[AMPS] input for 572b's

[nospam4me@juno.com]

Re: 52MHz dual 572b amplifier input circuits.    thoughts and more...

Hello there,

Awhile back I mentioned that I would be happy to share some directly
heated cathode input circuits from my hardcopy archives, in specific some
of my own and other builders 572 and 811 designs.  Others also requested
circuits for other zero bias triodes like the 3-400 and 3-500.

After a quick review of my printed archives, I decided to revisit some of
the more practical circuits for a revamp using updated parts and 
available components.  As my time allowed I've downloaded some tube data
sheets, amplifier circuits and parts supply information. 

Turns out this has become quite the eye opener.  Soon I hope to share
some of the issues I've been up against. These include:  printed math
formulas that don't seem to play properly , articles with fudged or
lifted circuits, tube data sheets which are rather vague and a lot of
head banging to make sense of it all. 

I've had to go back to amplifier 101school and work up circuits which
properly fit the requirements.  Where I'm at so far is the standard pi
circuit, a tapped hybrid tuned bi filar coil/choke and now a hybrid
toroid transformer.  Yes there are other input choices, but the
requirements are spec for a 52MHz  (mono band) two 572b circuit. A quick
reality check leans heavily for the pi circuit with the tapped tuned bi
filar next.

Other research through available online data sheets and projects turned
up a unique hybrid toroid circuit which I hope to play with as time
allows.

The pi circuit should be straightforward, right?   Nope... Turns out,
I've been running into some differences in mfgrs values for Cgk and the
curves don't quite match the graphs as shown. Remember this is 52MHz... 
So, I'd like to check out a few more things before I present what I've
found. All that glitters in not gold on the data sheets so the test
equipment is warming up.  The 572s I have here seem to have a drive
value/plate current distortion quirk which I'm trying to better
understand. I'd like to then verify this distortion occurs in all or
specific mfgrs tubes. Poor efficiency is another beast to cure, the test
fixture amplifier is less than 50%.  

The tapped bi filar coil/choke circuit is an old favorite with 811a
builders. I've used this circuit a few times already in HF operation. A
bi filar coil is made with copper tube and center stuffed wire, then
fudged into operation more on a lot of ballpark luck vs hard  theory. So
I'm trying to update this method using mini hard line to form a more
predictable coil which can be easily reproduced.  The physical limits
seem to be the larger two-tube heater current requirements.  Modification
of this circuit is a rather involved process, I've got various size coils
tossed everywhere.  Again the numbers don't seem to match the various
books I've tried to use.  

I'll probably check back in with some ideas and information about the pi
circuit first. It appears to be  the best animal out of the box.  I hope
to be at the tweaking stage shortly. Hopefully, a friend is going to lend
me a pair of newer (used) Svetlanas that I'll use to confirm or deny
another quirk of the 572 tubes I have here (surplus jan). 

cheers

skipp  

nospam4me@juno.com
http://sonic.ucdavis.edu


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