1970 Twin Reverb - Conductive Board (NO). SOLVED!

[bbmade]

UPDATE - Sluckey saved the day.  I cannot thank all of you enough and el34world forum.  The initial discovery of the oscillation was before I removed the push/pull boost that was added in 1972.  In the process of removing it, I misplaced the wires Sluckey saw in the picture.  I fixed that and the amp sounds fantastic.  This was much needed experience at the bench and I learned a LOT.  I'd like to say eventually I would have figured it out BUT, I was hours away from caving and ordering new boards for this amp.

THANK YOU TO EVERYONE who took time to offer guidance.

[/youtube]

I've done everything I can think of to stop this grid oscillation with the exception of moving the tone stack capacitors off the board and onto the pots to eliminate a bunch of grid wire.

I can measure an average of .45 - .5mV anywhere on the board.  At this point I think the boards need to go but I'm wondering how much is too much DC on the board.  I've searched the forum and it seems like anything less than 1 volt is nothing to worry about.

If I shunt pin 7 to ground on V4 while it's oscillating it stops.  If I pull V4 the oscillation will stop and V5 pull does the same thing.  V1-3 will not stop the problem.  It seems to be in the V4 circuitry but I can't get rid of it.

The amp is stable (again) at idle but if I tap the board it goes into oscillation even with the volume at 6 instead of cranked.

I can't play guitar through it past 3 on the volume without it going into oscillation.

At this point, I believe it's the board and am considering ordering Hoffman replacements.  I could have built three of these in the time I've spent troubleshooting.

[sluckey]

I would not be concerned with .5mV on the board. Heck, you may be able to read .5mV on the chassis.

It seems unlikely that pulling V5 would kill the oscillation. Does the amp oscillate with the footswitch plugs disconnected?

[DuaneOh]

While you are waiting for someone who knows the answer, here are a couple of thing to try:

Does the oscillation change when you change the vibrato speed? If it does you could be getting crosstalk.
With the amp off, turn the reverb all the way down and measure the resistance from v4 pin 7 to ground, should be around 150k. Make sure that the opposite end of the 220k that is connected to pin 7 is at ground. That point also grounds the cathode resistor for v5 oscillator and v4 cathode resistors. Touching up solder joints in that area can't hurt.

[bbmade]

I would not be concerned with .5mV on the board. Heck, you may be able to read .5mV on the chassis.

It seems unlikely that pulling V5 would kill the oscillation. Does the amp oscillate with the footswitch plugs disconnected?

It oscillates with or without the footswitch connected.  Pulling V4 stops it, grounding pin 7 on V4 stops it.

With all the tubes in, pulling V5 also stops it.

Good to know about the .5mV on the board.

The problem seems to be in the V4 circuit but any grid agitation send it into oscillation whether is playing the guitar or tapping the grid wires going to the tone stack or tapping the circuit board.
 
I’m open to any and all ideas.  I’ve reached the end of my troubleshooting abilities.

Thank you

[bbmade]

While you are waiting for someone who knows the answer, here are a couple of thing to try:

Does the oscillation change when you change the vibrato speed? If it does you could be getting crosstalk.
With the amp off, turn the reverb all the way down and measure the resistance from v4 pin 7 to ground, should be around 150k. Make sure that the opposite end of the 220k that is connected to pin 7 is at ground. That point also grounds the cathode resistor for v5 oscillator and v4 cathode resistors. Touching up solder joints in that area can't hurt.

Sometimes it will change it with the intensity control but not all the time.  The speed control never has an affect on it. 

Turning the volume down or bass control down did seem to tame it but today neither one would stop it.  I had to put it in standby to get it to stop.

I will check as you advised, and any other advice by Monday.  Thank you

[Esquirefreak]

I've had osc. issues when sharing K bypass caps in the preamp. The part where you mention not being able to turn up either the bass or volume was the same for me. Not that I'm certain that's the case here, just throwing it out there, that splitting the cathodes might be worth trying.

/Max

[bbmade]

I would not be concerned with .5mV on the board. Heck, you may be able to read .5mV on the chassis.

It seems unlikely that pulling V5 would kill the oscillation. Does the amp oscillate with the footswitch plugs disconnected?

Would it make sense to lift the board a little and see if the sound goes away?  Where the 3.3M resistor is in the reverb circuit seems to be the “hot spot”.  There has been a lot of soldering/modding in that spot.  I’m thinking maybe there’s excess solder between the boards.  There’s a wire there that squeled once before I resoldwred the junction of four resistors and a 10pf cap.

I plan to try every suggestion because I’m out of ideas on my own.

Replacing the board seems drastic but, it is starting to feel inevitable.

[bbmade]

I've had osc. issues when sharing K bypass caps in the preamp. The part where you mention not being able to turn up either the bass or volume was the same for me. Not that I'm certain that's the case here, just throwing it out there, that splitting the cathodes might be worth trying.

/Max

What’s the best way to do that since they share an eyelet on the control side of the board?  I’ll try anything at this point.  Thanks

[Esquirefreak]

Clip/loosen the jumper between the cathodes (pin 3/8) on v1/v2 (normal and vibrato ch first gain stages) and replace with a 22μF/1k5 resistor. That makes for the same bias as the current common 820R.

You could propably run them "flying" and ground at the same place on the board for testing purposes.

So in conclusion - four 22μF/1k5R instead of two 22μF/820R.

/Max

[bbmade]

Clip/loosen the jumper between the cathodes (pin 3/8) on v1/v2 (normal and vibrato ch first gain stages) and replace with a 22μF/1k5 resistor. That makes for the same bias as the current common 820R.

You could propably run them "flying" and ground at the same place on the board for testing purposes.

So in conclusion - four 22μF/1k5R instead of two 22μF/820R.

/Max

Thank you, I will give it a try.  New boards seems extreme and I’d rather find a way to fix it but, need any and all advice.  So far, I’ve got some great things to try.  Thank you all, keep any other ideas coming!

[bbmade]

While you are waiting for someone who knows the answer, here are a couple of thing to try:

Does the oscillation change when you change the vibrato speed? If it does you could be getting crosstalk.
With the amp off, turn the reverb all the way down and measure the resistance from v4 pin 7 to ground, should be around 150k. Make sure that the opposite end of the 220k that is connected to pin 7 is at ground. That point also grounds the cathode resistor for v5 oscillator and v4 cathode resistors. Touching up solder joints in that area can't hurt.

Pin 7 V4 measures 217K to ground.

Opposite end of the 220K resistor is at ground.

[sluckey]

The preamp jumper is connected between v1-8 and V2-8. Cut the jumper at V2-8. This leaves a 2-3 inch wire still connected to V1-8. Connect a 1.5K and 22µF to the dangling end of the wire that's connected to V1-8 and connect the other end of the 1.5K and 22µF to ground at the board eyelet where the other 1.5K and 25µF for V1-3 is grounded. Now follow the wire from V2-8 to the board where it connects to a 820Ω and 25µF. Replace that 820Ω with a 1.5K.

Now look at V4. Remove the jumper between pins 3 and 8.  Connect a 2-3 inch wire to V4-3 and connect a 1.5K and 22µF to that wire. Connect the other end of the 1.5K and 22µF to ground at the same eyelet as the 820Ω for V4-8 is grounded. Now replace that 820Ω with a 1.5K.

[sluckey]

Pin 7 V4 measures 217K to ground.

Should be about 150K to 160K depending on the setting of the reverb pot. Insure that the 470K that's also connected to V4-7 actually measures 470K (must lift one end from board). Check that the ground lug of the reverb pot actually measures zero ohms to chassis.

[sluckey]

Can you post another better resolution pic that clearly shows that 3.3M resistor? I believe that the brow and yellow wires are reversed. The brown wire that connects to the reverb pot should be connected to a 470K resistor. That short yellow wire (board jumper) should be connected to the 3.3M and 10pF.

[bbmade]

Pin 7 V4 measures 217K to ground.

Should be about 150K to 160K depending on the setting of the reverb pot. Insure that the 470K that's also connected to V4-7 actually measures 470K (must lift one end from board). Check that the ground lug of the reverb pot actually measures zero ohms to chassis.

Reverb pot ground is .11 ohms, continuity beeps clearly.

470K ohm resistor with one leg up, measures 500K.

[bbmade]

Can you post another better resolution pic that clearly shows that 3.3M resistor? I believe that the brow and yellow wires are reversed. The brown wire that connects to the reverb pot should be connected to a 470K resistor. That short yellow wire (board jumper) should be connected to the 3.3M and 10pF.

You were correct.  That was the area where they added a push/pull pot which seemed to be a boost of some sort.  I bet I goofed that up removing the mod.  First test with a dummy load and scope show no oscillation.

Taking the dog out of the shop so I can plug a guitar in and listen.

I feel like I've aged 7 years troubleshooting this amp but it's MUCH needed experience.  I will update after my test play.

I cannot possibly thank you, and everyone who chimed in enough.  I have to slow down even more when I get working on amps.

[bbmade]

Can you post another better resolution pic that clearly shows that 3.3M resistor? I believe that the brow and yellow wires are reversed. The brown wire that connects to the reverb pot should be connected to a 470K resistor. That short yellow wire (board jumper) should be connected to the 3.3M and 10pF.

Seriously...I cannot thank you all enough.  You saw the problem and as soon as I fixed it the amp worked exactly as it should and sounded better than it's ever sounded.  That push/pull mod created the initial problem and then I made it worse working too fast. THANK YOU