833a SE Peak A.F. Grid Voltage

[dominic_tollens]

Hello,
has someone data of the Peak A.F. Grid Voltage for the
833a in SE?
In the DATA Sheet stands only everything about Class b in it. I have the disk reconditions already:


voltage/plate resistance/current:
3 kV / 20K / 100 mA <
2 kV / 10K / 150 mA
1,5 kV / 8K / 200 mA
1 kV / 5K / 250 mA
650 V / 2K / 290 mA

I would use gladly 845 as driver over a 10:10k transducers.
See link.however I also the 10y or VT25 would like to do without, instead two 6SN7 or like use as preliminary stage.


MfG Dominic

833a Schematic

 

[PRR]

> Peak A.F. Grid Voltage for the 833a

Forget grid voltage; for the 833, it will be small.

Do you understand grid current?? If the grid is positive of the cathode, the grid resitance changes from very-high to very-low. This has a lot of effect on your distortion and driver design.

Your 3KV 20K 100mA bias point seems to give ample power (over 50 watts) without running into grid current. Peak positive grid swing will equal the bias voltage, about 60V. (Not counting the cathode feedback winding in the schematic you linked.)

All the others either give less power output, or require grid voltage to swing above the cathode voltage. Some of them even idle with grid current. This requires a much more powerful driver, more powerful even than your....

> use gladly 845 as driver over a 10:10k transducers.

To make full power with the 1KV 5K 250mA condition, you must swing the grid up to 75V above the cathode, where it will suck 100mA. The grid impedance is about 5K for small signals, 500Ω for large positive peaks, very-high for large negative peaks. A "10K" transformer will give very poor performance. An 845 driver, plate resistance 1.7K, will give poor performance.

On paper, the 3KV 20K 100mA condition is so much easier to drive, that all others look silly.

In real life, a 3,000 volt amplifier is VERY DANGEROUS. It can KILL. I believe that if you have to ask "how?", you probably should not attempt it.

 

[dominic_tollens]

Hello PRR,
thanks for its assistance. I understand understood "grid current".... Thanks also for the warning the 3000V to kill can do, that is clear me. I am a learned electrician in the range building and system and high voltage technology... know therefore thereby extreme caution am required.

Sorry, my English is not very good, most technical ones understands must I check....



MfG Dominic

 

[audiofenn]

Hallo Dominic,
sehe das du auch an einem 833 amp interssiert bist.
Bist di mit dem Projekt schon etwas weiter?
grüsse

 

[Tweeker]

If you are trying to drive grid current with a large HV tube, you would want a 2:1 or steeper step down transformer, not 10K:10K. More gain might be needed here though. Using 6SN7s rather than 10Y would help, but still may not be enough.

If using 1:1 you would do much better (but still not well) using something with lower plate resistance like a 300B rather than a 845.

If grid current is not needed then a 2A3 should be sufficient to drive the 833.

The insulation on the output transformer must be very good if 3kV HT is contemplated, it may fail in expensive fashion, even if no one is hurt. It is very hard to source a quality high impedance transformer that can carry DC. 20K is quite high. HF performance may be disappointing. This is the gotcha with this op point.