EF40 mc preamp

[el capitan]

I have been looking for a tube phono stage to build to pair to my DL-103, and after being given a pair of EF40 tubes, I found these:Pentode configuration.jpg

Triode 33dB configuration.jpg

Triode 38dB configuration.jpg

First of all, I have no experience with RIAA stuff, so for those who do, do these networks check out?

Also, they all look to have high output impedance. Would adding a cathode follower be a good idea?

Last, seeing as the schematics are so similar, I was thinking about putting all three in the same chassis and using a 3 position switch to move between them.

Thanks in advance for any wisdom,
Alex

[el capitan]

Any help would be really appreciated!

[SY]

Not what you'll want to hear, but nonetheless true: a good RIAA stage is perhaps the most tricky and difficult bit to design and build. The circuits you linked are incomplete, made for a different tube, and far from optimum. This is not a likely path to success.

The EF40 has low microphonics but relatively high noise- is that really the tube you'd want to use?

[el capitan]

Sy, Have you used the tube? From What I understand, it is the same as the EF86 with a rimlock base, and the EF86 is a relatively well liked tube, no?

About it being incomplete, I did think they looked a bit simple, but not having any real experience with phono stages, I couldn't really tell. What is incomplete about them?

[SY]

Well, you'd need at least one more stage and possible a buffer to bring the signal up to line level and drive volume controls or interconnects.

The characteristics of the EF40 aren't the same as EF86, but are in the ballpark. Unfortunately, the EF86 isn't very quiet either (see, for example, the measurements in Frank Bloehbaum's article in Linear Audio Vol 0).

Take a look at my article on MC phono stages ("His Master's Noise")- it's perhaps not the stage you'd want to build, being rather complex, but you can get a sense of the sorts of design analysis you'll need to do. This is also covered very well in Morgan Jones's "Valve Amplifiers," and in a more qualitative way in Allen Wright's Preamp Design Cookbook.

[barrymagrec]

My Mullard book gives the characteristics of the EF40 and EF86 as so similaras to to make no difference in practice. Noise figures are not quoted but I always understood the EF86 was the same valve in a B9A envelope. The EF40 was widely used in the late 40`s / early 50`s for low level preamps before the rush to standardise on B9A valves for signal purposes.
Two of the circuits given use the valve in Triode mode, this gives characteristics not far off an ECC82 / 12AU7 though not identical. I don`t know what this does for the noise but certainly a properly designed Triode preamp is going to be quieter than an EF40 / 86.
However, given that you have the valves and the rest of the components are cheap, why not give it a go, you may like the result. As already mentioned you will need additional gain / buffering to drive most amps though.

Barry

[SY]

An old timers' trick to get better noise performance out of EF86 is to use the screen grid as the anode. It's still not as quiet as later, better tubes, but more quiet than EF86 run in pentode. As a more conventional triode (screen and plate connected), it's more linear than 12AU7/ECC82, but the gain is low and the noise is still not great.

[barrymagrec]

Originally Posted by SY

An old timers' trick to get better noise performance out of EF86 is to use the screen grid as the anode. It's still not as quiet as later, better tubes, but more quiet than EF86 run in pentode. As a more conventional triode (screen and plate connected), it's more linear than 12AU7/ECC82, but the gain is low and the noise is still not great.

I`d not heard of that - interesting. I do know that the noise figure on a tape replay amp was some 6db better when the first stage EF86 was replaced with an ECC83.

As an aside , do you know whether the older EF37A was actually quieter than the EF40 / 86? The 1953 EMI BTR2 used them in the replay amp even though the later valves were available.

[SY]

Nope, never used an EF37.

The screen trick was unintentionally revived recently- there's an article in Linear Audio Vol 0 where that's used as the bottom tube in a hybrid cascode.

[el capitan]

thanks for the feedback. It sounds like this might be a viable project, if only for experimentation, but only after I have some better design skills