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trying to find a way in running one single 300B tube with a hammond 1640SE opt that has 1.25K primary, will it work? perhaps by adding some kind of resistor to the opt primary?
Follow Ups:
The trick is to match the operating point to the load impedance. At the low load, the tube must run fairly low voltage and high current to keep the distortion in check. At the maximum rated current of 100mA, I calculate about 300v plate to filament, plus maybe 55v bias.The WE data sheet shows a point at 250v, -45v bias, 80mA plate current, and 1500 ohms load.
Efficiency is lower at these "low and hot" conditions, so you might only get 6 or 7 watts output. Damping factor is also lower, maybe 1.5-2 instead of around 3 normally. Some claim better sound, though I am not yet convinced the comparisons are free of enough other changes.
Nt
If you connect 8 Ohm speaker to 4 Ohm secondary tap, the reflected plate load will be 5 K. Low frequencies will be rolled off, but then you can use it as mid-high amp from 200 to 20,000 Hz. You will need a powered sub for the low end.
I'd avoid this combination as quite high distortion will be
the result --therefore defeating the reason for using high
linearity DHT tube such as the 300B in the first place.Here is what can work alright: Parallel two 300B tubes and
set plate voltage and current/bias of EACH tube to the conventional
300B 2.5k operating parameters. you will obtain 12-14 watts of
fairly low distortion output this way.Sorry, but there are simply no attractive alternatives for a single
300B and a 1.25k OPT. Frankly, the additional resistor idea is
a prodigious power-wasting move, and a very backwards approach
--guaranteed to sound crappy. You will be displeased.Also: two parallel 2A3 will work optimally into such a load, and
give you a good solid 7-watts out.If you are dead-set on using a single 300B, then I'd order-up the
excellent Hammond 1627 unit with a 2.5K primary. Yes, the Hammond
5K primary 1628 model should give lower distortion figures in the
midrange and low end, but at slightly reduced power than the 2.5K
unit, and some competent builders have complained about resonances
and high freq rolloff with this unit.So the deal is:
(A) buying another pair of 300B and using orig 1.25k OPT for 12-14 watts out. (fairly low distortion)
(B) buying four new 2A3 tubes and using orig 1.25K OPT for 7 watts out
. (very low distortion)(C) staying with the single 300B config but buying a new pair of
Hammond 1627 2.5K OPT's for about 7-8 watts out (fairly low distort-
ion, but and likely more sonically refined than option (A))If the 300B tubes you would be needing for another matched
pair are in the approx $60 or so each range, then it might be a
tossup as far as price between option (A) and (B), with option
(C) costing somewhat more due to the shipping on new 11-lbs OPTs.Personally, I'd probably pop for a new pair of 1627s, and go
for option C. You can always ebay your 1.25k units to defer
costs, or keep for another project.Good Luck,
I was actually running 2x300B in parellel and looking for options that i can still use the 1.25K opt...
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