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dukep 14th October 2007 01:05 AM

cambridge p60 is alive
Well it has been years waiting patiently in a box. the pre-amp section is crippled due to corrosion of the pots, so I've decided to be an amp only. My vintage soundcraftman as the preamp.

I rebuilt the power supply (new caps/ rectifier/ bypass caps)
Then some flashy banana plugs and phono inputs. test.. hmmm-- sweet!

I had forgotten how well this amp performs. This cute little amp has a new life.

Anyone out there know the bias setting? I used the standard 5mv across the emitter resistor or ? (note:I don't have a distortion analyzer- but a good ear and 0-scope.)

Heatsink is a bit small and disjointed (4th transistor is on a 2nd section). It runs hot (after jamming 30min) and the 4th transistor runs cooler than its complement. Anyone know what the original specs were?

AndrewT 14th October 2007 10:15 AM

output stage is what?
Integrated darlington, or two stage EF, or two stage CFP, or somethng else?
5mV across the emitter resistor sounds like it may be CFP.

dukep 14th October 2007 01:34 PM

to-3 outputs
This has a pair per channel, mj802 & mj4502; bjt's
100v 30 amp outputs. Rail voltage runs +/-39v from a toroid xformer.

I slowly turned it up from 2mv to 5mv. the sound and crossover distortion improved.

Leolabs 15th October 2007 06:15 AM

Hi,anyone here got any information on the P50 MKII???

dukep 15th October 2007 04:21 PM

found the schematics
This helped me, maybe others too. The bias is set at 7ma q-current.
measure current with + fuse out (~20), turn until x+7ma.

Then do the other channel.

and maybe applicable to the p50.

Rolf Zetterberg 16th October 2007 06:15 PM

How do you know the current(which is what is the issue here) without knowing/telling the value of the resistor?
The P50 has TIP35/TIP36 output em followers,but with a little unusual arrangement of the resistors(0.15ohms).They are placed in the collectors and directly connected to + and -
I wish there was a standard using a 1ohm resistor to measure the current,and then the resistor is shorted in normal use.Would create a lot less errors,I believe.

dukep 18th October 2007 11:58 PM

bias never easy
I agree.

I usually measure the voltage across the emitter resistors which is much safer than pulling a fuse.
When you know that 7ma should flow through the emitter resistor, and it is 0.15 ohm ( this amp is a bit weird, with collector and emitter resistors). Run the calculation,, v=ir carry the zeros .....

I read the current by placing the multimeter on the amp output and emitter pin of the transistor, Mr. Mj802. .. but higher is not necessarily bad. Just closer to class A, but check the heat!
I'll be changing the heatsink anyway because my speakers are 4ohm.

I was once instructed to use a distortion analylizer ($$$) for a SAE amp. Don't have one. JUst tweek it and test, listen, and feel the heat. As we know, it is highly subjective. That's why it has us dIY's working so hard.


frankwm 3rd December 2007 05:36 AM

New Vintage Cambridge Audio site!!
I've, in the past seen some requests for info on the original Cambridge Audio gear - have P50 MkII / P60 / T55 / R50 myself - not all working (...).
Just over a week back I created a Google Groups site.

At present I've donated my Cambridge Audio P50 Service Manual - 80pp - contains all the info on the 3 types of that amp - there's a P60 circuit diagram - info/reviews on these + the T55 & R50 loudspeakers.
Stan Curtis is a member too!!

Hope that the site will be useful for info - also maybe some discussion on Cambridge Audio gear!

All best

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