Tremolo Help

[C Moore]

Tubes.....Martin -
I knew I was not smart enough to be stupid enough to decide 270k all by myself.
IF I am seeing this correctly.....I guess I got 270 from this old Ampeg schem.
Maybe I should change it anyway.?.....to match the other two 130k in the circuit.
Thank You

[martin manning]

I'd leave it; most likely that is the correct value, and if you lower it you'll probably lose some of the speed range off the low end. I was wondering how you ended up with that value.

[tubeswell]

Or try it with all the resistor values in the R-C network the same (except of course for the pot)

[C Moore]

Or try it with all the resistor values in the R-C network the same (except of course for the pot)

.
I guess with the other two at 130k, and the grid resistor at 270k...I could jumper another 270 across it and be close enough to 130k to observe the effect.
Is that what you are suggesting.? (more or less. )

[tubeswell]

yeah

[C Moore]

I really cannot believe this.
I have lost the Trem.

Was waiting on a head box to arrive.
That got here...and I plugged into the amp (still out of the new box)...and there is no Trem.

Take out the shorting plug (from the foot switch jack) and the Trem comes back on.
Do it again...and there is no Trem
.
So I start chop sticking the Trem Circuit components...and a slight touch to the plate resistor area brings the Trem right back.
So I re-flow the solder joints...same problem.
So I replace the jumper wires in that area...same problem.

Seems like chop sticking the caps in the circuit makes it cut in and out...so I replace those...and the cat resistor and by-pass cap.
Now I have No Trem at all.
I do not know why (obviously)...and do not really know how to trouble shoot this.

I have tried tubes...no help.
With the Trem turned on.....
1.8 VDC across the cat resistor
150 VDC at Plate One
Plate Two oscillates between about 120-220 VDC.

I need help (possibly psychiatric help as well).
Do I need to get back on the scope...and follow a signal through this somewhere.?
Thank You

[martin manning]

Sounds like the oscillator is working, but I think I would use the scope to verify that and have a look at the waveform, then follow it through to the power tube grids.

[Firestorm]

Oscillators sometimes have trouble getting started; they need something to "kick start" them, a voltage surge or something. Leaky caps in the phase shift network can hamper this, as can a too large bypass cap. Testing the LFO with a meter may mislead you because the act of connecting the meter may turn it on.

[tubeswell]

So I start chop sticking the Trem Circuit components...and a slight touch to the plate resistor area brings the Trem right back.

I'm guessing the plate resistor may have crapped out. Try a new plate resistor (I recall this happening to me one time after I had managed to wreck the resistor by moving the lead too many times)

[C Moore]

You know...the plate resistors are the only parts I did not replace.
Will give it a shot.
...and do the best I can with the scope.
Will tell you what I see.....to the best of my ability.
Thanks

[C Moore]

Oh Boy -
I need some advice/opinions.
I replaced the two 220k plate resistors...and the Trem came right back
Took out the foot-switch plug...put it back in...and the Trem was still there.
Did it a third time and the Trem was gone...but it came back in 10-15 seconds.
Took the plug out again...put it back in...and No Trem.
So I chopstick the board in the Trem Circuit area...and got all kinds of crackling...like with a short of some type.
Not sure if you can see in my pics...or remember them...but there is not much room in this chassis.
I hunted around the best I could with an inspection mirror...and everything looks fine, but obviously it is not.
Will be a fair amount of work to pull the baord and look...but I do not see that I have a choice at this point.
If I pull the board, and everything looks OK...I do not know what to do.
I think it is time to build a new Trem Circuit (tubeswell suggested this a long time ago...but i thought i had this problem solved)...this thing just may be cursed... for lack of a better term. This has really gotten frustrating.
What Trem Circuit would you guys put in this amp.?
Suppose I was just starting...what circuit would you guys suggest I copy...with my dual bias set up.
I am hoping something new will change my luck.
Thank You

[martin manning]

So I chopstick the board in the Trem Circuit area...and got all kinds of crackling...like with a short of some type.

I think this is likely, or a bad joint. Nothing wrong with the trem circuit IMO.

[C Moore]

Hey Martin -
Thanks for all of your help.
Even if a new circuit was the way to go.....I think I would be foolish (at this point) to not pull the board and take a GOOD look. Something is wrong somewhere.
Will be a chore.....but what else can I do.
I will tell you guys what I see when I see it.
Thanks

[Firestorm]

I have no scientific basis for this, but maybe try splitting the cathodes in the oscillator/driver stage. In the M12 schematic (which I assume you followed) the cathodes are coupled and they share a cathode resistor (cut off by the switch). What if you split them? The Fender Tremolux ran them seperately (and it didn't always work), but considering how finicky LFOs can be, maybe try it. I'm just grasping at straws here.

[C Moore]

I do not know (electronically) what the implications of that change would be.
Does it help with "starting" the oscillation.?
I do know...at present...there is something amiss with my board and/or wiring.
Something is not right somewhere...and I have to pull the board up unfortunately.