Ampeg Reverberocket 12RT Troubleshooting

[Steelwitch]

Hi everyone,

I have an ampeg 12RT on my bench that had a cracked 22k resistor off the cathodes of V3. I pulled it and replaced, but now there is I guess what folks refer to as "motorboating". It doesn't start until the variac is at 110vAC. I did a pretty careful solder job, the only thing I can imagine it would be is that the leg of the cracked resistor was wrapped in a way that made it very difficult to remove without damaging the pin. I managed to j hook the new resistor to it and there should be a strong connection. It's embarrassing since this wasn't happening before I replaced the microphonic resistor, but full transparency is the best way. I need some tips on how to troubleshoot this safely without damaging anything else and get this amp up to 100% with a very low noise floor. I'm not beyond judgement from anyone who's been doing this longer than I have, but I guess mistakes happen. I appreciate all of your help!

[Steelwitch]

Maybe I’m getting ahead of myself... low frequency oscillation should be coming from B+ line, right? Which shouldn’t have anything to do with the resistor on V3 that I just changed. I also checked on a scope, and I believe my connection on that resistor is solid. At least prohibitive noise from the cracked resistor is gone.. my question still stands! What should I be checking and how?

[The Dude]

Schematic?

[Enzo]

Don't try to out think yourself. It motorboats. Find out why. Don't sit there wondering how your resistor figures into it. It will either become irrelevant or it will reveal the connection. But as a repair tech, just fix the problem and let the rest fall into place.

[Steelwitch]

https://www.thetubestore.com/lib/the...-Schematic.pdf

[Steelwitch]

Don't try to out think yourself. It motorboats. Find out why. Don't sit there wondering how your resistor figures into it. It will either become irrelevant or it will reveal the connection. But as a repair tech, just fix the problem and let the rest fall into place.

Thanks, Enzo. I really needed to hear that!

[The Dude]

Often motor boating is caused by a bad filter cap. Try temporarily clipping a known good cap in across the existing filters one at a time and see if that cures it.

[g1]

That schematic (Piazza) is no good. The grounded plate at pin2 of V1 was my first clue.
Have a look at this one for accuracy: https://music-electronics-forum.com/...l=1#post494895

Does the motorboating frequency change if you adjust the trem speed? If so, it's a trem 'thump' issue rather than true motorboating.
As far as the resistor, maybe it was bad and hiding the motorboating because it was not working. Just a thought.

[Steelwitch]

That schematic (Piazza) is no good. The grounded plate at pin2 of V1 was my first clue.
Have a look at this one for accuracy: https://music-electronics-forum.com/...l=1#post494895

Does the motorboating frequency change if you adjust the trem speed? If so, it's a trem 'thump' issue rather than true motorboating.
As far as the resistor, maybe it was bad and hiding the motorboating because it was not working. Just a thought.

It is not affected by the trem speed at all. Thanks for the original schematic. unfortunately, it feel off this one a long time ago. Well, the original complaint about why it came to me was a popping sound periodically, but not like this. You might be totally right.

Is there a way to pinpoint the source on a scope? I checked the high voltage on the output tubes and they're stable, no dips or messed up behavior. They were about 400v. My guess is it's in the power section, and the only original caps in this amp is in the can cap. Is the best way to test voltage across it?

[g1]

You can check for AC volts (ripple) on the cap can terminals. Or you can temporarily clip caps in parallel with the can terminals, if you have caps that meet the voltage requirement and are similar value capacitance.

[Steelwitch]

can cap has been replaced, but was not the source of the issue. They original can cap was in there so it had to go anyway, but it's the V4 tube socket. I tried retensioning it as best I could, but I think this socket needs to go.

[Chuck H]

Hi everyone,

I have an ampeg 12RT on my bench that had a cracked 22k resistor off the cathodes of V3. I pulled it and replaced, but now there is I guess what folks refer to as "motorboating". It doesn't start until the variac is at 110vAC. I did a pretty careful solder job, the only thing I can imagine it would be is that the leg of the cracked resistor was wrapped in a way that made it very difficult to remove without damaging the pin. I managed to j hook the new resistor to it and there should be a strong connection. It's embarrassing since this wasn't happening before I replaced the microphonic resistor, but full transparency is the best way. I need some tips on how to troubleshoot this safely without damaging anything else and get this amp up to 100% with a very low noise floor. I'm not beyond judgement from anyone who's been doing this longer than I have, but I guess mistakes happen. I appreciate all of your help!

The stuff in bold goes about five different directions at once. You're over thinking this as Enzo has noted.

Odds are that you increased gain by repairing the cracked resistor and that demonstrated a flaw in what existed BEFORE you fixed that part, but didn't manifest before gain was restored.

Motor boating is usually a consequence of incorrect grounding in the power supply or failing power supply filter capacitors. I'm banking on failing filter capacitors. The need to repair this just was't apparent before gain was restored to the preamp.

EDIT: P.S. How do we know there was a "microphonic resistor" ??? If you don't have a solid reference for that problem you should let it go and replace the filter caps.

[Steelwitch]

The stuff in bold goes about five different directions at once. You're over thinking this as Enzo has noted.

Odds are that you increased gain by repairing the cracked resistor and that demonstrated a flay what existed BEFORE you fixed that part, but didn't manifest before gain was restored.

Motor boating is usually a consequence of incorrect grounding in the power supply or failing power supply filter capacitors. I'm banking on failing filter capacitors. The need to repair this just was't apparent before gain was restored to the preamp.

EDIT: P.S. How do we know there was a "microphonic resistor" ??? If you don't have a solid reference for that problem you should let it go and replace the filter caps.

I was definitely overthinking a simple issue, the original filter caps have been replaced, which was required whether they were still “functioning” since they were bound to cause problems eventually. The microphonic resistor I replaced was found as a source of noise on a scope. I’m 100% certain that it had to come out. However, the source of motorboating itself is coming from a bad connection from the V4 12ax7 tube socket. When given pressure pushed inwards, the problem is completely eliminated. I have to retension this socket or replace it. Hopefully there is a way to salvage it! Anyway, simple problem haha

[Chuck H]

Well if there is spotty contact for a tube socket you should be experiencing other symptoms as well. On that microphonic resistor... It's very likely that the tapping the resistor was just transferring vibration to a part more likely to be microphonic, like a tube or a capacitor.

Sometimes tube sockets can be re-tensioned. The tube pin contacts can be cleaned and fine pointed tool can be used to wedge between the pin hole wall and the contact sleeve to push the metal in slightly. There is more information on line if you look around.

[Steelwitch]

Very unfortunately for me, after retensioning the tube socket and getting a pretty good connection, and cure of this oscillation issue. It came back to me with the same complaint. It's interesting because when I send a 1k test tone through on a scope, it doesn't happen, but if I plug into the speaker and play with enough gain, it happens. It seems like a burst of AC voltage is triggering this. why would my test signal come through totally clear in all circumstances and then once there is dynamic signal THERE IT IS. Can someone help me out? I don't think it's the power supply since I replaced the can cap in this thing, and voltages on the output tubes look ok. Maybe I need to check further down the B+ line? where would you look?