Twin DHPs

If they glow, this is the place to be
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Cressy Snr
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#1 Twin DHPs

Post by Cressy Snr »

These look interesting for a differential driver stage for my amp project.

Direct heated double pentodes from the mid 1930s

KLL32, QP21, 240QP, and 1E7G

These have an amplification factor of 370 according to the data sheet for the 1E7G. Vade Mecum lists the KLL32 etc as equivalents,
so any of these could take a large amount of feedback, and operating at 135V, -4.5V grids and 7.5mA idle current seem
just the job for a two stage all British, direct coupled, pentode push-pull amplifier

Concerns I have are the 2V supply that would be needed for each valve and possible noise and microphonics, negating any good sound quality.

Anyone with experience of battery valves like to comment? :)
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IslandPink
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#2

Post by IslandPink »

DHP's are close to my heart .
It's a bit late to check out those particular ones you mention ( will try tomorrow ) but I've just spent most of the evening listening to a GK-71 driven by a 4P1L (DHP) . Pure magic !
You will have to be careful about common-mode noise into the cathodes in PP, but it's not as difficult as running DHP single-ended , although in this case it's only a matter of using Andrew's DHT current-reg supplies .

370 gain may be the case in the datasheet, but look at the specific conditions. As usual with a pentode, gain's roughly the transconductance x load resistor, if by-passed on the cathode ; maybe 60% of that if un-bypassed .

There are quite a lot of small DHP's out there - I noticed when I was looking at interesting options for a pentode/triode Aikido .
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steve s
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#3

Post by steve s »

I would guess i have some of these steve, they would take some checking out

Proably the best of them all would be the early ac/pen out put valves
i would lend you those, but i dont really want to part with them...
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Cressy Snr
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#4

Post by Cressy Snr »

IslandPink wrote:
370 gain may be the case in the datasheet, but look at the specific conditions. As usual with a pentode, gain's roughly the transconductance x load resistor, if by-passed on the cathode ; maybe 60% of that if un-bypassed.
Yes, I forgot about that...which is not good as it rules out all of the ones I found.
Insufficient gain, which figures really, as they are all small power pentodes.

Martin's wartime SP61, top cap grid IDH pentodes or EF37A (if Steve has any) are looking like the best way forward.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Alex Kitic
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#5

Post by Alex Kitic »

Almost as if I were reading this topic... I was posting about how I like the various direct heated pentodes (and beam tetrodes), and how the 1619 seems to beat the 2A3 in "overall" performance and tonal richness.
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