Ampeg Reverb Rocket R12R

[J Luth]

Nope, spoke to soon it's back. Who's bright idea was it to put silicone on everything? What a pain getting of all the tube socket lugs and ever where else.

[J Luth]

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.

Mick

Tried your suggestion and pulled all tubes except rectifier and power tubes and it's quite when tapped. As soon as I install the PI and tap it, it starts. I have 2 PI tubes and they both do it. Socket and pins look OK and are clean

[Mick Bailey]

That narrows things down quite a bit, so at least you can target further investigation around the PI circuit. Have you chopsticked around the PI tube and associated components? I would re-rension the PI tube socket contacts, but be careful not to break them. You could still have a bad tube - I have quite a few of these octal tubes lying around that are pretty bad. The problem is that there are a lot of faults that can produce the same symptoms; a bad solder joint, an internal connection inside a component breaking down, a broken wire.

When tracing a vibration-related fault I will often try to stabilize individual components or connections/joints by applying slight pressure with a chopstick and then re-checking to see if tapping anything else still induces the problem. Sometimes I'm lucky and this tells me which part is playing up.

I've also noticed with these amps that sometimes the solder joints are not 100% - maybe even from the factory. As though the component leg was contaminated when the amp was originally built. There's evidence of poor solder flow - no proper fillet around some of the component legs. These need to be desoldered, cleaned up and re-soldered (I use a little extra resin flux). If your eyesight is like mine a little extra lite and a magnifying glass is useful.

[J Luth]

That narrows things down quite a bit, so at least you can target further investigation around the PI circuit. Have you chopsticked around the PI tube and associated components? I would re-rension the PI tube socket contacts, but be careful not to break them. You could still have a bad tube - I have quite a few of these octal tubes lying around that are pretty bad. The problem is that there are a lot of faults that can produce the same symptoms; a bad solder joint, an internal connection inside a component breaking down, a broken wire.

When tracing a vibration-related fault I will often try to stabilize individual components or connections/joints by applying slight pressure with a chopstick and then re-checking to see if tapping anything else still induces the problem. Sometimes I'm lucky and this tells me which part is playing up.

I've also noticed with these amps that sometimes the solder joints are not 100% - maybe even from the factory. As though the component leg was contaminated when the amp was originally built. There's evidence of poor solder flow - no proper fillet around some of the component legs. These need to be desoldered, cleaned up and re-soldered (I use a little extra resin flux). If your eyesight is like mine a little extra lite and a magnifying glass is useful.

Yes I poked around with a plastic stick when it is happening, it doesn't matter what I touch. I pulled all the silicone off everything as best I could what pain. There was on a lot of the tube socket connections also that needed to be removed before soldering. Inspected all the tube sockets, re-soldered and retentioned the sockets. Reflowed the connections on the board also. Going to do a bit more testing and tracing if I don't find it I'll give it back. From what I was told this has been an ongoing problem and may have been looked at by others. If so I know who they may have been and they are good tech's.

[Mick Bailey]

There are only a handful of components in the PI circuit. With no preamp tubes installed then the main reason for excessive noise is usually down to voltage interruption through the PI tube. I would monitor the plate voltage for each half of the PI and see if that varies with tapping. A flaky plate load resistor could be the cause, or something intermittent in the cathode circuit.

Don't let the history of the amp get in the way of you finding the problem. Good techs or bad, everyone can miss a simple problem and even the most experienced can fail to see something that the person could spot a mile off. In the 70s I built a synthesizer from a kit and it was a pretty complex build (at the time it was the largest single-pcb synth that had been produced). It took a whole week to populate the PCB and in the end it didn't work at all. I spent another week trying to locate the problem. My Father, who has zero electronics knowledge, took a look and instantly said "Is that supposed to be that way round?" Out of all the components on the board he saw a printed transistor outline where the component was installed backwards. Problem fixed.

[J Luth]

Where I'm at today.

Still does it but when I push hard on the board in the area of the 2, 20uf radial caps it stops. The board does not flex or move a lot just slightly. All solder joints are good, all components in that are check out fine. A bit leery about lifting the board as it is very brittle. It was actually broken and missing a big piece behind the 3, 10w resistors. I repaired that area with small terminal board. Unfortunately I epoxied it to the original board and the wood behind the board to stabilize that area.

Forgot to add that when I am pushing on the board and taping with my other hand, I can not get it to make the noise when tapping the board or any components. I am using long plastic pieces to tap so I don't get zapped.

[J Luth]

Mick

Appears you may have been correct about a flaky resistor.

Tried tapping everything in the PI circuit very lightly until I found what was the loudest. The 510K resistor was very loud when lightly tapped. Changed it out and so far the noise is gone. The resistor checks OK both in and out of the amp and does show any defect when the leads are flexed. It still needs a set of good tubes, they are not very good and the slight noise I am hearing may be from them.

Now hopefully the noise does not come back.

This was a learning experience for sure.

[Mick Bailey]

Nice one!

[drewl]

Found the intermittent connection on this R12
Pin 4 of one of the 6V6's and negative feedback wire from OT.

Another thing to be careful of is to not have a long component lead go through a wire under the board.

Changed PI coupling caps and hit a high voltage wire putting high V on input grid.