2A3 Build

[Alonzo]

Just needed to keep my hands busy so got this done in time to spin tunes while catching up with some other vets.

[Alonzo]

This 2A3 Stereomour had some hiss in my horns.  To correct it I ordered a variety of 431's to try to get it lower.  After 3 try's I went with 665 ohm cathode resistors (I think that's the correct term for the resistor from 3 and 8 to ground on the 12AT7).  Still had hiss but after a dozen or so tube swaps I accept it, it is what it is.
Somewhere along the way I broke something, turning on the amp today after work gave a couple of loud tics and a loud squeal that got louder and louder till I shut down the amp.  Swapped power tubes, same result.  Swapped 12AT7's same result.
I'm assuming it's oscillation. It's new to me, I've had motorboating, this sounds different.  Anyone have a "readers digest" version of what oscillation is/does to the amp?
Going to re-wet all the new connections I've made since the last working version of the amp and hope that fixes the problem.  If not then will try the chopstick search for bad connections.  The amp worked well before the resistor changes but I'll check everything.
Another question, this amp is connected to a Smash, which has a 45 amp also connected.  When the 2A3 is powered up (and squealing) it dampens the 45 amp volume or the Smash volume.  I have to adjust the Smash a couple of clicks back and forth to get it back to my normal listening level.  What is the feedback from the 2A3 that is causing the Smash tubes to be dampened?
Thanks in advance for the help,
Alonzo

[mcandmar]

I tried the 431 experiment in my S.E.X. amp and gave up on it due to hiss.  I also bought half a dozen different variants but none of them were quiet enough.  What is the bias level you are aiming for as there might be an LED option out there you could try.

[Alonzo]

Just following the instructions from Paul J, the resistor is to drop the voltage 2.5 volts.  He even mentions led's in the forum response.

[Paul Joppa]

I see a lot of modifications, which means I don't know what the current circuit is, which means I can't make reliable guesses.

Oscillation is a broad term which can have many causes and many "features" - there is no condensed version.

Since there were no changes immediately before the squealing problem,  we'll have to start with a blank slate. We can start with what changes were made? You said the noise gets louder until you shut it down, so there is some risk in running it to determine the voltages. If I know the changes I might guess whether to make changes before further testing. At least we know it worked before this last problem!

[Alonzo]

The last working configuration was as a 2A3 amp with 12at7 input with C4S (still using the 431).  No selector switch and 100K resistors instead of a pot.  M4 grid chokes instead of the 249K grid resistor.  Ten turn 50 ohm hum pots, 15uF caps across pots.  Dowdy plate chokes instead or the PC-3, BH-5 output instead of the OT-2, 30uF parafeed cap (.3uF bypass) instead of the 3.3uF original cap.  In the power supply used the PC-3 at 270 ohms instead of the parallel 1200&360 resistors, Cree diodes instead of the UF4007.  Ran this way with 2A3-40 or 2A3 meshplates, different 12AT7's for a month or so.

Changes I made leading up to the squealing;
4 replacements of LM431's.  Each with a startup, play around 4 minutes (nickel hum disperses) then check for hiss.
Added 2uF PS bypass caps from HV+ to ground, checked for hiss.
Cut black wire from B3 and B8 at Kreg, added 665 ohm resistor from wire to ground, wire doubled on itself and twisted twice.  Started up, a little static pop and crackle, no changes to hiss, played a couple of hours.
Came in the next day, on startup, couple of pops/crackle and the squeal starts.  Fuse is good, swapped power and input tubes with no changes. 

So that's where I'm at today.  Thanks in advance for any/all help in diagnosing and repairs, but especially for explanations, its another opportunity to learn about amps.

 

[Paul Birkeland]

You'd need to post your voltages for us to have any idea what's actually going on.  Since your build is so customized, you'd want to list them as driver plate, driver cathode, driver grid, output filament, output plate, and output grid.

-PB

[Alonzo]

Paul,
Are you Ok with voltages without the  power tubes?  I've plenty of 12at7's so those can be sacrificed, I'm just afraid of blowing out some 2A3's because the squeal never peaked before I shut down the amp.  Halted Electronics is around the corner, I'll see if I can find some cheap 2A3's tomorrow.

[Paul Birkeland]

Without 2A3's in the amp, the voltage measurements aren't necessarily going to be super informative.

-PB

[Alonzo]

Hi Paul,
Here's the voltages;  using the Stereomour labeling where I can and adding part names where I know: all VDC
Output filaments:
A1  61   C1 63  A4 61 C4 63
output grids:
A3 0 C3 0
output plate:
A2 376 C2 374
B+ Right (IB) 397
B+ Left (IA) 397
Driver Plate (OA) 264
Driver Plate (OB) 133
Terminals:
1   376    15 = 375
2   263    14 = 138
5   60      11 =   63

16 60
17  0
18  0
19  0
20  63

Alonzo

[Paul Joppa]

Thanks for the summary of differences and the voltages - that will help a lot.

The 2A3s have good voltages, indicating that the squeal, if present, is not causing overloads on the tubes.

The only voltage this looks bad is the driver plate (OB). This looks like there is a bias problem on that section of the 12AT7. Some possibilities:

1 - the resistance from the driver cathode on that side is much less than the target

2 - the above resistance is zero (a short to ground), AND that half of the 12AAT7 has very low emission

3 - the grid of that half of the 12AT7 is floating - possibly a broken grid stopper resistor? - possibly a bad 100K input resistor that replaced the pot.

My money is on 3, but I'm not putting much of it up.

[Alonzo]

Thanks for the info Paul.  A little more reference points.
B8 and B9 were touching, I think from when I twisted the wire from B8.
Kreg reads .2 and .16 instead of 2.5

I'm going to re-wet the grid stoppers and 100K's and the Kreg connections. Then re-measure voltages.

[Alonzo]

Replaced driver tube, took out the 665 ohm resistors, connected 431 back at Kreg.  Now OB is at 197 and Kreg reads 2.5 on both sides.  Terminal 14 is still a little low, will confirm that lower connection is good and give it a speaker test tomorrow.

[Alonzo]

All's well.  Sounds pretty good but I'm back to the original problem.  May have to change speakers since I want to put some years on this amp.  I will leave it somewhat stock, well in my version of stock at least. 
Thanks for the help!

[Strikkflypilot]

Are the speakers very sensitive?