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-   -   ef86 based preamp (http://www.diyaudio.com/forums/tubes-valves/228282-ef86-based-preamp.html)

intensate 20th January 2013 08:58 PM

ef86 based preamp
 
Hello all.
I am trying to make use of 10 nos ef86 tubes I have.I also have 2 ecc82.
I have built a valve phono stage,so I want to make a preamp at line level.
I don't need tone control
Can anyone recomend a schematic
many thanks

jazbo8 20th January 2013 09:28 PM

Check out Mullard's preamp section:
http://usr.audioasylum.com/images/0/..._Schematic.jpg

tomchr 21st January 2013 03:46 AM

Funky with C1 and R3 across the anode load on V1. Looks like they may have had some issues with RF oscillations on the input stage. Or didn't want such oscillations to return in issues in the field.

Just an observation....

~Tom

jazbo8 21st January 2013 04:19 AM

Quote:

Originally Posted by tomchr (Post 3335398)
Funky with C1 and R3 across the anode load on V1. Looks like they may have had some issues with RF oscillations on the input stage. Or didn't want such oscillations to return in issues in the field.

Just an observation....

~Tom

I think you meant C1, R1... By funky, you meant classic, right? :D

"A CR network (C1, R1) shunting the anode load produces an advance in phase which increases the stability of the amplifier at high frequencies."

More here >> Mullard 5-10

grufti 21st January 2013 06:51 AM

R1 is the 1M resistor on the input. What does that have to do with the anode load?

jazbo8 21st January 2013 07:35 AM

Quote:

Originally Posted by grufti (Post 3335521)
R1 is the 1M resistor on the input. What does that have to do with the anode load?

I was referring to the schematic linked on my second post - it turns out it's different from the earlier schematic I posted, sorry about that... Same idea though, R1 & C1 refer to this Mullard schematic:

http://www.r-type.org/articles/5-10-b.jpg

Jaz

Shoog 21st January 2013 08:05 AM

I can't think of any situation where the gain of a ef86 in a preamp would be anything but a disadvantage. It then requires copious amounts of negative feedback to kill the gain - and a preamp is definiately one of those situations where there is little need for feedback.
This tube has no other place than a phono preamp in my opinion.

Shoog

reenberg 21st January 2013 08:06 AM

I would look into the quite interesting radford ( a vondeful amp by the way )
http://www.dhtsound.com/cgi-bin/tech...DSTA15-gif.gif

There is some papers for taming the high gain somewhere ( cant locate it right now ) and a great 2 vol article by Radford explaining the way it works.

barrymagrec 21st January 2013 09:33 AM

Quote:

Originally Posted by Shoog (Post 3335565)
I can't think of any situation where the gain of a ef86 in a preamp would be anything but a disadvantage. It then requires copious amounts of negative feedback to kill the gain - and a preamp is definiately one of those situations where there is little need for feedback.
This tube has no other place than a phono preamp in my opinion.

Shoog

I would agree (or a Mic or tapehead preamp) and even then It will be noisier than a decent ECC83 stage.
Why not strap it as a triode to reduce the gain?

DF96 21st January 2013 09:42 AM

The OP was asking about preamps, so why post circuits for power amps?

The network at the anode of the EF86 is to stop feedback loop instability (typically in the low ultrasonic region); the input stage itself is very unlikely to suffer from RF instability.

The EF86 has no place in a modern line stage, as it has far too much gain. Keep a few as spares (for your phono preamp?) and sell the rest so you can buy more suitable valves.


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