SFVR calls for 5UR4 but has 5AR4

[spiffpeters]

I bought the this 79 SFVR used and it already had the BF mod. It was loaded with an 5AR4 instead of the 5UR4.

Does the BF mod require the 5AR4? If not, does it hurt anything to run the 5AR4 rather than the 5UR4?

[JoeM]

Have you measured the B+? My SFSR (partially BF converted), calls for a 5U4GB and that's what I use. I can get a little more power with the 5AR4, but I believe B+ is actually higher than the typical BF B+ range. With the 5U4GB it runs around 465VDC.

How's it sound?

[greekie]

I'm using a 5U4GB with my SF Super Reverb that I've blackfaced, and I get around 410-415V B+, which is nice and low. It gets around 35-40W full on into a dummy load, and sounds amazing.

By the way, an interesting thing I've seen in several BF amps, that most Black Facing-guides don't mention: In a BF amp, the wires from the input jacks and the output of the volume pot are twisted together! That makes for a nifty crosstalk, that rolls of some of the highs when you turn it up loud. I've been doing that to all my SF amps, and it's definitely something to worth trying!

[JoeM]

geekie,

Do you know what year your SFSR is? My B+ is sure a lot higher than yours. My BF Princeton B+ is in the range of your Super Reverb. Possibly a different PT?

[greekie]

Hi Joe,

It's from 1974, but the power transformer was changed. I have around 228-234V in the wall here in Denmark, and with the 240V tap and the 5U4G (Russian tube branded by Mesa), I get the 405-415V B+. I think I had the Sovtek 5AR4 in before, and that gave me around 440V. What is your B+ with the 5U4G?

Kind regards, Jake

[JoeM]

Hi Joe,

What is your B+ with the 5U4G?

Kind regards, Jake

Around 460-465V depending on how hot the 6L6s are biased.

[spiffpeters]

Have you measured the B+? My SFSR (partially BF converted), calls for a 5U4GB and that's what I use. I can get a little more power with the 5AR4, but I believe B+ is actually higher than the typical BF B+ range. With the 5U4GB it runs around 465VDC.

How's it sound?

No, I haven't measured the B+. This amp several hundred hours on it with the 5AR4. I'm just not sure if running the 5AR4 is going to bother the PT or run the voltages too high in the long run.

The amp sounds great.

I'm just trying to find out if the BF mod requires the 5AR4 and if not, what damage may happen if I continue to run the 5AR4.

[JoeM]

...
The amp sounds great.

I'm just trying to find out if the BF mod requires the 5AR4 and if not, what damage may happen if I continue to run the 5AR4.

I wouldn't consider a BF mod to require a 5AR4 unless the stock voltages with the 5U4 were lower than that models BF version. Assuming the filter caps can take whatever voltages the 5AR4 produces, I dont see any problems, especially since you like the way the amp sounds. If it was my amp, I'd try the 5U4 too, (with a rebias) and see which rectifier I prefer.

[greekie]

You can lower it pretty easily with a MOSFET regulator and a zener diode. I've had great luck with RG. Keens circuits from his "MOSFET Follies"-article:

http://www.geofex.com/Article_Folder...osfetfolly.htm

Kind regards, Jake

[spiffpeters]

Thanks for the replies guy's. Picked up a 5U4. I'll play around with that. Measure some voltages and see what I find.