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Old 15th January 2009, 04:52 PM   #1
45 is offline 45  Italy
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Default Max supply voltage for small DHT's

I cannot find data about the max supply voltage (if using RC coupling) for all the old (and best, IMO) DH preamp triodes like the 26, 30 etc.

Datasheets only report the max anode voltage beacuse I think that in those days it was common practice to load such tubes with transformers when used for their max output voltage. This was not a problem as the required bandwidth was much narrower than nowdays. Typically 100Hz-8000Hz!

Do you have info or experience?
For example, do you think that a 26 can work safely using 360 supply voltage with 180V effective anode voltage?

Cheers,
45
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Old 15th January 2009, 05:59 PM   #2
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Isn't 180v right the edge of it's max plate voltage? I am sure it would run that way but I am not sure I would want to push a beautiful old 26 that hard. Others please chime in though, I am not a DHT expert. Why not choke or CCS load the tube, could then run a lower voltage and the tube would probably be happier load wise.

"...it was common practice to load such tubes with transformers when used for their max output voltage. This was not a problem as the required bandwidth was much narrower than nowdays. Typically 100Hz-8000Hz!"

True but transformer coupling isn't necessarily narrow bandwidth. Maybe then it was but now most Hi-Fi transformers will respond between 20Hz and 20kHz minimum. Some small signal types (though not limited to them) respond between 10Hz or better and 70Hz or better! I wouldn't rule out transformer coupling in what ever you are doing.

Using one of the above methods you could also drop that voltage with a nice CCS loaded shunt reg. Keeping 26's happy and quiet can prove to be a problem, so giving the excellent PSRR of a CCS loaded shunt and a choke load couldn't hurt.

A huge resistor dropping that much voltage would work, at least I don't see a problem with it.

Happy Listening!

Cheers

James
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Old 15th January 2009, 10:18 PM   #3
45 is offline 45  Italy
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Quote:
Originally posted by JPeitzman
Isn't 180v right the edge of it's max plate voltage? I am sure it would run that way but I am not sure I would want to push a beautiful old 26 that hard. Others please chime in though, I am not a DHT expert. Why not choke or CCS load the tube, could then run a lower voltage and the tube would probably be happier load wise.


With anode choke or transformer loading 180V anode voltage is the maximum but in my experience it is quite conservative.
Tubes like the 26, 30, 45, 46 are more rugged than one may suspect. Especially the 45.
Also the 30 was used in the output stage of many portable military receivers: push-pull in class B2 for 2W output power!!
This time I would like to use a small DH preamp triode to drive a power tube like the 45 or 2A3 but only as second stage voltage amplifier and the power tube biased by direct coupling to a mosfet source follower.
For example the 26, with 33K anode resistor and 30K effetive load, can swing plenty of volts with very low distortion and above all such distortion has very small high harmonics content.
A perfomance that any other indirect type (double or single) triode simply cannot match, IMO.


Quote:
Originally posted by JPeitzman

"...it was common practice to load such tubes with transformers when used for their max output voltage. This was not a problem as the required bandwidth was much narrower than nowdays. Typically 100Hz-8000Hz!"

True but transformer coupling isn't necessarily narrow bandwidth. Maybe then it was but now most Hi-Fi transformers will respond between 20Hz and 20kHz minimum. Some small signal types (though not limited to them) respond between 10Hz or better and 70Hz or better! I wouldn't rule out transformer coupling in what ever you are doing.
It is quite unpossible to find an IT transformer for PP to PP operation, suitable for 15-20K source impedance and at the same time extended frequency response, even today.


Quote:
Originally posted by JPeitzman
Using one of the above methods you could also drop that voltage with a nice CCS loaded shunt reg. Keeping 26's happy and quiet can prove to be a problem, so giving the excellent PSRR of a CCS loaded shunt and a choke load couldn't hurt.

A huge resistor dropping that much voltage would work, at least I don't see a problem with it.

Happy Listening!

Cheers

James
Yes James, however when possible I always go for the simple RC load when I don't need power drive. Sonically it has always been my preference, in the end.
PSRR isn't an issue with a well made supply.

To be honest I have found one preamp DH triode with both anode and supply voltage maximum ratings. It is the 841. It can work with 425V anode voltage and 1000V supply voltage. Unfortunately this is very rare (and would require 9.4 VA filament supply!...a bit too much for my purposes).
I think that everything depends on the internal construction and the conclusions are not so obvious for other tubes.

Cheers,
45
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Old 16th January 2009, 12:33 AM   #4
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20K 1:1 Push/Pull interstage with a claimed bandwidth of 5Hz - 110kHz 3dB down.

http://www.sowter.co.uk/specs/9525.htm

I never noticed that before about the 841 until you said something and I went back and looked at the datasheet for it, interesting. Sorry I couldn't be of more help, like I said I am not very versed with DHT's

Good luck and happy listening!

Cheers

James
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Old 16th January 2009, 12:35 AM   #5
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http://www.sowter.co.uk/specs/9100.htm

sorry linked to the wrong transformer, this is the push/pull 20k 1+1:1+1
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