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Old 20th January 2013, 08:58 PM   #1
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Default 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
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Old 20th January 2013, 09:28 PM   #2
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Check out Mullard's preamp section:
Click the image to open in full size.
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Old 21st January 2013, 03:46 AM   #3
tomchr is offline tomchr  United States
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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
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Old 21st January 2013, 04:19 AM   #4
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Quote:
Originally Posted by tomchr View Post
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
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Old 21st January 2013, 06:51 AM   #5
grufti is offline grufti  United States
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R1 is the 1M resistor on the input. What does that have to do with the anode load?
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Old 21st January 2013, 07:35 AM   #6
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Quote:
Originally Posted by grufti View Post
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:

Click the image to open in full size.

Jaz
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Old 21st January 2013, 08:05 AM   #7
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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
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Old 21st January 2013, 08:06 AM   #8
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I would look into the quite interesting radford ( a vondeful amp by the way )
Click the image to open in full size.

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.
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Old 21st January 2013, 09:33 AM   #9
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Quote:
Originally Posted by Shoog View Post
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?
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Old 21st January 2013, 09:42 AM   #10
DF96 is offline DF96  England
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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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