Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on June 07, 2023, 01:43:25 am
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I have a need to invert phase in a EF86 preamp.
Are there some good practices to do that without increasing the amplification?
The available resources for that are 1/2 12AX7 and EF86.
/Leevi
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Use the triode as CF
it will invert the signal in exit from the ef86
Franco
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Hi Franco, do you have an example?IMO cathode follower does not invert the phase.
/Leevi
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This works
/Leevi
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My fault, you are right
however you can solve with the same triode, arrange an anode follower with 1 or less of gain (if you don't want it)
may be you can have the same result using a mosfet instead of to add a triode
Franco
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I have a need to invert phase in a EF86 preamp.
Are there some good practices to do that without increasing the amplification?
The available resources for that are 1/2 12AX7 and EF86.
/Leevi
The Univox U-1221 uses a cathodyne (unity gain) after the pentode in the PI position. They used a 6AN8. May provide an option.
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If I've understand correctly Levi want only to invert the signal, not to have a phase inverter to drive a PP Power section
However using one of the exit of a concertina can give the wanted result
Franco
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This works
/Leevi
Yes. The cathodyne is the simplest way to get inversion with no gain. Ignore the cathode output. You could replace the plate resistor with a pot if it's important to adjust the gain to exactly 1.
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You could use a signal output transformer with enough connections, it would give you any phase you want and also provide galvanic isolation which might be a very nice thing to have in a preamp.
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If I've understand correctly Levi want only to invert the signal, not to have a phase inverter to drive a PP Power section
Yes, I'm going switch two preamp channels together and I want that the signals are in same phase.
EDIT: A common input is used for both channels (A and B) and you can switch between A, B and A+B
/Leevi
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https://www.zupimages.net/viewer.php?id=20/06/y6u6.png
Not an EF86 but similar implementation.
https://schematicheaven.net/bargainbin/dano_special.pdf