IT for 826

Design and use of the various types.
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prazza1
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IT for 826

Post by prazza1 »

Hello!

Before starting new amp project I need some advises.
I'd like to use the DHT 826 for the driver stage, like Josh Stippich of Electronluv with its 826 driving 2A3.
My question is: can it possible to make an 1:1 IT for this tube. I'd like to use the 826 with about 600V/20 to 40 mA. It seems that with these values the Rp is 10k.

Thanks
Ardee
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Post by Ardee »

If you can wait until next year, Lundahl are adding a transformer to fill the gap in their range between the LL1660 and LL1671. Apparently, it's designed for the 6BX7 so it might suit your application quite well.
Ed Sawyer

direct coupled

Post by Ed Sawyer »

I am pretty sure Josh direct-coupled that 826 to the 2A3. I have the schematic here somewhere.

Seems like the 1:1 tranny would need a fair bit of inductance if it's looking at an Rp of 10k.

neat tube though (826). I guess some of them have a tendency to become gassy quickly, perhaps due to the op point not being hot enough (?).

-Ed
dave slagle
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Re: IT for 826

Post by dave slagle »

prazza1 wrote:Hello!
My question is: can it possible to make an 1:1 IT for this tube. I'd like to use the 826 with about 600V/20 to 40 mA. It seems that with these values the Rp is 10k.
Thanks
i wouldn't suggest a 1:1 for this situation. The only 1:1's I suggest are bifilar and the 600V is a bit high for reliability.

Like ed suggested direct coupled off a stacked supply is always nice, or you could try a more conventional 1:1 with a bridging cap.

dave
prazza1
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Location: France

Post by prazza1 »

Thank you to all.
I wanted to use only two stages, but it seems not to be the better way with the 826. Maybe I would have to do with three stages to have more choice. For example, why not use 10Y:IT:45:IT, with the 45 like driver. It's not far from the gain of the 826 alone.
Like ed suggested direct coupled off a stacked supply is always nice
Maybe. But I'd like to drive a follower for driving a 833. So it seems to me more difficult to stack.
I guess some of them have a tendency to become gassy quickly
Would it be needed to make an op with more voltage ?

Pierre
prazza1
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Post by prazza1 »

I want to share my thought with you about my project.

Dave:
If I come back with the 826 like driver, and if the IT is difficult to do, maybe I can use one time in my life a RC coupled stage. Using two stages and two IT in order to not use a capacitor, maybe it is too expensive.
In this case, do you think a choke can be wound for loading the 826 ? With 10k Rp, 160H seems to me to be a good choice.

Ed, you said:
(826...) I guess some of them have a tendency to become gassy quickly, perhaps due to the op point not being hot enough
Do I need to burn the tube? What do you think about 950V / 50-60 mA. Max dissipation is 60W and max DC is 1000V?

Pierre
Ed Sawyer

thoughts

Post by Ed Sawyer »

950v should keep it hot enough! Probably doesn't need quite that much. See if you can chase down Gary Kaufmann (www.the-planet.org), he has used the 826 in a Shishido design and has more experience on the ones that tend to get gassy.

I think like a lot of the transmitting tubes of this type (VHF/UHF, and pulse applications) they often were made of self-gettering materials like the Eimacs and such. Sometimes they had traditional getters though. for the self-gettering types, sometimes they really need to get some heat into the plate to get the self--gettering to function due to the temperature properties of the plate materials, et al.

the other thing is with the 826, the socket and shape of the tube makes for potential for stressing the pin-to-glass seals. These are tungsten pins and quite brittle/hard, so they dont' have as much forgiveness as the smaller baseless tubes (ala 9-pin stuff).

Another neat tube in this format, although not a DHT, is the 3C33. Alas they are quite a bit more rare than 826.

good luck and let us know what you come up with.

you could also probably LC-couple the tube, with a nice big plate choke and small teflon coupling cap. (like the russian teflons)

-Ed
DowdyLama
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Post by DowdyLama »

826s do tend to be gassy - I can only speculate that it's the early Septar basing combined with brittle pins and glass.

They sound pretty good, but not quite as good as the 3C24/25T to my ears.

FWIW: I've never run any of these tubes much higher than 450 vdc.
prazza1
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Post by prazza1 »

They sound pretty good, but not quite as good as the 3C24/25T to my ears.
I need 50V (100V P-P) output from these stage to drive the last stage.
I didn't knew this tube.
Are they more good then 826?
What can be operating point?
What is Rp (curves of Eimac data sheet are not so easy to read).
What can be the value of the choke load.

Pierre
DowdyLama
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Post by DowdyLama »

I personally prefer the sonics of the 3C24/25T vs the 826; plus, in my experience, the 3C24/25Ts tend to have less problems with gassiness.

Please note that I've never experimented with any of these tubes above 450 vdc - it's possible that the 826 would really shine with 500 to 600 volts on the plate!

3C24/25T op points: with a B+ of approx 400 v, and an R(k) of 820 ohms, you should be drawing about 6 mA and R(p) will be 12 k [this is with a fully bypassed R(k), of course]. I've used plate chokes in the 400 Hy to 600 Hy range with great success.
prazza1
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Post by prazza1 »

3C24/25T op points: with a B+ of approx 400 v, and an R(k) of 820 ohms, you should be drawing about 6 mA and R(p) will be 12 k
What do you think about more juice, like 750 v, 20 mA, to have less Rp.
I've used plate chokes in the 400 Hy to 600 Hy range with great success
Why so high choke ? Why not use 200H for example. With this we can have low frequency of 10Hz (therocaly) with 12k Rp. I have read somewhere that each time we lower choke value sound is better.

Pierre
DowdyLama
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Post by DowdyLama »

More re: 3C24/25T:
> What do you think about more juice, like 750 v, 20 mA
No opinion; but I certainly encourage experimentation - heck, the 826 might sound great with 750 vdc!

> Why not use 200H?
My general rule of thumb is not less than 30 Hy per 1k of R(p)...I also used the plate chokes that were on the shelf
:-)
DowdyLama
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follow-up

Post by DowdyLama »

I realize this is an old thread, but I wanted to post more recent experience with the 826:
I'd like to amend what I said earlier, the 826 sounds every bit as good as the 3C24/25T ('tho I would not say it sounds better).
The 826 does indeed need at least 500 vpk to begin to shine...and the lack of a plate cap is a definite plus!
However, my above caution about the 826 having problems with gassiness remains.

Please note that I am using the 826 as an A1 driver in my SE GM70 amp; take the above comments within that context.

Jim
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