EF86/ECC83 Preamp

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jsefer

Well-known member
Joined
Dec 24, 2009
Messages
111
Location
Orange County, Ca USA
I just built myself a preamp out of an old M8, I practically gutted and rebuilt the units using a new design. Took some ideas from other sites and added my own flavor. Coupling Caps are russian PIOs, Power caps are Sprague and Russian PIOs. Tubes are telefunken. Tran-x are cinemag.


Im getting some oscillation at high levels, and the ecc83 seems to distort very early as a bus insert but as a DI seems to sound really good, a little muddy on the bass side, but other wise its a very usable unit.

EF86 is quiet with about a -60db noise floor.

Im posting the schematic to see if there is anything anyone sees that I might be able to try and keep redefining the unit. So far the only issue I see is that it has no negative feedback so I asume thats why its a little bit noisy on the ecc83 side.

Im no expert and this is my first tube project, so Im looking for ideas. I had posted the schematic before and got some good ideas that I implemented so with the new ideas, Im trying to see if theres any other possible things i can try. This unit is going to live as my mic preamp and bus insert to color anything i run through it.

If anyone has any comments or tips, I very much appreciate your time.
 

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> Im getting some oscillation at high levels

Layout.... and very high impedances.

Althoough when connected to moder 22K line inputs the gain should go-away, be near impossible to squeal.

There really oughta be a B+ filter cap at the left end of R20.

C30 100uFd seems extreme compared to what it drives, 500K pot. Even if that pot is full-up into a 22K load, 1uFd is plenty. And 100uFd is liable to leak enough to show large DC (fraction of a volt) across a 500K pot.

> it has no negative feedback so I asume thats why its a little bit noisy

NFB will not change hisss (except indirectly by changing gain).

I don't understand the output at R15. Gain from first grid is over 500. That's FAR too much for any line-level. And it is clipping even when the obvious output at R14 is swinging only 1.3V peak.

I don't understand how 3 small bottles in a rack-box need a fan.
 
PRR said:
> Im getting some oscillation at high levels

Layout.... and very high impedances.

Althoough when connected to moder 22K line inputs the gain should go-away, be near impossible to squeal.

There really oughta be a B+ filter cap at the left end of R20.

Just noted i forgot to include that in the chematic. Ive got a 20uf/600PIO in there...


PRR said:
C30 100uFd seems extreme compared to what it drives, 500K pot. Even if that pot is full-up into a 22K load, 1uFd is plenty. And 100uFd is liable to leak enough to show large DC (fraction of a volt) across a 500K pot.

Will definately change this one. thanks. I was under the impression that larger the bypass cap the wider the freq range.

PRR said:
> it has no negative feedback so I asume thats why its a little bit noisy

NFB will not change hisss (except indirectly by changing gain).

I don't understand the output at R15. Gain from first grid is over 500. That's FAR too much for any line-level. And it is clipping even when the obvious output at R14 is swinging only 1.3V peak.

I don't understand how 3 small bottles in a rack-box need a fan.

Im sorry for not understanding this last statement... I asume you mean reducing the value of the output VR, or is it eliminating the output from r15???

Im sorry but im still learning as much as I can about tube preamps.

Thanks for your input, its actually 6, that picture is for just 1 half. as for fan might be over kill but my equipment rack gets hot.
 
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