EL41 PP power supply questions

[VMUNIX]

Hello community, I have plenty of these old rimlock socket tubes and I was planning building this little amp.


I'm wondering what's the purpose of the cathodes connection to the S7 filament winding mid tap , is it for hum reduction ?
Also why is the filament winding for EZ40 connected to the +B and why is it using a separate winding S6 (I'm guessing it's the 5V for type 80, although the EZ40 has 6V filament).
What about the power transformer voltage S5 ? the usual 2x 375V 100mA would do ? apparently the plate of EL41 is rated for 250v max. Any advice ?

[v4lve lover]

the 6.3 volt winding connected to the cathode of the rectifier tube suggests it has separate live windings . this deals whit the issue of rectifier cathode breakdown . ive had cases where i was shocked by the AC fillament supply

much safer to use diodes . but that is my biased opinion .

the other winding seems to be the heather winding . EG there is a common cathode resistor wich shares ground whit the chassis and the center of the heater supply much like a hum pot . this does not matter as there is now only little potential on the heater windings

valve

[SY]

Originally Posted by VMUNIX

I'm wondering what's the purpose of the cathodes connection to the S7 filament winding mid tap , is it for hum reduction ?
Also why is the filament winding for EZ40 connected to the +B and why is it using a separate winding S6 (I'm guessing it's the 5V for type 80, although the EZ40 has 6V filament).

Heaters are often biased positive with respect to the cathodes (of the input and phase splitter tubes) to reduce noise coupled from heater to cathode. Often it's done with a voltage divider from the B+, but the designer here saved a few parts by using the output tube cathode bias voltage, a clever idea.

Indirectly heated rectifiers usually have the cathode internally connected to the heater, so the heater is several hundred volts above ground- that's why it's run off a separate winding.

[VMUNIX]

Valve, Sy, thanks you for your answers
what about the 10uH choke, I don't think I'll be using it, can I replace it with a 200ohm resistor? (perhaps using slightly bigger capacitors)

[DF96]

You mean 10H choke? Check the EZ40 data sheet before increasing C9.

EL41 has a reputation for overheating and becoming gassy, giving grid current and thermal runaway. However, if you have lots of them this will not be a problem.

[VMUNIX]

Originally Posted by DF96

You mean 10H choke? Check the EZ40 data sheet before increasing C9.

EL41 has a reputation for overheating and becoming gassy, giving grid current and thermal runaway. However, if you have lots of them this will not be a problem.

I didn't know that, though I knew that the socket particularly had trouble with internal deposits eventually shorting the pins was replaced by the 9 pin socket.
bought these tubes for 50 cents

And yes you are right about C9, I'll make sure to stay below the maximum.

So what do you think all in all, can I go wrong with this for 10W ?

[DF96]

You might find that the input stage has too much gain and noise for modern sources. Try running in triode connection to reduce both.

[VMUNIX]

Originally Posted by DF96

You might find that the input stage has too much gain and noise for modern sources. Try running in triode connection to reduce both.

Good idea, I will try that eventually.
One more thing , per your recommendation I checked the EZ40 sheet and found that it's rated at 350V each diode, and I have a 375 + 375 transformer, do you think it might hurt the tube ?

[DF96]

No, unless you stress it in other ways too. Rectifier limits tend to be a bit fuzzy, so you can break one to some extent provided you are nice to it in other ways. For example, a bit over voltage but less current or smaller caps than specified. Unless you really stress it, we are talking about lifetime not sudden failure.

[VMUNIX]

Originally Posted by DF96

No, unless you stress it in other ways too. Rectifier limits tend to be a bit fuzzy, so you can break one to some extent provided you are nice to it in other ways. For example, a bit over voltage but less current or smaller caps than specified. Unless you really stress it, we are talking about lifetime not sudden failure.

Thanks you for your help, I appreciate it hopefully in my next thread I will post the project completed and my views about it, may help others who want to experiment with tubes other than the usual suspects.