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In Reply to: RE: Defective 300B Tubes? posted by Devils Advocate on August 16, 2015 at 11:04:26
I believe I remember your posts when you bought it. The Cary does run tubes a bit hard but what you described is odd. I'd call Cary and talk to them before using the replacement tubes. There are some upgrades that you may be interested in. No sense in frying more tubes.
It's a nice piece. I had the 300SE monoblocks in 1994.
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I plan on contacting Cary tomorrow to discuss having the unit checked out and possible upgrades. Apparently replacing the coupling caps with either Jensen or Clarity caps and hexfred rectifiers are popular upgrades so maybe I'll get that done as well.
Do you know what caps are in there now? The Hexfred upgrade will relieve your rectifiers of some work, the bass will be tighter, but the rectifiers will still be there. That info came from someone on another forum who is having Hexfreds installed on an SLI-80.
I took the bottom plate off to take a peek, mostly out of curiosity of the design/build, and also to check for leaking caps & burnt resistors (considering age of the amp). Everything looked good, and I was impressed with the layout & build quality, even to my untrained eye you could tell Cary did a good job building these amps. And to answer your question, the unit is stock with no modifications.
One thing I want to bring up when talking with Cary's tech is possibly having a power supply delay board installed, I'm not a tech but perhaps the tubes would fare better if they were slowly exposed to voltage during the warm-up process (see 'Power Supply Delay Action Board' under 'Transformers/ Chokes/Parts'). Not sure if it would help but maybe worth discussing.
That may already be in the circuit but you would have to ask Cary to be sure. Do you recall if the coupling caps were a dull silver tone?
It was a couple of months ago when I looked under the hood so can't be sure, but I vaguely remember them as you described but again not 100%.
Then they may be Cary's Audio 1 caps which I believe are Vitamin Q's.
Vitamin Q?
Vitamin Q's are a paper in oil capacitor. They can be very nice in the right application.
SETdude brings up a good point, is it safe to run the tubes @ 160mA, and what's the benefit of running them at the higher amperage?
I would ask Cary about the bias point. That may be an issue with the particular 300B tube you are using.
I just looked at the manual for your amp. It's not tube rectified..no big deal. A bias setting of 160ma is what the manual says but that may exceed the limits of your particular tube. Look up the data sheet. The Hexfreds would be an upgrade. Capacitors? What you would prefer would likely hinge on your speakers.
Tube dealer recommends 120mA so that's what I've been using. Speakers are Merlin TSM.
That is 20ma more than the data sheet for the tube recommends.
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