Univox 1221 - tremolo circuit won't oscillate

[tubegeek]

Hello all - long time no see!

I was helping a friend today and it was very frustrating. He has a Univox 1221 head which seems like a pretty decent amp, and his is exhibiting a problem which I was unable to fix. The schematic is here -

http://www.prowessamplifiers.com/sch...univox1221.pdf

Or see attached.

univox1221.pdf

The tube at the bottom left is the tremolo oscillator. As I understand the circuit, the trem oscillator sends a varying voltage into the grid of the 12AU7 which is biased off the negative bias supply to be a few volts below ground (measured -1.6.) When the oscillator swings positive, the 12AU7 turns on and sinks the dry signal path to ground. When the trem oscillator swings negative, the 12AU7 turns off and the dry signal path passes unchanged.

So far so good? I'm not completely sure but that's what I believe the principle of operation is. Chime in if I'm wrong, please!

In my friend's amp, the trem oscillator won't oscillate. As a result, when the trem intensity control is turned up, the signal is weakened but doesn't vary.

We hunted for blown caps, miswiring, and we replaced a couple of caps that seemed ugly-looking. etc. Swapped tubes, pots, etc. I checked the parts I could check in-circuit and everything was right at spec. Nothing improved with any of our changes.

My next idea would be to pull the caps around the oscillator section and measure them for drift. Anybody have any better ideas, or specific experience with this problem?

Thanks,

-j (tubegeek)

[Jazz P Bass]

If the tube is good and the plate & cathode voltages look good, it most certainly is the caps.
All it takes is one bad one & the oscillator circuit is dead.

[Enzo]

It won't be about blown or drift, it will be about leaky.

If the values drifted, all that would do would be shift the speed of the trem.

Blown? Nah, no stress on them.

Replace the 0.02uf and the couple 0.01uf ccaps with some nice film or ceramics. Replace the 10u/50v cathode bypass cap. Verify the lemme see, five resistors, and make sure the speed pot is not open or unwired. Disconnect the 0.1uf so the oscillator stands alone. You should have oscillation now, if B+ is there and the 12AX7 is OK.

Once the trem LFO works, then go ahead and swap out the 0.1uf. Verify the resistors down to the intensity pot, and the pot itself. With trem off, I calculate about -2.09v of bias from the drawing, but -1.6 is probably close enough.


A personal mission of my own is to slow the damned trems down. Most start out too fast at the bottom end and turn up to a useless buzzing speed. I think the slow end should be a sexy throb. Increasing one or both of those 0.01uf feedback caps will slow down the LFO.

[tubegeek]

A personal mission of my own is to slow the damned trems down. Most start out too fast at the bottom end and turn up to a useless buzzing speed. I think the slow end should be a sexy throb. Increasing one or both of those 0.01uf feedback caps will slow down the LFO.

I REALLY like this idea Enzo, thank you - I'm going to look for an existing hole to put a switch - I'll wire up a second pair of .01's in/out where "in" will slow it down - I, too, like a slow-motion trem.

Any suggestions for mods to get a different sound out of the non-effects channel? I have a couple of nice JFETS here - maybe cascode the 2nd input for some CRAZY gain on the second channel? That'd sure be DIFFERENT!

Also I intend to rewire the heaters with shielded twisted pair to reduce the hum a tad - any other ideas while I'm in there?

Thanks so much. A friend told me "those grey oil caps are the leakiest caps of all time." I didn't suspect them immediately because, typically, oilers in my experience are good quality. Live and learn.

I'll report back to this thread once my Mouser order gets here and I go back in. THANK YOU to ENZO and JAZZ P BASS!

[Enzo]

You don't need switching. I'd start with .02 instead of .01 in those spots. WHat it does is move the range from something like 5Hz through 500Hz and moves it down to 1Hz through 200Hz or something like that. The top end will still be too fast to use, but the bottom will be nicer. In other words, enlarging those caps won;t take anything useful away, it just adds some more useful slow end.