Dual Rectifier Tweeds

[rp]

I see some builders offering these old styles and DYI cloners too. Is this just an anachronism that the development of the 5AR4 obsoleted or does building with 2X 5Y3s or 5U4s really bring something unique to the amp that a 5AR4 doesn't? Is it that much stiffer? Or, are builders just using dual rectos for vintage rigor/marketing rather than feel/sonics? Any one try the same circuit with dual rectos vs 5AR4? If so, what'd you find?

I really like the 5AR4 - it's the Goldilocks porridge of rectos.

Just curious if I build an other tweed, it would seem best to forgo the dual recto, especially the PT requirements.

[JazzGuitarGimp]

I have wondered about the same thing. It will be interesting to see what the general consensus is here.

[Structo]

Isn't the dual rectifier tube done for higher current handling?

[rp]

Isn't the dual rectifier tube done for higher current handling?

That would make sense if they'd only done it going from the 2 power tube models to 4 in the Twin but they played around with it among the 2 power tube models (see the 5C8 and 5D8). And is there really much difference in current demand btwn any of the tweed Bassman styles, or the black or blonds for that matter, i.e. anything a single 5U4 couldn't handle? And, wouldn't the last tweeds like the 5F6A, the 45W fixed biased models, have been the ones befitting the most from the dual rectos as they would be pulling the most current? I'm guessing it was done in an attempt to stiffen up the feel for bass players or accordions, but not sure. They sure dumped the idea quick when ss diodes and the 5AR4 came along.

[Firestorm]

Fender's logic in using the dual rectifiers is a little hard to figure out. The 5D6, 5E6 and 5E8 models were choke-filtered, so I would guess they wanted to reduce the rectifier impedance to help overcome the B+ voltage loss the choke imposes. I can't imagine what they were thinking in the 5D8, which had no choke and used two 5Y3s instead of a single 5U4. The 5F8 Twin was the first high-power model and used a single 83 rectifier, which is an oddball that may have delivered enough current (although, the OT was supposedly not up to the task of handling four 5881s). And strangely, the 5F8-A used only a single 5AR4, when really it should have used two of those (or two 5U4s). Go figure.

[martin manning]

That's a good point rp, 5U4's and 5Y3's drop a lot of voltage. A stiffer supply and more B+ and output power for the same HT could be had by paralleling two of them. 5AR4 has low voltage drop, so a pair wouldn't be much advantage. No surprise that Fender opted to go back to one rectifier when more efficient designs came out, saving a bunch of parts and labor cost.

[Tweedguy]

I built a 5E8-A Low Power Twin with the two Rectifiers. I can tell you that using two 5U4s does sound and feel different than a single 5AR4. It's a fatter, thicker tone and it holds together longer before breaking up. The 5AR4 is a thinner sound and with a bit more immediate attack to the note; a sharper sound.

As far as the history of Fender designs goes, all my research (7 years worth) indicates that Fender was simply trying to keep their big amps as clean as possible. The dual rectifier design was the best option before the 5AR4 became readily available and had been tested in guitar amps. It was cheaper to convert to a single rectifier tube once they new the 5AR4 was reliable.

Most people just build the amp with the dual rectifier chassis and proper power transformer and then use whatever sounds best to them. One 5U4, two 5U4s, or one 5AR4. I've personally enjoyed the versatility of trying all three different setups. It's almost like having three amps, one is soft and dirty, one is big and round, and one is sharper and a bit brighter. My personal favorite however, is the two 5U4s.

[martin manning]

Interesting... Did you happen to get some measurements of the B+ voltages with those three configurations?

[Tweedguy]

The Main B+ on my amp is about 455 Volts and will drop about 5 Volts with one rectifier. But the main thing that affects the different feel and sound is the SAG from using just one 5U4. They have a lot of sag, which is why some people like them and some don't.

With the 5AR4, the B+ is about the same as two 5U4s, but it all depends on the particular tubes you have. Any type of rectifier tube that I've tried seems to have a variation of about 10 Volts when trying them out one at a time. So you just have to listen to each one.

...and I can tell you beyond a shadow of a doubt, the rectifier tube DOES affect tone.

[martin manning]

The Main B+ on my amp is about 455 Volts and will drop about 5 Volts with one rectifier. But the main thing that affects the different feel and sound is the SAG from using just one 5U4. They have a lot of sag, which is why some people like them and some don't.

With the 5AR4, the B+ is about the same as two 5U4s, but it all depends on the particular tubes you have. Any type of rectifier tube that I've tried seems to have a variation of about 10 Volts when trying them out one at a time. So you just have to listen to each one.

...and I can tell you beyond a shadow of a doubt, the rectifier tube DOES affect tone.

I agree with you, and of course the idle B+ is only part of the story. These things act like non-linear resistors in the power supply which will lose about 1/3 of their resistance from idle to max current draw, creating a compression effect that is unlike a simple linear resistor. After studying the characteristics a bit I believe that in general a pair of 5Y3's would act like a single 5U4, and a pair of 5U4's would be similar to a single 5AR4.

[rp]

Thanks tweedguy, it's what I was looking for. Guess if I have the room I could just wire up for two sockets, though sourcing and fitting a PT with 6A on the 5V isn't appealing at all.

[Colossal]

...and I can tell you beyond a shadow of a doubt, the rectifier tube DOES affect tone.

Completely agree! The Mullard GZ34 is the benchmark and wow, what a sound. I have some good old 5U4s, 5V4s, etc and yes, they all have their own tones and feels.

[martin manning]

With the 5AR4, the B+ is about the same as two 5U4s, but it all depends on the particular tubes you have.

How does a pair of 5U4's compare tone-wise to a single 5AR4? The data sheets suggest that the sag characteristics would be quite similar.

[Tweedguy]

Read my first post for a tone comparison.

[rdjones]

I agree with you, and of course the idle B+ is only part of the story. These things act like non-linear resistors in the power supply which will lose about 1/3 of their resistance from idle to max current draw, creating a compression effect that is unlike a simple linear resistor. After studying the characteristics a bit I believe that in general a pair of 5Y3's would act like a single 5U4, and a pair of 5U4's would be similar to a single 5AR4.

Something else that may contribute to variations may be the difference in 5V current draw. 1.9A, 3A, 4A, 6A.
I wonder how much this is a factor on regulation.
Surely with 6A being pulled at idle it would cause the B+ to be slightly less than that with a 1.9A rectifier.

rd