Sag Resistor

[d2camero]

Has anybody ever tried a power supply sag resistor in your express, rocket or liverpool? I am curious as to what values produce a tube rectifier type sound.

thanks
Don

[Luke Duke]

I'm curious to know as well, I'm thinking an express in stock form would be too stiff for me.

Luke

[jimipage]

Haven't built a 'Wreck (yet), nor an amp with SS rectification, but I did recently just finish a Spitfire with 6v6's and a GZ34. I built it with a 10W 100ohm sag resistor. Didn't realize how much I liked it until I removed it. Probably reinstall it, maybe at 110ohms. I'm just taking a guess here, but I think with a SS rectifier I would imagine a little more resistance would be needed, say 120-200ohms. It's such an easy component to switch in and out, I say give it a try. In my experience, it works well.

[mhuss]

A GZ34 has an internal resistance of about 175 ohms, so adding that inline with your SS diodes should get you in the ballpark. As jp suggested, you might like more or less, so feel free to experiment.

Note that this resistor will dissipate 7 or 8 watts, so use a 15 or 20 watt flameproof resistor, and keep it away from anything else you don't want to slow cook.

--mark h

[Tube_S_Cream]

An annealing to simulate the characteristic of a GZ34 ist a SAG Resistor of 100 to 150 Ohms. The GZ34 is lightly nonlinear: For very low currents it's like a 180R Resistor decreasing to 100 R at full load.
So 100 R ist quite a good value.
I've tested it with some DALE 20W resistors up to 150R.
The amp is compressing a bit more than normal, but i must say my PT delivers a B+ of approx 430V (quiet op.)
With a 150 Ohm sag it will break down to approx. 360V.
Nice "Brown Sound" but less defined.
If +B is much lower i'd prefer 100R max.
Also remember that a tube rectifier uses smaller caps (max 47µF)
To have a authentic SAG yo may want to experience with the first storage capacitor which is originally 80µF . A larger capacitor may take longer time to reload, which may cause a significant pumping effect (comparable to the attack/release times of a compressor).
Smaller values act faster but increase AC ripple on +B.

Greetz

Tube_S_Cream

[d95err]

To have a authentic SAG yo may want to experience with the first storage capacitor which is originally 80µF . A larger capacitor may take longer time to reload, which may cause a significant pumping effect (comparable to the attack/release times of a compressor).
Smaller values act faster but increase AC ripple on +B.

I thought it was the other way around:

Large filter caps work as a reservoir for current, so the tubes can draw current from the caps when pushed hard and the powersupply can't deliver. This counteracts the voltage drop. A smaller cap drains faster which causes the voltage to drop.

As an example, modern Vox AC30s have a "Smoothing" switch where you can select "Modern" (44uF) or "Vintage" (22uF) filter cap. This gives a tighter ("Modern") or looser ("Vintage") feel.