View Full Version : 5U4G or 5U4GB??

02-29-2004, 06:57 PM
I'm looking to try a different rectifier tube in place of the stock 5AR4 in my Norh SE-9. Is there any difference between the 5U4G and 5U4GB or could I use either one? I love the fat bottle look of some of the 5U4G's and would really like to try one.


02-29-2004, 07:30 PM
They *can* be used in place of a 5AR4, BUT they will limit the output power. Case in point: A '59 Fender Bassman:

5U4/5U4G = 35-40 wrms output (used for "sag")

5AR4 = 45-50 wrms

SS Diode "Cheater Plug" = 50 wrms

02-29-2004, 08:15 PM

I have a spare 5U4GB I can throw in with your other tubes if ya want to try it.

02-29-2004, 08:23 PM
5AR4 is a soft start and was the prefered tube diode by Dynaco, Fisher, Heathkit to name a few. I don't know why you would want to try a 5U4 but they are cheap and it won't hurt anything. Constant use will shorten the lives of other tubes and possibly some other componants.
There are a very large range of sizes in 5U4GBs with the shorter being not too much larger than the larger 5AR4s I have used the 5U4Gs just because I like the ST shape but in vintage gear haven't noticed any difference in sound from rolling 5U4s. With the tubes, amp, cables and MikE's ears I believe there are some.
Since your amp is a SE EL-34 which according to my GE book will use a plate voltage of 250V, unless there are some major step down resisters to lower the voltage after the rectifier a 5V4 should be OK. It is also a soft start like the 5AR4, is in a ST bottle like a 5U4G and are not expensive. But you do need to know the voltage and amperage that hits the plates of the rectifier tube before trying it.

02-29-2004, 08:24 PM
The 5u4g is the earlier "ST" bottled-shaped glass and the 5u4gb is the later straight-bottled shaped version. They are very similiar electronically. The Tungsol and the Sylvania 5931 [military-spec 5u4gb] are two of the more respected 5u4gb types. I have both. You're right, the earlier version is prefered. Though, I have a bunch of the "ST" 5u4g types I ended up with an early Raytheon 5u4gb... that was untill Tim sent me that Mullard gz32, which is a 5v4g. The Tungsol 5u4g VT-244 are highly sought, as are the Ken-Rad, Raytheon and Sylvania VT-244's.

Disclaimer: I can not recommend that you follow my lead and sub the 5u4 with a 5v4. Or for that matter, a 5u4 for the 5ar4. Depending on specs of your output transformers it may only be able to handle a MAX 2A draw, and the 5u4, 5v4 family have a MAX 3A draw. This could cause your PT to die a premature death. In my case, the Electra-Print transformers in my amp are overbuilt and rated at 3A. MikE

02-29-2004, 08:42 PM
I got the okay for the 5U4 swap for the 5AR4 from the technical guru at Norh. He said it would work just fine and in fact could perhaps sound better. It's the look I'm really after. I'm snagging a set of fat bottle 6CA7's to replace the straight bottle EL34's that come with the amp. Is there a ST shaped 5AR4?

Grump if you haven't shipped out those tubes yet then please do send me a 5U4 as I'd love to hear one.


03-01-2004, 10:29 AM
No ST shaped 5AR4 that I have ever seen or heard about :( Keep in mind that tube life along with electrolytics will be deminished compared to the 5AR4 because of the slow start feature as mentioned above. The 5AR4 is much easier on components.


03-01-2004, 11:17 AM
I don't know Mike, seems you might want to at least consider what the guys who know there stuff here are saying. Is it really worth giving up the slow start-up character of certain rectifiers to use something that "looks good"? With the price of good NOS tubes these days, I'd want to use a slow start rectifier (CV378 in the case of my 300B amp) if your amp/pre-amp doesn't have the ability to pre-heat the filaments.

I've got a few NOS CBS 5U4GB's I'm going to try in my 45 amp but I'm designing in the ability to delay B+ till the tubes are warm; after laying out the bucks for RCA UX245's and WE 417A's, I think it makes sense.


Chad Hauris
03-01-2004, 11:34 AM
These are just my comments on the subject:

One problem with subsituting 5U4 for 5AR4 is the filament current draw. 5U4 draws 3 amps for fil. and 5AR4 is 1.9 amps. If doing this subsitution, feel the power transformer to make sure it's not getting hotter than it should.

The drop in power output remarked upon by another poster is due to higher internal resistance in the 5U4. This causes the dc output voltage to be lower under load. I have plenty of equipment that specs the 5U4 or 5U4 GB and have not noticed any degradation of components due to the nearly instant B+ of the 5U4. You just have to make sure the filter caps are rated for the surge voltage available before the amplifier tubes start to conduct. Also when using 5U4, the first filter cap after the filament/cathode should be 40 mfd max to prevent damage from excessive initial charging current.

Equipment designed to use the 5U4 takes all this into consideration...for a 5AR4 it might not...that is the input filter capacitor might be larger in mfd and not rated at high enough of a surge voltage for the operating conditions of the 5U4.

5V4 is rated for less current draw than 5AR4. The analogy is, that 5Y3 is to 5V4, as 5U4/GB is to 5AR4. (Direct filament vs. cathode sleeve).

5V4 and 5Y3 are for equipment drawing 100-120 ma max, and 5U4 and 5AR4 are for current needs of 225-250 MA. The 5U4G is rated for less current draw vs. the 5U4GB.

03-01-2004, 11:45 AM
I would check back with the Norh guy and ask about 5V4s. Since there isn't near the voltage used for SE 6CA7s that is with them in PP a 5V4 might work fine, you will have the ST shape, soft start and good sound.
The typical gear that I have pulled 5U4s out of have been built like tanks with PIO caps only under the chassis and electroltics only in the PS. Hammond organs and such. Or it has been the opposite end of the spectrum where the parts were all cheap and the 5U4 was in there because it was cheaper to buy.

03-01-2004, 02:34 PM
Appreciate the additional info. My particular amp was designed for BOTH 2a3 or 45 use. That with 2a3 it uses 5ar4 and with 45 a 5r4 or 5y3. I know some people think I'm flirting with trouble but in the last two years haven't run into any problems with 5u4 or 5v4.


03-01-2004, 02:49 PM
Real coolness factor in Mercury Vapor rectifier tubes. Probably large current draw for them, but they have the coolest blue/purple glow I have ever seen in a tube.

03-01-2004, 03:30 PM
Okay I'm heeding your advice. I emailed Michael Barnes at Norh and inquired about using a 5V4s and about any problems with using a 5U4GB.

Thanks guys


Jack G
03-01-2004, 04:13 PM
If your amp came with a 5AR4, I would just bite the bullet, and buy a decent Mullard(early version). It will cost, but it will last many years, and quite possibly sound better than others.

03-01-2004, 10:26 PM
I believe the mostly highly regarded 5ar4's are the Mullard "fat base" and Amperex "metal base". I have both. I'll flirt between the two - WHEN USING 2A3 - but honestly didn't experience the jump in performance as I did with most any 5u4 or 5v4. Because of this I was especially surprized by the results from the Mullard gz32 5v4. My new reference. The other plus is that the Mullard 5v4 are less sought after [cheaper] than the 5ar4.


Jack G
03-02-2004, 06:38 AM
Not all gear deals with swapping rectifier types very well. It can be hard on the gear and tubes. One must remember, that 5U4s, 5V4s, and 5AR4s are not direct replacements.
Your Moth is an exception. None of my gear is that flexable. Changing 5AR4s from a crap sino or russian tube to Mullard or Amperex, is quite dramatic.


John Day
03-09-2004, 11:18 PM
I'd pop it in without fear and see how it sounds.
It is true that the 5U4 draws 3 Amps compared to the 2 Amps of the 5AR4, but its higher internal resistance is easier on the power transformer, so the total flux dissipation in the power tranny won't increase. However, you will lower your voltage an your power by doing this. That will be GOOD for tube life, offsetting the mildly faster warmup than the 5AR4. It will lower your power output, probably not the main reason you have a tube-amp, but... The input cap in the power supply should not exceed 40 uF for either a 5AR4 or a 5U4, so that should be no problem.
One of the scrimpiest power transformers ever, on the Dyna ST-70 was made for a 5AR4, and I got one 34 yrs. old with the original tubes all still working, sporting a 5U4GB that the original kit builder had substituted at the outset. Good arguement to dive-in, I'd say...