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Old 16th June 2004, 07:41 PM   #1
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Default Rogers Cadet III - balance control

I have managed to get an old rogers cadet III working which I bought from Ebay with help fro a schematic posted elsewhere on this site. However the circuit does not include the balance control. Does anybody have a diagram of this. It is possible that one of the ECC807 valves doesnt work properly although I am not sure. I have heard these are difficult to source - is it possible to use ECC83 and change the heater circuit?


A further question is about the operating light on the cadet - it goes on and off after the amp has started - is this normal?


Thanks for any help


Tim
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Old 16th June 2004, 09:34 PM   #2
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The bad news: ECC807 is virtually unobtainable. The good news: It's highly unlikely to be faulty. The sockets in old valve amplifiers often become dirty and the fingers that grip the valve pins lose their tension. A squirt with electronic cleaning solvent such as isopropyl alcohol will help, or if all else fails, a careful poke with a large needle can re-tension pins if it's really necessary. It's not just that the pinout is different, the ECC807 has 50% higher mu than ECC83...

The operating light is just a very small light bulb screwed into a cheap and cheerful socket. I expect it's loose. It ought to be on permanently.

The balance control is trickier. I do have a photocpy of the Cadet III circuit, but it's very poor quality, and I doubt that it would survive a scan. Essentially, the balance control is centre-tapped to earth and changes the relative values of a pair of cathode resistors coupled by electrolytics to the cathodes of V2. You probably need to replace all the electrolytics in the amplifier before it works properly. (I did when I refurbished one recently.)
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Old 16th June 2004, 11:13 PM   #3
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Hi,

Quote:
ECC807 is virtually unobtainable.
Yup....unfortunately.

OTOH, one of our members, Suppersready, may still have a few.

He had them on sale in the "Trading Post" section quite a while ago and someone enquired about them recently.

Should he not have any, I still have some myself but I can only sell you a pair or so.

Another valve with the same pinout is the E283CC but it would require some drastic changes to the circuit to say the least.

Some of these amps go for as little as UKL 18 which is probably less than a pair of ECC807 alone would go for.

Cheers,
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Old 17th June 2004, 09:33 PM   #4
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Thanks for all the help. I have replaced all the electrolytics apart fromsome of power supply ones /? surge protection. I still can't get the balance control to work but the amp sounds good. If you could try scanning the schematic I would be very grateful as I can't find any other full diagrams (the ones on this site are incomplete). The amp I bought had been repaired (badly) and I cant quite work out the PSU layout either whether it is a half wave or full wave voltage doubler... the way it was wired on my amp was as a half wave voltage doubler with one 100uF and then a number of 8 / 16uf caps which I replaced as 24uf as I think they were for surge protection.

I hope you will excuse my ignorance - but I really am a novice with valve amps at the moment

Thanks

Tim
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Old 18th June 2004, 12:57 AM   #5
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To keep the files size down, I've scanned only the pre-amplifier part of the diagram. As you can see, it's terrible quality. I think my copy must be an nth generation photocopy. The balance control is in the middle. If the balance control doesn't do anything, I would suspect that the pot's wiper may not be contacting the track.

I couldn't seem to find the thread where I posted a readable version of the power amplifier and voltage doubler, but if you have found it, it's correct. You need to replace those two 100uF electrolytics that are part of the doubler with parts intended for switched power supplies having the highest ripple current rating you can find. Don't worry - they will still be smaller than the originals.

We all started out ignorant...
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Old 18th June 2004, 08:05 AM   #6
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Thanks I will study that carefully. Changing the caps to 100uF has reduced mains hum - is there anything I can do to reduce it further? THe voltage across the diodes is also up to 277v but less than 300v on the other diagram.

When the amp is plugged in the power on light comes on straight away - before turning the switch. Is this normal?

Thanks again


Tim
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Old 18th June 2004, 09:55 AM   #7
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Quote:
Originally posted by Tandrews
When the amp is plugged in the power on light comes on straight away - before turning the switch. Is this normal?
No, that's not normal at all. That rather suggests that the whole amplifier is in operation the moment it's plugged in, and that the mains switch has been bypassed.

As for the hum, is it 100Hz or 50Hz, and does it sound buzzy or smooth?
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Old 19th June 2004, 10:25 AM   #8
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Thanks I have sorted out the switch so it nolonger comes on whenjust plugged in. The mains hum has much reduced with new electrolytics on the PSU. I would say it now sounded like 100HZ and a bit scratchy but is only evident when the volume is turned up to about 75%+


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Old 19th June 2004, 11:34 AM   #9
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Hmmm. That sounds like HT hum. There's no problem with replacing the 24uF smoothing capacitors with larger values, doubling each one to 47uF would make a very large difference.

The other possibility is that the volume control and associated signal wiring is picking up hum directly from the mains wiring going to the switch on the back of the control. (Although this would cause 50Hz hum.) If I remember correctly, the mains switch wires are just straight wires that pass quite close to the audio circuitry. A better implementation would be twisted pair running along the corner of the chassis as much as possible.

Of course, hum is a subjective thing, and the Cadet was probably one of the cheapest decent amplifiers available at the time, so quite a few corners had to be cut, meaning that yours may already be working as new.
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Old 9th July 2004, 07:36 PM   #10
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Thanks. I think I will leave the hum for a bit as it is not too bad. The balance control seems to work when I dont use the tape input and from the circuit diagram I think this fits.

I have a further question about distortion which seems quite noticeable with 91db speakers at higher volumes and lower frequencies. Is this just a limitation of the system? I have changed the output valves - with no difference. Should I try biasing the output valves? I can't find any instructions about this on the net and don't have a service manual. Maybe its just the 40 year old output transformers as well. I am just using the 3-5 ohm connection although they are 8 ohm speakers (It sounds even worse when I use other windings - although I havent tried all possible connections

Despite all this I really like the sound compared to transistors and I quite enjoy tinkering. However, it may just be better to buy a better amplifier. Would something like an Ella Kit from Hong Kong just be so much better that I would be better going down this direction??!

All advice is very welcome


Thanks


Tim
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