SF Twin Reverb - Tremolo disappears at slower settings

[waspclothes]

This is new to me..
I have a SF Twin - tremolo works, but as you're turning the speed lower once it gets past a certain threshold the tremolo effect will fade away and then disappear. It will slowly restart again when the speed knob is turned back up.

I replaced the 3 ceramic capacitors in the tremolo oscillator section, but the problem is still happening.

Any clues what else to check?

[glebert]

Have you tried a different tremolo tube?

[Enzo]

Those are the phase shift feedback caps, but how about the cathode bypass cap?

Do circuit voltages agree with the schematic? Scope it. Does the LFO signal reduce in amplitude as the effect fades? And any chance the output cap is leaky?

[waspclothes]

Have you tried a different tremolo tube?

Just the trusty 12AX7 that's been on my bench for years to swap into amps to see if things work. But I can go grab another and test..

[waspclothes]

Those are the phase shift feedback caps, but how about the cathode bypass cap?

Do circuit voltages agree with the schematic? Scope it. Does the LFO signal reduce in amplitude as the effect fades? And any chance the output cap is leaky?

I didn't swap the cathode bypass caps, but they were original so i definitely should. The voltages did all seemed correct though. I was only scoping the output at the time watching the effect fade. I can do another check of the voltages, scope the LFO signal and see.

What capacitor are you referring to as the 'output cap'?

[Enzo]

Sorry, thinking of aanother schematic, where there is a separate cap from teh LFO plate to get the LFO signal to the next stage. In this the "output" comes from the phase shift string..

[Enzo]

A quick calculation (someone please check me) 25uf cap and 2700 ohm resistor, means about 2.4Hz So if that cap is aging poorly that freq might rise making the LFO less reliable at lower end of freq range. Just a thought.

[pdf64]

A good application for LED bias.

[J M Fahey]

That circuit has just enough gain to start oscillation and a little bit more.

So anything that lowers gain: weak tube (which will still work in a preamp), out of tolerance caps, dry cathode caps, out of tolerance resistors (carbon composition types drift a lot) will make it "lazy".

[Chuck H]

That circuit has just enough gain to start oscillation and a little bit more.

So anything that lowers gain: weak tube (which will still work in a preamp), out of tolerance caps, dry cathode caps, out of tolerance resistors (carbon composition types drift a lot) will make it "lazy".

This was my first though reading the thread title. Rebuild it all snuffy with all spec components (as already addressed) and it'll probably be fine. 12ax7's of all types can be high-ish or low-ish gain. The published spec of "100" is just a generalization. So a durable and high-sh gain example should be chosen for the circuit to offer any time on the repair.

JM2C

[Chuck H]

A quick calculation (someone please check me) 25uf cap and 2700 ohm resistor, means about 2.4Hz So if that cap is aging poorly that freq might rise making the LFO less reliable at lower end of freq range. Just a thought.

This is the -3dB point? (didn't do any calculations myself ) Since the trem operates in the few oscillations per second range I really don't see any harm in bumping the bypass cap to 47uf or even 100uf. Unless there's something about the time constant with a longer initial charge time that fouls this consideration.

[waspclothes]

A quick calculation (someone please check me) 25uf cap and 2700 ohm resistor, means about 2.4Hz So if that cap is aging poorly that freq might rise making the LFO less reliable at lower end of freq range. Just a thought.

You Nailed it Enzo! I swapped the 25uF cap and the tremolo is now working at even the lowest speeds.