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Old 12th February 2005, 08:20 AM   #11
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Quote:
Originally posted by Searcher
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.
You can be sent to hell for covering up the glow of valves. For all we know there could be a forest of silicon growing inside there!

Quote:
Originally posted by Searcher
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.
Great idea! Though you will may run into problems re-routing wires from the old to new sockets. And kill any resale value...

Quote:
Originally posted by Searcher
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?
If this is truly a simple design you could probably trace it out yourself. More than likely it isn't that simple - I suppose just open it up and have a look inside. Sorry, I don't know of any sources of Copland schematics.
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Old 12th February 2005, 11:58 AM   #12
mcs is offline mcs  Denmark
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Quote:
Originally posted by Searcher
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??
A push-pull amp that runs in pentode mode does need quite a lot of feedback, to get the output impedance down to a resonable level. In triode mode the output impedance will be a lot smaller, so you don't need a lot of feedback (or any). But does the pentode/triode switch just change the operating mode or also the feedback? You'll have to study the circuit (or get a schematic) to find out.

Quote:
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.
The only problem would probably be the PCBs - modifying a p-to-p design would be a lot easier...

Best regards,

Mikkel C. Simonsen
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Old 12th February 2005, 12:37 PM   #13
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Default My ECL82 just died on me!!!

I didnt watch it during the last moments of its life because the amp was left at my parents place. Anyway, now it is playing softly with tons of distortion and cracking. Sort of like a rock guitar sound.
Visually, you can tell a dying tube from its faded glow.
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Old 13th February 2005, 07:16 AM   #14
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All right now, I have to check the level of nfb in “triode” mode as this is the only mode I use my power amp.

The only remaining questions are as follows:
Do I have to use a tube tester circuit when changing the 6550 C output tubes or do you get readily matched pairs or is this a matter of no bothering?

Is there any need for mods If I would go for the KT 88?

Regards Peter
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Old 13th February 2005, 10:11 AM   #15
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Default Re: Re: Tubes lifetime and sound

Quote:
Originally posted by audiousername
(I know of some which have been used in hifi amps for nearly fifty years)
I've often pull tubes out of 40-50 year old gear that test as good -- sometimes better than new.

dave
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Old 13th February 2005, 10:14 AM   #16
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Quote:
Originally posted by Searcher for that work I would need the schematics on the equipment. Is there anybody out there having them available?
I always feel it is a good excercise to just trace the circuit -- a little harder if it's on a circuit board...

dave
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Old 21st February 2005, 03:17 AM   #17
amperex is offline amperex  United States
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Default Tube life

Tubes near the end of their useful life sound flat and may produce higher background noise.

Preamp tubes normally operate from 8000 to 15000+ hours. Power pentode tubes normally operate from 5000 to 8000+ hours. 6550s have a lot of cathode emissions due to large filament wattage, thus should function well to 8000 or more hours.

Some amps run the tubes very hard, thus examples for less than minimal hours listed above do exist, but that is not too common.
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Old 21st February 2005, 07:32 AM   #18
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Default Re: Tube life

Quote:
Originally posted by amperex
6550s have a lot of cathode emissions due to large filament wattage, thus should function well to 8000 or more hours.

Hi there.........that's not what I'm finding......6550's don't have high heater wattage....compared to KT88's when it comes to cathode area ....I find 6550's mighty sensitive to heater current variations.......vary the heater volts +/- 10% on JJ 88's EL34's and emission says the same.....whereas on 6550 family it varies.
Funny enough all orig power tube spec sheets quote 6.3V +/-10%
but recent made tubes are often skimpy on this. Svet 6550B's are notorious on emission variations.....otherwise that tube is actually well sounding. life expectancy of 8000hrs is too optimistic...

richj
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Old 21st February 2005, 01:24 PM   #19
amperex is offline amperex  United States
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Default 'do not find wattage higher'

I was referring to more common and earlier design tubes such as 6V6GT & 6L6 having .45 and .9 amps filament current.

Granted, the 6V6 & 6L6 are tubes of lower output, but I would prefer a 6550 at 35-watts plate and 10-watts of filament vs a 6L6GC at 25-watts plate and 5.7 watts of filament for longer life. Almost double the filament power vs plate dissipation of 1.4 times higher.
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Old 21st February 2005, 01:37 PM   #20
amperex is offline amperex  United States
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Default 'life expectancy too optimistic'

I am refering to NOS US manufactured tubes with superior oxide filamants & higher vacuum unlike Russian & other new manufacture tubes. Per E. Barbour, Sovtek were only evacuated to 10-5 Torr where as NOS tubes were at least 10-6 Torr. Better vacuum, longer life.

Many 6550s from the early '60s are still functioning well. Although exact data on audio tubes does not exist to my knowledge, 'sweep' tubes such as the 6LF6 with simular design features of high filament power vs plate disipation as well known to operate 12000+ hours within manufactures design ratings.
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