ef 86 / ecc 81 mic pre

suggestions welcome.
sorry for the bad quality pic.



edit: bias resistor 1M

The bottom triode on the right needs some kind of grid bias R.

thanks for your quick reply.
like 1 megaohm to ground?

That sounds about right.

thanks.
some things that come into my mind :

there is no feedback (?)
highpass filter (like 22 nF in series is enough?)
a 20 dB pad


edit:

ok, i will try to build that thing.
input will be 1:10 neutrik NTE 10/3 and output 2:1oep A262A2E

[quote author="ioaudio"]some things that come into my mind :

there is no feedback (?)
highpass filter (like 22 nF in series is enough?)
a 20 dB pad

ok, i will try to build that thing.
input will be 1:10 neutrik NTE 10/3 and output 2:1oep A262A2E[/quote]

You could leave the EF86's cathode resistor unbypassed for some padding and feedback. As it stands, I'd expect the high gain to produce clipping a little too soon.

If the coupling cap from the EF86 to the pot really is 22nF (I can't tell whether that's an N or a "micro" sign) then your -3dB frequency should be 72Hz. I'd make it 15nF for 100Hz, then bypass with 1uF for 1.6Hz.

Oh, it might be a good idea to hoist the filaments to about 85V DC, just to avoid problems with the upper cathode on the ECC82. EDIT: Er, make that ECC81. Read the thread title, Paul.

Peace,
Paul

i will build it as is, trying with the feedback later (maybe i´ll try something like the feedback circuit from the redd 47)

coupling cap should be 0.22 µf , but i will also try what you suggested; learning and listening by doing is my plan...

[quote author="ioaudio"]suggestions welcome.
sorry for the bad quality pic.



edit: bias resistor 1M[/quote]
Hi, :grin:
It's interesting schemo,and I think it will sound nice.you have a SRPP output stage with the ECC81,great I've build the same with a 12AT7 and it works fine with a OEP.

Good,
A+
B2.

I could be wrong but I thought the EF86 needs 6V heater and the ECC81 needs 12V. You probably have the right power supply to get both voltages.

collecting parts...
this neutrik input transformer is really... tiny ... maybe i check also oep.

two possible ecc 81 output caps: wima 4.7 µ or this beautiful sel with 10µ (only 125 v- is that enough?)

[quote author="tk@halmi"]I could be wrong but I thought the EF86 needs 6V heater and the ECC81 needs 12V. You probably have the right power supply to get both voltages.[/quote]

hmm. in the original layout of the old tape recorder both heaters for ef86 and ecc 81 are connected together, so i didnt see a problem there.
i will check that, thanks for the hint.

edit: the old tape recorder where i got the tubes&supply from.

Just run the ECC81 heaters in parallel at 6V. Very common. It's probably what you saw in the tape machine circuit.

thats exactly what i found out. excellent.
lets go.

ok, works.



vintage breadboarding








canning the input transformer







replaced ef 86 with ef 83 - less noisy,less gain - maybe still enough for my needs.

my first preamp




some sleep, next i´m going to improve the psu (dc heater... regulation for b+ ?)
after that i´ll play around with feedback. but maybe i like the fact that it works without it...

i´m really surprised : powered up and heard my voice :grin:
thanks to all, i think i fell in love with diy !!!

And how is it soundwise???

Jens

i have to listen today.finished soldering at 7 a.m. , now after some sleep i´m going to test.

sounds very nice for my ears, especially with the ef 83.

noise is still an issue, have to layout a better psu i guess. its unregulated, just rectifier, caps and a choke and 290 instead of 250 volts.

the ef 86 is quite microfonic, maybe its worn out, i will replace it with another one. lots of gain with the ef 86, maybe nice for ribbons (i dont own any)

very nice distortion at highest input levels with the ef 83, will test it for git/drums recording.


i guess its a really simple project, the sound seems to be quite unique and coloured; some might call it vintage; i dont have any serious preamps lying around for a-b, so i compare it to what i´ve been working with in studios so far.

very cheap project indeed:

10 ? tube tape recorder from the flea market - tubes and psu
broken di-box for the output transformer
12 ? input transformer
15 ? resistors, caps, wires


today i was in viennas flee market again, bought another tape recorder, labelled "stuzzi stereo recorder 202" for 5?.

so i´m going for a stereo version next...


oh, one thing to mention: two month ago i didnt know nothing about valve/audio stuff.
i learned everything fom this wonderful place, and i´m just about getting into it.

thank you so much for sharing!

keep it going!

update:



dc-heater done, still having hum problems.

stupid question: the amp is not racked jet. would this solve the problem?

very cheap project indeed:

10 ? tube tape recorder from the flea market - tubes and psu
broken di-box for the output transformer
12 ? input transformer
15 ? resistors, caps, wires

Click to expand...

That´s the real spirit of DIY

great :thumb: :thumb: :thumb:

Jens

I have a couple of suggestions:

For better high-end response, you might consider putting a small resistor at the grid of the lower ecc81 triode. (Say, 27k to 68k.) It won't impact gain but the grid-to-cathode capacitance (about 2-4 pF) would not roll off high frequencies. (I don't know much about cascode topologies, so maybe this is not useful. I'm curious what others think....) You could do the same thing with the EF86, which seems to be wired in triode mode.

If you think you have hum being caused by the heaters, you could consider tying the heater voltage to a higher DC potential. Do that by running the center tap of your filament secondary to a point between two resistors (say, 100k and 47k), with the other end of the 47k going to ground and the other end of the 100k going to B+. A nice thing about this is you don't have to rectify the heater supply.

Also, it appears that you have the pentode wired in triode mode. If you wire it in pentode (or have a switch set up to switch between the modes) then you don't need to worry about interelectrode capacitances as much.

Any corrections would be appreciated.... I am thinking of putting together an almost identical design.