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
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
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
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
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.
When you're ready, consider building an EL84 amplifier, perhaps by rebuilding a Leak Stereo 20, or perhaps using new transformers.
Missing Component
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.
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.
Re: Missing Component
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.
jkeny said:
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.
component sifferences
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?
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?
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.
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.
Oops
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.
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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