Hoffman Plexi 50 power supply

[Sergiouliana]

Hello friends,my name is Sergio I'm from Brazil and is my first post here.I need help from anyone because I have a doubt.I'm buiding a Hoffman Plexi 50, here I'm posting the diagram of power supply.Can someone check if the values of capacitors and resistors are OK for 360Vca transformer output?..thanks in advance..regards for all.

[pdf64]

C1 and C3 could share the same balancing resistor, likewise C2 and C4.

[Sergiouliana]

Hi friend, thanks for your reply.You said C1 and C3, C2 and C4 share one resistor for two capacitors, could you make a drawing please?..Also, my biggest doubt is about "voltage", output transforme 360 - 0 - 360vca, capacitors are 400Vcc, do you think is OK?

[EL34]

I have this diagram listed under one of the documents for the Plexi 50
https://el34world.com/schematics.htm#Hoffman_Plexi_50

[Sergiouliana]

Yes, I sew this on the library documents, but my biggest doubt is the voltages.I have transformer output 360-0-360Vca, the capacitors are 400V, in serie they will be 800V.Output after diode retifiers 720 x 1,41=1015vcc.
Please let me know if I'm right or no?

[sluckey]

Yes, I sew this on the library documents, but my biggest doubt is the voltages.I have transformer output 360-0-360Vca, the capacitors are 400V, in serie they will be 800V.Output after diode retifiers 720 x 1,41=1015vcc.
Please let me know if I'm right or no?

You are wrong. The voltage out of the diodes is 360 x 1.41=507. Your cap voltage rating is fine. See pic for sharing balance resistors...

[EL34]

Here is the 100 watt power supply diagram

[pdf64]

Such a high HT voltage of 500V may turn out to result in shortened valve life.
The HT voltage for push pull EL34 with a load around 3k5 is supposed to be 400-430V.
As HT voltage is increased above that, unless the load impedance / screen grid voltage is adjusted to compensate, anode and screen grid dissipation at high audio power outputs will increase near exponentially, to well above their limits.

https://tubedata.altanatubes.com.br/sheets/010/e/EL34.pdf

[Sergiouliana]

Yes, I sew this on the library documents, but my biggest doubt is the voltages.I have transformer output 360-0-360Vca, the capacitors are 400V, in serie they will be 800V.Output after diode retifiers 720 x 1,41=1015vcc.
Please let me know if I'm right or no?

You are wrong. The voltage out of the diodes is 360 x 1.41=507. Your cap voltage rating is fine. See pic for sharing balance resistors...

thanks a lot friend

[Sergiouliana]

Such a high HT voltage of 500V may turn out to result in shortened valve life.
The HT voltage for push pull EL34 with a load around 3k5 is supposed to be 400-430V.
As HT voltage is increased above that, unless the load impedance / screen grid voltage is adjusted to compensate, anode and screen grid dissipation at high audio power outputs will increase near exponentially, to well above their limits.

https://tubedata.altanatubes.com.br/sheets/010/e/EL34.pdf

I will check the voltages,but the point is: I'm building the amplifier following the original project I hope it works.

[pdf64]

… I'm building the amplifier following the original project I hope it works.

Transformer voltages such as your 360V can be specified at no load 0mA, full load eg 250mA, or some point in between.
A transformer’s secondary voltage will decrease as the current drawn from the winding increases.
The point being that 360V might be fine if it’s the no load voltage, but may result in valve stress if it’s the full load voltage.

[Sergiouliana]

Hi friends, a fells days ago you helped me to check the project Plexi 50W and now I'm finalizing the circuit,here are the voltages I have on the circuit (without the valves) and I will be very grateful if you help me, the output transforme is 360-0-360Vac, please see attached file.Thanks in advance, regards.

[sluckey]

How may we help you? Be specific.

[Sergiouliana]

Hi sluckey, I need to know if these voltages are Ok(higher ou no) for 12ax7 and EL34 valves? I'm afraid it might damage the valves.

[mresistor]

Those voltges will drop once the tubes are installed and the circuit is under operational load.


PDF64  the 420 PV rating for EL34s in PP is where they are operated in most amplifiers but EL34's can take a lot more PV without being harmed.

[Sergiouliana]

mresistor, thanks, so I can go ahead with the project.One more doubt: for a Dual input(HI and LO) I have this drawing, what do you think?

[mresistor]

should look like this disregard the red color markups.

[Sergiouliana]

mresistor, very nice my friend, thanks a lot!

[sluckey]

for a Dual input(HI and LO) I have this drawing, what do you think?

It's perfect.

[Sergiouliana]

My thanks for all!..best regards

[pdf64]

Those voltges will drop once the tubes are installed and the circuit is under operational load.

Indeed, but we lack the detail to know how much the drop might be.

PDF64  the 420 PV rating for EL34s in PP is where they are operated in most amplifiers but EL34's can take a lot more PV without being harmed.

Yes, theoretically 800V, though a 500V limit for the screen grids.
But whether the EL34 might be overly stressed by whatever electrode voltages will be governed by the load and other circuit factors.
And EL34 in guitar amps tend to be used with a 3k5ish load, ie only 875ohms when the signal level pushes the output stage into class B operation.
So very high peak anode current unless the HT voltage is sensible.