Building PRETOR toroid preamp

[revintage]

Couldn´t resist to build an preamp for the toroid poweramps. Very simple and minimalistic. The chassi is less than 5"*5"*1,5"! It will be fed by 12V and contains a 12-210V RECOM DC/DC converter(black can on top). I will try with a pair of 1,6VA Talema 115+115:18+18V toroids to get close to 5:1 in parafeed.

[kenpeter]

I assume you know already what happens
if you were to lift C2 halfway up into R2?

[revintage]

You would sabotage the parafeed circuit?

[oldeurope]

Originally posted by revintage
You would sabotage the parafeed circuit?

Hi revintage,

I think you should say "ultrapath".
From Radio News Feb. 1930: “A condenser, C... ,
is needed to form a local signal circuit in the output circuit ...”

BTW: You did the same mistake Broskie did.
( The input should be cathode referred.)

Kind regards,
Darius

[revintage]

Hello Darius,
And you did a mistake participating in this thread. Make your own thread if you want to discuss Ultrapath/Parafeed! Not here please!

[Michael Koster]

Originally posted by kenpeter
I assume you know already what happens
if you were to lift C2 halfway up into R2?

You get a balanced-current series push-pull output?
Clever.. maybe not quite 1/2 but offset by 1/Gfs...

Lars:

That's awfully cute. It would go with what I'm building
right now. I assume you'll buff the box to a nice luster.

How did you find that Recom converter? I'm using a stack
of +/-15V for B+ because I didn't know they made any HV
outputs. It's not easy to find on their website.

How do you plan to mount the converter? One method I use
is solder it to a rectangle of veroboard, etc.. larger in one
dimension and screw that down to the chassis inverted so
the converter is in contact with the chassis as a heatsink.
The little bricks like a lot of cooling.

Cheers,

Michael

[oldeurope]

Originally posted #3 by revintage
You would sabotage the parafeed circuit?

Hi Lars

No, of course not. It'll still be parafeed.

Kind regards,
Darius

[kenpeter]

Ultrapath bypasses cathode to low impedance B+, and may have
PSRR issues with a ground referenced drive. Unless you are clever
(as oldeurope sometimes is) and drive with careful consideration
for how this problem may be turned to an advantage. He knows
some very good tricks along those lines.

But here we do not have an ultrapath to B+. We have parafeed
from a triode ref'd close to GND, and high impedance - ripple free
- constant current sink on top.

I don't see how driving common to top or bottom of cathode
resistor in this topology makes for a hill of beans either way?
Everything below the current sink is GND ref'd, and therefore
common noise.

----------------------------------------------------------------

Yes, tapping R2 halfway up will totally "Ruin Parafeed".
Don't ever try that. We don't want anything extra for free.

[oldeurope]

Hello Forum,

...parafeed is just the short form for parallel feed.
The opposite is serial feed.
In a serial feed (Abb.77)circuit the load in in the DC path.
In a parallel feed (Abb.76) the load is DC free.
That's it, nothing mythical in there.

See attachment,
parallel-feed = Parallel-Speisung
serial-feed= Serien-Speisung

Kind regards,
Darius

[oldeurope]

Originally #8 posted by kenpeter
...

I don't see how driving common to top or bottom of cathode
resistor in this topology makes for a hill of beans either way?
Everything below the current sink is GND ref'd, and therefore
common noise.

...

Hello kenpeter,
you are right, I had Broskie's in my mind.
To answer your question, please have a look there:
http://www.nutshellhifi.com/library/Tube_Fest_Talk.html
Lars is making something equal to the Western Electric (output).
The problem is that the current source is imperfect and the
output transformer adds an unknown impedance to the anode.
This generates a 'noise' current in the cathode resistor and adds its 'noise' voltage drop to the input.
This problem is solved by referring the input to cathode, see Western Electric (repeater amplifier).

The LED in Lars's circuit has a low impedance and makes the cathode ground referred. But what about R4?

Kind regards,
Darius