A Little Help With Rectifiers Please

[frankie5fingers]

I'm now up to three 5E3 types. One, a cathode bias A/B circuit with beefier trannies etc., uses a 5U4GB (2X5881). I'd like to try other rectifiers, maybe to tighten up the sag a little, but I'm not sure which would plug and play and what the differences might be (if any). Any help?
Thanks

[smolder]

Mike's probably on the phone talking about tubes for some amp, sooo

https://www.kcanostubes.com/content/rectifier-comparison-and-interchangeability-chart

[frankie5fingers]

According to the info from the charts - the 5U4GB draws 3 amps of filament current, which, according to the chart would make it compatible with the GZ37? I was always under the impression that you could go to a "higher" letter e.g. 5V4 or 5Y in this case, but based on the current draw, that wouldn't be the case.

[GearHeadFred]

The 3 amp draw is noted on that chart as a warning... the others draw less (2 amps).. so if you're running a 5U4 already, your PT can handle the worst case.
You can go to the 5AR4 (aka GZ34)... but realize that the B+ in the amp will increase (by as much as 50 volts).

[TweeDLX]

What GHFred meant to say was that you should at least CHECK YOUR BIAS after a rectifier swap which causes a drastic change in B+. :)
Before swapping rectifiers, be aware of their ability to handle capacitance. You can trash a 5Y3 by using it with a 47uf or higher first filter cap.

[frankie5fingers]

OK, got that. Now can I draw a correlation with sag in rectifiers like I can with mu in other tubes, i.e. 12ax7 = 100, 5751 = 70? Could we rate the sag at 100 for a 5Y3, 5R4 at 90 etc.?

[TweeDLX]

Probably not numerically, but you can see from Mike's list what sags most & least. You can rate them numerically for voltage drop, which is what most people do. Do a google search for "rectifier voltage drop". There used to be a nice little chart out there with the V drop for each tube. Might have been at Billy Penn's site. Here 'tis! (http://www.300guitars.com/articles/rectifier-tube-voltage-drop-chart/)

[Chrome Dinette]

OK, got that. Now can I draw a correlation with sag in rectifiers like I can with mu in other tubes, i.e. 12ax7 = 100, 5751 = 70? Could we rate the sag at 100 for a 5Y3, 5R4 at 90 etc.?


Mu is an actual characteristic of a tube that is listed on a data sheet and can also be figured out for a given circuit by looking at tube curves and doing some math.

Sag seems to mean different to different people. You may want to look at the data sheets for the various rectifier tubes and see what the voltage drop is for a given current, what the maximum allowable current is, what the maximum size of the first filter cap is, etc.

[frankie5fingers]

Okay so... am I right that the "most" suitable replacement for the 5U4GB would be the 5V4? If that's the case, would I realize much (any) difference - it doesn't look like it.

[Blue Strat]

Okay so... am I right that the "most" suitable replacement for the 5U4GB would be the 5V4? If that's the case, would I realize much (any) difference - it doesn't look like it.

5AR4 would give the most voltage and clean headroom. It will also increase the bias current the most which might not be safe unless you rebias. 5V4 may help a bit, but it can't SUPPLY as much current as a 5AR4 (and not likely as much as a 5U4 either). That aspect isn't covered in my chart but it readily available when doing a web search on "5V4 data", etc.

[TweeDLX]

I find the 5V4 to be the perfect rectifier for many of my amps. It's a matter of personal taste. Not as stiff as a 5AR4, and the 2A draw doesn't tax my PT. I've gotten several different brands from Mike, and all are to my liking. I haven't noticed a big difference in voltage between the 5AR4 & the 5V4. Some, but not a lot. Try one!

[smolder]

I was thinking the 5u4 and the 5v4 are fundamentally the same, but with a 2 amp draw instead of 3... so there would be little if any difference in tone?

[Blue Strat]

It depends on what type of power tubes are being used. With 6V6s, you aren't drawing enough current (plate current) to hit the sag point of a 5V4 but with 5881s or 6L6s you can get into sag territory more easily.

Bottom line, no one can know without trying both. That's reality.

[killer blues]

OK, got that. Now can I draw a correlation with sag in rectifiers like I can with mu in other tubes, i.e. 12ax7 = 100, 5751 = 70? Could we rate the sag at 100 for a 5Y3, 5R4 at 90 etc.?

no. they are apples and oranges. sag has to do with the flow of current, while mu is how high can the tube amplify the signal coming in. If you get sag before the signal is amplified to its capability, then you have either a design problem or component problem. sag usually occurs when you are out of the range of the tubes ability to amplify a signal without clipping. At this point all you are doing is drawing excessive current and dragging down the supply voltage. That is when sag will occur.

[frankie5fingers]

...and there you have it. I'll try the 5V4 but won't expect much difference; that may be all I was looking for anyhow, just a little change.
Thanks All, Frank