Dual Rectifier implementation/Copper Cap

[Bob M.]

I have a homemade amp where I'd like to have a choice of rectifiers (tube and ss diodes), which I've done many times before. Because of what I'm trying to accomplish in this amp, I wanted a rock-solid clean channel, using a solid state diode rectifier but also I wanted a looser, lower voltage brown out sound as well. I thought I use a 5R4 rectifier tube (I liked its high mAs and big voltage drop) but I was dissatisfied with its filter capacitance limitation at the first filter position so I bought a plug-in copper cap emulator from Weber (WR4) instead.

My question is this: Normally, with a rectifier tube and ss diodes, I use a switch to select the output (either diodes or tube) at pin 8 of the rectifier socket. The solid state diodes will swamp out the rectifier tube when connected to pin 8 (DC output) using a SPST switch. But what will happen with the copper cap in place instead? I assume it's completely solid state inside. I could use more switching poles to switch not only the rectifier output but also the input (3PDT) and, of course, I'll need another pole for the bias tracking (4PDT). It's starting to get complicated. I'm looking for a better way to implement this. Has anyone had experience with this type of problem and with using a copper cap and ss diodes? Does anyone know the capacitance limit of the copper cap WR4 at the first filter position? The last fact is that the power transformer is much happier (and cooler running) not having to use the 5 volt winding to power a rectifier tube.

Thanks,

Bob M.

[ThermionicScott]

WR4 is just diodes, resistors, and thermistors inside, so you can use however much capacitance you want.

My speculation is that the low capacitance limit (4uF on most of the older data sheets you can find, but RCA had uprated them to 20uF by 1961) on 5R4 rectifiers is due to the high-voltage service for which they were intended. With the higher voltages, the current spikes will be higher, so the input capacitor needs to be smaller to keep from damaging the cathode. So in a typical guitar amp with a 600-700VAC secondary, you can probably push it up a little, but I wouldn't go too crazy with it.

- Scott

[Pedro Vecino]

Itīs probably easier to bypass the resistor and the thermistor... (are in series in the copper cap). There is no more to switch...
Some versions of copper cap are to be installed on the chassis, not on a octal socket

[Bob M.]

Some versions of copper cap are to be installed on the chassis, not on a octal socket

... But not my version.

[Bruce / Mission Amps]

Super easy

Here's an easy way to do this... it's an old thing but sometimes I use it in Harp amps to give the player a variation in power output and some "expected" rectifier sound differences.
The 10w resistor saves the rectifier tube from a big current surge when charging the first filter cap.
I wonder if a Hi-V 1A steering diode in the tube side would make any difference?