Tube Preamplifier Heater Methods

[Lampie519]

Usually if i really need a DC power source for the heaters i buy off the shelf products as these are easy and fast to replace in case one fails (have redundant PSU's just in case).

 

[Tubetec]

Tell us a bit more about the above circuit Lampie ,
is it a phono preamp ?

 

[Lampie519]

It is a preamp for a D/A converter (buffer stage) that replaces the opamps (I/V) as the converters i use are current output converters only.
The amp is still in use occasionally as it sounds ok, just the pencil tubes are very microphonic, something to be aware of.

 

[dmp]

Next step is an AC heated microphone.

I wonder how well you could do with AC heaters in a tube mic. The heater wiring sharing the cable with the signal is the obvious issue. You have two twisted pair running parallel for ~20'.

 

[rock soderstrom]

I wonder how well you could do with AC heaters in a tube mic. The heater wiring sharing the cable with the signal is the obvious issue. You have two twisted pair running parallel for ~20'.

Yes, it's not without its problems. One of the few microphones with an AC heater that I know of is the RFT CM-7151, where the entire power supply was built into the gigantic microphone body. This concept was not very successful and that microphone is not exactly famous for its low hum.

Note the humdinger and the separat AC winding for V1.

 

[dmp]

What is the value of the separate AC winding for V1? The V76 has this as well.

 

[rock soderstrom]

What is the value of the separate AC winding for V1? The V76 has this as well.

Good question, I'm guessing 6V as well. This type of dedicated winding for the V1 tube was widely used back then. Even in consumer reel2reel tape recorders this was done for the first tube (very often EF86) in the signal chain.

Grundig Tk30/TK35

 

[dmp]

It must improve noise performance somehow, by isolating, I'm not sure why though. (this is what I meant by 'value')

 

[rock soderstrom]

It must improve noise performance somehow, by isolating, I'm not sure why though. (this is what I meant by 'value')

Okay, my mistake. I guess due to the dedicated winding less current flows in this circuit which improves the SNR for V1.

But I am not sure either.

 

[Lampie519]

Something is mentioned here on this forum:
https://repforums.prosoundweb.com/index.php?topic=32557.0

 

[emrr]

Not dissimilar to using a separate rectifier heater winding. Keeps any effects local.

 

[rock soderstrom]

Something is mentioned here on this forum:
https://repforums.prosoundweb.com/index.php?topic=32557.0

"Oliver has answered on the other forum saying that it's because the V77 has a split-winding transformer to achieve lower noise (hum-bucking sort of configuration)"

That doesn't really convince me. Usually the two windings for the heaters are asymmetrical (in terms of current). V1 gets the small winding exclusively. The rest is supplied via the second winding. This can be anything from a second identical tube to a complete mini PA including power amplifier.

Maybe it's just about the individual setting of the humdinger for V1.

 

[dmp]

Not dissimilar to using a separate rectifier heater winding. Keeps any effects local.

Interesting - So for example the 5v AC heater winding 'floats' in a gz34 to the B+ voltage (if I understand you).
If both heater windings are tied to ground through a CT or virtual CT, why do they need to be separate?
For the humbucking idea, you could still have two windings connected together in parallel supplying all the tubes and have humbucking at the transformer to reduce radiated emf. Wouldn't need to split the windings to have V1 on a separate winding.
It must have to do with the interaction of the V1 heater with the other tubes which pollutes somehow to increase the noise at V1. Would be interesting to test.

 

[emrr]

RHD3 and 4 can answer all this, pretty sure. You never see a 6V rectifier on the same winding as audio in anything old and professional, it's always separate.

I ran into an interesting one with a rack of modular pre's and an outboard PSU that needs some help. If you ran 2 pre channels and mix them into the same audio stream (at any point) there's no hum, you mute one and there's huge hum. The hum in each is reversed and cancelling.

 

[Tubetec]

The second stage injects a certain small amount of unbalanced noise via the heater ,
the first stage heater allows you cancel out that ,
You see that same thing on some guitar amps , at a mid volume setting hum cancels better via the hum dinger .

 

[Tubetec]

another way of saying it is ,
the best position of the humdinger pot depends on the position of the volume control ,
You can always make the 'dinger' pot a back panel control if you want , it doesnt have to be recessed for 'tech' usage only .

 

[My3gger]

In this context, I am talking about a regulated and well-filtered DC voltage. Specifically, I had an LM317 power supply with ample RC filtering compared to the AC variants mentioned by dmp.

Edit:

I would summarize my position this way. You can get very good results with AC heating with the right tubes and careful wiring and layout in terms of residual hum.

This was also a stated design goal of the manufacturers at the time, to optimize certain tubes for this.

DC heating relaxes the situation somewhat, one achieves very good results faster and more reproducibly, even with ordinary tubes in demanding applications.

The disadvantages of regulated DC heating are the increased component costs, the thermal load on individual components and the somewhat reduced reliability in the long term.

Recent books about AC and DC heaters confirm AC heating is fine for sensitive input stages. The only thing texts lack is comparison between special tubes like those having double helix filament and more common designs.
I will measure differences between two options when remaining trafos meant for DC are gone. My HT supplies are designed to work without heatsinks up to 40mA, it would be nice if heater didn't need them either.
Btw, microphone with tube preamp in it would be gigantic, heavy )