Tremolo Help

[martin manning]

Oops, I meant put the probe on the output stage of the trem oscillator, on the 6SL7. What you are seeing there is the B+ ripple.

Looking for the term oscillator output with no load on it, and then the effect of loading it up by connecting it to the dual intensity pot.

You should always have a dummy load or speaker connected to the amp's output if the power tubes are in!

[tubeswell]

The double-ganger intensity pot following the plate-driven 'follower' is contributing to the weak trem wiggle - because the two pots are in parallel in terms of the AC load that the plate is seeing. (The bias voltage 'source' is effectively a ground return point from the point of view of AC, because it is decoupled to ground with the bias filter cap).

What's more double-gang intensity pot is in-turn in parallel with the plate resistor of the 'follower' stage (because HT and ground are one and the same from the perspective of AC).

So you have a highish output impedance from the 6SL7 feeding a lowish input impedance for the trem wiggle, and that's a recipe for AC attenuation, let alone what happens when you get to looking at the tube current in the 6K6 output tubes (that the trem wiggle is trying to drive into cutoff). However, dropping the double-gang pot will do away with your individual biasing.

That's why I suggested converting your plate-driven follower to a cathode follower (as per a 5G9-type LFO wiggler). This would dramatically lower the output impedance of the overall LFO circuit, making a better impedance bridge, and thereby reducing the lossiness-to-AC that is currently built in to the circuit.

[C Moore]

Oh Jeez.....sorry.
meant to say I have a Resistive Load...No speaker connected.

OK...on the output of the Oscillator.....I have about 200 mV p to p.
But the sine is jumping all over the screen. No matter where I turn the trigger control, the sine moves all over the place. Not a dual trace, but it jumps up and down a lot. Do not think I could take a picture...it is moving too much.
Will connect the speed pot and report back.
Thanks

[C Moore]

tubeswell -
I am definitely open to changing the circuit. I hate to keep wasting Martins time.
Let me get this connected again and scope it one more time.
Thank You

[martin manning]

Tubes, that is what we're trying to prove and/or confirm.

HH, don't worry about it... you're doing all the work! Yes reconnect the speed pot. Having confirmed the 6SL7 is functioning correctly you want the oscillator to run now.

Re converting the driver to a cathode follower, the Vh-k max for the 6SL7 is only 90V, so DC coupling to the 150V on the plate of the oscillator won't work, at least not for long!

[cbass]

Hi guys I'm just kinda jumpin in here I haven't even read all the posts.But something that helped me troubleshoot a tremolo was to temporarily replace the cathode resistor/cap with an LED.Then you can visually confirm if the oscilator is working.

[C Moore]

I hope this is going to make sense.....
With the oscillator connected again, the output at pin 5 still jumps around, but not as "violently".
It is kind of hard to tell, but it seems like the p to p voltage is a bit less now. Maybe 150mV instead of 200 when it was not connected.
Is that making sense to you guys.?
Thank You

[martin manning]

So 200mV on the scope with the driver stage disconnected from the intensity pot and 150mV connected? WIth the x10, 2V and 1.5V?

[C Moore]

So 200mV on the scope with the driver stage disconnected from the intensity pot and 150mV connected? WIth the x10, 2V and 1.5V?

Sorry...I steered you wrong somewhere.....
Disconnected it looked like 4 Divisions at 5mV per and a x10 probe.
Connected it looks like 3 divisions at 5mV per and a x10 probe
So that would be 200mV and 150mV......??
No Volume Bass or Treble...they were at Zero

If I hit the foot switch and turn the Trem On I am seeing 200 Volts p to p

[martin manning]

If I hit the foot switch and turn the Trem On I am seeing 200 Volts p to p

This is what I wanted, both connected and unconnected. Is that 200V p-p with the oscillator connected to the intensity pot?

[C Moore]

yes it is connected back.

[martin manning]

That sounds good. What does the waveform look like, and with the intensity pot all the way up what do you see on the grid?

[C Moore]

The 200 Volt Sine wave looks clean..."normal".
I will scope the grid.

[C Moore]

OK...the grid of the Oscillator...right.?
The sine wave looks good.
0.2 x 3 x 10 = I guess that would be 6VAC p to p on the Trem Grid.

[martin manning]

No, sorry I guess I wasn't clear which grid. I was looking to see what trem signal amplitude reached the grids of the power tubes.

The 200V p-p at the 6SL7 driver stage plate seems like plenty though, since I believe you only need to get about 45V p-p trem signal at the power tube grids (same magnitude as your -45V bias voltage). That will vary the bias around a -45V mean, going from -67V to -23V.

Let's confirm what you have now:

- From the 6SL7 driver stage plate (pin 5) you go to a (jumpered) 130k resistor, then split into two 0.1uF caps, which go (separately) to the tops of the intensity pots.

- Each of the intensity pot wipers goes to a bias feed resistor and a grounded 0.01uF, and the bottoms of the pots go to the 22uF caps in the bias supply.

Correct?