Preamp DC filaments and capacitance

[trobbins]

Without photos, could you clarify some topics please:

Do you have a 6J7 grid stopper right at the top cap, and then use screened cable from the top cap to the grid leak, and ground the screen at the grid leak and cathode bias 0V common node ?
Do you have the standard steel can shield on the valve?
(Some example photos in this restoration https://www.dalmura.com.au/static/Ph...0amplifier.pdf )

What 6J7 cathode resistance and bypass cap is in circuit, and what cathode bias voltage is present? You may want to try increasing the bypass cap, to better short the cathode to 0V, or if the voltage is practical, then use a LED for cathode bias.

I'd try again to use the humdinger on the DC side (rather than across the winding), as 0.68uF is circa 2kohm impedance, so the DC heater terminals are still bouncing around at 120Hz with respect to the 6J7 cathode, and so the leakage capacitance and resistance between 6J7 heater and cathode has still got quite a high 120Hz signal across it, and that may not be completely bypassed by your cathode bias bypass, and so presents itself as a cathode to 0V hum signal, and hence as an input grid hum signal.

You may want to also try reducing the heater DC filter cap to say just 10uF - a volt of ripple is still much closer to DC than 6.3V of '60Hz ripple', and if that reduces the higher frequency rectified pulse harmonics flying around then that could be noticeable.

[EFK]

Yes, the preamp 6J7 and 6F5 are completely encased in can-type tube shields like in the linked article. The grid caps are ceramic with the grid stoppers soldered directly to the 'clamp' inside each ceramic cap, and there is screened/shielded cable running directly from the input jacks through the chassis and up to the grid caps. Each can-type shield has a top cap that fits over top the whole grid cap arrangement, so each preamp tube is entirely shielded from base to top. I ground the grid wire shielding to the chassis but not to any signal grounds, as it's shielding for RF and 'junk' much like the chassis itself or the tube shields and I believe I read one of Aiken's papers long ago that advocated for keeping any shielding grounds separated from signal grounds. At least, I think it was Aiken.

I've tried both 2.2K and 1.5K cathode R, both using a large 100 uF bypass cap. There doesn't seem to be any hum difference with either cathode resistor. I do have some red led's and they would probably provide a close enough voltage, so I can pretty easily give one of those a try.

If I lift the dual .68 caps off the DC side, and use either a humdinger or a pair of resistors, should I lift the dual 100 ohm resistors on the AC side? I'd like to try a pair of 100 ohm resistors on the DC side but don't know if I can do that while leaving the dual resistors on the AC side as well without burning something up! I have seen diagrams elsewhere wherein people have used the virtual center tap on the AC side and a pair of smaller (i.e. 39 ohm, if I remember correctly) resistors on the DC side, but I've never tried that.

I will also try reducing the size of the large cap across the bridge - I have a few different values between 15000 uF and 10 uF so I'll work my way down and see what happens!

[EFK]

I don't have one but I ordered one to try. The problem is that the bases on those metal tubes and the smaller glass GT types are larger diameter than the base on the glass tubes, and they won't fit through the tube shield base. I'm going to make a "socket extender" with an octal base and socket, so I can run metal 6J7 and 6F5 without having to remove the shield bases, just haven't gotten around to it. I have heard that the metal tubes are supposedly a bit quieter than the big old glass tubes.

[EFK]

Lower levels of capacitance cause the hum to get louder so it actually seems like 15000 uF is the sweet spot. Also tried a red led, cathode voltage stayed pretty close (1.8 volts vs. 1.6 volts w/ resistor) but the hum level was unchanged.

Because the 6F5 is so quiet, I keep focusing on the tube itself. I wonder if the 6J7 - being a fairly sensitive pentode - could be somehow picking up something from the chassis in a microphonic way? I would expect 60 hz in a situation like this but maybe there could be a source of 120 hz of which I'm not aware. I do not have the 6J7 socket isolated from the chassis with grommets, which I've often done with other preamp pentodes to soften microphonics.

[trobbins]

Perhaps redo the test with a direct short from input grid to cathode bias 0V node, as a benchmark. If you have a spectrum analyser scope (or PC and software), then that would be your benchmark noise+hum spectrum.

That then allows assessment of the input wiring and grid leak and input socket shorting mechanism - as that input circuit scheme should give the same spectrum as the benchmark.

If the benchmark has too much hum and noise, then the spectrum will identify the dominant contributor. Are you operating in pentode or triode mode - if in pentode mode then is the screen bypassed to the cathode?

[EFK]

It's running as a pentode, screen is bypassed to the same ground node as the cathode.

Is there some advantage to bypassing the screen to the top of the cathode?

In an old thread here, take a look at this diagram:



What is the purpose of the 1K resistor across the DC side of the bridge? Also, why use 100 ohm resistors on the AC side, but only 39 ohm resistors on the DC side? Why not just use 100 ohm off both?

[trobbins]

Pedantically, the screen voltage is being decoupled, and the screen voltage is screen to cathode. In the old days it would often have been advantageous to ground the decoupling cap to chassis rather than bring the negative terminal back to the cathode - those decoupling caps were often the largest part by miles in that area of circuitry as they had to be circa 0.5uF and 400-600V and so were the waxed paper style.

The 1k would discharge the cap, but the 39 ohm humdinger will do that too. I wouldn't follow that graphic.

[Dave H]

[ATTACH=CONFIG]48510[/ATTACH]

What is the purpose of the 1K resistor across the DC side of the bridge? Also, why use 100 ohm resistors on the AC side, but only 39 ohm resistors on the DC side? Why not just use 100 ohm off both?

Isn't one of those artificial centre taps redundant?

Have you tried it with just the DC supply cap -ve grounded without using any artificial centre taps?
That's how it is in my Valve Junior and it has no audible hum.

[EFK]

I'm not using a setup modeled after that graphic, I'm simply trying to understand the purpose behind the methodology.

I think I've got it as quiet as it's going to be without going to dedicated, regulated setup. It's certainly quieter than just running it on AC. I increased the dual .68 caps to ground (through the elevated output cathode voltage) to a pair of closely matched 10 uF caps. No particular reason, just playing around. With the volumes maxed out, input jacks shorted, I measure 5 mV of AC noise at the speaker on the 6F5 channel (pretty much all hiss, no hum that I can really hear) and 13.5 mV of AC noise on the 6J7 channel (this is 120 hz hum). This is listening to it and measuring it with a speaker right on the bench next to the chassis. When I actually hook it up to a speaker cab and stand a few feet away, it's actually a lot less noisy than any of my old MArshalls so maybe I'm being a bit anal. I think ultimately the 6J7 is just a super sensitive tube - especially when using the big st shape glass version - when run as a pentode and set up for a good deal of gain. I really appreciate all the help and education!