Dual Rectifier Tubes

[Fletcher Munson]

Can anyone explain the dual (tube) rectifier concept? I'm looking at the old 5D8 Twin, thinking this might be my next build, and trying to understand the circuit. Why use two rectifiers? Is the idea to provide more current? Does the DC current coming off the dual rectifiers require less filtering?

[tubeswell]

2 rectifiers are wired in parallel to increase current and voltage for the B+ supply. You need a filament winding with twice the current rating of a single rectifier of the same type.

The immediate advantage is that you can easily get more sag and lowered B+ by pulling one of the rectfiers out.

[Fletcher Munson]

The immediate advantage is that you can easily get more sag and lowered B+ by pulling one of the rectfiers out.

Cool idea. I wonder if you could put one of the 5Y3's on a switch. A sort of a "sag" or "brown" switch... might be interesting.

So when Leo Fender designed this circuit, he most likely incorporated the dual rectifiers as a way of eliminating sag and getting a faster, more immediate response from the power section. Is that correct? Any thoughts on how parallel tube rectifiers would differ in sound (feel) and performance from diode rectifiers?

[tubeswell]

Tube rectifiers drop more voltage (in comparison to the VAC on the HT) than diode rectifiers.

Take a winding of (say) 300-0-300VAC

A 5Y3GT will produce about 330VDC B+

A 5U4G about 360VDC B+

A 5AR4 about 390VDC B+

A pair of 1N4007 (full wave) about 424VDC B+

[Pedro Vecino]

HT voltage in Dual Rectifier with diodes is 445V. With 2x5u4 is 400V
Using 4 6L6 you should maintain always the two 5U4 since the amplifier supposes too many load for one alone 5U4 and would finish failing.
If you want to obtain something good using the tube rectifiers you should automate the bias adjustment when these are activated.
Regards

[Bruce / Mission Amps]

I'm going to mildly disagree with some of the 5U4 opinions here...

I think the use of dual 5U4s was only to keep the B+ rail better regulated and voltage drop lower across the rectifier (rectifier as a complete "circuit", not to increase the current rating of the rectifier circuit.

Why? Because any decent 5U4 can handle a huge amount of current from anode to cathode.
Most 5U4s are actually rated at nearly twice what you'd guess for a tube rectifier.
As an example, a 5U4GB is actually rated at around 1500v and 1 amp!! Brother that is 1500 watts but of course nobody would ever demand that of a tube rectifier.
So, it is not the current rating, it is the voltage drop at high current that is the Achilles heal in the 5U4.
High current through the saggy rectifier means a continuation of lower B+... lower B+ means lower idle and output current through the power tubes... so guess what.... momentarily the B+ rises back up again because there is less current demand with lower B+.... hence all the conversation about sag, tone and other subjective arguments.