Changing the operating point

[johnsonad]

I've read that the 300B can have a 45ish sound with the operating point of 350v plate to cathode, 60mA idle current and a 5k plate load. Could this be achieved with the Paramount without too much trouble? I was thinking for iron using an EXO-050 for the OPT.

Thanks!

Aaron

[Paul Birkeland]

You'd want a different plate choke - 50H or more at 60ma. 

I see a 350V P-K, 50ma, -76V bias, 5K load operating point, maybe that's what you' want to try?

This would call for a 1.5K cathode resistor.  You could use two 750 Ohm tubular resistors and put them in series to get this.

Depending on the DC resistance of your plate choke, you migt have to increase the power supply dropping resistor a little bit to bump the voltage down, but it might work as-is.

[johnsonad]

Thank you for your help Paul.  I've seen both 50mA and 60mA though I would like to run it at 50mA and have a little headroom.  Gordon wrote about it nearly a decade ago and Thorsten had made a couple of post about this OP on AA.  Have you tried it before?

[Paul Birkeland]

I haven't tried it. 

If I wanted to mess with the Paramount 300B operating point, I'd be tempted to run the 300B in fixed bias and go for the 18 watt operating point.

You'd need the gain of the 12AT7 to get there, and a small 1:1 power transformer to generate the bias voltage, along with a 20 watt 2K single ended transformer (huge piece of iron!).  Interestingly, you might be able to use the stock plate choke and poke around to find a suitable 2K/20W parallel feed output transformer. 

I think Sowter or Magnequest could produce such a transformer pretty easily, though getting a 25W Edcor XSE transformer would make the experiment more affordable.

[xcortes]

If one drops a 4.5K opt in the same point the power is 12.5W and the distortion numbers are better. But more importantly you can get away with a Robin Hood Parafeed which is only $99 ea or with exo 050s (from 150 to 325 ea depending on core material).

Or how about a tfa 2004 at 3K. That gives you15 watts.

How does one design the bias power supply in psud? Or more specifically, what current goes through the grid?

[Paul Birkeland]

Ideally nearly no current goes through the grid.  On really hard transients, the grid may begin to pull some current, but without some serious mods to the amp, the distortion will drastically increase when you hit this point. 

-PB