12AT7 different gain

[caetano]

the 12AT7 has a gain factor of about 60.

Is it possible that some of the variations - like the E81CC or 7062 or 5965 - have lesser gain then the 12AY7 or 6072 which have a gain factor of about 45.

if looking for less gain - say between 60 and 40 - what would be the best choise for a microphone or preamp?

 

[idylldon]

12AU7 = 17
12AY7 = 40
12AV7 = 41
12AY7 = 44
12AT7 = 60
5751 = 70
12AX7 = 100

Hope this helps.

Cheers,
--
Don

 

[Marik]

[quote author="caetano"]
if looking for less gain - say between 60 and 40 - what would be the best choise for a microphone or preamp?[/quote]

You don't specify some rather important considerations such as should it be a double triode or single would be OK (for example EF86 stripped as a triode), should it neccessarily be 9 pin, or octal, nuvistor, subminiature would be OK, as well?

 

[caetano]

I'm sorry, I forgot to be specific. It has to be a double triode - 9pin. it's for a DIY adaptation to an existing design. As many are doing, I am going to modify my Nady TMC1150. I know many, even Telefunken - RFT, switch to the 6072a, but I want to explore other possibilities...

I'm doing this because I think I have a reasonable shot at making my Nady more Telefunken then the Telefunken...

...its a long story, but I have a Neumann K49/47 that needs a good home

http://recforums.prosoundweb.com/index.php/m/127795/14086/#msg_127795

I know that everyone who are doing the mods still complain about the 'harch' Chinese capsule sound.

I hope that by doing all of the high-end mods - changing the x-fo, tubes and caps - + adding the neumann I will get a decent mic.

So as the Nady is a very 'hot' mike, I'm looking for good (sounding) alternative tubes.

 

[Marik]

First, you don't want to sell your K47/49, don't you?
Use it in your Nady! Swapping the capsule itself will bring the mic to completely different level.
If you want to discard the "ribbon" body please contact me.

If you want to stick to 9 pin double triode, I am afraid the 6072 would be the only choice more or less "suitable" for microphone use. There is a good reason why only this tube from 12xx7 family was used in many classic mics. The GE 5star is one of the best.

I assume you found the Nady1050 and 1150 mods threads on this board.

If you want to explore, then triode connected EF86 would be another choice. In this case you will need to completely rewire the thing.

 

[Gus]

this is a funny thread

 

[clintrubber]

[quote author="Gus"]this is a funny thread[/quote]
Just be patient, it'll make more sense once combined with the '12AT7 different plate resistance' thread. :wink:


But serious, I guess Gus hints at the fact that you just can't compare tubes by only looking at their voltage gains (mu).
At least the plate resistance Rp and anode resistor Ra need to be incorporated.

 

[caetano]

I understand.
The knowledge I bear with me is not great - but I'm learning every day.
I mean, I know that there are physical and audible differences in all tubes -hence the different names.

But I was building upon my knowledge of replacing 12ax7 tubes in the preamp stage of some of my tube amps. I was foreseeing that this wouldn't be the same with a microphone due to the much more precise S/N demands. Still, to find out I was willing to buy and test some types...

...or find out by one of you guys :wink:
Is there a short way to explain why this works with preamps and won't work with microphones?

thx

 

[caetano]

btw, I think I'll stick with the capsule...(at least till I tried it out)
this will probably be the closest I'll get to a U47 for years :green:

I will however - probably - be losing the EV body.
Marik, you'll be the first to know!

As for the EF86, I'd like to, but that would meen modding the design and the PSU...

I'm not technically up to that at this moment :sad:

 

[clintrubber]

[quote author="caetano"]I understand.
The knowledge I bear with me is not great - but I'm learning every day.[/quote]
It may have sounded a bit smart*ss, those two postings of Gus & my reaction. Just kidding, please ignore. Have a look around in the Meta's, lot's of great info & links to gain some insight in all things tube.

I mean, I know that there are physical and audible differences in all tubes -hence the different names.

The resulting gain from a such triode stage is:

mu * Ra / (Rp + Ra)

(ignoring the 1M load of the subsequent stage)

The figures in the first post above only addressed 'mu'.

But I was building upon my knowledge of replacing 12ax7 tubes in the preamp stage of some of my tube amps. I was foreseeing that this wouldn't be the same with a microphone due to the much more precise S/N demands. Still, to find out I was willing to buy and test some types...

...or find out by one of you guys :wink:
Is there a short way to explain why this works with preamps and won't work with microphones?

thx

I understood you're after changing gain. Whether it's in a gtr mic amp does not matter, a stage simply has gain determined by the type of tube & the surrounding components.
So if you'd plug in an ECC82 i.s.o. a ECC83 (hey, it's Europe here),
with say a 100k anode resistor you'd definitely get less total voltage gain from a single triode stage because the resulting figure from the mentioned equation is lower.

 

[hagtech]

If you want to lower the gain of your 12at7 stage, easy trick is to remove the cathode bypass capacitor. It'll sound better, too.

Also, it helps to refer to the 'tube gain' as 'mu', since the actual gain of the circuit will be something far less. Only by loading into a current source can you realize such a high value. Swapping with a 6072 is also probably ok.

jh