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[trobbins]

50uF cathode bypass is capable of keeping the heater elevation pretty quiet for mains and above frequencies, given that the elevation's main aim is to suppress...

[trobbins]

I guess one option could be to connect the heater CT to the output stage cathode, as a simple form of DC elevation. That may possibly also benefit a 6X4...

[trobbins]

I'm with Greg - it's a legacy from vintage diode heatsinking, so no issue with a modern power diode like a 1N4002. At least that schematic has the positive...

[trobbins]

That stands to reason.

Also note that any vintage power diode from the late 1950s to even up to the early 70's has likely been exposed to stressful...

[trobbins]

No probs - it's meant to be a publicly availably resource - I just add stuff as I get around to it.

[trobbins]

Using open-circuit or capacitive loading has been found to often impose the worst-case stability conditions on a wide variety of valve amps with global negative...

[trobbins]

Most squarewave testing to adjust high frequency (ie. circa your 100kHz) resonance response would typically use a resistor load first to optimise the parallel...

[trobbins]

What compensation methods did you try? I'm guessing you added some different small value caps across the feedback resistor and observed the ringing response...

[trobbins]

The only preference is what is on a shelf near you. I went to a nearby large linen/haberdashery store and got Helmer product - below link.

[trobbins]

The material needs to be flexible but also robust in 'stretch' - so a strong 'klennex' (multiply) may be ok. Another option is a swatch of thin dress or...

[trobbins]

I've used an acid-free PVA type glue that dries clear and flexible, won’t yellow, and is meant for dense paper (Helmer) - you can use tissue paper or...

[trobbins]

Perhaps it could be 'normal'. The thread started with high frequency noise. Do you now hear anything abnormal with the amp, or by looking at the speakers...

[trobbins]

Vibrato intensity is not vibrato speed - different pots.

[trobbins]

See if the instability level changes when vibrato intensity is varied. If not then the issue likely relates to 'C' supply rail. If it does vary then likely...

[trobbins]

Post #4.

[trobbins]

I personally don't like the anode to B+ measurement because it is a hazardous measurement, and it requires a DC only responding meter (given that there will...

[trobbins]

This is likely one issue that could benefit from a soundcard+software bench tool to provide a cheapy but powerful spectrum analyser plot that can very easily...

[trobbins]

I would have thought that any such influence would relate to how well the loadline (given you are using a resistor load for testing) approaches the knee of...

[trobbins]

Is the amp showing nominal circuit idle voltages, and are the output tubes acceptably balanced (you would need an add-in bias tester or add in cathode sense...

[trobbins]

There are solid-state versions of the vintage Magnatone vibrato phase shift circuit. Bjorn Isheden used jfets instead of valves, and BAS70 silicon diodes...

[trobbins]

Temperature, time and physical re-location/vibration history are all plausible reasons for that query. Swapping out all valves also may help wipe the socket...