AC10 clicky trem

[Alex R]

I'm doing a general overhaul on an early 60s AC10. Schem below. There's a loud click on the tremelo, which goes along with the wobble. On the scope, on the plate and grid of V3B, a jagged interruption appears at the bottom of each tremelo waveform. This is visible elsewhere in the amp but at its sharpest here. I am assuming that's the click.

I changed out C17, C19 and C20 with no effect on the click. I wondered if the tube was biased wrongly, as the scope image reminded me of when you see the ripple at the top or bottom of the waveform when a SS power section waveform hits a rail (vague hunch I know, based on weak knowledge I also know ). Increasing the value of the cathode resistor shared between the V3B ECF82 pentode section and the EF86 V1 did bring down the clicking a bit, but only when the oscillation is also reduced. Adding screen bypass caps on V3B from screen to cathode does stop the clicking, though not until the values go over about 1uF.

I don;t know why this clicking is happening, nor do I understand why the cathode bypass cap cures (or masks) it. Anyone want to educate me?

[MarkusBass]

Isn't it related to poor capacitor in the power supply section for the oscillator (which is common to input valves power supply)? I'm talking about C10A capacitor (8uF). Have you check it? Or, simply solder 22uF in paralel to it (just for test).

Mark

[Chuck H]

It sounds like the oscillator is hitting cutoff badly. This could be because of the cathode loop. I would check the shared cathode circuit cap and resistor.

[Alex R]

Thanks guys. The filter caps and cathode bypass caps have all been replaced. Clipped in 22uF over those 8uF preamp filters - no change.

There is a mighty swing in the oscillator, but it doesn't like oscillating at slower speeds. Once it starts, it then clicks, with a helping of thumps coming in as the speed increases, all in time with the trem. The screen bypass cap wheeze stopped working after a while, ie it started clicking again. Sigh. So I put it back to stock.

I tried separating the ECF82 pentode-side cathode from the EF86 cathode - stuck an LED on the ECF82 cathode. This created a hum on the vib channel without stopping the clicking. Larger cathode resistors didn't stop the clicking or the hum. Back to stock again.

Replaced C14 for luck. No change. Resistor values in the osc/mod circuit are correct.

Tried varying cathode resistor and plate resistor values to change bias and move the swing around - the effect was to increase or reduce the trem but without removing the click. It seems that any attempt to limit the swing in the oscillator turns the oscillation off before it removes the click.

EF86 and ECF82 both test good. Swapping in EF86 spares makes no difference. I've ordered an ECF82 to try as I don't stock them.

scoped the click again, it is the oscillator all right. Sometimes it looks like a sudden voltage drop - the oscillation is sine wave but at the bottom of the wave there's either a little jagged bit, or just before the wave hits the lowest point it drops vertically for a little distance.

Bit stumped really...


with trem off and no signal
the "+255v" supply is 279v
ECF82 plate is at 102vDC
ECF82 screen: 50vDC
ECF82 grid: 0v
shared cathode: 2.32vDC

eta C5 is ok too

[Joel66]

I have a 1965 AC10 Twin with the same issue and have been unable to troubleshoot it. Did you find a fix yet?

[Joel66]

I have a 1965 AC10 Twin with the same issue and have been unable to troubleshoot it. Did you find a fix yet?

Replaced ECF82 and the ticking went away. DONE!