• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

One more 4P1L SE

This is how I'd rebuild my previous 4P1L PSE with fixed bias and A2 drive:

4P1L-PSE-A2-Amp-v01.jpg


The driver is the 6e5p (or better 6e6p-dr) which is very linear and works well with a bit of degeneration in the cathode. As the stage is a hybrid mu-follower (i.e. gyrator load) the reflected cathode resistor in the anode isn't impacting the output impedance. The follower is a classic MOSFET (high GM, low Crss) follower using a symmetric +/-80V to provide enough headroom. The HT is a bit marginal for the driver but provides good headroom for the 4P1L.
The rest of the circuit is self explanatory
As I don't need the A2 power, I end up with the 4P1L PSE with filament bias I have now. Probably will rebuild this version as the follower drives well the PSE pair. I'd go for 3 valves next time.
Hope this helps.
Cheers
Ale
1 tube of 4p1l fixed bias is -10.5v 4ma then 2 tubes fixed bias is -21v 8mA right? If screen grid resistor 1M feeding -21v the current will be too low? 0.02mA not 8mA?
 
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1 tube of 4p1l fixed bias is -10.5v 4ma then 2 tubes fixed bias is -21v 8mA right? If screen grid resistor 1M feeding -21v the current will be too low? 0.02mA not 8mA?
"1 tube of 4p1l fixed bias is -10.5v 4ma then 2 tubes fixed bias is -21v 8mA right?"
No.
Each /paralleled/ tube fixed bias is -10.5V.
There are -practically- no DC current (uV region) trough 1M (FET gate) bias voltage resistor.
 
"1 tube of 4p1l fixed bias is -10.5v 4ma then 2 tubes fixed bias is -21v 8mA right?"
No.
Each /paralleled/ tube fixed bias is -10.5V.
There are -practically- no DC current (uV region) trough 1M (FET gate) bias voltage resistor.
if feeding -10.5V through 1M bias resistor, only some uV will pass through the bias resistor and current will be very low at 0.01mA.

Then, what 4mA screen grid in Tube data sheet related to? Thank you.
 
Sorry, I don't understand that what's the relationship between FET's gate DC current (if it even really producing) and (trioded pentode) power tube's screen current?
Sorry, due to my little knowledge. I would like to change question. What is the purpose of the datasheet information of -10.5V 4mA screen grid of 4P1L use for (if not for the purpose of fixed bias)? Thank you.
 
"1 tube of 4p1l fixed bias is -10.5v 4ma then 2 tubes fixed bias is -21v 8mA right?"
No.
Each /paralleled/ tube fixed bias is -10.5V.
There are -practically- no DC current (uV region) trough 1M (FET gate) bias voltage resistor.
https://www.bartola.co.uk/valves/2016/03/10/russian-pse-in-steroids-01a-into-4p1l-part-iv/

from website of Mr. Bartola https://www.bartola.co.uk/valves/2016/03/10/russian-pse-in-steroids-01a-into-4p1l-part-iv/

the screen voltage is -24VDC (not uV). However there is no 1M bias resistor and 330R grid resistor.

That is the point I do not understand why if feed -10.2V to 1M bias resistor there is no DC (uV only) but another -24VDC has no 1M bias resistor and feed -24VDC instead of uV.
 
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If are you read carefully, these numbers are valid at penthode mode, at specific Ua, Ug, Us values.

The linked (made by Ale) schematic contains triode connected pentode tubes, with other subject to conditions.
Triode connected pentode tube's -summed- anode current contains both currents (Ia, Ig2), so at development -in itself- does not need to be considered Ig2.
 
"1 tube of 4p1l fixed bias is -10.5v 4ma then 2 tubes fixed bias is -21v 8mA right?"
No.
Each /paralleled/ tube fixed bias is -10.5V.
There are -practically- no DC current (uV region) trough 1M (FET gate) bias voltage resistor.
I borrow KT88 PP fixed bias in this forum and try to test -10.2V pass through 1M bias resistor.

the result is voltage is little drop from -10.2V to -10.199V and current is about 1.02nA. Is it correct, Mr. Euro21? Thank you very much in advanced.
 

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