300B operating points

[vetmed]

I have some very nice iron for the 300B, made by Sowter. It is 3.5k primary. I have read a lot of stuff on the merits or lack thereof of running this tube at higher current as opposed to low. The WE data sheet has several operating points that are appropriate for this transformer, I favor the 400v, 60ma, -87v bias. Comments and criticism gladly accepted

[Paul Joppa]

That's a little higher-distortion point than I like, but not unreasonable - I'd do maybe 360v at 65mA for a 3500 ohm OPT. My preference is for resolution over warmth. YMMV.

[vetmed]

Just the sort of response I was hoping for, thanks. At that voltage and current the bias would be...?

[Paul Birkeland]

Which Sowter do you have? 

360V/65mA would be about 75V of bias. 

If at all possible, you could develop your own tastes for operating points if you can source a vintage high voltage power supply.  If you have monster Sowter transformers that will take lots of DC current happily and support tons of power, then you might also want to try the higher power operating points.  It does sound a bit different.

[Deke609]

If at all possible, you could develop your own tastes for operating points if you can source a vintage high voltage power supply.  ...

An Edcor "Tinker Box" might help with playing around with different operating points (no affiliation): http://www.edcorusa.com/tbpwr-1

cheers, Derek

[vetmed]

Its the SA-08. 100-150ma quiescent current, 25W, 15Hz to 50kHz +/- .25dB. And they are humungous

[Paul Birkeland]

Hell, try 400V/100mA and just run EH tubes until they die.  You could get 14W there.  Definitely a good excuse to find a 500V/200mA bench supply!

[vetmed]

Wow, what to make of this? Not something to do with more expensive 300Bs. EML make tubes that would work (I think) but not sure I would want to be stuck with only one tube type, which is one of the appeals of tube amplification, being able to try different varietys 400v and 100ma what would be the bias, fixed would be required I believe.
thanks

[Paul Joppa]

The 300B was rated for 40 watts absolute max in the thirties, though the 1950 data sheet says 36 watts. I believe I've read somewhere that WE said it would lose around half of it's nominal lifetime at "maximum conditions". But the data sheets list as max operating conditions only those over 400v.

Max rated current is 70mA in fixed bias, 100mA with cathode resistor bias.

Bias 80v at 400v p-k, 100mA. My target plate load would be around 2000 ohms at that point though, so 100mA is overkill for the Sowter.

I would not hesitate to do 400v at 70mA into 3500 ohms - bias around -85v (1200 ohm cathode resistor). That will still get you 10+ watts.

[Paul Birkeland]

Bias 80v at 400v p-k, 100mA. My target plate load would be around 2000 ohms at that point though, so 100mA is overkill for the Sowter.

There was a fair bit of extra room on the top side of that load line!

[vetmed]

You would need a B+ around 500 volts, which is the upper level of my comfort zone. Fixed bias just adds an additional supply and makes things more complicated Thanks for all the suggestions.