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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

45 -> 813 Interstage

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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 ?

tubelab_813SE.jpg



Thanks guys !
Steve
 
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...
 
JoshK said:
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.
 
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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.
 
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Steven-H said:
Gents,
Thanks for the help ! I made a mistake on my schematic; there will be 300v B+ - not 400 as shown.
As far as the gain goes; I am thinking of building a passive preamp; so I don't think it will be much of an issue ?

Thanks again,
Steve

Hi Steve,
Passive pre should work fine in this application. I still recommend a slightly lower plate voltage (250V) for the 45 unless you are using cathode bias in which case 300V is just right. In my application they sounded a bit strained at 275V, and yes they measured better, but the 3rd harmonic came up relative to the 2nd which also decreased slightly relative to the 250V operation and overall it actually sounded considerably worse. (I'll admit this was into a 4K load however. IIRC this was the case both for constant dissipation and constant current comparisons. ) Fine tuning is a good idea.
 
You guys are awesome. I'll check out the 5687 but I guess it is a twin triode - so I would have to rewire the socket to work on the board (which is fine, as I was assuming to mount the sockets to the chassis with short leads to the board.....). I really would like to get more use out of my soon to be preamp :D

Thanks again,
Steve
 
Steven-H said:
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

Nahhhh .... You don't need any transformer and forget caps and diodes. Grab a bunch of 12V batteries (size MN21) and you have got a clean cheap and easy negative rail. I made it a number of times and it works. Never mind, the batteries will last long (you are not biasing that 813 in A2, aren't you?). OH! Those big transmitting triodes really like to have the cathode at ground.

Ciao
Gianluca
 
Interesting Stuff ! How do you keep tabs on your battery voltage though ? I would imagine that as the batteries age - that your bias would change ?!

For the filament supplies, I bought the TentLabs 5A supplies. So I suppose that I would connect them to the cathode, then ground one side out right at the tube pin ?

Also - while I have your eyes - I thought that the IT needed to be loaded somehow ?

Lets see if this schematic looks correct....
 

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