ef 86 / ecc 81 mic pre

[ioaudio]

hi blandman,

heating is rectified in the meanwhile, but it didnt solve the hum-problem.

i want to try a different (canned) input transformer, because hum is present even when i unplug the circuit from the mains.

nice to hear someone working on a similar thing! oh, your first post: welcome to this diy temple !

 

[ioaudio]

update:

after improving the psu (~250 volt ht, better rc filtering, dc heaters)
and adding a more decent(canned!) oep input transformer
hum problems are gone.

 

[ioaudio]

after testing with different mics:

signal seems to distort in a small amount, but constantly. no matter how much (output-)gain, also a pad didnt help, nor a feedback loop.

i´m afraid the output load of the ef 86 into the 100k pot doesnt match.
should be higher like 230k, i guess.

can someone give me a hint how to match that?

edit: 100 k before the output cap of the first stage did it.
gain lowered, but its still too much.
will be working on the feedback next.

 

[blandman74]

Take a look at page 4 of this datasheet:

http://tdsl.duncanamps.com/sheets/ef86/ef86_01.pdf

It has some sample values. I would think that lowering the load resistor (i.e., at the anode) to 47k would eliminate some of the clipping. But that advice may be just the blind leading the nearsighted ....

 

[pstamler]

[quote author="ioaudio"]after testing with different mics:

signal seems to distort in a small amount, but constantly. no matter how much (output-)gain, also a pad didnt help, nor a feedback loop.

i´m afraid the output load of the ef 86 into the 100k pot doesnt match.
should be higher like 230k, i guess.

can someone give me a hint how to match that?

edit: 100 k before the output cap of the first stage did it.
gain lowered, but its still too much.
will be working on the feedback next.[/quote]

Try removing the cathode bypass cap on the EF86. That will cut the gain at the place where the clipping is happening.

Peace,
Paul

 

[ioaudio]

thanks for your replies.

thats actually the fun part of diy - tweaking the design.
i´ve been playing a lot with input pads, feedbacks, tryed to remove the cathode bypass cap and changed certain values all over the circuit.
no final version yet...

in the meanwhile i build an external power supply with phantompower (baumann, thanks, works perfectly!) to provide several tube-mic-pre boxes,
which i´m planning to build. sort of modular system.

 

[ioaudio]

update:
(sort of) finished.

changed the output tube to ecc83. couple of other changes, will post the final schematic soon.

short wav audiosample:
http://groupdiy.twin-x.com/displayimage.php?pos=-436

pics of my incomprehensible housing:
http://groupdiy.twin-x.com/albums/userpics/10055/housing.jpg

inside:
http://groupdiy.twin-x.com/albums/userpics/10055/inside.jpg

psu:
http://groupdiy.twin-x.com/albums/userpics/10055/psu.jpg

need to go to a friend for the mechanics

 

[analag]

I did such a pre, about 9 nine years ago, but I had the pentode cascade into a triode then a White cathode follower. I notice that you triode connected the pentode, but you could do a pentode connection and run the screen at a much lower potential than the plate for a nice sound.

Here it is....