Hum in directly-heated single-ended output valves

[GrimJosef]

Originally Posted by G6Tanuki

... For balanced mixers a 3-30pF trimmer is usually used as the variable capacitance - at audio frequencies you'll need something a bit larger ...

I think you've hit the nail on the head here. If I've done the sums right a capacitor which would pass the same current (90 degrees phase-shifted of course) as a 100ohm resistor (half the pot) at 100Hz would have a value of about 16uF. The values you mention give much smaller overall phase-shift than that, but given that I can do a pretty good nulling job already I suspect that a much smaller phase shift might be all that would be needed to affect the null.

I'm struggling a bit to believe that there is even a degree or two of valve-to-valve phase-shift variation at 100Hz going on. If there was, and if it affected the normal operation at all, then that would quickly become pretty exciting when the valve was operating at 20kHz, which it does do. But this experiment wouldn't be hard to carry out, so when I get a minute I might try it.

Cheers,

GJ

[trobbins]

As an extra comment just for hum issue, have you been able to confirm there is no hum induced due to any/all measurement equipment being mains ac powered?

Also I wouldn't discount the tuned choke method for reducing 100Hz B+ ripple, very simple, and likely providing more 100Hz attenuation than doubling the choke inductance.

Is that choke suitably distant from the OT, and with the choke 'noisy input' terminal as the innermost winding terminal?

[Diabolical Artificer]

Some interesting stuff here about filament supply + hum - http://www.4tubes.com/Lost-Websites/...ch/humbal.html The lads over on DIY Audio usually use a dedicated filament supply, whose details I can't find at present or some of the articles I've read that pertain to your problem.

Will keep looking. Andy.

[kalee20]

Originally Posted by GrimJosef

Imagine one end of the filament - say the negative end - is connected to the 1k bias resistor. When the filament supply is at minimum voltage (9V) the average filament voltage will be 4.5V above 57V. When the filament supply is at maximum, 11V, the average filament voltage will be 5.5V above 57V. Simplistically that's a 1V pk-pk signal fed into the grid-cathode circuit (in practice the finite cathode impedance of the valve will reduce this, but not by much). 1V g-k will generate a lot of hum at the load ! The job of the 200ohm pot is to 'centre' the 1k resistor on the filament and thereby null out this signal. It's a very commonly used arrangement with DHT's.

Yes, I can see that, if the filaments were supplied with AC. But the sketch in your original post showed a DC filament supply. If the DC is pure, the 200Ω should make no difference to hum.

Originally Posted by GrimJosef

If we imagine rotating the pot to the ends of its travel I think it's clear that the resulting 100Hz hum at the speaker will go from one very large value to roughly the same very large value but with the opposite polarity (180 degree phase change). Somewhere in between, actually very close to the centre of the pot's rotation, the hum from this source must either go through zero amplitude or through a 90 degree phase shift. There are no other ways from plus to minus.

True. And knowing the frequency of the hum fundamental would be really helpful. It may be good to revert to pure AC on the filaments, and using your spectrum analyser tune the pot for minimum 50Hz. If this is near zero, excellent. If it is not zero then effects as Radio Wrangler suggests could be kicking in.

Originally Posted by Radio Wrangler

Could there be a mismatch in the relative positions of the centroid of the emissive surface and the centroid of the heater voltage?

There will be a significant phase-shift between heater voltage and heater temperature, due to thermal lag anyway.

If the 50Hz component is null-outable to a satisfactory extent, then the residuals (100Hz etc) are down to non-linearities and it won't be possible to null them by the pot. You have to make the HT supply 'blameless' with lots of μF (even if not feasible in the final design, you can connect capacitance to prove a point).

It sounds very inelegant, but if push comes to shove you may be able to cancel the hum (100Hz) by finding some 100Hz from a FW diode bridge, putting through a phase adjust RC network and an amplitude adjust pot, and injecting into the grid circuit to cancel what you can't fix.

[GrimJosef]

Originally Posted by trobbins

As an extra comment just for hum issue, have you been able to confirm there is no hum induced due to any/all measurement equipment being mains ac powered?

Also I wouldn't discount the tuned choke method for reducing 100Hz B+ ripple, very simple, and likely providing more 100Hz attenuation than doubling the choke inductance.

Is that choke suitably distant from the OT, and with the choke 'noisy input' terminal as the innermost winding terminal?

As far as instrumentation goes, the test set (HP3561A and HP8903A, inputs both floating) don't generally pick up much noise. The most sensitive test I've done with this amp was to disconnect the primary of one of the output transformers and replace it in the 211 anode circuit with a resistor. I wanted to see how much magnetic flux leakage from the mains transformer was entering the output transformer directly. The signal on the secondary (6ohm load) was about 0.3mV RMS total with 50Hz at about -72dBV and 150Hz 2-3dBV lower. The 100Hz peak was much lower still (can't remember exactly how much, maybe 10-20dBV ?). So I think the instrumentation's clean, at least as far as 100Hz goes.

Likewise there's not much leakage from the current HT choke since it's outside the amp altogether and connected just by a length of twisted pair. I've tried rotating its core axis to point along three orthogonal directions to see if that changes the measured 100Hz. But it doesn't.

Thanks for reminding me about tuning the choke. A quick test has shown that I can get about 3dB improvement in the bad valve and rather more (not that I need it) in the good one. Perhaps this suggests that the higher level of 100Hz in the bad valve isn't coming completely from the HT ripple. So when I reduce that ripple a good deal something else is still contributing to the 100Hz in the bad valve.

Cheers,

GJ

[valvekits]

Hi GJ,
I suppose it is quite possible that you are the first to observe the valve to valve difference and as such there isn’t an answer to your question. Tube cad article gives some insight into this type of problem and as often is the case, has a rabbit waiting in the hat.

Eddie