resistor and the two grid resistors. Do these values appear to be within normal ranges?

[dscottguitars]

No hummmmmmmmmmm!!!1

Hi all,

I found the problem: grounding the filament heaters with a ~50 resistor. I put in a pot to balance the hum, which is not very much, and it is ghost quiet. I had to put my ear down to the speaker to make sure it was on when I first plugged the power in.

Yeehaw...

Thanks for all your help, now I just need to dial in some tone issues. What a great feeling!

[Chuck H]

I know that feeling... (power on... standby flipped into play) "Oh shit, now it's not making any sound at all. What could I have done wrong?"

Maybe you even shut down for fear you might melt something. But you have to power it back up to get an idea of what your dealing with. Then, "Oooohh... It's just virtually noiseless... Cool!"