Magic modify of converting cheap PP EL34 Amp's to 300B SE
------after all is dr. Klimo 300B circuit (OPT with
Degauss coil inside )
Below detail just usage one pole of P-P (push-pull)OPT coil to do Degauss
coil function.
hyper-link ....
http://www.audioasylum.com/cgi/t.mpl?f=tubediy&m=76018
Document...
I have now invested some time into this lark and a few things become
clear.
If a typhical EL34 PP Amp is used with one EL34 as pure "Ballast" you can add
your 300B on the other side.
The Impedance loading the 300B becomes 1/4 of the A-A impedance of the EL34
OPT. Typhically this will give between 1K - 1K5 nominalimpedance.
If we declare the 4 Ohm tap to be 8 Ohm and the 8 Ohm tap to be 16 Ohm we are
fine(ish). If you need a < 8 Ohm output impedance you are nearly out of luck
if your amp does not have 2 ohm Taps (few cheap chinese EL34 PP Amp's do). BUT,
you can use a trick and connect your < 8R Speaker between the 4R & 8R
Tap. This will give a tap with a very low nominal impedance, which would limit
available power a little (which might explain thge rated 5W for the
Beltaine).
BTW, I got a copy of the readers letter to CHF with the Beltaine schematic .
Some interesting tricks. The EF184 driver pentode gets it's screen voltage from
the 300B Cathode, 300B has LCL filtered DC heater. The EL34 uses an unbypassed
cathode R and is indeed merely "ballast", balancing DC. The rest of the circuit
is very traditional, EF184 drives 300B in classical Pentode driver, Triode
output self biased circuitry .
So, converting any "surplus to requirements" cheap EL34 PP Amp to 300B SE in
Klimo Beltaine style should be easy enough.
This is based on re-cycling a Dared Tube VP-80 which prooved way to
unreliable for the guy who bought it.... Looks nice though:
Heaters for the 300B's will DC (probably LT1085-5 Reg) from the 6.3V Output
Heaters and shared with the EL34 which will run AC.
The 6SN7's stage on AC, lifted to around 70V (in other words EL34
Cathode.
Some ideas are filched from Klimo (Screengrid/cascode upper grid bias, NFB
switchable), some are mine.
To improve the way the DC magnetisation cancellation in the OPT tracks
varying 300B's and to reduce differences between various EL34's the EL34
"Ballast" valve gets the Cathode voltage of the 300B as Gridvoltage, the cathode
Resistor is suitably increased.
The Frontend uses a 6SN7 cascode to approximate a pentode, you can of course
use a real pentode. The idea for me is to use only stuff I have "around" and
good octal Pentodes are non too common in my stash.
The PSU substitues a Mosfet"Voltage Follower" for a choke, smaller (size is a
premium).
Anyway, this should make an interesting little amp, gor for the usual WE
Standard 7 - 7.5W....
In document we know Dared Tube main land China VP-80 clone already.
Principle ...
Old SE 300B connect high voltage on OPT coil cause Hysteresis curve
floating up (磁滯曲線飄起來) to O2 point.
Audio AC signal trasfer on the Hysteresis curve then cause Audio AC
signal floating up too.
Meantime audio AC signal Loss signal sensivity and linearity (just
partial saturation).
Here dr. Klimo 300B usage Degauss coil to pull back Hysteresis curve
as below.
Driving Degauss Tube as EL34 in circuit (as red line
function).
If use Mosfet to replace EL34 then your need big heat sink(because
always have DC current cross over Mosfet's D & S pole ).
Dr. Klimo is a really master.
Sense of hearing
Old Tube amplifier always have good man-vocal but woofer and
tweeter seem is less a point .
But Dr. Klimo circuit have something different just
opposite.
The issue is cause tube ampifier RIAA play-bake curve have a drastic
changes.
