Posted by Jay Rogers on May 11, 1998 at 09:08:24:

In Reply to: Re: Super Reverb post AB763 conversion posted by Rob Livesey on May 11, 1998 at
05:35:36:

: : After converting my '73 S.R. to the AB763 circuit I was wondering if you guys stick with the
5U4 and lower voltage (about 450vdc on the plates) or do you run the higher voltage 5AR4 (about
472vdc on the plates)? Mark

: Hi,

: I have a '65 Super Reverb with a '69 mains transformer, which has the higher voltages. I like it
with the GZ34/5AR4. I have 480V on the plates. I tried a GZ37 which gave me about 450V and I
liked it at home, but in a gigging situation I like the GZ34 be
: Rob.

Hi Mark,
This is what I have on hand quickly that goes to your point. I get this same question a lot, so I'll put
the whole thing in here so others can get it at the same time. It is from Weber's Desktop Reference.
Others can add more "real world" experence to the following. 

"Generally speaking, 5U4s and 5AR4s are interchangable but not always. Some Fender Deluxes
(and other amps with 6V6 output tubes) used a 5U4 and developed about 410 to 420 volts on the
plates of the output tubes. If you changed to a 5AR4 in an amp such as this, you would most
definitely have a problem keeping power tubes from shorting. In amps that were originally
designed with a 5AR4, installing a 5U4 could possibly cause the power transformer to overheat
which would ultimately result in transformer failure. This is due to the fact that a 5U4 draws
almost twice the filament heater current. The good news is that almost any amp that originally used
a 5U4 will take a 5AR4 and since most people prefer the 5AR4, why not try it?"

Additionally, in the TRAINWRECK PAGES (in same book), there is more info:
" The common rectifier tubes used in guitar amps are the 5Y3, 5AR4/GZ34 or 5U4. These tubes
are directly interchangable in most guitar amps. However, each tube produces a slightly different
tonal characteristic in the amp. The 5AR4/GZ34 produces a sound closest to solid state silicon
rectifiers that are most common in amps today. This is because this rectifier has less of a
characteristic known as 'sag'; that is, as the amplifier draws current, the voltage across the tube
drops (there is a voltage drop across the tube that increases with current draw)."
"The 5AR4/GZ34 has the least loss of voltage with current draw. The 5U4 tube has slightly more
voltage loss with current draw and therefore has the 'sofest' sound. Also, if you have an amp that
'eats' output tubes, it will be easest on the amp to use a 5Y3. One can substitute these tubes and
judge for one's self which one provides the basic tone that one likes."

There is more about solid state replacement of these rectifier tubes and some general warnings on
the use of solid state rectifiers on certain models because of voltage ratings.

Hope some of this helps. I put in all this info to help others with similar questions. Anyone with a
tube amp doing this kind of switching and such should get this book: A Desktop Reference Of Hip
Vintage Guitar Amps by Gerald Weber. My copy includes a nice photo of ace local amp guru Jeff
Bober (Budda Amps) looking like a German terrorist --
-- Jay