1959 Ampeg M12 Distorted, Raspy Tremolo

[64deluxe]

I am working on a 1959 Ampeg M12 w/ 12AX7's and 6V6GT's and after replacing the cap can and some wonky resistors it sounds quite amazing, very Deluxe-like but when I switch on the tremolo it works but sounds awful, distorted with a weird scratchy artifact like the ticking of an optoisolator but, of course, there isn't one.

I replaced all the resistors and caps that are in the tremolo circuit on the schematic, re-heated the solder joints, cleaned the pots thoroughly, remove the old footswitch [but not the wire itself] but no change at all.

There are no voltages on the schematic so I am not sure what to look for in terms of voltages.

I have successfully repaired many Fender amps but this is my first Ampeg and would appreciate any help or advice.

[Enzo]

trem on, but dial the intensity down to zero. Does the amp sound normal that way?

Just for science, did you try a different trem tube?

Scope the signal at the top of the intensity pot. Does it look like a smooth up and down? Does there appear to be any DC offset?

When you replaced all the caps in the trem circuit, did that include the 0.1uf cap from plate to the intensity control?

[Prairie Dawg]

I've got a 1961 Mercury and I love it to death. It had three new Tung Sol reissue 6SL7s in it and I figured I'd put some NOS tubes in it ...trem no workee. Back in went the TS reissues with a promise not to disturb anything ever again. One thing for sure is if the amp's been bodged it might have been wired up wrong. I have a gut shot of mine that may be of use to you. and I canb always take some detail pics if needed. Come to think of it it'd give me an excuse to remove all that RTV silicone in there.

I recently repaired a Gibson amp that had had the trem wiring bodged years ago and it caused a lot of heartburn until I figured out how to make it work properly. The owner said he never uses trem, and I told him it was now time to start.

[64deluxe]

FIXED!

Eureka, it's fixed!

Thanks, Enzo, for focusing me in on the solution.

I turned the Intensity pot down with the Tremolo on and the distortion went away. Since I already had tried a different tube and replaced the .1 cap you mentioned, and haven't yet learned how to use an oscilloscope, I looked for DC on the Intensity pot with a DMM and as I turned the pot up the DC suddenly came on strong. Since I had replaced everything in the other direction I followed the wire from the wiper to the middle of the PI, at least I am assuming that is what that group of caps and resistors is. And am I right in thinking it is an "accordion" PI as it has pattern of going up and down between the eyelets like the folds on an accordion?

I replaced the two .02uf caps there, and the two 270K resistors, which were more than 20% over spec.

Beyond them on each side were two 120k 1/2 watt resistors where the schematic of the M12 6v6 model that's on the Net shows 47K 1 watt. Could this be because the one I am working on is the 12AX7 model and the other has 6SL7's in the preamp?

Anyhow, I replaced them all and the amp now sounds perfect and the tremolo is superb.

[64deluxe]

Now that it's fixed this amp really blows me away. It is very versatile and the interaction of the controls is wild. It is like the best vintage Fender Deluxe, especially as this one is 6V6's and 12ax7's, but with Tremolo like a Magnatone. I was very sad to return it to the owner this afternoon....

[Enzo]

Your 0.02uf caps were leaky, the ones coupling the PI to the power tube grids. The pair of 270k resistors are your power tube grid returns, which normally go to ground. (or to a negative bias supply in fixed bias) Here the ground connection is through the intensity pot, which takes that large low frequency trem signal and lets it vary the grid bias. If those caps were leaky, then there would always be some unwanted DC on those grids. And when the trem tried to operate it got in the way as well.

If you find a coupe leaky caps in an amp that old, it is prudent to replace them all.

I usually see the term concertina PI. That refers to a single triode used as PI. It sends signals out both the plate and cathode, at opposing polarity. Hence the voltages alternately close in on the triode in the center and then move away from it. Like the ends of a concertina. Your phase splutter has two triodes, and so would not be one of those. The physical layout of a circuit would not be the same in all amp models, so I doubt the name had anything to do with the layout.

[Prairie Dawg]

Pretty interesting fix. I'll have to put that in my bag of tricks. Mine's got a swampy trem that's just...delish, as my sister would say about some tasty morsel on her plate she'd just speared. I'm wondering if the later model is a little more gainy than mine? Not complaining, you understand.