Another tube for the Gyraf G7 mic?

[Zepdave]

Hi all!!

I'm thinking about building the G7 tube mic from Gyraf. I have an older microphone body that I want to use for this project... the problem is its size to fit the electronics (the Lundahl transformer and the EF86 tube).

I have a bunch of JAN 7737 from Amperex (mini E186F type). Can I use this tube in the G7? I know that the output impedance of the EF86 is 15K... don't know the 7737 one. I know that the factor amplification is higher in the last one. Could I modify the electronics to fit the new tube in the project? I don't know much about the 7737 pentode and this is why I ask here.

Thanks in advance to all and for the great supporting forum.

David.

PD: sorry for my poor english.

 

[ioaudio]

yes should work - probably very good, if noise is not overwhelming.

 

[rodabod]

Might be worth estimating Rp when strapped as a triode, and maybe also checking that the G7 bias resistor value is reasonable.

PRR explained about sketching estimate triode curves from plate curves in the past by selecting a single reference point which correlated with the triode curves (I think it was G2 = B+) and then drawing a line to the origin. Or you could just experiment.

If Rp//plate R is over 15K then I'd considering running the Lundahl 1538 at 10:1 instead.

 

[Zepdave]

Many thanks ioaudio and rodabod!!

I'll try to experiment with the 7737 instead EF86... What I don't understand is the ratio of the Lundahl 1538, from its specs it's 1+1:5. There's another 1578 with 1+1:10 which could like "seeing" more impedance from the tube output.

ioaudio: do u think that the 7737 is more suited for noise problems?

Cheers. David.

 

[rodabod]

You're quite right; the 1538 will only do 1:2.5 and 1:5. I thought it was a 1:5 / 1:10 but alas it is not. The latter may be a safer bet.