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Hello,
I have a Cary 300SEI integrated amplifier (IMHO a fine amp), and by process of elimination I've narrowed down the problem I'm having to what I suspect is a defective 300B output tube(s).
Here is what's happening... since the tubes are relatively new (approx. 100 hours) I have a Cary bias meter hooked up to check amp bias, and when I turn it on with the suspect tubes installed the meter immediately pegs the needle 'hard & fast' to the end of it's sweep, the needle physically hits the far end until it cannot go any further. Scale markings are graduated to 300mA and the needle swings beyond these markings. I'll also hear a 'pop' coming from the speakers before I can shut off the amp which I do as soon as I see the needle peg.
As I've said, I'll turn the amp off PDQ (within a couple of seconds) for fear of causing damage to the tubes and amp. I've tried removing & reinstalling the tubes and swapping which socket they're installed to see if it's an issue with the amp, I've also tried lightly tapping the tubes with a pencil to see if that might alleviate the problem but to no avail.
I bought the tubes 2 months ago and followed the tube manufacturer's recommendation of setting bias @ 120mA and up till a few days ago they worked perfectly. The problem first emerged when I returned from a week long vacation and I went to turn on the amp and saw the bias meter peg. Luckily the meter was still plugged in or the amp/tubes probably would've blown.
Today I tried once more reinstalling the defective tubes to see if they would cause the same problem and they did. In the meantime I've installed another set of 300B tubes (same manufacturer) and so far have not had any problems (knock wood).
I'm going to send the defective tubes back for a warranty exchange, but in the meantime I'm curious what would cause a set of tubes to all of a sudden go bad like this, I'm relatively new to tubes so please enlighten me.
Thanks.
Follow Ups:
I have a good friend who runs a Cary SEI300b in his system and I have repaired it for him on several occasions. The heater supply to the 300Bs is a joke. It consists of a silicone bridge and a cap. Failed twice in my buddies amp. Even worse, the voltage was well over 5V. 300Bs don't take too well with over voltage on their filaments. I would measure the filament voltage being supplied to the 300Bs. If significantly over 5V I would suggest adding a resistor between the bridge and cap. Look up the current draw of the 300Bs to estimate the value of the resistor for the needed drop and determine final value empirically.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Are you 100% sure that you didn't accidentally insert the tubes improperly?
Having the tube rotated 90 degrees by accident would peg your bias meter immediately.
A "dead" 300B that pegged your bias meter would need to have a pretty catastrophic problem (there should be metal parts loose inside the tube at that point).
How long have you had the amp? Did you buy it used?
You don't mention what brand of 300B, but I'll hazard a guess they were Gold Lion.
Tubes do just fail. This why I always recommend a reputable dealer.
Dan Santoni
Tubes are of Chinese origin but are not Gold Lion, and were purchased direct from reputable stateside dealer. I had a problem with their 6SN7 tubes before and they replaced them under warranty.
At this point I'd rather not say who the dealer is because I think that would be unfair to them by not giving them a chance to resolve the problem, and quite frankly I don't think it's all that important at this point.
What has me puzzled is how a relatively new tube which was working fine one day suddenly turns into a ticking time-bomb. I shudder to think what could've happened if I didn't have that bias meter plugged in, dodged a bullet there...
I suspect Gold Lions (Russian) because I ran into a rash of their 300B's that were bad right out of the box. They couldn't stand the shaking from shipping, and it was with two different Cary 300SEI amps that they came with. One amp amp even had the replacements bad.
On to your problem. There are many ways a tube can fail. When they fail suddenly like that often it is there own heat causing a deformity, or a bad internal connection. They're fine until they cool down, then heated again the connection finally breaks.
Dan Santoni
I had something similar happen with a Tung Sol KT150. The tubes were packed O.K. but there was nothing placed inside the individual tube boxes and the tubes were allowed to 'move' inside the box. I believe one of the filaments was damaged because one channel was distorted and swapping tubes did not help. They were also purchased from a dealer that does additional testing.
I contacted the dealer and they indicated that they will replace tubes under warranty.
The dealer wrote the following in their email "I have serious concern about your amp" and also stated "there is a very big chance that your amp is out of specification, as a result, caused the tubes to failure prematurely" and went on further to say "I would like to request you to have you amp check to see if there is any issue".
I understand the dealer's position as these tubes are expensive to replace, a 6SN7 tube ($100 per tube) failed about a month ago and now the 300B tubes ($1,200 matched pair) just a couple of days ago. If I were in their shoes I might also have a concern there may be something going on with the amp but... their tubes are the only ones that failed. I've had JJ, RCA, Sylvania, Ken-Rad and not had a single issue with any of them. That's not to say I never will as anything is possible but as far as I can tell the amp is working just fine with bias rock steady @ 120mA as per their recommendation (Cary advises 160mA), there are no strange noises, smells, etc... just wonderful music.
So my question is this... should I heed the tube dealer's request and ship the amp back to Cary to have them check it out, or are the failures nothing more than tubes behaving like tubes?
snip: I understand the dealer's position as these tubes are expensive to replace, a 6SN7 tube ($100 per tube)
You could buy a bunch for that $....and be happier.
"I can't compete with the dead". (Buck W. 2010)
Some of the new 6SN7 tubes are quite good. Have you used them? I have a buddy with boxes of old production and he prefers the Treasure CV-181.
I have well over 500 6SN7s and variants. I don't have any need for modern production tubes. They don't interest me at all.
"I can't compete with the dead". (Buck W. 2010)
Edits: 08/19/15
Good for you.
You're right. It is. Excellent in fact.
"I can't compete with the dead". (Buck W. 2010)
I've seen that exact same response twice before from Sophia. In both cases I checked out the customer's amps (one of them was a Sophia amp) and there was nothing wrong.
This is a common response from them trying to cover their butts.
With that said I highly doubt there is anything wrong with the amp, but if you're not sure have it checked out.
In both previous cases Sophia replaced the tubes, and there have been no problems. One replacement had a piece of glass rattling around inside but the tube worked perfectly well otherwise. I'm inclined to think it's a quality control issue more than anything. But they do stand behind the product.
Dan Santoni
How long have you had the amp? Did you buy it used? If so, how old is it?
Edits: 08/16/15
purchased on Agon, 1994 vintage.
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.
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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