| stephen_chrol |
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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| HollowState |
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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| EC8010 |
| 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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| stephen_chrol |
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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| jkeny |
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?
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Opinions much appreciated
John |
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| jkeny |
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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| EC8010 |
| 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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| jkeny |
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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| poynton |
| 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
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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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| EC8010 |
| Our recycling centre has a sign saying that you're not allowed to take stuff away... |
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| jrevillug |
| quote: | | Our recycling centre has a sign saying that you're not allowed to take stuff away... |
Ditto- I asked about a very nice Thorens TD160 MKII at my local 'Recycling' centre, the bloke said that he was not authorised to sell items. The UK Govt's message on waste it 'reduce, reuse, recycle'.
Humph. :)
James |
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| EC8010 |
| quote: | Originally posted by jrevillug
Ditto- I asked about a very nice Thorens TD160 MKII at my local 'Recycling' centre, the bloke said that he was not authorised to sell items. |
That's disgraceful. I imagine you won't bother asking next time... |
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| poynton |
| quote: | Originally posted by jrevillug
Ditto- I asked about a very nice Thorens TD160 MKII at my local 'Recycling' centre, the bloke said that he was not authorised to sell items.
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You should have said 'I wasn't going to pay for it, I was just going to take it away !'
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I used to live near Farnham in Surrey.
Farnham recycling centre is Surrey County Council and does not permit sale of items.
4 miles away is Aldershot tip, run by Hampshire Council. They have a large shed where reuseable items are put for sale at a nominal price. A very popular place !!!!
Andy |
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| jkeny |
I don't ask, I just take - I'm always dropping off something for recycling myself so it's not as if I wander in to have a look.
I was prevented when I asked staff to plug in a scrapped Mac G4. They then said that nothing could be taken out of the recycling centre - I argued that I was truly recycling but to no avail. Since then I just remove what I want & don't bother with the staff. |
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| jkeny |
Checked the electro caps, so far all seem to be well (using a multimeter with uF reading) - so hum & hiss is probably inherent to the design. Checked the wiring runs & signal wires seem to be well seperated from PS wires - so problem not here then.
I have two spare 220uF 400V caps & I figured to parallel the existing Cadet Sprague 47uF (this is what I measure on the multimeter) smoothing caps with these - these are a higher value to the Cadet III component values list (16uF 350V) or the schematic drawn by dld in 2004 (24uF 450V).
I presume any cap above 300V rating will be fine here as the voltage test points on the schematic show 265 & 260V.
Other suggested ways of reducing hum are to try star grounding instead of plane (chassis) ground which is in the Cadet. Looking at the Cadet wiring this is probably a lot of work but does anybody know if it would be worthwhile?
One more mod suggested| quote: | | 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 than these mods does anybody have a good schematic which uses the 3 ECC807 & 4 ECL86 tubes?
John |
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| jkeny |
| Component values list |
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| jkeny |
| Schematic drawn by dld 2004 |
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| EC8010 |
| quote: | Originally posted by jkeny
Other than these mods does anybody have a good schematic which uses the 3 ECC807 & 4 ECL86 tubes? |
You won't want to design anything using ECC807. They're unavailable and there is no equivalent. Rogers Cadet owners will rip your arms off to get them.
I would advise against increasing the value of the smoothing capacitors unless you also replace the rectifiers... |
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| jkeny |
Ah good point re smoothing caps - I didn't consider the extra stress on the rectifiers, EC8010.
Do you consider it worthwhile to change from common cathode bias resistor (130R with paralled cap) to individual bias res 270R with biased cap?
Another area that occurs to me is the power supply - it seems fairly basic - any ideas for upgrading it?
I take it ultra linear operation requires an ultralinear output transformer?
Don't get me wrong, I don't think this is a bad amp (especially for free) I'm just keen to get the most out of it.
Thanks for all your help
John |
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| EC8010 |
| I don't think there's much to be gained from modifying a Cadet, although separate bias resistors might help. You should accept it for what it is, a nice sounding, but very cheap amplifier. The output transformers are far too small to be able to manage bass with low distortion. They make a nice treble amplifier in an active crossover system, though. |
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| jkeny |
Thanks for the advice EC8010.
I'll probably go the Baby Huey amp route using the ECL86 tubes from the Cadet. Just looking into how much the OPT will cost.
John |
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| poynton |
HI.
If it's all complete and working, I would sell the Rogers as it is.
As EC8010 says, the ECC807 are very rare and you should get a good price for it.
Andy |
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| jkeny |
yea the Cadet is all complete & working but I thought of using it's tubes, trafos & chassis to rebuild something sonically superior such as baby huey. I figured that if I sold it and then bought the parts for a baby huey amp I would be out of pocket, besides I like the retro look & it's use of wood blends nicely with a Rega planar II deck & Rogers LS3/5a speakers.
I know I'll need to source two better output trafos (icluding Ultralinear taps) but wondered about rewinding something myself - could the Cadet OPTs be rewound to something better or is it the lack of iron that's the problem? Where is a good cheap local (UK & Irl) source for these trafos - remember I'm in experimental stage so don't know if I will be staying with tubes so don't want to shell out for Lundahl's?
This brings me full circle to an my original post| quote: | | 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. |
So any ideas how good it's preamp stage is using the ECC807 tubes? I've searched here & the net but found no schematics just refs that the tube was used in Cadet & VTL 'minimal' preamp - anybody got schema?
I know I'm full of questions but I'm just a tubes noob so be gentle
John |
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| EC8010 |
The RIAA stage isn't too bad for the era. It's the lack of iron that's the problem with the output transformers, so rewinding them wouldn't help (and it's the winding that is the expensive bit). Replace the electrolytics with low ESR types such as Elna RSH, replace the coupling capacitors with film/foil polypropylenes and check each resistor to make sure its value hasn't changed. Enjoy the amplifier for what it is.
When you're ready, consider building an EL84 amplifier, perhaps by rebuilding a Leak Stereo 20, or perhaps using new transformers. |
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| jkeny |
I did some more testing of Cadet III components last night and I couln't find the 6k8 ohm resistor which appears at the bottom of schematic (runs from the 16 ohm tap on the opt to pin 2 of tube).
I could find the 390pF styrene cap but not the res. Is the schematic correct? Am I safe to put in this res? I wonder how it was left out?
I am connecting my Rogers Ls3/5a's to the 16ohm taps as they are rated at 15ohm so this might well have affected the sound I aqm hearing. |
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| jkeny |
| Here's the schematic |
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| EC8010 |
| quote: | Originally posted by jkeny
Is the schematic correct? |
Yes, it is. It was my original sketch (on the back of an envelope) that p10 redrew. I took the diagram from the manual and by looking at a Cadet III on the bench. I'd say someone deliberately removed that feedback resistor. Look hard and you'll find the cut leads or freshly melted solder. Real loudspeakers aren't resistors, so you "15" Ohm LS3/5a won't bother the 16 Ohm output of the Cadet at all. You might like to try the 8 Ohm output, although the reduced power may make the problem of a small amplifier and inefficient loudspeakers even worse. |
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| jkeny |
Thanks EC - one other difference I noticed - the R3 resistor (the one which goes to pin 2 & is bypassed by 40uf cap)on all schematics is 1.2k but mine measures 4.55K & is coded as 4.7K (yellow violet red silver).
Anybody confirm this? What effect would this change have? |
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| jkeny |
| Should I change these components to the values on the schematics - put in the 6.8K feedback res & change R3 to 1.2K ? Could this cause the failure of a component or overheat tube? |
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| EC8010 |
| 4k7 instead of 1k2 will bias the input valve to a different point, with maybe a small change in distortion or noise. You could change it if you liked, but I'd be prepared to bet you won't hear the difference. Fitting the 6k8 resistor will make a big difference. It will reduce the gain of the power amplifier, reduce its noise, reduce its distortion, and reduce its output impedance. Do it. |
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| jkeny |
| Thanks EC I will do put in 6.8K feedback res & report back |
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| jkeny |
EC one other difference in the amplifier section - the 6.8pF cap between pin 6 & pin 9 doesn't exist on original schematic but is on your schematic & exists in the circuit. Can you say what this does?
Thanks again for all your help |
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| EC8010 |
| That's right, that 6p8 capacitor was what made me draw the circuit from the real amplifier in front of me. It applies positive feedback at HF, but only from one side of the push-pull output pair, so I imagine it's there to tidy up the square wave response due to the output transformer having unbalanced leakage inductances from each anode terminal. I ought really to have tested it (by removing it) when I had the amplifier on the bench but I found it impossible to get a good square wave response due to the tone controls that couldn't be set perfectly flat. |
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| jkeny |
I made a mistake - the 6.8K feedback resistor is in the circuit already - when I measured this res, I measured it in circuit, got 220R value & assumed it was the R4 resistor on the input valve. When I checked it's position again & checked the res colour code this showed it to be 6.8K feedback res. Testing it out of circuit confirmed this. So feedback res is fine.
R2 1M ohm res from R1 to ground is missing from circuit ( & your schema) but is in Rogers schema. Not significant I presume?
I will change R3 on the input valve from 4.7K to 1.2K to hear what affect it has, otherwise it seems that all components measure correctly. |
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