45 -> 813 Interstage

[Steven-H]

Gents,
It seems that there are many good things to be said about the Electraprint. Im looking at their 1:1 5k:5K IT for my application of a 45 to drive my 813SE. Just curious if anyone has any other suggestions before I pull the trigger....

Thanks !
Steve

[Gluca]

Electraprint irons? 45? 813? Interstage coupling? All of them sound good to me. Strap the 813 as a triode. What other tubes will you use in that amp?

Ciao
Gianluca

[Steven-H]

Hello !
I am going to use the tubelab DHT-SE board. It uses a 5842 as a driver into the 45.

[Steven-H]

Gents,
I ordered up a set of 5K 1:1 IT's from Electraprint...So am I going about it the proper way here ? I am also thinking about maybe going fixed bias on the 813 - but that would require yet another transformer in the PSU; so I'll have to think about that some more....If I do go that route - I have seen where the negative voltage gets fed directly into the secondary of the interstage - sound about right ?




Thanks guys !
Steve

[Brett]

Originally posted by Steven-H
Hello !
I am going to use the tubelab DHT-SE board. It uses a 5842 as a driver into the 45.

That sounds OK to me, but a bit too much gain, especially if running U/L. Triode only needs about 80V Vg at Va=900V. U/L will need less.

[Steven-H]

Brett,
Understood - I was under the impression that I needed quite a bit of swing though ?

[Brett]

Originally posted by Steven-H
Brett,
Understood - I was under the impression that I needed quite a bit of swing though ?

Only double Vg. Good to have lots more linear swing capability here, but the gain is unneccessary.

[JoshK]

I don't see how it is too much gain. 5842 yields about 35 and the 45 another 3. So we have 105x for this combo. 80 Vg is ~160Vpp, so that gives us 1.5Vpp input sensitivity of (1Vpp = 1/(2*sqrt(2))Vrms right?) 0.53Vrms input sensitivity. A bit much maybe I guess. So I guess I recant...

[Brett]

Originally posted by JoshK
I don't see how it is too much gain. 5842 yields about 35 and the 45 another 3. So we have 105x for this combo. 80 Vg is ~160Vpp, so that gives us 1.5Vpp input sensitivity of (1Vpp = 1/(2*sqrt(2))Vrms right?) 0.53Vrms input sensitivity. A bit much maybe I guess. So I guess I recant...

He's using it U/L which will need less. 80V Vg is for triode.

[kevinkr]

Using an IT with the 45 I absolutely would not run it at more than 275V and would recommend something like 250V at 32mA or so which seems to be a sweetspot with most vintage 45 I have encountered. Grid bias voltage at this current and plate voltage would be about -48V.

I would probably run the 813 in triode mode, but that's just me.

I'm currently working on a design using 5842/45/845 as a possibility for my next amplifier. I'll be using an IT, but the 845 requires significantly more drive than the 813 so I don't expect quite the gain issue you have, although you could treat this as an integrated amp in which case the available gain would seem suitable.

I used custom made Electra-Print transformers in my headphone amplifier and am very pleased with the result.