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Old 1st November 2006, 03:44 PM   #1
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Default Rogers Cadet III Power problem ?

My amp began to smell of burning today, the 470 ohm resistor in the power line was obviously very hot and discoloured. There was also a distinct hum which I've never had problems with before.

Being new to Tube Amps could someone kindly point me in the right direction ?

(I have some electrical background).

Thanks in anticipation.
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Old 1st November 2006, 06:47 PM   #2
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Hi Stephen,

Without being there or seeing a schematic, I can only give you the basics of what to look for in general order of preference. And I assume you mean a 470 ohm resistor in the B+ supply line. Not the AC mains.

1) Bad tubes, especially the output tubes.

2) Leaky coupling capacitors at the grid of the output tubes causing lowered bias and excessive current through the tube. (assuming you have fixed bias) Look for signs of tube overheating like a plate turning dull red.

3) Failing negative bias supply causing the same condition.

4) Leaky/failing power supply filter capacitors draging down the voltage level.

Hope this helps.

Victor
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Old 1st November 2006, 07:45 PM   #3
EC8010 is offline EC8010  United Kingdom
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It's most likely to be leaky electrolytic capacitors. I refurbished a Cadet III a couple of years ago and after the first five electrolytics tested faulty when removed, I replaced the lot without wasting any more time in testing.
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Old 3rd November 2006, 07:48 AM   #4
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Chers chaps, did some checking last night and yes the Electrolytics are leaking like a seive !

I'm now changing out the lot.

Thanks again.

steve
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Old 1st December 2006, 05:06 PM   #5
jkeny is offline jkeny  Ireland
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Hey, Posted this on another thread but thought I should post here to existing owners of the amp
Quote:
I just rescued this amp from a recycling centre. It's the integrated version with control unit & amplifier in 1 case (wooden - will post pics when I get a chance).

I don't know if it works yet but I wondered how its phono stage was rated in the grand scheme of things (the EL86 tubes were considered low noise & high gain I think). I need a phono preamp/amp & thought this would be a good place to start.

I am a solid state guy but always wanted to experiment with & hear a tube amp. So this is a perfect free intro (if I don't have to replace tubes - I know 40 yr old caps should be tested & probably replaced).

Before I spend any money on this is it worthwhile?
Opinions much appreciated
John
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Old 3rd January 2007, 08:49 PM   #6
jkeny is offline jkeny  Ireland
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Plugged in Cadet III - it works!!! much to my suprise as it had been thrown into the bottom of a wire cage in the recycling centre ( I had toclimb in to retrieve it).

Anyway, a worthwhile find although it is noisy (Hiss & Hum). One ECL86 tube is hotter than the rest - will test all electrolytic caps & replace.

Too soon to decide if I prefer tube to solid state. I can hear some potential (warmth,clear top end,etc) in the sound but maybe it's just not as precise as SS?
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Old 3rd January 2007, 10:00 PM   #7
EC8010 is offline EC8010  United Kingdom
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Default Now, that's what I call recycling...

Quote:
Originally posted by jkeny
... thrown into the bottom of a wire cage in the recycling centre (I had to climb in to retrieve it).
Good for you! Definitely replace all the electrolytics and possibly the ECL86.
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Old 4th January 2007, 12:27 AM   #8
jkeny is offline jkeny  Ireland
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Default That's what I call recycling

Thanks EC8010 - I also recycled from the same cage (although not at the same time) - three Samson Servo 550 solid state power amplifiers (all working), Ecler Smacpro40 mixer preamp, a Rega planar II turntable. I have only visited this recycling centre about 6 times & I often wonder what I have missed. It seems a shame that such useful & interesting equipment is being destroyed (I don't know what they actually do with it but I doubt it is refurbished).

Anyway, I will change the electrolytics & implement some of the changes you suggested on another thread - change 24uF smoothing caps for 47uF, change 100uF PSU caps for modern low ESR high ripple caps. There was also a mention somewhere about changing the biasing res from one shared 130R & bypass cap to 270R & bypass cap to each tube's plate(? - I'm not a tube guy so forgive me if this is the screen)

Other discussions I read concern the PU adaptor - I plan to try to run the Rega planar II with high output moving coil cartridge into the Rogers Cadet III & using my Rogers LS3/5a speakers - might well be a nice system - it looks nice & retro anyway with all that wood on display.
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Old 4th January 2007, 08:37 AM   #9
poynton is offline poynton  United Kingdom
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Default Recycle and Reuse not Scrap!!

Quote:
Originally posted by jkeny
I have only visited this recycling centre about 6 times & I often wonder what I have missed. It seems a shame that such useful & interesting equipment is being destroyed

It's a pity that so-called recycling centres don't live up to their name and RECYCLE.

Most seem to through away!!!!!!!!!!!!!!!!!!

Andy
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Old 4th January 2007, 09:15 AM   #10
EC8010 is offline EC8010  United Kingdom
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Default How ludicrous can this be?

Our recycling centre has a sign saying that you're not allowed to take stuff away...
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