Troubleshooting bias-vary tremolo tick

[BlisterFingers]

Hi all,
First post here and looking for some pointers to troubleshooting steps for a ticking bias-vary tremolo on a Fender-ish circuit.
Hoping this is the right board to post to.

First up, the amp is a point-to-point wired, push-pull, cathode-biased circuit, loosely based on a Tweed Deluxe but with a GZ34 rectifier and pushing about 20 Watts. Additionally the amp has reverb and a bias-vary tremolo. I bought it second-hand in Australia about 15 years ago, and according to the builder it was put together for someone trying to get their Neil Young on but, with a bit more oomph and clean headroom than a standard Tweed Deluxe. It's currently housed in an open backed combo cabinet with a 25W Greenback and it rocks hard

It's always had the trem tick and I've lived with it, but now I have a bit more amp circuit knowledge I'm finding it's annoying me more than it used to and I have a burning need to fix it

I've traced the schematic, and drawn the layout using DIYLC. copies attached (but noting some electrolytic cap value errors in the DIYLC file and I'm still in the process of measuring voltages)

Here is the oscillator circuit



The wiper from the Depth pot is connected to the control grids of the output valves.



For reference, here is the valve layout:
V1B - 1/2 12AX7 -1st preamp stage after input
V1A - 1/2 12AX7 - Preamp stage for reverb signal
V2B - 1/2 12AX7 - Reverb recovery
V2A - 1/2 12AX7 - Preamp - recovery before phase inverter
V3B - 1/2 12AX7 - PI (cathodyne)
V3A - 1/2 12AX7 - Tremolo oscillator
V4 - 6V6GT - Reverb driver
V5 - 6V6GT - Power
V6 - 6V6GT - Power
V7 - GZ34 - Rectifier

Troubleshooting steps to-date
The caps in the oscillator circuit are all Panasonic film caps, I haven't pulled them to test values but I don't have any reason to believe there's any issue - this isn't a 60-year old relic. Not directly related, but I've pulled and tested the power supply filter caps on an LCR meter. Capacitor values and dissipation (tan-delta) are well within manufacturer specs.

I've changed out all valves with new ones and it hasn't changed anything so I'm suspecting something parasitic.
I'm not an expert but the lead dress looks good and doesn't stand out to me as a likely issue, but I'm open to be convinced otherwise. The gut shot below shows the lead dress.



Hoping to find new paths to investigate and silence the noise.

Thanks in advance

[Stevem]

Hello there!
With the Fender amps it's 95% due to the wire layout to the Trem circuit.
I do nor have my mod info with me at the moment, but if you do a search here or on line you should turn up the mod info.

[xtian]

Very pretty PTP layout. Is that pot, third from right, with the pink and white wires, one of the tremolo controls? If so, unzip the pink wire from the blue and green ones and move it away from them. If this is not the case, you'll probably find the solution is similar--move the tremolo control wires away from signal wires--that's probably where your ticking is coming from.

[BlisterFingers]

I got it hooked up to my scope today to narrow down where the LFO signal bleeds into the audio signal.
The tremolo LFO and the Phase Inverter share two halves of a 12AX7 and there seems to be some crossover there.
The signal wires and tremolo wires were cable tied together at one point and I've now separated them and put some distance between them, but it's made no difference.

The pic below shows the wiring into the 12AX7



The effect is most noticeable on the signal coming from the PI plate. The clip below shows a 1KhZ sine wave captured at the PI plate.
You can see the waveform oscillate as I crank up the trem intensity control.




When I hook up the scope to the PI cathode, the effect is barely noticeable so it's mostly affecting the inverted half of the waveform coming from the Plate.
I'm truly at a loss now I'm wondering if there's some kind of parasitic crosstalk occurring 'in the glass' between the two 12AX7 triodes. Would a few pF of capacitance between the plates be a candidate here?

Any suggestions truly welcomed.

[Tony Bones]

I would double check what's going on here compared to the layout you provided.

[Stevem]

One thing I spen a bunch of time on when thinking about a build or making a built amp better is laying things or re-laying out things so the grid wires are as short as possible because they are the antenna's for trouble and I don't give a Rat's Ass if my build does not look as neat as it could because of it.

Extending the length of plate wires before the placement of the coupling cap helps in shortening up the grid wires by a bunch many times and taking this course on the first two gain stages is always my priority in thinking about a layout.

One thing besides what I posted about to try on your build is to use a sheilded wire on that Tremolo grid and hook the sheild of that wire up to the tubes plate and then heat shrink the unused end so no shorts take place.

[tubeswell]

Looks like a gain spike from an overdriven cathodyne. This happens when the cathodyne starts to overdrive the output tubes. As the cathodyne input grid input signal swings more positive, the cathode signal follows it. Eventually the output tube fed by the cathode output will be driven sufficiently positive that it starts to draw grid current, which will in-turn clamp the output tube grid, and prevent its input signal from rising any further. This in turn clamps the cathode of the PI. As such, the PI cathode voltage is no longer changing, and this is effectively the same as if the cathode load resistor were bypassed by a large capacitor - i.e. all the gain in the cathodyne stage becomes transferred to the PI plate, causing a gain spike at the peak of the signal (which you see as the bottom peak on your scope).

The fact that its happening when you dial the trem intensity up suggests that the output tube bias (as its being altered by the trem) reaches a point of low bias voltage, which causes the output tube grid current to clamp, thus resulting in the cathodyne gain spike. The fact that you can see it in the signal path with no guitar signal, is that its modulating the usual hiss (signal) in the amp.

Anyway, that's my theory. YMMV. This should be able to be confirmed by scoping the output tube grid that is on the side that is being fed by the cathodyne's cathode output.

The cure would be to increase the output tube bias voltage. Also, to limit the onset of output tube grid blocking, you could also add some largish (up to 47k) grid stoppers on the output tube grids.

[Stevem]

Could you post up the whole schematic with the power supply ?