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Searcher 11th February 2005 08:51 AM

Tubes lifetime and sound
 
I have a concern about my tube equipment.
The equipment consists of Copland CTA 501 power amp. and Copland CTA 301 MKII pre amplifier.
Dunlavy Athena speakers.

1.
How do one recognise the very first audio impressions when power amplifier tubes (in this case 6550 C Svetlana) are reaching the end of their effective operating lifetime?

It is easy of course if they just stop working, but I suppose this is not the case always.
I have lately being concerned with a sound that seems to more and more lack


2.
Is there a really big difference between different tube brands?
Can I reach a new level of high-end experience by changing the tubes to some other manufacturer?

Would be truly grateful for your response in this matter.

Peter

audiousername 11th February 2005 09:02 AM

Re: Tubes lifetime and sound
 
Quote:

Originally posted by Searcher
It is easy of course if they just stop working, but I suppose this is not the case always.
I have lately being concerned with a sound that seems to more and more lack

Valves don't usually just stop working. They can - that means something has catastrophically gone wrong, but they usually don't. Most failures will occur when the valves are just installed (infant mortalities), and after much use (because they're just old). That being said, valves can last many decades (I know of some which have been used in hifi amps for nearly fifty years) - but that depends greatly on the circuit in which they operate.

I have never heard of any of your audio equipment before, so I can't really comment on how the valves are run (though I do know that some modern manufacturers run their valves in conditions which will make them unhappy...)

What do you exactly mean by the sound "seems to more and more lack" - is the volume reducing? And, how long have the valves been used?

Quote:

Originally posted by Searcher
Is there a really big difference between different tube brands?
Can I reach a new level of high-end experience by changing the tubes to some other manufacturer?

There are differences between different manufacturers of valves, because they all make them in slightly different ways. However, in my opinion, the differences aren't usually enormous. Be very wary of anyone trying to sell you something at an exorbitant price, and promising that it will take you to some sort of new level of enlightenment.

EDIT: I noticed this is your first post, so welcome :wave:

thoriated 11th February 2005 05:39 PM

I've found that output tubes in transformer coupled tube amps will eventually start sounding 'flat' with a progressive loss of maximum power and corresponding distortion increase as their emission and transconductance starts to drop. That would be a good time to replace them, although you could maybe somewhat extend the period of satisfactory use at reduced power by rebiasing.

This subjective 'flat' sound doesn't seem to occur with my DC Coupled OTL design (even with a number of 17 year old tubes in it, even though maximum power has dropped off somewhat over the years), suggesting to me that the output transformer's open circuit inductance and capacitance itself influences this characteristic.

Sch3mat1c 11th February 2005 06:49 PM

Re: Tubes lifetime and sound
 
Quote:

Originally posted by Searcher
I have lately being concerned with a sound that seems to more and more lack
How long have you had the amp? It could be just losing its novelty, as a wife does after being married a few years. :devilr:

Quote:


2.
Is there a really big difference between different tube brands?

No, but despite the scientific fact I'll still get called tone deaf here for saying so.

Tim

richwalters 11th February 2005 10:40 PM

Re: Tubes lifetime and sound
 
Quote:

How do one recognise the very first audio impressions when power amplifier tubes (in this case 6550 C Svetlana) are reaching the end of their effective operating lifetime?
Hi there....How long have you been operating the amp for ? (Very roughly)
Svet 6550C's are newcomers to the market place compared to other tubes. If I said a life of 10,000 hours with a B+ of 250V at 60mA quies then that may be reasonable. I might get told-off at this...However, many amps using this tube operating around with B+ 400V ballpark at 50mA quies then perhaps only 1000hrs. One can strike it lucky, for example one of my 30W amps still uses an original set of coke bottle KenRad 807's made in 1954 and is in constant weekend use.

Some tubes simply fade away as emission drops....as per comments ...however, a high power amp using fixed bias and high gm tubes strung at 600V B+ may go out in style with anodes orange-hot and self destruct.
The only way to obtain a long life is to use generously rated tubes with modest operating conditions. A KT88 with larger anode area put in lieu of a 66 or 6550C will last longer in the same circuit with nearly no change in operating conditions.

rich

mcs 11th February 2005 11:00 PM

Re: Tubes lifetime and sound
 
Quote:

Originally posted by Searcher
Is there a really big difference between different tube brands?
Can I reach a new level of high-end experience by changing the tubes to some other manufacturer?
If this is a "normal" Copland, it's probably a push-pull amp with some/a lot of feedback. That would make the differences in sound between different tube brands smaller, compared to an SE amp with no feedback for instance.

Best regards,

Mikkel C. Simonsen

jlsem 11th February 2005 11:19 PM

Quote:

How do one recognise the very first audio impressions when power amplifier tubes (in this case 6550 C Svetlana) are reaching the end of their effective operating lifetime?
They go euphonic, i.e. more distortion as normal listening levels are maintained.

Quote:

Can I reach a new level of high-end experience by changing the tubes to some other manufacturer?
They will sound somewhat different, but no epiphany will occur.

Unless there are mitigating circumstances like a new beautiful woman in the room.

John

fdegrove 11th February 2005 11:29 PM

Hi,

Quote:

If this is a "normal" Copland, it's probably a push-pull amp with some/a lot of feedback.
Nice looking amps, tons of feedback making them sound as flat as a pancake....
As Mikkel said, it would not surprise me one bit if various tube brands don't even make an audible difference.
The Copland preamp is even worse, measures well but sounds boring. Not my cupper....

I doubt the output tubes are worn out.
Unless the heater voltages are out of range they should last at least a decade before replacement is in order.

Cheers, ;)

P.S. Speakers aren't bad though...:D

Searcher 12th February 2005 07:23 AM

2 Attachment(s)
Thank you for the fast response I got.
I think the equipment is about 2 –3 years old. I bought them used from an audio store about a half a year ago. The store had used them as demo examples so they could have been switched on for a maximum of 7-8 hours / working day for a couple of years.

I missed a word in my original problem description; what I meant was that the sound seems to be more and more thin and flat with lack of low-end bass. Uppers dynamics are ok. , but low end just seems too lean.

As I like the look somewhat the Copland’s physically heavy duty design (please see attached picture) accept that one doesn’t have the possibility of see the tubes beautiful glow in the dark.

So what I am going to do is some mechanical modifications to get at least the power tubes sticking out in the free air above the standard housing lid.

The Copland brand is a product of Denmark and designed by a Danish designer. I understood that he has a very well reputation on designing specially tube equipment with simple (KISS) designs with minimal or no feedback.

This I heard is not the case with latter equipment made by Copland as I think they have left the tube arena and also got some dispute with this designer.
So what I thought I had bought was about zero feedback!!
And now MCS from Denmark is telling me “If this is a "normal" Copland, it's probably a push-pull amp with some/a lot of feedback”.


What I know about the design:
I think it is indeed a push-pull design but what makes it interesting in my ears is that one can switch the power amp to work in “triode” mode with just by turning a switch on the front. Doing this makes the power specs go down from 70 w to 30 w but that is not an issue with my Dunlavys.
Still, having push-pull and output transformer doesn’t state that I have to have a lot of feedback, or??

I have done a lot of DIY projects being electronics engineer myself. So I don’t see any problems with redesigning or modifying the original design if any improvements would be available. But for that work I would need the schematics on the equipment. Is there anybody out there having them available?

And secondly I would need your opinions on these design issues.

Regards Peter

richwalters 12th February 2005 08:51 AM

Quote:

[i]Originally posted by fdegrove

Nice looking amps, tons of feedback making them sound as flat as a pancake....

Hi there.
Yes ...well said........the best sounding amps are those using 5-15dB nfb and using simple circuits....tubes under such conditions are also the least likely to effect the sound before they die of emission. Having said this, an amp using only modest feedback is more able to accept other o/p tubes in lieu without serious sound degradation.

rich


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