Help Please 6G15 Reverb Feed Back

[Willabe]

Well I did it. I think I made an oscillator instead of a reverb.    

I thought all was fine, but I had the 12AT7 swapped with the 12AX7. When I put them where they belong the verb starts to feed back with the dwell on 3, mix on 3 1/2 and tone on 7. Hit an open G and it starts. It's not a high pitch squeal and it's not motor boating. It sounds like a mid range note feeding back.

I disconnected the feed back loop, nothing changed. Changed the 12_ _ 7 with several known good tube, both NOS and new, sane thing. Tried 5751 and 12AY7 for the 12AX7 only changed the point at which it starts to feed back to a later/higher setting on the mix pot.  

Here's the voltages I'm getting;

12AT7;

1A-  Plate 112.4, Cathode 2.1, Grid 0
1B-  Plate 130.7, Cathode 2.3, Grid 0

12AX7;

2A-  Plate 191.6, Cathode 1.3, Grid 0
2B-  Plate 286.3, Cathode 125.3, Grid 139.5

6V6;

Plate 310.3, Screen 302.4, Cathode 23.1, Grid 2.4mv

B+ supply nodes, 6V6 Pa 318.2, Ps 303.1, 12AT7 Pa 290, 12AX7 Pa 286.7

DC stand off for heaters 68.8dcv

I think it's in the layout. I chop sticked around, nothing. It sounds great (to me) except for the feed back. Maybe I'll have to re-do the tag board layout?


                      Thanks,   Brad        


Edit; I took the tank out of the cab, no difference, tried different tank, no difference.

  

[Willabe]

I find it strange that using a 12AX7 for the pre/driver stage (instead of the 12AT7), results in no feed back. I wonder if the extra gain from the 12AX7 is fighting off the oscillation caused from bad layout of the .02 coupling cap going to the 6V6 grid stopper that's going right past the OT secondary?

So, I'm gonna try moving the coupling cap from the 12AT7 (pin 6) off the board and right to pin 6 of the 6V6 and see if that solves the problem.

If not I'm going to try a 10K grid stopper on pin 2 of the 12AX7.


                         Thanks,     Brad  

[Willabe]

Well I think I fixed it.      

I moved the .002 coupling cap from the driver plate off the tag board and put it tight to pin 6 of the 6V6, helped some but only a little.

Then I ran a piece of shielded cable from the verb input/return chassis RCA jack (verb tanks output) with a 10K grid stopper tight to the 12AX7 (1a recovery stage) pin 2. I also rewired the +/- between the RCA jack and the 1/4" foot switch jack with a tightly twisted pair of 22g dressed down on the chassis. Much better, almost all feed back gone.

Then I reconnected the 6V6/driver feed back loop (33K to the cathode) and it's gone. I can turn the dwell, mix and tone full up and it's fine now, not that any one would play with it set like that.      

I should have wired it this way to start with since it's the smallest signal (recovery stage) in the unit. I didn't think that only about 3" of grid wire would be a problem.

I'll have to sit down and play through it for an hour or two to see if it stays as it is now.

Do these stand alone units have a tendency to feed back when the controls are maxed? I should say I'm using an Accutronics 3 spring, long dwell (4 years old, made in USA) tank.

I really love this verb unit with my 5E3 clone, strat and Tubby Tone 12" hemp cone speaker. Gets some pretty good sustain and tone. Real nice slide set up with my Paul (humbuckers) too.    


                     Thanks,     Brad

[Willabe]

Ya know, I wonder if this was happening because I dressed the recovery stages grid wire up in the air instead of down on the chassis?

I did that because I didn't want to run the grid wire across the heater wires which I had dressed down on the chassis. (You can see it in the pics in the first post.)


                      Brad      

[tubeswell]

pan feeding back?

[Willabe]

pan feeding back?

Hi Pete,

I changed pans, no differance at all. I'm using the sponge rubber grommets that come with it, _ fat _ side to the cab, set the nuts looser, set them tighter, no differance. I took the pan out of the cab and had it on the bench (with the rubber grommets still in), no change.

The pans are 4/5 years old USA made Accutronics that I bought new and stored away in the house. They should be fine. They are 3 spring, long dwell thou.

It seems to be fine now, I'll see if it's true.    


                       Thanks,     Brad