5F8-A PT & Rectifier questions

[jly56]

Hi everyone..


I'm building (or will be) a Hoffman 5F6A the will have 4 6L6 power valves...  Basically a 5F8A and I am confused about the PT and rectifier.  The original Fender schematic shows a single GZ34 and the PT specs look close to a standard 5F6A PT.   Then I found Weber's layout and they use their WZ68 SS rectifier (2 GZ34s).  I gave also seen variations that use two GZ34s!   


So what are the advantages/disadvantages of using a 1 or 2 rectifiers? 


The 8A PT shows 300 VAC and the 6A shows 325 VAC inputs.  Any preference.  I thought I saw one Fender cross reference shows the same replacement PT for both... Could I use either the Hammond 290DX sitting here!


Thanks for your help!


Jeff

[HotBluePlates]

... I am confused about the PT and rectifier.  The original Fender schematic shows a single GZ34 and the PT specs look close to a standard 5F6A PT.

Except double the output tubes, so double the current on the high voltage winding, and near-double current on the 6.3vac winding.

... I found Weber's layout and they use their WZ68 SS rectifier (2 GZ34s).  I gave also seen variations that use two GZ34s!   

So what are the advantages/disadvantages of using a 1 or 2 rectifiers?

Today, it's just silly to use 2 rectifier tubes.

In the 50's, Leo had a problem. He needed big output power from a quartet of 6L6's, but solid-state high voltage rectifiers weren't yet available (and the lower voltage selenium rectifiers had significant voltage drop in use, not unlike a rectifier tube). If Leo stuck a single 5U4 tube in his power supply, the current of four 6L6's creates a significant voltage drop when pushing max power (remember, voltage = current * resistance, so much more current = much more voltage drop across the rectifier tube).

Cheat #1 was to use a pair of 5U4's in the 5E8 Twin (a pair of 5U4's to feed 2x 6L6's). Power supply voltage would sag most when the amp is trying to make maximum output power, which then drops screen voltage and limits output power. No good for the purpose of this amp. Two rectifier tubes means their individual plate resistances are in parallel, so you get half the voltage drop for the composite rectifier.

Then Leo upped the game to 4x 6L6. Double the current for the output section, so he needed something with less voltage drop than a pair of 5U4's. Or at least something with a non-changing voltage drop in the face of increased current draw. Cheat #2: Enter the 83 mercury vapor rectifier for the 5F8 Twin. Mercury vapor rectifiers have a pretty low voltage drop by rectifier tube standards, but their big attraction is the drop stays largely unchanged from around firing voltage up to the current limit of the tube. Good for the task at hand.

But mercury rectifier can be twitchy and unreliable if mishandled. Especially upside down, when the data sheet generally says to mount them upright, and there is liquid mercury in the tube to be vaporized at turn on. So Leo needed to ditch the 83. Cheat #3: Get a GZ34 already! I'm not old enough to know the details of it, but while American companies listed a 5AR4 in their manuals, I've seen an aweful lot of British GZ34's in old Hi-fi gear and tube test equipment, carrying the American hi-fi brand's logo. I assume Mullard and Philips had good marketing for some of their products over here, and it stands to reason Leo used the GZ34 in the 5F8-A Twin.

The GZ34 doesn't have the high current rating of the 5U4, but it has the lowest voltage drop of the common 5v rectifier tubes. So that's why you see it in the bigger amps.

Cheat #4: As soon as solid-state diodes were available, Leo switched to them in the biggest amps. It's silly to pay for 2x GZ34's and 2x sockets, wiring and additional 5v current capacity when you can set up a string of cheap(er) diodes.

[HotBluePlates]

Just note that you don't need 6x diodes total, as you see in the blackface amps. Leo needed them at the time, because the diodes he could get didn't have to 1kV rating of the 1N4007. If you're concerned a single 1N4007 per side isn't enough, use a pair in series on each side of a rectifier board. Then you'll have a 2kV rating per leg @ 1A. You can use the massive overkill 1N5408's (rated @ 3A) if it makes you feel better, but you won't hear any difference.

My opinion is the only reason Weber offers anything that has 2 rectifiers or approximates 2 rectifiers is because someone asked for it. I imagine because they didn't really understand why it was done that way to begin with, and the safe choice for "Tone!" is to copy as closely as possible.

Could I use either the Hammond 290DX sitting here!

No, you're gonna be pulling 4.5A from a 4A 6.3v winding.

And then there's the high voltage winding...Assume you're shooting for 80w of output, and the OT is 2kΩ plate-to-plate. You can estimate the plate voltage swing on 1/2 the primary as √(80w*1kΩ) = ~283v RMS. To get 80w out, you need 80w/283v = ~283mA RMS of current (Check: 283v * 0.283A = 80.09w).

But can your power section swing this? 283v RMS is 283v * 1.414 = 400v peak plus you need another 40-50v across the 6L6's. So 283mA is plenty if you have a B+ around 450v at full-tilt boogie.But even using solid-state diodes, a 300-0-300v PT can only give 300v * 1.414 = 424.2 - ~2v = 422vdc.

Since your B+ is lower, if you want to hit the full 80w, you'll need a bit more current, so 300mA+ is a safe bet for your PT high voltage rating. 422vdc is still a bit more than the original 5F8-A spec, but only because you're not dropping 22-27v across a GZ34.The Hammond 272JX is probably a good match if you stay in that line, although the 8A 6.3v winding is massive overkill. My guess is the radical under-loading of that winding may allow you to pull a bit more than the rated 287mA from the high voltage winding.

[PRR]

> American companies listed a 5AR4 in their manuals, I've seen an aweful lot of British GZ34's

I think US makers felt the 5U4 was "good enough". And all the US rectifiers are 1933 designs (basically the Type '80).

Europe developed better rectifiers such as GZ34. I think 5AR4 is just the JEDEC issued US number for the GZ34 ("our" rectifiers start with "5").

Hi-Fi makers were pleased to use the GZ designation because it looks sexier than 5AR4, and the funny-number makes it likely the owner would get a replacement from the Hi-Fi company instead of the Rexall.

Weber likes to sell stuff, so they are pleased to sell you two diodes and a resistor in a neat little can. And it may be much more reliable than old/abused bottles or the mediocre Asian tubes.

> 4 6L6 power valves...

Then use the "100 Watt" PT and OT sold for Fender Replacement. Our host sells them, good price, fast shipping:
http://hoffmanamps.com/MyStore/catalog/parts1.htm
Output trans Twin reverb
Power trans Twin reverb

[kagliostro]

If I can say my 2c

Using those WU4GB (this is the name you can see on the schematic) that are build with SS diodes they are actually putting solid state diodes in parallel

and usually isn't a good thing to do that

in this WU4GB they use big SS diodes the 1N5403 so one WU4GB will be enough for the amp PS

Seems that Weber put the two octal sockets only in order to permit to those who want to have the 2 Vacuum Rectifier version as to be more near to the original version of the amp, in fact on the schematic you can read that who want to use a pair of 5U4GB tube must add a Weber 5V3A filament transformer as to supply the filament of one of the two rectifiers

K

[jly56]

WOW.... great information everybody!

HotBluePlates - thanks for the history lesson.  I really find it interesting to see what our amp forefathers (Leo in particular) had to do to compensate for the equipment he had available at the time.  So it's a single GZ34 for the project!  I was hoping to use the 290Dx, but now that I look closer at the specs, I can see (with your enlightenment) that it would be below the specs I need.

PRR - I'll look into our host's PT & OT... was hoping to use something on my shelf, but as HotBluePlates showed, it would be inadequate for my needs.  The only downside is I wanted a multi-secondary for the OT.   And yes, marketing is a funny thing... you can charge more for sexy names!

kagliostro - I've always thought it heresy to use SS rectifiers when there is a tube available.... but that's just me. 

Thanks everybody!

Jeff