5U4GB vs. 5R4GA vs. SS rectifier?

[Lectroid]

Hello all,

I'm finishing up my single-channel BFDR clone using Hammond AO-43 iron.  Many of you have helped me with it this year. I'm  ready to start it up and test voltages.  I'd like to get as much plate voltage as possible out of it in case I want to run 6L6s later.  (I may be dreaming, but that's the plan.)

My issue today is that I don't have a 5AR4/GZ34, but I do have both a 5R4GA-JAN and a 5U4GB. Would rather not shell out big buck$ for the 5AR4 if I can avoid it.

On a thread of mine from April 26, 2022, vampwizard said of the AO-43 PT:

"Field manual on the PT says
6.3V - 5A
5V - 3A
315-0-315V - 250mA (1/4A)"

I'm getting just shy of 330-0-330V from my PT.  I've been studying posts here, and reading the spec sheets.    Consulting the rectifier tubes cheat sheet, it looks to me like the 5U4GB would be the best choice if I used a tube rectifier.

Any comments on running the 5U4GB  vs.  the 5R4GA?  OR-- do I just need to bite the artillery shell and buy a 5AR4? 

Schematic attached, just in case.  Used Hoffman's one-channel AB763, with some additions: sluckey's pre-PI master volume from his TDR, and a modified Marshall-like bias supply taken off one side of the PT primary.  Also lowered the P/S dropping resistors for more voltage.

Thanks for your time,

[nandrewjackson]

My 2 cents


Out of the +/- 10 amps I've built, I've only built 1 with tube rectifier and it was the "sweetest" sounding one.


It had 2-6V6 output, "tweed" style output section.


As far as possibly running 6L6, that may require a different -VDC bias than you're getting now.
If you can achieve -VDC for both 6V6 and 6L6, you'll still hear the difference between the two types irregardless of the plate voltage.

[tubeswell]

5R4 and 5U4 are directly heated and so will come up to full power faster than a 5AR4. This won’t be a problem if all your coupling caps are rated for the voltage drop you’ll get on startup.

[Lectroid]

5R4 and 5U4 are directly heated and so will come up to full power faster than a 5AR4. This won’t be a problem if all your coupling caps are rated for the voltage drop you’ll get on startup.

Right.  I didn't build in a standby switch b/c I was counting on the slow warm-up of the 5AR4.  But that could  also limit the types of rectifier tubes I could use.  Dang, didn't think of that.

Luckily, my coupling caps are all 630V.  Could I get away with one of the directly heated tubes?  What do you think?

And if I put in a SS rectifier plug, that would also put full power into the circuit almost instantly, right?

Leaving out the standby is looking more and more like a bad idea.

Thanks,

Rich

[jordan86]

I’d probably drop the “big bucks” $22 for a JJ GZ34 before hassling with a standby switch. Will provide the highest plate voltage of all rectifier tubes with a slow warmup. It solves many of your “problems”.

[pdf64]

5R4 and 5U4 are directly heated and so will come up to full power faster than a 5AR4. This won’t be a problem if all your coupling caps are rated for the voltage drop you’ll get on startup.

Right.  I didn't build in a standby switch b/c I was counting on the slow warm-up of the 5AR4.  But that could  also limit the types of rectifier tubes I could use.  …

I don’t see how?
Just need caps to be sufficiently rated, which they should be anyway, eg in order to accommodate the 6.3V heater supply failing.
And to use5U4, the PT rectifier heater 5V winding would need to be good for 3A.

[Lectroid]

I’d probably drop the “big bucks” $22 for a JJ GZ34 before hassling with a standby switch. Will provide the highest plate voltage of all rectifier tubes with a slow warmup. It solves many of your “problems”.

@Jordan86,
thanks for the splash of common sense.  I hear so much trash talk about various new tubes on the forums, it's good to hear actual hands-on experience.  I'll order the 5AR4.

I don’t see how?
Just need caps to be sufficiently rated, which they should be anyway, eg in order to accommodate the 6.3V heater supply failing.
And to use5U4, the PT rectifier heater 5V winding would need to be good for 3A.

@pdf,
Understood, and I do have 3A. available, according to vampwizard. 

My other question:  Assuming my caps are all rated at 630V, would a solid-state plug-in be an acceptable substitution?  Will the SS rectifier allow a current spike that might damage the reservoir cap, or other filter caps?  I'd like to try it to see what extra B+ voltage I can get, but not at the expense of blowing things downstream.

Thanks to all for the helping hands, you're all great.

[dude]

I see no issues with using SS rectification which would increase the B+ voltage. SS will give you a tighter tone, tighter bass and less sag. Some say a tube rectifier sounds better but you can use a sag resister from the SS diodes to simulate a tube's sag. Say 150 ohm R, at least 5 watts,  better 10 watts if you use 6L6's. BTW, the Hammond AO-43 PT can handle a pair of 6L6's without overheating. I have even used EL34's for hours with no build up of heat off that Hammond PT but don't recommend using EL-34's with that PT, 6L6's yes.   

[Lectroid]

@dude,

Thanks for pitching in--always good to hear from someone who's been here before.  When you ran the 6L6s with this PT, do you recall what plate voltage was going to the power tubes?  And did you have to go to the EE rectifier to do it?

[shooter]

my coupling caps are all 630V.

pick'n tube or SS effects the PS filter caps, Ecaps.
Had one build that started at 520vdc, 15 seconds later it finally started takin the load, settled ~ 440vdc
so the moral of the story, spec the PS caps about 50-100vdc above your load vdc. stacking, in series, counts

[Lectroid]

@shooter,

I did assume he meant coupling caps, and shouldn't have.  Thanks for the correction.

Today I formed the caps and took voltage readings and all looks good so far.  Also breadboarded a -35V Marshall-style bias supply circuit and got it working--my first fixed bias supply.  I have a 5AR4 on order, when it arrives I'll do a final startup sequence and see what I've got.

[dude]

Can’t remember exactly what the voltages and bias was, but l think with SS l had about 385v on the plates with the 6L6’s biased at 48mA. Sounded good.

[Lectroid]

Great.  Thanks very much!