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

Showing 3 responses by atmasphere

The bottom line, though, after listening to many different pieces of music, other changes accompanying what sounded like improvements in clarity started surfacing. Upper mid and higher frequencies have become more sharp to shrill which doesn’t appeal to me.
@broadstone
When you make changes like this, you’re dealing with a set of compromises and you may have upset the apple cart. I find that good solid state rectifiers are better because you can support heavy bass passages better since the power supply sags less. But I’ve also seen rectifiers affect the sound since often solid state devices can make ’diode noise’ (which is actually caused by a resonance between the inductance of the power transformer and the capacitance in the junctions of the rectifiers; this can take a bit of work to snub the noise effectively) which can increase the IMD of the amp (resulting in harshness). In this regard, the HEXFRED rectifiers are some of the best we’ve heard but even they can have noise issues.

FWIW, when you google Yellow Jacket rectifier, all the hits are a bit sparse on information. This leads me to suspect that they employ silicon rectifiers, which are the mostly likely to have noise issues. So I would not write off solid state rectification until you’ve done more research. In effect, trying the Yellow Jackets, not liking them and then giving up on solid state rectifiers is akin to trying out a tube amplifier, finding you don’t like it and then give up on all tube amps. There’s a lot more than meets the eye with rectifiers!
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.
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.