Rectifier modification: from SS to valve
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?
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.Originally Posted by JayGee
FS Marshall 2266c, TC Nova System
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.
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.
FS Marshall 2266c, TC Nova System
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.
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Two or three alternative resistors selected by switch(es) might be practical though...
Even if it were practical to fit a (fekkin' 'orrible!) valve rectifier you would seriously "de-power" the amp since the transformer will be wound for a lower HT AC output due the the vastly greater efficiency of solid state rects. Then, I don't have the circuit of that amp handy but it probably uses a bridge rectifier, single winding, which you cannot implement with valves (well, not easily, you can however make a silicon/valve "hybrid" bridge).
The commonest valve rect, GZ34 has a "resistance" of around 50 Ohms, as good a starting point as any to go for sag.
Dave.
I did experiment with a high-power rheostat as a 'rectifier control' in a Champ once, and it gave some interesting results at high resistance values, but if you're using it at typical rectifier valve values the differences are too small to be worthwhile making continuously variable, and two or three switched values is much more practical.
Not seriously. A worst case voltage drop is about 20V (200mA maximum through 100R) which in a B+ of about 420V is not a lot... 5% on voltage, 10% on power or thereabouts.
You're right - I had thought it was a centre-tapped winding, but it's not. You could still do it with a valve and two of the diodes though, which gives effectively the same result.Then, I don't have dthe circuit of that amp handy but it probably uses a bridge rectifier, single winding, which you cannot implement with valves (well, not easily, you can however make a silicon/valve "hybrid" bridge).
I'd go higher - the GZ34 is the lowest resistance and 'least valvey' rectifier valve, and if you want to hear more of the effect I'd go for something more like a 5U4. 100 ohms is probably a good starting point.The commonest valve rect, GZ34 has a "resistance" of around 50 Ohms, as good a starting point as any to go for sag.
There's still no way I'd consider a real valve in an amp which doesn't have the right transformer or chassis space to take it, though. The resistor will get you 95% of the effect for 5% of the work.
Last edited by ICBM; 4th October 2012 at 11:18 PM.
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I've done a fair bit of "trying" different rectifier valves with various amps I have, mainly due to being unable to detect any sag at all with GZ34's! I should say that I've also been careful to make sure that the rectifier that I was substituting in would work in terms of current, voltage and impedence with the amp and not put too much stress on either the amp or the rectifier valve.
Fender bassman reissue, notice no real difference between a solid state "plug" and GZ34. However, a 5U4 definitely introduces some noticeable sag and in a very nice way. Need to watch the heater current with 5U4's though as they draw 3amps as opposed to 1.9amps for a GZ34 so you need to be sure that your mains transformer is up to the increased heater load.
I've also had very good "saggy" results with 5V4G as a replacement for a GZ34. It draws 2V so shouldn't stress a mains transformer any more than a GZ34. Worked well in a Marshall Bluesbreaker reissue and very well in an old British Miles Platting head converted to take 5881 power tubes.
Should also say that I've used NOS rectifiers (Mullard GZ34; Tung-Sol and Amperex 5U4; and Brimar 5V4G), as when I started looking at this there were so many horror stories about the newer equivalents. From what I hear however, some of the newer ones seem to have got better in the past couple of years with some actually meeting the original design specs! ... which is good as the price of NOS rectifiers has gone through the roof!.
Table here: http://www.300guitars.com/articles/r...ge-drop-chart/ provides a good summary of valve rectifier specs and what drop you might expect.
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