6d22s heater referencing question

thomas · Post by **thomas** » Wed Mar 05, 2014 11:34 am

Hi all,

I'm now vaguely contemplating a GM70 build (gulp!) using two transformers I got at the audiojumble a while back, which are 750v, 180ma. I was going to choke input them, rectifying with pairs of 6d22s, which I want to voltage reference, but, how?!

I could just tie a heater pin to a cathode, like a GZ type valve does, but does this not put a 50Hz ac into the HT?

Would it be better (less noisy) to tie heater and cathode with a resistor, say 1K or even 100K for that matter?

Or, is there any point in going the whole hog; 'centre tap' the heater supply with 1k resistors and tie this to an HT potential divider, decoupled, which is the sort of way I reference heaters at the moment?

Explanations welcome!
Thanks
T

pre65 · Post by **pre65** » Wed Mar 05, 2014 11:52 am

On my old 845 amp, I used a hybrid bridge with two 6D22s damper diodes.

The heaters were referenced to full HT @ 950v.

I use a similar arrangement on 833a, and thought I'd done the same , but having just checked, seems I didn't.

EDIT.

Thinking about it, I seem to remember Nick saying that it was not necessary to reference my 6D22s heaters to anything, but I can't remember the reason.

I'm sure the 833a were built with referenced heaters but later removed.

thomas · Post by **thomas** » Wed Mar 05, 2014 2:54 pm

Ta Phil, but how exactly did you reference the heaters to 950v?!

pre65 · Post by **pre65** » Wed Mar 05, 2014 3:29 pm

thomas wrote:Ta Phil, but how exactly did you reference the heaters to 950v?!

950v connected to one side of the 6.3v feed.

My 6.3v winding was for the 6D22s only, and according to the transformer manufacturers (Antek) the insulation was good for that voltage.

Paul Barker · Post by **Paul Barker** » Wed Mar 05, 2014 5:05 pm

No problem with that.

thomas · Post by **thomas** » Wed Mar 05, 2014 6:16 pm

OK......!
Just wondering if there was a theoretically better way......
I'll be worrying about cap ratings next....

IslandPink · Post by **IslandPink** » Wed Mar 05, 2014 6:25 pm

Well if you look at the PSUD info for ripple after the diodes, I'd say the 6.3V AC from the heater supply is almost lost in the noise . If you want to reduce it, as you suggest, a virtual centre-tap with resistors would be OK . I'm not sure you could let it float, because there's a limit of -600V from cathode to heater . Nick will have to jump in and inform me if he knows something I've missed .

pre65 · Post by **pre65** » Wed Mar 05, 2014 6:42 pm

thomas wrote:OK......!
Just wondering if there was a theoretically better way......
I'll be worrying about cap ratings next....

On 833a I use three caps in series on the PSU, 3 lots of 3 per monoblok.

That's 18 caps.

thomas · Post by **thomas** » Wed Mar 05, 2014 6:45 pm

IslandPink wrote:Well if you look at the PSUD info for ripple after the diodes, I'd say the 6.3V AC from the heater supply is almost lost in the noise . ......

Ah I was wondering if that were the case or not.....

I must try installing PSUD on my computer at work, doesn't work with my mac

Post by **Nick** » Wed Mar 05, 2014 8:03 pm

Still trying to work out why you would want to do this. Just let the heater float to where it likes. Giving it a path to ground or any voltage you chose just increases the chance of a insulation break down.

pre65 · Post by **pre65** » Wed Mar 05, 2014 8:07 pm

Nick wrote:Still trying to work out why you would want to do this. Just let the heater float to where it likes. Giving it a path to ground or any voltage you chose just increases the chance of a insulation break down.

I remember now, that's why I took the HT reference off.

thomas · Post by **thomas** » Wed Mar 05, 2014 9:14 pm

Nick wrote:Still trying to work out why you would want to do this. Just let the heater float to where it likes. Giving it a path to ground or any voltage you chose just increases the chance of a insulation break down.

Hmmmm so it doesn't need to be 'referenced' and can float? Didn't know that. I thought any valve's heater needed to be referenced. But this is a 'diode' here. Or something. Perhaps.

Thanks for jumping in, anyway, Nick!

Post by **Nick** » Wed Mar 05, 2014 9:29 pm

The normal reason you reference a valve heater is to prevent noise from the heater supply coupling across the capacitor that is formed by the ceramic insulation between the heater and the cathode being coupled into the cathode.

It helps for two reasons. First the lowish impedance from the heater to "ground" damps down any switching noise from being coupled from the other secondaries of the transformer into the cathode. The second is more cunning, and its why you normally reference it some volts away from the cathode. The insulator between the heater and cathode is non linear, most coupling is when there is no voltage between them, as the voltage increases the insulator is biased and conducts less, so less heater noise (and hum) is coupled across the insulator when there is a voltage differential.

In a rectifier its all (IMHO) a bit pointless. The cathode is moving up and down 20v or more, much more if its a choke input, so a few mv of hum and noise from the heater is neither here or there. Also the cathode is moving so much, what to reference the heater to? You could connect it to the cathode, so it behaves like a GZ32 is. But the damper diodes allow the heaters to live in their own because if you want, and the voltage difference is not more than the max hk voltage you can use them in a bridge or voltage doubler with a shared heater supply.

thomas · Post by **thomas** » Wed Mar 05, 2014 9:48 pm

Nick wrote: ... The second is more cunning, and its why you normally reference it some volts away from the cathode. The insulator between the heater and cathode is non linear, most coupling is when there is no voltage between them, as the voltage increases the insulator is biased and conducts less, so less heater noise (and hum) is coupled across the insulator when there is a voltage differential.

In a rectifier its all (IMHO) a bit pointless. The cathode is moving up and down 20v or more, much more if its a choke input, so a few mv of hum and noise from the heater is neither here or there. Also the cathode is moving so much, what to reference the heater to? You could connect it to the cathode, so it behaves like a GZ32 is. But the damper diodes allow the heaters to live in their own because if you want, and the voltage difference is not more than the max hk voltage you can use them in a bridge or voltage doubler with a shared heater supply.

Ah so that's why referencing the heaters of most e.g. driver valves +30Vh-k or so is supposed to reduce hum....I'd read it, but not known why...
And yes thats interesting, a bridge could share the same heater supply (albeit rather hefty it would have to be...) Interesting....
Thanks for explaining it!

Mike H · Post by **Mike H** » Wed Mar 05, 2014 11:22 pm

So therefore, you can just connect one side to cathode as per GZ-whatever, like you said

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