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Rogers Cadet III - balance control

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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
 
Ex-Moderator
Joined 2003
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.)
 
diyAudio Senior Member
Joined 2002
Hi,

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,;)
 
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
 
Ex-Moderator
Joined 2003
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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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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Ex-Moderator
Joined 2003
Tandrews said:
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?
 
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%+


Tim
 
Ex-Moderator
Joined 2003
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.
 
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
 
Ex-Moderator
Joined 2003
Bass distortion: You are hearing the inadequacies of the tiny output transformers. Most commercial amplifiers have inadequate output transformers and can only deliver full power down to 50Hz, but the Cadet is particularly inadequate. I think you have to accept it for what it is, a nice little budget amplifier, but definitely not an earth-shaker.

I'm not familiar with the Ella Kit, so can't comment. If you like the Rogers, then you will probably prefer push-pull to single-ended. Good quality output transformers for EL34 and EL84 are readily available (bigger is better, here), so perhaps you should consider building a Mullard 5-20 or Williamson (but with EL34). Be warned, good output transformers contain a lot of iron, and are expensive.
 
Ex-Moderator
Joined 2003
Sadly, the "specifications" offered at those sites are not worth the virtual paper they're printed on. One of the key specifications for an output transformer is the maximum output at a specified low frequency for a specified distortion. Thus, 10W @ 20Hz for 1% THD would be a useful specification. A continuous secondary with taps is usually a bad sign. Better transformers tend to have four separate secondary windings which are connected in series/parallel combinations to match the load. Try Sowter or Hammond.
 
Owner's manual

Hi Tim

Didn't see this thread before. If you want a photocopy of the whole of the service manual send me an email with your address. I'd send it as a scanned document but I recently 'upgraded' to XP and, just my luck, there are no XP drivers for my scanner, harumph!!

Regards

Paul
 
Modifying Rogers Cadet

Recently I added a 'Millenium' output transformer to one of the channels of my Rogers cadet. Its primary resisrtance is 6.6k rather than ?8k for the cadet - but it actually sounds quite good - much more bass and power! and not much audible distortion. The Mains hum (50hz) as got louder on this channel probably because of the better frequency response?

Any suggestions about tweaking the push pull circuit to compensate for the bigger transformer and still using ECL86s

i was thinking about designing a new power supply and mounting away from the amp The only mains transformer I have gives an out put of 250ac and therefore around 350 for the HT which is quite a bit more than the 295v recommended

Transformer came from here:-

http://www.livinginthepast.demon.co.uk/xformer.html


Does this all seem a bit crazy - should I just build a new push pull from scratch - It would seem difficult to change the Rogers to a star earth configuration which would I guess improve the hum

I am still learning but enjoy experimenting

Any suggestions welcome - particularly a good schematic of a suitable psu using valve rectification - I was thinking of using a 5Z4 - as I have some - are there limits on the size of smothing capacitors for valve rectifiers?

Tim
 
Tim,
Take up Suppersready's offer of getting the copy of the Operators manual. It contains full schematics. With a little work the Cadet III is a decent little amp. If I had to make a critisism of it, it would be that it had subjectively too much bass, so if yours appears not to have enough bass I would be checking cathode bypass caps, coupling caps etc. - of course it maybe the Vintage Warfedales I run mine into. This arrangement together with an Electronic Timer and an old Philips FM Tuner is my "clock radio" which wakes me up each morning.

Cheers,
Ian
 
Ex-Moderator
Joined 2003
Re: Modifying Rogers Cadet

Tandrews said:
Does this all seem a bit crazy - should I just build a new push pull from scratch - It would seem difficult to change the Rogers to a star earth configuration which would I guess improve the hum

You'd do better to build a new amplifier from scratch. The Cadet is a set of compromises that match. Start modifying and you're likely to show up other weaknesses. You might like to try SY's "Red Light District" amplifier as a good first project.
 
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