Yoda wrote: ↑Sun Jan 09, 2022 7:20 pmI’ve built several amps with Rob Robinette’s EF80 power section...
I'm curious about the MV and the lack of a cathode bypass on the power stage. On his site sometimes those features are there, sometimes not. What was your experience?
I used bypassed cathodes on all my amps. You would have to ask Rob R. why some of his designs don’t have bypassed EF80 cathodes.
So with 320Ω Rk, that means 4.2/320 = 13.13 mA, where about 0.75 x 13.13 = 9.84 mA is plate current and 13.1 - 9.84 = 3.26 mA is screen
(from EF80 data sheet, screen current is 25% of cathode current at Va = Vk = 250V)
Plate dissipation is then 9.84 x (253-4.2) = 2.45W vs. 2.5W max, and screen dissipation is 3.26 x (251-4.2) = 0.81W vs. 0.7W max
Load line output power is ((253 - 4.2) - 75.4)) x 0.031/2 = 2.69W
Screen dissipation may be a bit high. You can check your screen current by measuring voltage across the screen resistors, and maybe increase those a bit or add a shared screen dropper to get the screen dissipation down to 0.7W or less.
martin manning wrote: ↑Mon Jan 10, 2022 12:45 pm
So with 320Ω Rk, that means 4.2/320 = 13.13 mA, where about 0.75 x 13.13 = 9.84 mA is plate current and 13.1 - 9.84 = 3.26 mA is screen
(from EF80 data sheet, screen current is 25% of cathode current at Va = Vk = 250V)
Plate dissipation is then 9.84 x (253-4.2) = 2.45W vs. 2.5W max, and screen dissipation is 3.26 x (251-4.2) = 0.81W vs. 0.7W max
Load line output power is ((253 - 4.2) - 75.4)) x 0.031/2 = 2.69W
Screen dissipation may be a bit high. You can check your screen current by measuring voltage across the screen resistors, and maybe increase those a bit or add a shared screen dropper to get the screen dissipation down to 0.7W or less.
Ah, great. So much of it is in de datasheet of these tubes.
If you look at it like that it is indeed a little rich.
I just measured the voltage over the screen resistor. It's at 1,39V / 1kOhm = 1,39mA
So can I then say?; 1,39mA x (251-4.2) = 0.34W which is wel below 0,7W
ok, that's good, but seems far from the expectation. The plate current must then be 13.13 - 1.39 = 11.71, and plate dissipation is 11.71 x (253 - 4.2) = 2.9W
martin manning wrote: ↑Mon Jan 10, 2022 1:17 pm
ok, that's good, but seems far from the expectation. The plate current must then be 13.13 - 1.39 = 11.71, and plate dissipation is 11.71 x (253 - 4.2) = 2.9W
I think 13mA going through the Cathode resistor is the current for both tubes.
So per tube it would then become:
13,13/2 - 1,39 = 5,175mA
plate dissipation would then be 5,175 x (253-4,2) = 1,28 Watt
Ah, yes! So that means my original assessment was 2x too high for both!
So with 320Ω Rk, that means 4.2/320 = 13.13 mA, where about 0.75 x 13.13 / 2 = 4.92 mA is plate current and (13.1 - 9.84) / 2 = 1.63 mA is screen per tube
(from EF80 data sheet, screen current is 25% of cathode current at Va = Vk = 250V)
Plate dissipation is then 4.92 x (253-4.2) = 1.22W vs. 2.5W max, and screen dissipation is 3.26 x (251-4.2) / 2 = 0.40W vs. 0.7W max
Last edited by martin manning on Mon Jan 10, 2022 2:02 pm, edited 1 time in total.
martin manning wrote: ↑Mon Jan 10, 2022 1:53 pm
Ah, yes! So that means my original assessment was 2x too high for both!
Exactly, But thanks for your insight. I really enjoy it!
I know so little still of tube amps. So my take on it is just start copying some excisting ideas an modify them.
So when someone like you starts firing some questions, that's a good place for me to learn
You can decrease the cathode resistor to get more idle dissipation, as you are less than 50% now. That will lower the voltages, too, though. I would experiment with that for sure.
martin manning wrote: ↑Mon Jan 10, 2022 2:11 pm
You can decrease the cathode resistor to get more idle dissipation, as you are less than 50% now. That will lower the voltages, too, though. I would experiment with that for sure.
I will do so. I set off building this with the original idea of feeding these tubes 300V. But in the end it became somewhat less.
To be causious I was planning to put in a 300 Ohm resistor for the setup with 300V. But only one I could get a hold of was the 320 that is in place now.
You can parallel something on the 320Ω to get a lower value. Adding another 320 would result in 160, which should bring the plate dissipation up to 90%, IF the voltages stay at 250. That's really close to the 250V operating point in the Philips data sheet.
martin manning wrote: ↑Mon Jan 10, 2022 3:17 pm
You can parallel something on the 320Ω to get a lower value. Adding another 320 would result in 160, which should bring the plate dissipation up to 90%, IF the voltages stay at 250. That's really close to the 250V operating point in the Philips data sheet.
Oke, if have got exactly the same resistor laying here. But if I was to guess, I 'd say that was to low. But I'll give it a try!
Last edited by kuDo on Mon Jan 10, 2022 7:34 pm, edited 1 time in total.
Another thing you can try is lowering the primary impedance on your 125B output transformer. There is a 17,600 option with an 8Ω load (violet and green secondary leads) which might sound good. This and the output stage idle bias are things that you should use listening tests to make the final selections. In my opinion it's easiest if you can get a really good player to help you out with that.
