Uncontrollable feedback

[Trigger]

I have had my Express running for some time now. It has started to create feedback that is way out of control. I have swapped out V1 with all the 12ax7's and 5751's I have, they are mostly JAN Philips ECG's to no avail. If I run it through my Airbrake at the lowest setting the feedback is no issue. I have tried many fixes over the past couple of days grid stoppers on the preamp tubes, choppsticking, re-routing wires, screaming, etc... and nothing has worked. I used cloth covered pvc wire and had read that Mr. Fischer had said that the cloth created transiant voltages that caused problems. I have ordered some new wire to redo this thing properly. Not sure if that could be some of my problem. I put a 100pf across the PI as I do not have a 47pf. I even tried going to a 500pf that seemed to make things worse. Anyway what is it that makes V1 so sensitive? I would love to have this thing working properly but I seem to be at this huge brick wall that I can't get over. Any help would be great. This picture is without my cap across the PI at the moment.
Voltages:
V1
1. 183
3. 1.46
6. 208
8. 1.93
V2
1. 254
3. 3.76
V3
1. 179
2. 31.5
3. 46
6. 185
7. 30.2
8. 46
V4
3. 415
4. 404
5. -32
V5
3. 415
4. 403
5. -32

I have the bias set at 43ma.

[billyz]

Did you check your Negative feedback loop ? Make sure nothing is open.
I don't see the cap across the PI, not that it is necessary. I guess you have some kind of MV thing going or it is a cut control. I would take a close look at the circuit around the PI.

[Trigger]

Did you check your Negative feedback loop ? Make sure nothing is open.
I don't see the cap across the PI, not that it is necessary. I guess you have some kind of MV thing going or it is a cut control. I would take a close look at the circuit around the PI.

The cap is out at the moment. I do have a cut control. I disconnected the cut control and no change. I also disconnected the presence at the imp. switch and it had no effect. I did not try disconnecting it at the board though. I will poke around the PI some and see if that helps.
Thanks

[billyz]

Disconnecting the NFB would cause quite a bit more gain and possibly the uncontrollable feedback you describe. Check the 150k resistor to ground from grid of the V2, If it is not connected or measures very high you have a bunch more gain as well.

[Trigger]

Disconnecting the NFB would cause quite a bit more gain and possibly the uncontrollable feedback you describe. Check the 150k resistor to ground from grid of the V2, If it is not connected or measures very high you have a bunch more gain as well.

I measured it at 149.3k, I installed a 100k to try and lower the gain some and it did not seem to improve any. I've tried so many different things I've lost track. I should have written down what I had done and the effect it had, my own little amp journal. Hindsight.

[billyz]

it is grounded ?

[Trigger]

It is grounded.

[billyz]

Do your tone controls work? Check the ground on the middle control.
OK, some one smarter than me should jump in.

[fishy]

Or maybe just someone else

Nice looking build by the way....

Have you checked for leakage across coupling caps?

[markr14850]

If I run it through my Airbrake at the lowest setting the feedback is no issue

Is the amp on top of the speaker cabinet?

Does the speaker cable happen to pass anywhere near the preamp section?

Do you have a closing plate?

[Trigger]

Do your tone controls work? Check the ground on the middle control.
OK, some one smarter than me should jump in.

Tone controls work, not alot of control on the bass but I've heard that is normal. Mid is grounded. I thank you for all your suggestions. How many times is a problem caused by an oversight from the builder, too many times to recount.

Or maybe just someone else

Nice looking build by the way....

Have you checked for leakage across coupling caps?

Thank you, Here I will show my ignorance. How do you check for leakage?

Is the amp on top of the speaker cabinet?

Does the speaker cable happen to pass anywhere near the preamp section?

Do you have a closing plate?

I have tried the amp as far away as apprx 6-8' from the speaker cab.
Speaker cable goes from the amp down to the cab.
No closing plate yet. I need to go and get some material to get that done.

Again I appreciate all the help.

[skyboltone]

The only departure from normal lead dress that I can see is that the two wires coming from the bass pot should be as close together as possible and stay on the deck flat all the way back to the board, where they come 90' straight up and on to the eyelet. The other kinda bad deal is that the OT primary passes directly under the OT secondary switch and under the grids. I assume you've chopstuck that but OT primarys are ALWAYS suspect.

[fishy]

A link for info on checking coupling caps.

http://www.geofex.com/ampdbug/coupling.htm

[Trigger]

The only departure from normal lead dress that I can see is that the two wires coming from the bass pot should be as close together as possible and stay on the deck flat all the way back to the board, where they come 90' straight up and on to the eyelet. The other kinda bad deal is that the OT primary passes directly under the OT secondary switch and under the grids. I assume you've chopstuck that but OT primarys are ALWAYS suspect.

Thanks Skyboltone for the help. I made the changes but can't test it out until tomorrow. The wife, kids and neighbors may not be as excited as I am to hear it at this late hour.

Fishy, thanks for the link I will give that a try tomorrow as well.

Again, thanks for all the advise. You guys are great.

[rooster]

Man, if I were you I would check the ground buss connection on your pots? It looks like you came up with your own design here, not a good version from what I can see, either. Is the Treble pot even attached to the ground buss? Get that looking typical with #14 solid copper wire first off - across all the pots. Then check out your Presence control hookup? the cap legs look odd - you do not have the cap legs attached to the outside lugs of the pot, do you? This should not be the case. One leg is on the wiper, the other on an outside leg.

Remember also that some builders suggest to remove the Presence pot from the ground buss attachment altogether, and create a separate ground to the chassis right in front of it instead. Follow this? I run like this because it makes a lot of sense, actually.

OK, that's all I got. Very neat work in so many ways, but the ground buss looks like it was done by a newbee. Find some pics and copy what has been done because there is a reason. Oh, one more thing? I am very sure you have attached the .1 coupling cap that takes the signal to the phase inverter - because you wouldn't have any sound if you didn't. The thing is, you have run it underneath the board - which means there are two high voltage wires laying right on top of the cap connecting wire - not a good thing. Put it on top of the board to avoid this, this was KF's layout. Good luck, you'll find the problem.