# Ideas input for a Interstage driven 300B SET

#### euro21

Paid Member
"The schematic above, did you build that in Spice? again impressive how quickly you turned that around."
I made many thousand simulations (very much with tubes) in the past (almost 20) years, so building new (if the appropriate models presents) with available blocks is relatively easy.

In most cases the "more linear" operation (which goes together lower distortion) mode coincides the "better tone" feeling, so if you don't chase more and more power, higher loaded power tube is better solution.

I use (more than ten years) 5k OPT loaded (CCS loaded D3a, FET SF direct driving fixed bias power tube) 300B SE amplifier which is have 1.1 .. 1.2% THD near at A1 border (6W).
Due to the beefy driving at 10W the distortion only 1.5%.

IMO the driving of power tube is one of the most critical designing mission.
If it possible, I use direct driving (CF or SF) solution: driver low impedance/driving capacity is one of the main design parameter.

1 users

#### Dorin Bodea

A couple of years ago this was my idea of a 300B SET output stage driven by interstage xfmr, but not only an idea because the second graph shows the practical frequency response. The next immediate step was adding an input xfmr (1:2, 1:4) with exactly same response, all xfmrs made by me

@Matthew78 - So, keep the faith, it's doable

#### Matthew78

Thanks for the encouragement! In both schematics I see a resistor parallel with the output side of the interstage, what is the function of that, to have some current running through the interstage? Is the value of that resistor critical? I assume that resistance is also seen in on the input side?
Thanks

#### Dorin Bodea

I see a resistor parallel with the output side of the interstage, what is the function of that, to have some current running through the interstage? Is the value of that resistor critical?
It sets an AC load for the driver tube/xfmr, it has nothing to do with the quiescent current set by RC group in the cathode of the driver tube, as in bias setup.
The value is more or less approximate, is chosen according to capabillities of interstage xfmr; in some cases can be ommited entirely, in other cases may be doubled by a Zobel network, but in all cases the final tube's grid sees always the lower RDC value of the secondary winding, which translates in better stability.

#### Matthew78

Hi Dorin, you I might have phrased it poorly but that was indeed what I had in mind (because of the load there is AC Current flowing), one day I might actually start to understand these things Thanks for clarifying, much appreciated!

#### Dorin Bodea

@Matthew78 - then excuse my basic level of explaining too... in a more advanced analysis that resistor can influence bandwidth, distortion, so it's not superfluous there