Using solid state rectifiers on Cary SLI80 Amplifier


One might wonder why experimenting with such a high performing amplifier as this SLI80 Signature is but I would like to try this to see if doing so will tighten bass and/or percussive sounds from things like drums and piano. I’ve tried raising the bias above the Cary recommended 75mA and the sonic difference was significant but, because the higher resulting voltages to the tubes would be continuous while operating, Cary recommended against it primarily because of predicted shortened tube life. I’ve taken their advice and adjusted the amperage down to 80 from the 90mA that I was trying. Now, based partly on Cary’s having switched to solid state rectification in their newer amps, I thought it made sense to try it on this older amp and need advice concerning exchanging the tube diodes with solid state replacements. What I’ve learned from several sources is that the resulting instantaneous power to the tubes and power transformer could cause damage or, at least, blown fuses. Without adding circuit protection that delays the impact, does anyone have experience with this modification or insight otherwise. I do have a variac to lessen the initial impact of this experiment if I decide to do so.  Thank you. 
128x128broadstone
What I’ve learned from several sources is that the resulting instantaneous power to the tubes and power transformer could cause damage or, at least, blown fuses. Without adding circuit protection that delays the impact, does anyone have experience with this modification or insight otherwise.
@broadstone First, the inrush current will not be a threat to the power transformer so you can cross that off the list of concerns. Its unlikely you'll be blowing fuses too.

The concerns are these: the power transformer was designed to make the right DC voltage when used with a tube rectifier. When you go solid state the voltage drop will be less (and you get better power supply regulation since there is less voltage sag when the amp is at higher power). The problem is that voltage might be more than the tubes, filter caps and transformers might be able to handle. The primary of the power transformer won't care much since the filament load of the rectifier was removed. So there's that.

But the real issue IMO is the extra B+. There are solid state rectifiers packs that are meant to replace tube rectifiers. Some of these have a voltage drop to mimic a tube rectifier, but they don't act the same way. I think using solid state rectifiers is a good idea since the voltage drop that tube rectifiers have contribute to distortion in the amp and extra distortion is never good. If you really want to do this right the way to do it is get the newer power transformer and replace your old one so you get the right operating voltages.

I know that's an expensive solution but if you want to do it right, that's the best way to go about it.
Atmosphere, over the years I’ve learned much from your posts and I appreciate this response which I think I mostly understand. Anyway, because I’ve already bought them I’m going to try the ss rectifiers simply out of curiosity. I have one question, though; for those ss diodes that are designed to mimic tubes, if part of that feature is to more slowly ramp up the application of power, will that not also affect these diodes’ reaction speed overall and negate the reason for their substitution in the first place? Im probably overthinking this and a bit over my head technically but this all started when I went back to using the relatively low sensitivity Martin Logan CLS2 speakers and was looking for a bit more dynamics still keeping the Cary in the system. 
As usual Atmasphere is spot on. I like the bass with SS rectification much more than tube in general. Check the voltage spec of your power supply caps and be sure they can handle the higher voltage per Ralph’s comments. New transformers spec’d to deliver the needed B+ voltage is certainly reasonable. Yes,  very costly. 
if part of that feature is to more slowly ramp up the application of power, will that not also affect these diodes’ reaction speed overall and negate the reason for their substitution in the first place?
Yes, to a certain degree. You don't have to worry about the tube rectifier going bad... For my part, I'd put the amp on the bench and see how bad things are when a set of HEXFREDs are installed. Measure the power supply voltages at idle, compare to the ratings of the caps (usually if you exceed 85% you're risking over-voltage under certain conditions).


One thing that can be problematic is the power supply is charged up long before the tubes are so there is no load. The no-load voltage is much higher! Some manufacturers (and I've no idea about Cary in this regard, I'm not calling them out), because a tube rectifier is used, will have lower voltage caps in the power supply as a result. But with a solid state rectifier, the caps will be in a no-load situation and so will be much higher than when the power tubes are warmed up.

If that no-load voltage is too high you can destroy the filter caps- and it won't just be the input caps to the power supply that are that high- **all** of the filter caps will be at this voltage until the tubes warm up. So you really want to be careful! It can be really upsetting when a filter cap blows its guts out.


If any of this sounds intimidating, good- if that's the case, you really might want to resist that urge to see what can be done. OTOH, if you feel like *perhaps* also replacing the filter caps maybe it won't be any problem. But at some point you might want to consider building the amp up from scratch.