PS tube rectifer question

[johno9]

I have completed a 5E3 build and was going to follow up with a CJ11. The schematic for the CJ11shows a center tapped 5V winding for the GZ34 rectifier, with the HV coming from the center tap. On the 5E3 there is no center tap on the 5V winding, and HV is taken from Pin 8 of the 5Y3GT rectifier.
Back in 2011 there was a discussion here of the 5Y3 vs 5AR4, detailing the construction,voltage drop, and current differences.
Is the center tapped 5V used to have less AC filtering (2.5V vs 5V) on the HV? Or lower HV?
My PS transformer does not have the center tapped 5V. Would it be safer to substituted the 5Y3 and take HV from Pin 8?

CeeJayEleven.jpg

[nworbetan]

That seems odd to me, running the entire current of the amplifier through the power transformer twice. I'm asking purely out of ignorance, so please forgive me if I'm wrong, but how confident are you that the schematic is correct?

[johno9]

The schematic has been posted by builders of the amp. You know what "assuming" it is correct can bring...

[nworbetan]

I saw Sluckey's response over on the other forums, and it makes sense. I guess I've just never put enough (any) thought into tube rectifiers. I was assuming the B+ current through the 5vac winding would interfere with the line voltage similar to an autotransformer. As it turns out, interfering with the line voltage similar to an autotransformer works better if it's done in a balanced way with a center tap.

Edit: Going back to your original question, I want to say it doesn't matter whether you use pin 2 or 8 but as soon as I say that someone will probably come along and point out why one is better than the other. With a 20uf cap, you can get away with using either a 5Y3 or a 5AR4 in that amp, depending on whether you want lower or higher B+, respectively.

[Stevem]

No biggie, just wire that standby switch such that it opens and closes the HV center tap ( red / yellow ) which is what Ampeg did a lot, and it's just as quite with no D.C. Pops and the switch last forever since no D.C. Is on it!

Every time over the years I have needed to dive into one of my many Fender amps I rewire there standby switch this way.

Here's a interesting mod to do that eases the load that is placed on a single recto tube in a high powered amp ( above 60 watts rms with a 5ar4 recto) and yet still has some nice sag to it, and if the recto does go bad you just yank it out and run on the diodes just fine after you do the likely needed fuse replacement.

[martin manning]

I saw Sluckey's response over on the other forums, and it makes sense. I guess I've just never put enough (any) thought into tube rectifiers. I was assuming the B+ current through the 5vac winding would interfere with the line voltage similar to an autotransformer. As it turns out, interfering with the line voltage similar to an autotransformer works better if it's done in a balanced way with a center tap.

The issue is where the the superimposed 60 Hz heater voltage is placed in the circuit. For rectifiers with directly heated cathodes, using a CT heater winding will equalize the charging pulses, and for rectifiers with indirectly heated cathodes, using the CT will make adjacent charging pulses unequal.

Edit: Going back to your original question, I want to say it doesn't matter whether you use pin 2 or 8 but as soon as I say that someone will probably come along and point out why one is better than the other. With a 20uf cap, you can get away with using either a 5Y3 or a 5AR4 in that amp, depending on whether you want lower or higher B+, respectively.

The HT should be taken from pin 8. Otherwise the 120 Hz charging pulses will be unequal due to the 60 Hz filament voltage.

[nworbetan]

I had never noticed that 5AR4/GZ34 had an indirectly heated cathode until now, thanks for pointing that out. (I thought the EZ81 was the only indirectly heated cathode rectifier in common use.) Also, heaters and pin numbers are omitted from so many schematics that I didn't even realize that it does matter.

[nworbetan]

Sorry for double-posting. It's either this or another ninja edit and I'm picking this poison this time. (A lot of times a technical issue will kind of simmer on the back burner of my brain for a little bit and a little while later it'll come back to my attention ready for more stirring.)

Back to my original gut feeling that the schematic is maybe not 100% correct. As drawn, the circuit is optimized for a 5Y3. A GZ34's life will be shortened if used as drawn because of the B+ current passing through the heater (which is omitted from the schematic). I learned these things before I was even done drinking my coffee today. That means I can take the rest of the day off, right?

[pdf64]

The hot switching standby is very much not optimised for any tube rectifier

[johno9]

Thanks all for the discussion and information.

[pdf64]

No biggie, just wire that standby switch such that it opens and closes the HV center tap ( red / yellow ) which is what Ampeg did a lot, and it's just as quite with no D.C. Pops and the switch last forever since no D.C. Is on it...

Breaking the 0V return of the HT CT allows the reservoir cap to discharge, hence this is a hot switching type standby and so a bad idea for use with tube rectifiers, such as the amp being asked about in this thread.
Dunno why you think there's no dc? I suggest to try measuring the Iac and Idc with the switch closed, and Vac and Vdc across the switch contacts with it open
Agree that the pulsating nature of the dc will probably be easier on the switch.

...Every time over the years I have needed to dive into one of my many Fender amps I rewire there standby switch this way...

It also seems a bad idea if there's a fixed bias supply taken from a tap on the HT winding eg any AB763, as the bias supply will be subjected to the Vdc measured above, ie possibly somewhat over the ratings of the components used.

...Here's a interesting mod to do that eases the load that is placed on a single recto tube in a high powered amp ( above 60 watts rms with a 5ar4 recto) and yet still has some nice sag to it, and if the recto does go bad you just yank it out and run on the diodes just fine after you do the likely needed fuse replacement.

Have you checked this, ie the relative current division between the tube and solid state rectifiers? Maybe ok for a 5Y3 but I suspect a GZ34 would take the lion's share of the current, and so get overstressed when the loading was much over its 250mA limit. Hence a series protection diode and / or fuse in series with each plate would beneficial. Even better 2 tube rectifiers to share to loading *, or just use solid state rectification only, with a sag resistor if that's the goal.

* eg https://el34world.com/charts/Schematics ... _pa100.pdf