ef86 based preamp

intensate · 20th January 2013, 08:58 PM

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:

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

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?

"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

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:

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 )

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

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.

20th January 2013, 08:58 PM	#1
intensate diyAudio Member Join Date: Jul 2012	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

21st January 2013, 03:46 AM	#3
tomchr diyAudio Member Join Date: Feb 2009 Location: Calgary	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 __________________ Modulus-686: 240W/8Ω @ <0.00025% THD. Modulus-86: 40W/8Ω @ 0.000054% THD. DG300B & Other Tube Circuits. Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right

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20th January 2013, 09:28 PM	#2
jazbo8 diyAudio Moderator Join Date: Jan 2011 Location: In Transient	Check out Mullard's preamp section:

21st January 2013, 06:51 AM	#5
grufti diyAudio Member Join Date: Jun 2004 Location: Hillside	R1 is the 1M resistor on the input. What does that have to do with the anode load?

21st January 2013, 08:05 AM	#7
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	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

21st January 2013, 08:06 AM	#8
reenberg diyAudio Member Join Date: Jun 2005 Location: Dk	I would look into the quite interesting radford ( a vondeful amp by the way ) 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.

21st January 2013, 09:42 AM	#10
DF96 diyAudio Member Join Date: May 2007	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.