57 Twin w/ dual 5U4s - why?

[jhundt]

Another thread got me to looking at old Twin schematics. I see that the old 5e8 Twins used 2x 5u4 rectifier tube. What was the reason for this, and what was the result?

I have noticed this before. I always assumed it was some Fender idea that didn't pan out in production, because it was only used for a few amps for a short time.

So I'm wondering why they brought it back for the re-issues? Is it just to be circuitally-correct (notice my new word) or is there any real reason/advantage/sound to be had with a twin rectifier?

 

[Wally]

The dual rectifier provided for less sag/compression...higher foltage. IF one wanted to increase the compression, one 5U4 coudl be removed. One might want to look at the bias
when doing this. The voltage will decrease when pulling one of the rectifiers.

 

[jhundt]

The dual rectifier provided for less sag/compression...higher foltage. IF one wanted to increase the compression, one 5U4 coudl be removed. One might want to look at the bias when doing this. The voltage will decrease when pulling one of the rectifiers.

Reducing voltage sag makes sense - that's why Fender originally switched to the GZ34, isn't it?

Did players remove one rectifier tube to influence their sound back in the 50's?

So do you figure they re-introduced dual 5U4s is to reproduce the original circuit AND allow the user to determine sag?

I'm thinking about copying that circuit, that's why I'm asking...

 

[skeksis]

keep in mind that most of what I've learned, I learned on the internet....

fender was primarily looking for higher voltages (louder amps w/ more headroom) and lower costs, so experimented with a number of configurations. eventually, they settled on SS rectifiers for the big amps. GZ34 was a favorite, but was expensive to make. 5U4 was relatively cheaper -- which is probably partly why you saw a switch back to that tube in the cbs circuits.

I can't imagine the 2x5U4 rectifier does anything too special for the sound - fender probably kept it to maintain vintage authenticity.

 

[Del Pickup]

"I can't imagine the 2x5U4 rectifier does anything too special for the sound - fender probably kept it to maintain vintage authenticity."

I think if you check out the big thread about these amps over on the Fender Forum (horrible place - mush nicer here!!) you'll find a variety of people who would tell you that there is a difference between having one or both rectifiers in the 57 Twin.

If they were only interested in maintaining authenticity they wouldn't have added a bias adjustment pot to the reissue.

Also, having recently had the chance to use one of these amps and having read the handbook that came with the amp there is no reference there to having to rebias the amp if removing one of the rectifiers. So presumably if they don't mention it then it shouldn't be needed.

 

[jhundt]

skeksis - yes, this is kind of how I interpreted the changes in rectifier design. I must admit that I sometimes wonder about the logic. They say that at a certain point GZ34s got too expensive for Princeton amps - so Fender switched to 5U4s to save money - but then they had to buy different transformers too. Why did a GZ34 suddenly become too expensive? And how much more expensive was it than a 5U4? And do you suppose that it was really cheaper to install and wire up 2 tube sockets than to just put in a GZ34 in the first place? I don't dispute this common knowledge, but sometimes I wonder about it!

 

[Bob Arbogast]

A relevant question may be this: "When did the GZ34/5AR4 rectifier go into production?" I have heard that the GZ34 was not available or was not yet proven when the 5E8-A was issued. Not being a tube historian, however, I can't comment on the accuracy of that statement.

Bob Arbogast

 

[Wally]

Bob, you are correct on that one. IN fact, the 5F6 Bassman and the 5F8 TWin carry an 83 rectifier as opposed to the dual 5U4's in the earlier Bassman/Twins. The
5F6A BAssman adn the 5F8A Twin went to the 5AR4/ZG34 when it became available.
This must have been in '56, right? I don't the tube history, either. I would think that the 5AR4/GZ34 was not available in '55 , or Fender would have used it in the 'E' series('55) rather than using the 83 rectifier.
Biasing....there may be no pressing need to re-bias when pulling one tube. It is a fact that when the voltages change, the bias point changes. Players who are into the finer aspects of sonics probably would find re-biasing interesting. Maybe not. IF I had one, I certainly would be curious enough to know what goes on to take mesaurements and compare that to what the ear hears. Also, FEnder is not interested in having people rebias these amps because they probably have them biased cold so as to reduce warranty issues. So, if one were to pull a rectifier and the voltages go down, the current draw would probably still be in the safe zone.

 

[mad dog]

Whatever Leo Fender's rectifier reasoning, I'm really happy they did it this way. The '57 tweed twin RI is turning into one of my favorite amps ever, and that dual rectifier is a big part of why. It changes the feel, for the better. I've played with one pulled, didn't like it nearly as much. Earlier breakup, a little too much sag. With both rectifiers in, the amp still has some sag, enough and no more. It gains a definite "bounce", especially noticeable on right hand palm muting. Individual notes pop out in the nicest way. There's a certain softness to the initial attack, just enough to cushion the muscle underneath. I'm used to playing responsive tweeds, but this one is truly something special.

 

[marshman]

I dunno about 30 years ago, but I know I can get 5U4GBs for a few bucks at any hamfest, but you'd be hardpressed to find a 5AR4 for much under $50 nowadays.

 

[Wally]

I had a 5E8A back in the '90's. Very special amp. I was sad to see it go....and it left with 2 '59 Bandmasters.....back home to CA from TExas.