1960s era ampeg jet j-12-a has tremolo problem

[lectricmandan]

My vintage Ampeg Jet that I bought new in 1965 has developed a problem with the tremolo circuit . When I turn on the tremolo it just takes off pulsing the speaker at a constant rate, regardless of where you set the knob .This all came about after I recapped ( I did use .047 caps in place of the .05 it calls for) the amp, new preamp tubes and installed a new pot/switch combo for the tremolo control . I have been through it wire by wire making sure it is correct but can find nothing wrong . I do have a schematic with all of the voltage readings of the amp attached

[Enzo]

OK, verify: the speed control is 500k ohms? Measure.

[Jazz P Bass]

"This all came about after I recapped ( I did use .047 caps in place of the .05 it calls for) the amp,"
This may be a hint.
Not sure.
Go back again & check your work.
Why did you do the work in the first place.
You have not said.

[lectricmandan]

I did verify the pot, it ohms out at 480k.The reason that I changed the pot is because it was mech broke as in the switch did nothing and then it wouldn't move after it clicked but if I jumped it out the tremolo would work but there was no speed adjustment because of a mech issue with the switch.The .047 caps have been in it for 8 yrs. - no problem. I did change the .02 coupling caps along with the 2 6SL7 tubes because I was having a distortion problem . I also changed the 3 electrolytics - 25uf - 25volt which is what was in there when new. So now I have no more distortion and a mindless tremolo

[lectricmandan]

i have now replaced everything but the power and output transformers with no improvement. When I turn on the tremolo the speaker just pulses away at about 9 hz and is unaffected by the speed pot . I think that maybe the B+ is too low as the print calls for 350vdc and I only have 309 vdc.

[g1]

I did verify the pot, it ohms out at 480k.

That says nothing about whether the wiper of the pot is working. Measure from the middle lug to one end lug. Turn the pot as you measure. Repeat from middle lug to other end lug.

[Jazz P Bass]

9 Hz is the hint.
That is the trem frequency.
It is not being controlled.

[lectricmandan]

IT is a brand-new pot from Fliptops and it does work as it should as I have checked it several times. One end of the pot is not used and the other end is connected to ground with the wiper being connected to a120k resistor. One strange thing that I have observed is with one lead on the open end of the pot and one lead on the wiper side of the resistor ,it will read from 490k to 5 as I turn the pot - if I move the lead to the cap side of the resistor and read from the open end it reads 125k all the time regardless of where I turn the pot. It kind of matches the problem but I can't make any sense out of why or if the two .05 caps have any affect on it . I have changed the resistor 2x . You would think that if the pot reads 490k and you put a 120k in series with it then it should read 610k !! Like I have said the speaker is actually pulsing in and out regardless of any of the controls as soon as I switch it on.

[Enzo]

Two 0.05uf caps and the 120k resistor come together above that pot. Measure resistance from that point to ground. Not to points on the pot. Rotate the pot. The reading should vary from about 100k to about 600k. If it does this, forget the weird pot stuff. If it doesn't then we work on the pot.

We are assuming the trem is doing this to the amp. But what if the amp is motorboating? 9Hz is a reasonable motorboat expectation. It may have nothing to do wwith trem. We don't really know yet.

Does turning the trem off at the trem switch make it stop? If so we know the trem circuit is involved, but not yet HOW. Scope the plate of the trem tube, is it pulsing at the 9Hz? Strong pulses? Or does it look different? Now look at the B+ supply to that plate resistor, node B on the supply. Is that bouncing at 9Hz?

You replaced a lot of parts. Is the resistor between B and C actually 22k? There is a 680k resistor to ground from the trem line, shunt across it to make the power tube grids return directly to ground. DOes that stop it?

[lectricmandan]

Measured the resistance to ground while rotating the pot - meter just stayed at 126k from end to end. So I then took the new pot out and put in a spare 500k I had laying around (with no internal switch) .Now the trem was adjustable but the pulsing was still there but not as bad and it now measured 7 hz .It only motorboats when then trem is switched on. Will try and exchange the switch at Fliptops if possible, thier only 5 miles away .The resistor between B and C is new and is 22k . Will continue tomorrow .

[Enzo]

SO new pot was either bad or somehow miswired.

You say the tem is now adjustable but it still pulses at 7HZ? I think we are using different terms. TO me the trem is the actual effect oscillator. If you mean the resistance from that point now varies, that is cool. If I read you right, you mean the latter but the motorboat pulsing remains and not adjustable?

You can disable the trem oscillator without shutting down that tube by shorting to ground either the gris (short acros the 270k resistor ) or short to ground that point we were reading resistance from. Either of those will stop the trem tube from oscillating. If the pulsing contnues we know the tube is involved, but is not oscillating from the circuit.

[lectricmandan]

Got a pot that works everytime now. When the trem is switched on it does work, as you turn it up the speed increases as it should BUT you also get the motorboating at the same speed as the trem is running. I tried shorting the 270k resistor but that takes everything away. Maybe it's got too much intensity ?

[Enzo]

SHorting the grid allows the tube to continue to conduct, so it is a test. It makes all this stop, so I think your initial diagnosis is correct. - it is trem related

Yes, too much intensity is a good theory. You might tack in a 1 meg pot in place of the 680k output resistor so you can dial down the intensity see if that helps

As it sits, your trem oscillator sends a strong signal. How large is that signal?

[Helmholtz]

Output pulsing with LFO frequency is typically due to unbalanced output tubes.
As bias is not adjustable, these amps need matched tubes.

The LFO output is fed to both power tube grids as a common-mode signal.
So it won't appear at the output of a perfectly balanced PP stage.

[lectricmandan]

As far as the output tubes go, they are supposedly a matched pair and I swapped them with the old ones that worked for decades with no change in the motorboating. It only motorboats when the trem is switched on. Tried swapping a 1meg for the 680k output resistor but it made no difference. Tried adding a 470k to the screen and that reduced it so went with a 680k and that reduced it more but it's affecting the intensity big time. The wave form is good, but I can't tell how strong it is as I have an old tectronix scope and I'm not real familiar with it .For reading frequency I use my Fluke 87 DMM.