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suggest circuit with gain of 60

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I need to design an input stage with a total gain of 60. the stage will have a 2:1 output IT so overall gain is 30.

very low output impedance of stage.

The only thing is that I NEED to use tubes that are made today. i.e. 6SN7, ECC99 etc....

Maybe 2 stage DC coupled...

ideas???
 
input stage of mosfet.

mosfet with over 1000pF of input cap.

so thinking like a 12B4 or similar with a 2:1 step down as an IT.

input impedance to the MOSFET less than 500 ohms is good.

Need at least 30v swing on the MOSFET though with a 1v input to tube stage.

kind of want to keep to tubes in current production.
 
The 12AT7 IS in production. The drive capability of a Gary Pimm CCS' mu follower O/P is EXCELLENT. The GP CCS uses FETs.

If you buy 'T7s with tightly matched sections, you can run them in parallel. Parallel sections lower the mu follower O/P impedance in 2 ways. The combined Rp of the 2 parallel sections is 1/2 that of a single section and the doubled current in the CCS lowers the mu follower's O/P impedance too.

I think that a GP CCS will drive that NASTY gate capacitance well.
 
>>So what is the advantage of an IT then?<<

A trafo constructed for parafeed service (no standing DC) is physically smaller and has better all around performance. The coupling cap. is a small price to pay, IMO. FWIW, both MQ (US) and Sowter (UK) manufacture parafeed IT "iron".

For the project being discussed, the importance of the 2:1 step down IT (regardless of construction) is the 4X reduction of O/P impedance and the 2X increase in O/P current.
 
Ok Eli,

That’s clear ;) But if it is for what I think, a choke loaded source follower as in this thread, then coupling caps are not done to drive the fet, either before the trannie, either after the trannie. In case of clipping the coupling cap will be charged quite an amount away (due to the gate protecting zeners) resulting in huge recovery times and LF output bumps.

A source follower has mainly C_dg (app. 90 pF for an IRFP240) as its input capacitance. Is that really as large as 1000 pF? With proper fets it is much lower. A balanced PP drive is then also an option with a 1:1:1 IT. A long tailed pair or x-coupled pair loaded with an PP IT will do also then.

Cheers ;)
 
yes it is

it is for the source follower...

i was going to use a IRFP240n as the MOSFET... which has high input cap. any better ones out there??

do you think I could get away with a 1:1 IT??? personally I like the idea of a lot of current drive on that mosfet.

The IT works good because A) I like the sound of IT's and B) I can bias the mosfet using the secondary of the IT.

so... I am thinking of a 4K:1K or a 2K:500 IT tranny... which one though?????

need a gain of 60 on my tube section using current production tubes...

how about the JJ 6922??... with what though...

hey, is the 12B4 in current production?

Thanks for all the help.
 
A 1:1 IT should work just fine, it just requires a gruntier tube in front of it. Or to be correct, the mosfet follower requires a gruntier tube infront of the IT.

If I were about to build this amp I´d try a 2:1 IT driven by a PC86
at about 150V and 12-15mA.
If that for some reason didn´t prove to be grunty enough I´d head for a two stage solution, say a trioded E81L (a bit odd but it´s very nice) followed by a 6C19.

12B4A would be a nice alternative to the 6C19 in such a circuit, but they are a bit less linear. No current production AFAIK.

By the way, why is it so important to use tubes that are currently produced?
I´m sure you can find a lot of nice tubes that are cheap and available enough to stock up enough for a lifetime.
 
No current production as far as I know, but you shouldnt have any problems finding as many as you need. Look for EC86 too, same thing except for the heater voltage.

Honestly, I don´t know how it sounds but I can´t see any reason why it wouldn´t sound perfectly good. Better linearity than many "hifi"-tubes.

E81L and 6C19, yeah... If you get tired of the mosfet, just replace the IT´s with some descent output trannies and keep playing:)

I´d go for 4k:1k. Any specific type/brand you´re thinking about?
 
E55L would be very good for this application, not very cheap though.
For a 1:1 IT you can also look at E81L without 6C19, it should be grunty enough by itself.

Have you looked at Lundahl IT´s?
A lot of different models with endless (well, almost) possibilitys when it comes to different configurations and turns ratios.
 
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