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In Reply to: RE: If you own a cary,you answered your own question. posted by Michael Samra on April 18, 2010 at 00:42:08
That a company of this stature would design such an obviously flawed power supply, and offer zero support to fix the problem.
Jeff
Follow Ups:
when I realized that the string of NOS pre-war and mil-spec 5U4G I have gone through were sacrificed, expensively, for this design flaw. I had thought it was the tubes, or me, and here it is the fracking design.
Lots of good tubes lie dead in the tube burial ground. I really am sorta pissed at my dealer and Dennis, as they surely knew about this--and my amp even got a Cary recommended modification to ease the start-up problem. I guess it wasn't enough.
Sidebar: I recently talked to Cary about this and they insist on me using directly heated rectifiers like the 5U4G that I have been using, and not the indirectly heated ones like the 5V4--which was the original spec'ed rectifier. Whatever. Dennis, want to buy an interesting rectifier sculpture??
Jeff,
The 5V4 is even less tolerant of a large 1st filter cap. value than the 5U4 is. It's a miracle that any vacuum rectifier survives even 1 turn on cycle in that PSU design.
What you can do is to install 1200 PIV Silicon carbide (SiC) Schottky diodes, along with resistors, in an Octal plug everywhere a 5U4 goes in those questionable designs. SS diodes are not much faster turning on than directly heated vacuum rectifiers are. Schottky diodes are just as quiet as vacuum diodes. The part you need is made by Cree. It's DigiKey stock # C2D05120A-ND and the cost is $9.32 per diode. Yes, that's expensive "grit", but it's a heck of lot less costly than blown 5U4GBs, NOS or current production, are.
Is a schematic available for perusal?
Eli D.
It seems to me that the circuit not only demands too much of the rectifiers at switch-on, but also at switch-off.
Unless PIV is an issue all the time in operation, I actually forget.
;-)!
valve rectifiers sound better though, eh!? Sag and stuff?! ;-)!
Tim B
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
http://www.theanalogdept.com/tim_bailey.htm
Best to find a location to put in a 10uf Solen and a 50 ohm resistor before the existing B+ string.
___
Long Live Dr.Gizmo
I said 1200 PIV Schotkys, with resistors wired in series. The idea is to roughly emulate 5U4 forward drop, while having large 1st filter capacitance tolerance.
One way or another, the rail voltage has to come in right. On that point, we are in agreement.
Eli D.
But no luck.
Jeff
Greetings- In this particular power supply, there is a .22 uf cap before the 1200 uf cap. Do you think that is enough capacitance to soften the "blow" of the big 1200 uf??? regards, wpod
Wow! Talk about asking for trouble...
The .22 uf is in parallel with the 1200 uf, so it will actually add a tiny bit more to the rectifier's woes.
And a standby switch - boy, when you throw that switch with a discharged 1200 uf cap and a hot CV378 - it's amazing they don't pop every time!!
Plus - do I see the 450 volt caps are exposed to 465-470 volts??
Essentially all that is limiting the inrush current is the power trafo secondary winding resistance. And it's not just on startup that large current pulses will occur.
This PS needs some re-engineering if you intend to use a tube rectifier. Don't waste those expensive RCAs!! Keep in mind too that a recto failure can cause other damage.
If this schematic and the voltages on it are accurate it is a totally irresponsible and wreckless design IMHO. I find it hard to believe any commercial amp maker would employ this circuit. There is much more wrong with it than right with it!
Hundreds of these lovely tubes have burned up prematurely in these poorly designed amps. Grrrrrrrrrrrrr!
Jeff
Trolling around the web revealed a few posts from people having the same problem. One even said that their questions to Cary Audio went unanswered. Now that's costumer service for ya.
You should send them the bill for all those roasted rectifier tubes. I bet you won't get a reply.
Jeff
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