@Bluesystems,
(Apologies to OP for this side-track, but I hope it is generally in line with the original question and of interest to some followers).
Thank you very much for your informative reply. Very interesting to my, since I have a large stash of 6P15P-ev tubes I acquired well before Russia invaded Ukraine. Nice project you have, and I have to admit enjy for your well-equipped electronics lab.
Thanks also for sharing your design’s schematic. One question: You mentioned keeping G3 to about 28V above the cathode to limit the screen current. My questions is about the schematic that shows a G3 supply voltage of 33.8 V and cathode sitting at 6.94 V leaving G3 at 26.86V above the cathode. It is close to 28 V but I wanted to make sure I understand correctly. Second question is G1 shown as -2.78 V potential and I don’t see a grid resistor to ground shown. I expected to see G1 at circuit ground potential. Am I missing something?
Are you familiar with the so-called Hazen Mod that connects G3 to Cathode via a capacitor (0.1 uF) for power tubes with G3 and Cathode brought out to separate pins, like 6P15P and EL34. See: https://www.decware.com/newsite/paper146.html
Decware’s Steve Deckert in the article listed the G3 voltage sitting at 26.8 V (by coincidence), but that is when the EL34 cathode is at 27 V. I don’t understand why the Hazen Mod works, but it seems to have a similar effect than what you did with G3.
Again, thank you for sharing the details of your project.
(Apologies to OP for this side-track, but I hope it is generally in line with the original question and of interest to some followers).
Thank you very much for your informative reply. Very interesting to my, since I have a large stash of 6P15P-ev tubes I acquired well before Russia invaded Ukraine. Nice project you have, and I have to admit enjy for your well-equipped electronics lab.
Thanks also for sharing your design’s schematic. One question: You mentioned keeping G3 to about 28V above the cathode to limit the screen current. My questions is about the schematic that shows a G3 supply voltage of 33.8 V and cathode sitting at 6.94 V leaving G3 at 26.86V above the cathode. It is close to 28 V but I wanted to make sure I understand correctly. Second question is G1 shown as -2.78 V potential and I don’t see a grid resistor to ground shown. I expected to see G1 at circuit ground potential. Am I missing something?
Are you familiar with the so-called Hazen Mod that connects G3 to Cathode via a capacitor (0.1 uF) for power tubes with G3 and Cathode brought out to separate pins, like 6P15P and EL34. See: https://www.decware.com/newsite/paper146.html
Decware’s Steve Deckert in the article listed the G3 voltage sitting at 26.8 V (by coincidence), but that is when the EL34 cathode is at 27 V. I don’t understand why the Hazen Mod works, but it seems to have a similar effect than what you did with G3.
Again, thank you for sharing the details of your project.
Good spotting. Yes the voltage on G3 is a little low as the zener came form my junk box and was only "close".
Likely the screen current is a little bit higher than idea. Life goes on.
The 6P15P G1 does not go to ground. It connects to R19 and R41 (301K) through named nets PENGN and PENGP to the fixed bias network.
The design uses hybrid bias, mostly cathode bias and a bit of fixed bias.
This allows the exact bias value to be "dialed" in (RV4) , allows the bias value between tubes to be balanced exactly (RV3) and also reduces the power loss from full cathode bias while still providing some over load and bias stability of cathode bias.
It is a compromise. I am Canadian after all.
I do tend to use a lot of named nets to make connections. A habit from past design work where schematics possibly a 100 pages or more long where making connections with lines on the page becomes just a silly idea.
Take care.
Likely the screen current is a little bit higher than idea. Life goes on.
The 6P15P G1 does not go to ground. It connects to R19 and R41 (301K) through named nets PENGN and PENGP to the fixed bias network.
The design uses hybrid bias, mostly cathode bias and a bit of fixed bias.
This allows the exact bias value to be "dialed" in (RV4) , allows the bias value between tubes to be balanced exactly (RV3) and also reduces the power loss from full cathode bias while still providing some over load and bias stability of cathode bias.
It is a compromise. I am Canadian after all.
I do tend to use a lot of named nets to make connections. A habit from past design work where schematics possibly a 100 pages or more long where making connections with lines on the page becomes just a silly idea.
Take care.
😀 Thanks for the explanation. I overlooked the connection from PENGP to R41 and PENGN to R19.
Re:"I don’t understand why the Hazen Mod works,"
I did not like the "floating" idea. I want a deterministic voltage ratio G3 to cathode so performance is predicable across tube samples and over time.
I found this link on pentode suppression provided me a good understanding of G3s operation.
http://www.r-type.org/articles/art-004c.htm
Varying the G3 to cathode voltage varies it effectiveness and so changes the suppression level in the pentode.
That changes the Ia and Is current ratios at low anode voltages in a pentode.
I wish more power pentodes had G3 pined out separately. It provides some flexibility in setting the suppression levels as suppression levels needed vary based on the screen and anode voltage used in the amplifier design.
I did not like the "floating" idea. I want a deterministic voltage ratio G3 to cathode so performance is predicable across tube samples and over time.
I found this link on pentode suppression provided me a good understanding of G3s operation.
http://www.r-type.org/articles/art-004c.htm
Varying the G3 to cathode voltage varies it effectiveness and so changes the suppression level in the pentode.
That changes the Ia and Is current ratios at low anode voltages in a pentode.
I wish more power pentodes had G3 pined out separately. It provides some flexibility in setting the suppression levels as suppression levels needed vary based on the screen and anode voltage used in the amplifier design.
