Can I use a 5R4 GY in place of a 5AR4?

[mr mojo]

Howdy folks,

I know the ratings on the 5R4 GY are all much higher than the 5AR4, I know my power transformer can handle the extra .2 amps for the pair in my amp.

My question is: Since the 5R4 GY has much higher ratings for current and plate voltage (@ 750 vs. 550 and 160 vs 250 for current) would I expect my power supply voltages to soar since I would be pulling a much lower ratio of max current with the 5R4GY vs. the 5AR4?

I guess it's just plain old curiosity for one. Two, since the amp uses 7591s I can't do any easy output tube swapping without rewiring the sockets and lastly, although this is a pretty shallow reason, asthetically I'd like to see the tube rectifiers sticking up above the pair of 0A3s-it's just bothersome to me to be using a pair of tube rectifiers only to have them tucked behind the 0A3s to where they can't really be seen unless looking at the amp up close.

Silly reasons, I know, but I'm still curious.

Anybody got an answer?

Best,
mr mojo

[tubelab.com]

I tried this in my 845SE amp hoping to gain a few volts, but I lost about 50 volts! I even tried those cool looking military tubes from a B52 bomber, same deal. I am using a two tube voltage doubler off of a 480 volt industrial transformer to make 1100 volts, which is pretty tough on the tubes. I have tried just about any rectifier that will fit in the socket and oddly enough I get the most voltage with Sovtek 5AR4's. This can be verified by looking at the tube voltage drop data in the tube books, but I had to try them any way since all of the new tubes don't have much real data.

There is another reason why you don't want to do this. The 5AR4 takes a long time to warm up. This way your 7591's are already hot by the time the B+ comes up. A 5R4 starts to put out voltage in a few seconds and your 7591's are still sleeping when they get hit with plate voltage. This is really not good for them, and good 7591's are hard to get. If you have (and use!) a standby switch to warm up your amp, then this is not a problem.

If you use the standby switch, and can stand to loose a few watts, then you may find that different rectifier tubes will lead to a different sound character, especially if you turn it up to eleven!

[Eli Duttman]

Tubelab's experience dovetails well with some known information. The spec's looked at DON'T tell the whole story.

The 5AR4 and 5R4 are at opposite ends of the spectrum, in so far as forward drop is concerned. The forward drop in a 5AR4 is unusually low for a vacuum rectifier. OTOH, the forward drop in a 5R4 is unusually large. Perhaps damper diodes will do better in the forward drop dept. than the 5AR4.

Also, look at the max. allowable value when a cap. I/P filter is employed. The 5AR4 is unusually tolerant of capacitance, while the 5R4 is anything but tolerant of capacitance.

[Joel]

5AR4 has a k value of 3.21
5R4 has a k value of 0.456

Voltage drop is (I/k)^(2/3)

So, for example, for a 100mA load the drop across the 5R4 is 36V. For the 5AR4 it's only 9.9V.

Joel

[mr mojo]

Fellas I really appreciate your help on this stuff-this is without a doubt THE BEST PLACE online for info like this.

I guess I can live with those little old 5AR4s "buried" behind the 0A3s. I knew there had to be more to it than I knew-especially since my knowledge is so limited!

Things like the forward voltage drop and the filter cap are issues that, at least in my application, can't be overlooked.

Like I said-a big part was plain old curiosity, and thanks to you folks I've got my answer.

Best,
mr mojo