Ampeg Reverb Rocket R12R

[J Luth]

Got one here that's acting up.

Has a bad intermittent noise and sensitivity problem. When the problem happens, anything that is tapped or rubbed, including the cabinet makes a lot of noise through the speaker. Found a few bad resistors that I changed. The 3, 10w resistors are reading bad. Need to replace them before I test for the noise. Question below.

Question.
The 10w resistors are 1k, 250R and 5k. I have 1K but I don't have 250R and 5k handy right now so wondering if the 5k can be replaced with a 4.7k and the 250R replaced with a 220R or 330R?

[Mick Bailey]

Bear in mind the original resistors have a tolerance - possibly +/- 10% in an old amp. The 5k can be replaced by 4.7k. It may increase your preamp voltages slightly, but we don't know the exact measured value of the original resistor. The amp is cathode biased and the 250R resistor affects the current draw of the output tubes. I would go to the next highest available (330R), rather than lower.

When you say the resistors are reading bad, what values are you getting? Did you lift one end to check them?

[Justin Thomas]

Mick is correct. 10% is standard in vintage Ampegs. Rarely have I seen 5%.

Is this an original R12-R? Or an A, B, or one of the other iterations? New or old? (Please don't use the term "reissue" cuz they never were! ) Schematic?

Justin

[J Luth]

The schematic glued to the back of the back cover is R-12-R. it's in rough shape so I got this.



https://elektrotanya.com/PREVIEWS/63..._sch.pdf_1.png

The PDF attachment didn't work.

[Justin Thomas]

Keep looking for a period-correct schematic in your spare time, as the Piazza schematics have a history of discrepancies from origibals. Or maybe someone will be able to find a good copy of an original. And to think, I believe these were the first ever amps with on-board reverb, predating Fender by 2 whole years...

Justin

[Justin Thomas]

Is this your amp? Schematic in first post...

https://music-electronics-forum.com/...ght=Ampeg+r12R

Justin

[J Luth]

That schematic looks correct, a bit hard to read on this laptop.

More info, I picked up a new meter and rechecked the resistors.

250r and 1k read OK, so much for the other meter.
The body of the 5k can be spun on the leads. Did not bother take a reading on that with the new meter just want to replace it.

So the question is if the 4.7k would be OK in that position.

[Mick Bailey]

As per my previous post, the 4.7k is an acceptable substitute for 5k, given the original likely tolerance of the original. If you want to be doubly sure, Either lift one end of the resistor and measure the actual value, or measure the voltage on the preamp side of the original resistor and then compare the with the replacement. A small increase won't matter.

[J Luth]

What I found so far on the amp.
5K 10w resistor open replaced with the 4.7k 220r resistor on the cathodes of the PI open, replaced, the 10K resistor from the PI cathode to the output transformer was a 4.7K replaced with a 10k. It still has the old can cap installed. No I did not do any of the cap work or other changes on the amp only did what is listed.

Now for the 2 main problem. The amp was brought here for me to try and find the problem below. Others have tried and can't find it. I am also at a loss.

On the reverb / dimension, when the amp is turned on as it warms up with all the controls turned down all the way the amp is fairly quite. When the reverb control only is turned up to any level after a short time the amp begins to make noise similar to feedback and it will increase in volume over time. This is with all other controls on 0. It is not in the video below.

In the short video below you can hear the noise the amp makes when it is tapped. In the video in the beginning you can't see it but I am tapping on the chassis. Also tapping on the everything else including the cabinet. I am not hitting it hard. This is an intermittent problem. It does not do it every time the amp is turned on and may go hours or days or weeks before it does it. It did it when dropped off and did not do it again for 4 or 5 days.

[J Luth]

Thanks Mick. More above on this amp.

[The Dude]

That sounds MAYBE like an arcing socket pin. I'd clean all the sockets and check that wiring to the sockets is well soldered.

[Mick Bailey]

You could have tube that's playing up. I would remove all the tubes except the output pair and tap them. Then install the PI and tap that. Then the reverb tube (my suspect). And so on, working backwards. I've had a very similar problem with my own amp and a single tube swap fixed it, well - it did for a short while and it came back again. Guess what, the NOS replacement tube was faulty as well so I went with a Russian version thats been fine since.

[drewl]

Yeah, those octal preamp tubes are noisy.

I have an R12 that was sounding great on the bench, with the preamp tubes a little microphonic.

Put the chassis back in the cabinet and I'm getting a similar distortion at certain frequencies.

The vibration is wreaking havoc with the 6SL6's.

[J Luth]

May have found it when I turned it over. I shook it around to see if any loose solder blobs fell out. I found an old internal tooth lock washer floating around in there. Was it shorting something out, maybe. Might explain why the problem would come and go I looked at all the pots and jacks to see if they all had washers which they did but one of the input jacks had a new one under. All the others were old. I guess someone lost one when working on it and couldn't find it. Pulled out the lock washer and the noise stopped. Also found that when I moved the input wire for the reverb pan where it connects to the board it would make considerable noise and I could stop it by moving it. I disconnected it and pulled it out from behind the board, cut it back some, routed it in front of the board and wires and soldered to the board. So far no noise. Also pulled as much of the old silicone off of everything as best I could. All 6SN7 and 6SL7 tubes are noisy and microphonic need to be replaced as well as probably the old filter can cap. It's on and no loud distortion noise so far but will have to see if it stays that way.

[J Luth]

Mick
I'll try that next if the problem comes back. He bought and brought over a few replacement tubes that may have been NOS and all of them are noisy. Tues are GE and Raytheon. The worst microphonic are both of the Raytheon 6SN7's. The GE tubes are slightly but crackle once warmed up.