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Swapping rectifier tubes; 5AR4 vs 5R4 vs 5V4 vs 5Y3

mherrcat

Member
Messages
22
This has probably been covered here before, but...

My Allen Accomplice Jr. currently has a 5AR4 rectifier and RCA 6V6GTA power tubes in it. I want to see how different rectifiers affect plate voltage, cathode current and sound, but don’t want to do anything to damage the amp. (Duh!)

I found this site with a lot of specs on various rectifier tubes:

http://www.fourwater.com/files/fullrect.txt

Admittedly I do not know what a lot of these numbers mean, except that using a rectifier that draws a higher filament current could cause transformer damage, so I was looking at tubes with ~2-amp current.

Here are the specs of the tubes I was looking at:



Here is what the current setup looks like. The first plate voltage is with maximum negative bias voltage applied. The rest of the numbers are after the bias voltage was set and is pretty self explanatory.



So here’s the question: Considering the specs on the 5R4 and 5V4 and 5Y3 rectifiers, how will the numbers I am seeing in my amp change using each of them and how might it affect the sound?
 
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wyatt

Member
Messages
4,169
Of those, the 5Y3GT is ill-suited for the application for two reasons. 1) The Accomplice uses a 40uF first node filter cap, the 5Y3GT datasheet recommends the tube be followed by no more than 20uF. 2.) It has supplies a maximum current of 125mA (MaxPmA), which is borderline when cranking the Accomplice, and probably not enough if you switch to 6L6GC's. The tube will burn out prematurely if the amp demands more than 125mA on a regular basis.

The Vdrop is the voltage drop across the rectifier, those numbers (-17, -25, -60, -67) are over-simplified, the actual Vdrop will depend on the actual incoming AC voltage becing rectified. The 5AR4/GZ34 has the least Vdrop, then the 5V4, then the 5R4 and finally the 5Y3. Higher voltages mean more headroom and tighter feel. Because the plate voltage changes, you should check bias after swapping rectifiers.

A side-effect of the voltage drop is the sag it adds. The more Vdrop, the less efficient the rectifier is, meaning it takes longer to supply the current. This adds Sag. When you pick a note or strike chord, the amp demands more current and the less efficient rectifiers can't supply it right away, so you're note/chord sort of drops in volume before ramping back up again. The more the amp is cranked the prominent the sag.

Ultimately, the differences will be subtle.
 
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mherrcat

Member
Messages
22
So, the 5R4 would have more "sag" but still be OK in terms of the current it can supply? David Allen did say the 5V4 would be OK in that amp, but I didn't ask him about the others at the time.
 

Diablo1

Member
Messages
620
So, the 5R4 would have more "sag" but still be OK in terms of the current it can supply? David Allen did say the 5V4 would be OK in that amp, but I didn't ask him about the others at the time.
Yes, the 5R4 would be OK to use, as it can supply all the current and more than your amp could use. The amount of sag is a function of the voltage drop at the current rating. The 5Y3 would have the most sag, because it's 60 volt drop occurs at .125 amps. The 5R4 drops 67 volts at .250 amps. So, if we assume the voltage drop is roughly linear with current, the 5R4 would drop 33.5 volts at .125 amps. When you measure the B+ voltage in your amp, you'll be doing so at idle, when the amp draws something like .05-.075 amps. Expect to measure less voltage drop because you're drawing less current at idle. Play the amp to judge the sag effect with each rectifier.
 

mherrcat

Member
Messages
22
Thanks! I ordered a couple of different brands of 5R4 and 5V4 to try. Also looking at different 6V6 tubes as well. If anyone can point me to a comprehensive comparison that would be great!
 

zenas

Member
Messages
8,872
Lots of amps run 5y3s with 40uf filters. 60uf like Ampeg did right before they went to diodes is a bit much.
Actually as I recall the 5r4 is only supposed to be good for like 10uf but have a history of working fine way over that.
Go with old stock on everything except maybe the GZ34 because those ain't cheap.
 

pdf64

Member
Messages
8,152
That's a really good site for anything about vacuum tubes
That page contains some erroneous info, eg that 1kV is an adequate rating for silicon diodes when used as replacements for tube rectifiers.
Go with old stock on everything
I don't have much experience with current production tube rectifiers, but suspect that bad circuit design (eg hot switching standbys) is a primary reason for their bad rep. Sort stuff like that out and I suspect they will generally give good service life.
 

zenas

Member
Messages
8,872
"I don't have much experience with current production tube rectifiers, but suspect that bad circuit design (eg hot switching standbys) is a primary reason for their bad rep. Sort stuff like that out and I suspect they will generally give good service life."

Could be the case. Only new production ones I use are a few GZ34s, so far so good. Otherwise 5y3s, 5u4s and such are so cheap NOS, or used I just don't bother with new ones. Only had one new 5y3 come in so I checked the voltage and compared to the old USA stuff that tube was higher. Don't recall how much or if it was a JJ, Russian or Chinese tube now.
There was a Russian 5u4 in my '73 Super Reverb when I got it 20 years ago. Had one pulled out of a free Baldwin organ, it's been in there ever since. Maybe the Russian one would've stood up too, I don't know.
 

teleman1

Silver Supporting Member
Messages
15,382
I have a few 5v4 NOS rca's. One thing for sure, they are the best looking of the rectifiers being a bottle style.
 






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