[bobluis]

Hello, I would like to know your comments regarding 6J7 tube as 300B driver. I have studied the WE91 classic circuit to build my first SE amplifier.

As a first try I will stay on a pentode driver like the WE amp. I have some 6J7 samples (GE) and also 6SJ7, which is a later similar valve. 310A tube is very difficult to obtain for me, and the suggested subs are 6J7, 6SJ7 and even 6C6 which was used in the early WE theater amp. Later will think in other driver possibilities but for now I like the WE pentode idea as a first try.

Some points:

1- the image below shows GE plate curves. I have drawn 90K DC load line, which is the typical driver operating point for some WE91 based amplifiers including SP Joe Roberts adaptation with 370V B+. Understand that those resistors are for 310A. 6J7 is a bit different regarding max current, anode voltage, etc.

So I have drawn another line with lower load (around 50K). My concern is Miller effect on bandwith, so a lower Ra would help but my limit seems to be in the max current capability and max dissipation available with 6J7 (red dotted line).
Do you think from a purely theoretical point of view that 50K will work better?

2- Grid bias remains around -3 volt and on AC I still have more than +/-70 volts output, enough to make 300B work.
6J7 anode resistance in triode mode is 11K aprox. Using this value I calculated SG resistance around 55K:

rsg= ra(triode) x (Ia+Isg) / Isg

This value is important to design the SG voltage divider used in the classic circuit. Correct me If I’m wrong. Classic WE91 circuit uses 75K upper resistor and 30K lower resistor in voltage divider. To calculate voltage divider (B+ 420V) for desired SG DC voltage (around 100V) I should considered lower resistor in parallel with SG resistance.

I have discrepancies here. If SG draws about 0,5 mA at 100V, rsg is about 200K, not 55K // 30K. What is wrong with my reasoning?

Sorry for so lengthy post But I think could be usefull for other members too.

[WardSmall]

I have drawn 90K DC load line, which is the typical driver operating point for some WE91 based amplifiers including SP Joe Roberts adaptation with 370V B+. Understand that those resistors are for 310A. 6J7 is a bit different regarding max current, anode voltage, etc.