Can I put a tube rectifier after a PT that ain't center tapped? (6x4 -> Epi Valve Jr)

[R.G.]

But how about his original question? Make a solid state bridge, then feed the resulting DC THROUGH a tube rectifier - a series sag element.

DOH!!

See what all those speed reading classes do to you?

@OP: Sure. You can do it. The sag for both elements paralleled will be less than the sag in a CT configuration. Using only one anode you'll have to be careful to limit the currents and first filter cap to not damage the cathodes with too much current in the charging pulses.

A resistor to fake the sag is a viable alternative and might be an easier solution in the face of needing heater power for a tube.

[Tom Phillips]

...The thing is I want to add a 6X4 rectifier in a Epiphone Valve Jr. If all I want is the tube sag...

I'm chiming in with those who recommend just adding a resistor instead of a tube. My additional input is a recommendation that you think about the recent comments regarding the effect you will actually get when you add additional resistance in the B+ supply of your Epiphone Valve Jr. Your stated goal was to add sag. However, what you will actually get is a predominantly constant voltage drop. (Because the current drawn by your class A amp does not change much when you play loud riffs) The constant voltage drop will just will lower the plate voltage throughout the amp reducing the power output by a little and possibly changing the tone and responsiveness of the preamp. All of these changes will most likely be insignificant. Note that dynamic sag is not on the list and, unfortunately, that was your main goal.

If this line of reasoning doesn't make any sense to you just ask and we can provide further explanation.

[R.G.]

And Tom, you're correct.

Sag depends on the variation of the current pulled by the output stage lowering the B+, which makes the output power smaller. Little or no variation, little or no sag; this with either a tube rectifier or resistor.

[R.G.]

As I hit "submit" this occurred to me.

It would be interesting to take the output signal, derive an envelope from it, a DC-filtered waveform which "connected the dots" between successive peaks of the output signal, and then use that to decrease the B+ to either (a) the output stage or (b) just the preamp. This would produce exactly the situation which causes sag in Class AB amps.

I mention B because if you can get your sag with low power techniques just on the preamp, that's easier to do.

Two ways come to mind: using the envelope to add additional power supply current drain, perhaps through a power device and a resistor to ground, and by using a series regulator on the B+ side to simply lower the B+. The first is more like the real situation, but I worry about adding more load to a (probably) minimally designed PT. The second would not add additional loading to the power supply, just sag the B+.

I'm fairly sure there are patents on something like this. I don't know if they're still in force. I have a hard time with patents on stuff like this. To me it's "obvious to one skilled in the art" and hence not patentable, but that doesn't seem to stop Randall.

[Tom Phillips]

Very interesting idea R.G. Automatic gain control type circuits could be used with changes to cause the sag effect rather than smooth out the high and low amplitude potions of the signal as AGC is traditionally used.

[Steve Conner]

Congratulations, you just reinvented the tube compressor! (No, I don't think it is patentable either )

[Steve Conner]

PS: The average plate current of a single-ended stage might not vary much between idle and full output, but the screen current varies a lot. So you could maybe introduce sag to the screen supply using an RC filter with a largish resistor.

The downside is that in a single-ended amp, any disturbance of the operating point is treated just like signal. You could expect to see your speaker cone flapping in time to the screen voltage. A push-pull amp can cancel the DC shift while keeping the variations in gain and clipping level that go with it. Indeed, all of the classic tube compressors used a push-pull circuit for this reason.

[Randall]

I'm fairly sure there are patents on something like this. I don't know if they're still in force. I have a hard time with patents on stuff like this. To me it's "obvious to one skilled in the art" and hence not patentable, but that doesn't seem to stop Randall.

Huh?

[Enzo]

Not you, he means Randall Smith of Mesa Boogie.


That Randall has a reputation for patenting just about everything, much of which ought not to be patentable.

[R.G.]

Sorry Randall - Enzo's right again - the Mesa Randall that periodically patents something right out of RDH4. No slur on you intended. I should have checked the members list.