May I use solid-state replacements for 6x4 rectifier tubes (Quicksilver Full Function preamp)?

As much as I love my Quicksilver Full Function preamp it keeps eating those 6x4 rectifier tubes at an alarming pace (it goes through 5 to 6 pairs per year). Since the cheapest of those tubes have almost doubled in price since 2015 I am thinking of either having the preamp modified to use one 5AR4 instead of two 6x4s (as in the later Quicksilver units) or using solid-state replacements. Having the unit modified by a technician will likely turn out to be prohibitively expensive - I'd probably be better off selling my preamp and then buying a used one from the later series. Anyone know if I can try those pin-compatible replacements without risk? I am referring to these „SS tubes“: 5 Pcs NOS National SSR-8 solid state replacement for 6X4WA / EZ90 Vacuum Tubes | eBay

Arne

 

This does not seem normal to me. Rectifier tubes should last a long time in a preamp. Have you contacted Mike Sanders at Quicksilver? I realize you are far away but I would ask him about the situation first.

 

If you are going through rectifiers that fast, you might have a deeper issue to deal with....like overvolting of the 6X4 filaments....or a random short in one of your power supply caps or some such. You will be even sadder when you start burning through vintage 5AR4 tubes.

Have your unit repaired. If it were my unit, I would consider replacing all of the power supply caps just as a basic precaution. If you do the job yourself it's a cheap bit of insurance.

 

The preamp is used for several hours daily - it is switched on at 7:00 a.m. and rarely switched off before 10:00 p.m. - I think that its „burn rate“ of 6x4 rectifier tubes is not that uncommon - there must be a reason why the manufacturer switched to the 5AR4 design. I had the unit serviced in 2018 and everything was found to be in excellent working order.

 

Whoever designed the power supply screwed up big time. No wonder it was changed to use a single 5AR4. Be interesting to dig into the schematic and see what's going on.

jeff

 

This thing is designed to destroy the rectifiers.



It uses the rectifiers in a voltage doubler to deliver a preposterous 760 volts. Likely because they couldn't otherwise figure out how to get the gain/bias high enough on the phono pre. The 160uF capacitors are in danger from this voltage if they charge at all unevenly and aren't individually rated for 800V. Then the voltage is regulated down a bit, but we have bad current flows there if the rest of the tube complement is unready to draw current.

Let's have a look-see at the 6x4 data sheet, shall we?


Capacitor value 10uF (some have said up to 40uF with small transformers); instead in the design we have inrush current filling a 160uF capacitor for each half cycle of turn-on as soon as these things start to warm up, and another 320uF after the choke.

The rectifier datasheet even advises input limiting with a coil; without that, just turn the preamp on and off fast a few times to make it go poof.

An inrush limiter thermistor on the AC line, and current-limiting resistors in both transformer output lines is almost mandatory if you don't want to watch tubes die.

Fixing this power supply by just yanking the tubes and sticking a 1000V diode into each socket solves that problem, but then the turn-on power is immediate to the rest of the tubes before they've warmed up. What this preamp needs is a standby switch inserted in one of the high-voltage transformer output lines. Only turn it on once the tubes are warmed up. That would allow a solid-state rectifier.

(we assume they didn't screw up the heater voltage, but that part of the schematic is not clear)

 

Holy Schiit, that's not what I was expecting at all.

What about changing the sockets and going with something like 2x 6BW4's, assuming the heater winding will supply the extra current needed?

Good on you for digging up the schem.

jeff

 

God I wish I understood even 10% of what @harby said.

 

Thank you very much! I also wish I understood the schematic by myself. Having someone explain it to you the way @harby does is priceless. He is really walking the extra mile for a fellow forum member.

I love the sound of the amp but will not continue to fork out $150 per year just to keep it running. Mind you, that's without replacing any of the signal tubes!

Would I like the sound of an Audio Research SP-8 or SP-10? Ideally, I would like my next preamp so sound like my beloved VTL Maximal but with greater dynamics and bass response.

 

I have never heard this Quicksilver preamp but I’ve heard a lot of their units and owned several pieces. The Full Function was highly regarded by many. I’ve heard and owned a lot of Audio Research too (still do). I really like Quicksilver but Audio Research is better. I wouldn’t hesitate to make the change although all the models we are talking about here are getting long in the tooth. An Audible Illusions Modulus 3 would be another good choice, with the caveat that getting service from the manufacturer is difficult at best.

 

Another question: How are you powering the preamp in Germany? Is this a full European version with euro power outlets on the back?

The 50Hz puts more demands on the filtering, and 230V wall spec (via the built-in euro transformer or your converter) may be putting more voltage out of the heater and B+ windings than expected. A external variac can turn down the wall voltage. It could be reduced until the DC voltage measured after the choke coil is at 650V or less, or reduced until just where the 640V output begins to drop or show ripple (or by ear), to reduce the regulator currents.

If we don't have other owners of this preamp also reporting abnormal tube failures, one might have a tube amp technician look into the preamp for problems, to ensure the zener diodes all have the 100V voltage drop across them, and that power supply capacitors and all preamp tube resistors and caps check out at their rated value.

Additionally, I would turn the preamp off if you are away from it. Power blips from trees in storms or short power failures, especially in the winter months, are a recipe for component failure.