Selecting Output Transformers for My WOT Line Preamp

[snoopyma]

I want to upgrade my WE437a WOT preamp (running at 25ma). I want wide frequency response and low output impedence as well as balanced outputs. I have considered the following Lundahl transformers:
(1) LL1671/30ma (Alt Q connection, L=35H, 4:1, no freq. spec.);
(2) LL1689/18ma (Alt P connection, L=90H, 18:4, no freq. spec.) to be gapped at 25 or 30 ma; or
(3) LL1660/18ma (Alt Q connection, L=100H, 4.5:1, freq. 11Hz-35kHz +-1db) to be gapped at 25 or 30 ma.
Grateful if anybody advise which of the above is the best for me? Or is there any more suitable counterpart? For options (2) and (3), should I specify the gap at 25 ma or 30 ma? What's the difference?

Any advice or information is appreciated. Thanks in advance.

 

[Brett]

If it were me, I'd whack a CCS on top of it and run it parafeed. I reckon it sounds better.
If running the Tx on the anode, use the smallest gap you can get away with for the anode current. It'll increase the inductance. There's probably not going to be a lot of difference between the Lundahls once they're gapped to a similar spec. Turns ratio will likely be the major difference and choose that depending on what you need.

 

[snoopyma]

Hi Brett,

Thank you for your advice. Could you elaborate the difference between parafeed and normal (i.e. with DC current) design in terms of sound quality and technical performance?

What's the suitable candidate (brand and model) for the parafeed output transformer? Will it be permalloy or amorphous ones? How to calculate the value of the coupling capacitor?

Thanks

 

[Brett]

Much cleaner sounding to me, less wooly and (not) vaguely distorted.
Any of the Lundahls will be fine, with no gap and whatever turns ratio you're after.

http://www.nutshellhifi.com/gary/index.html

 

[analog_sa]

I have used a 1660/18mA as 4.5:1 and really liked the sound.

Parafeed will certainly sound different and would greatly increase the possible choices of transformers with better cores. If i were to go the parafeed route i would make sure i have a nice plate choke at hand - just in case i can't swallow the CCS. And a VERY decent cap. OMG, this parafeed adventure may get expensive

 

[snoopyma]

I have used a 1660/18mA as 4.5:1 and really liked the sound.

What tubes that you are using and at what current? Thanks.

 

[analog_sa]

6S45PE but i really can't recall the current. Very possibly 15-17mA.

 

[dave slagle]

use the smallest gap you can get away with for the anode current. It'll increase the inductance.

it will also decrease the linearity and place you in a very nonlinear part of the BH loop.

I'd suggest just the opposite. Determine how much inductance you really need and use the largest gap possible that will still give you that value.

dave