Rectifier modification: from SS to valve

[knuckleberryfinn]

I just got a Fender Deluxe Reverb II. It sounds glorious, and is in top condition, but is very attack-y. It has a C10Q speaker in it, which I would like to change for an alnico (or possibly 2x10 alnicos - but that's another thread) in order to mellow it out a bit. But I do miss the rectifier sag of the SRRI I had a while ago. I am wondering, then, how difficult/easy it would be to mod the Deluxe Reverb II to valve rectification. And also, of course, how expensive. The perfect thing for me would be to have switchable SS/valve rectification as sometimes the very crisp cleans the amp produces with the SS rectifier are very welcome, though I'm not sure if this is even possible (I am a valve amp novice). So, what do you more knowledgeable people think?

[JayGee]

There'll be someone technically competent along at any moment to give chapter and verse but you should be able to get pretty much the same effect from a couple of hundred ohms or so of resistor.

Basically it's the internal resistance/impedance of a valve rectifier which generates the characteristic "sag" on the HT voltage when you draw current which "softens" attack and adds a hint of compression. A suitably sized resistor (I'll leave it to someone qualified to suggest values, power ratings and exactly where to put it!) on the DC side of a solid state rectifier will give pretty much the same effect and it won't be rocket science to make it switchable...

[John_A]

There'll be someone technically competent along at any moment to give chapter and verse but you should be able to get pretty much the same effect from a couple of hundred ohms or so of resistor.

Basically it's the internal resistance/impedance of a valve rectifier which generates the characteristic "sag" on the HT voltage when you draw current which "softens" attack and adds a hint of compression. A suitably sized resistor (I'll leave it to someone qualified to suggest values, power ratings and exactly where to put it!) on the DC side of a solid state rectifier will give pretty much the same effect and it won't be rocket science to make it switchable...

Good suggestion, I defy anyone to be able to tell the difference between a valve rectifier and a SS one with a sag resistor, a 100 ohm 5 or 10 watt resistor would do the trick, would cost 20p to do and to add a valve rectifier would most likely involve an additional heater transformer and prove to be quite a big job. I'm not familiar with the DRII layout, but I'm supprised there isn't one in there allready.

[knuckleberryfinn]

Thanks for the advice. I had read about the use of a resistor on the rectifier part of the circuit in Blues Jr amps (I think). It certainly sounds a great deal easier than adding valve rectification - in fact, it sounds like even I could do it. Presumably I could mount a pot on the rear of the chassis to control the level of resistance/sag? This would be even better than a switchable on/off sag.

[John_A]

A typical pot is 1/2 watt, you need something in the order of 10W for a sag resistor, so a pot really wouldn't be any use.

[ICBM]

You can tell the difference between a solid-state rectifier with a sag resistor and a valve rectifier if you A/B them in the same amp, but only under fairly controlled conditions. If you were coming blind to an amp and didn't know which it was, or fitting one to an amp that doesn't have a valve rectifier already, I think it would be impossible to say, so it's a moot point here.

Fitting an actual valve rectifier is pretty much a non-stater with this amp, you'd need an extra transformer for the valve filament supply and enough space in the chassis for it and the valve socket - I'm pretty sure this effectively rules it out. The DRII is laid out quite a lot differently to the DR, the power valves are closer to the end of the chassis.

I would use a chassis-mounted aluminium-clad resistor for the best reliability - I think there should be enough room to fit one. Although in theory the resistor only needs to be about 3W, if a power valve shorts the temporary current will be much higher than normal and you don't want the resistor to burn out before the fuse blows. The alu-clad type have such a high momentary overload rating that this is very unlikely.

[JayGee]

A typical pot is 1/2 watt, you need something in the order of 10W for a sag resistor, so a pot really wouldn't be any use.

Two or three alternative resistors selected by switch(es) might be practical though...