Hi:
I have heard or owned a number of single-ended 45, 2A3 and 300B amps. Of
these amps, I liked the 300B ones the least. With that in mind, I just
recently came across a website for a foreign amplifier manufacturer that
has
some interesting thoughts about driving the 300B.
I would appreciate your comments about their ideas. Here's the exact text
from the manufacturers website (NOTE: I am copying it as-is, it loses a
bit
in the translation):
What points we are interested in?
Perhaps there are so many 300B single ended amps that we could
not discern good ones from bad.
For its natural and mellow mid-range, 300B single ended amp
is loved so much but its anemic bass and weak high frequency
response drive out many tube amp enthusiast from entering in the
Single Ended World.
What are the problems?
Basically there has been no good driving force for 300B.
First, Western Electric's 91B circuit was improvised for
easy manufacturing (for delayed production of 86B) and higher gain for
a theater system.
It's driving tube 310B ( or 6SJ7 or 6SH7 ) is pentode, notorious for
its distortion (too sensitive of its screen grid condition ) and
cotton
sugar bass response and too high internal impedance (about 100K?!!
bad frequency response in low and high spectrums).
Second, there are two stage driver circuit. Some use 6SN7 twin triodes
one triode for 1st stage and 2nd stage for driving stage. Alas! 6SN7
is not so powerful for 300B and it's high gain of two stage is not
suitable
for 300B power amps, so they sell them as integrated amps.
Its major defect come from capacitor coupling, which can transfer a
few
percent of driving wattage to 300B grid, almost of its energy is
converted
into useless heat of 6SN7's plate resister.
There are also a power drive circuit using EL34 or 6V6 or we349A (triode
connection).
They are not really 'power' drive, why? They use big plate resistor
for
EL34 about
10k? 20w, but it generates big heat only and could not drive 300B
sufficiently due
to the next coupling capacitor, in theory it transfers only few
percentage of its
power.
There are only two methods to consider.
First, there is interstage transformer coupling and then plate choke
drive method
(which will be discussed in our 300B integrated amplifier). A
interstage
coupling
method is a ideal one for transferring driving power to 300B grid for
its full
operation.
Its driving wattage could be transferred about 93% (less some coil and
core loss of
interstage transformer). We need sufficient power to make 300B work of
its full
capacity.
You will be surprised to the difference between under driven operation
of 300B and
full driven one. But it is really difficult to get a good interstage
transformer.
There three points to check for interstage transformer.
1. Must be wide range. Minimum 20Hz - 20KHz FLAT at designated source
impedance.
2. Good response to 10KHz square wave for flexibility and musicality.
3. Low distortion at low frequencies.
What do you think? Thanks!
Gerry
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