Your thoughts on this SE EL34 schematic idea?

[markmalin]

Hi all!
I’m working on some ideas for a single ended EL34 amp and have a few questions.  Would greatly appreciate your input.


The preamp is pretty much nailed down.  What I’m doing now is mating that preamp to an SE EL34.  After looking at several ideas from other guys’ posts here and in AX84 I’ve come up with the hand drawn schematic attached.  I have a few questions, though:

• What size (henrys) choke should I use in the power supply section?
• Any obvious mistakes?
• Does the B+ supply section look OK?  (for example, 47uf/500v caps seem OK?  I will tweak the resistor values to get the preamp voltages I want once I get it up and running)
• Do I NEED a standby switch?
• Do I need an NFB resistor, or not?

My OT is a Mercury Magnetics SE-3K, and my PT is a Mercury P20JM (320-0-320, 6.3V ct, and a 120VAC primary)


Most humbly,
Mark

[tubeswell]

Merlin Blencowe has a page on SE EL34 design


http://www.valvewizard.co.uk/se.html


With an example of a CLC filter using 50uF caps on this page:


http://www.valvewizard.co.uk/smoothing.html

[markmalin]

Merlin Blencowe has a page on SE EL34 design

http://www.valvewizard.co.uk/se.html

With an example of a CLC filter using 50uF caps on this page:

http://www.valvewizard.co.uk/smoothing.html

Thanks!  That will be my light reading for this evening


Mark

[sluckey]

I would double up on the diodes, add an extra filtering stage, use one of the chokes suggested, and no NFB.

[markmalin]

I would double up on the diodes, add an extra filtering stage, use one of the chokes suggested, and no NFB.

Thanks, Slukey.  I was planning to double the diodes.  Thanks for the help with chokes!
Mark

[markmalin]

Slucky, if you don't mind, why is another filter stage needed/suggested?  And with only 2 preamp tubes and on power tube, would there be anything tied to the 5th filter stage?
Thanks!
Mark

[sluckey]

A single ended amp does not have any hum rejection characteristics like a push/pull amp. That's why you need the extra filter stage up front to reduce the B+ ripple as much as possible. Putting the choke up front means all current for the entire amp will flow through the choke. That's why you need 150mA capacity.

The fifth node (47µF and choke) is connected directly to the diodes and nothing is connected to it. The EL34 plate is fed after the choke by Node A. Screens are fed from Node B. Then tone stack is fed from Node C2 and finally the preamp is fed from Node C1. There's a discrepancy with your node labeling. The preamp should be fed from the very last node in the B+ string.

[markmalin]

A single ended amp does not have any hum rejection characteristics like a push/pull amp. That's why you need the extra filter stage up front to reduce the B+ ripple as much as possible. Putting the choke up front means all current for the entire amp will flow through the choke. That's why you need 150mA capacity.

The fifth node (47µF and choke) is connected directly to the diodes and nothing is connected to it. The EL34 plate is fed after the choke by Node A. Screens are fed from Node B. Then tone stack is fed from Node C2 and finally the preamp is fed from Node C1. There's a discrepancy with your node labeling. The preamp should be fed from the very last node in the B+ string.

Thanks for the clarification.  That makes total sense.
My bad - I was looking at the edited version you so graciously provided and didn't notice the extra filter stage you put in!    .  Sorry about that.

Mark