• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Clarke and Smith EL84 Amp

Hi Folks,

About to resurrect a Clarke and Smith Amplifier chassis which was used in a 1960s "Talking Book" outfit. These were used by the Institute for the Blind to supply subscribers with audible newspapers/books etc. I don't have the unit, just the amp.

Basically a tape player, magnetic pickup, first/second stage, volume/tone control and an EL84 used Single-Ended. EZ80 rectified power supply.

I thought I'd ask before I delve into it, anyone have a circuit diagram for these units handy?
Would save a bit of head-scratching time... :)

Alec
 
Hi Folks,


I can't believe it's been so long but never mind..... The dark evenings are now here and it's back to the indoor projects in the evenings!


Photos of the amp:
[IMGDEAD]https://photos-3.dropbox.com/t/2/AAA7baTVXdPYFtjKZEOfAP4bg7z7PDwEosVJsexGt7YqIQ/12/621785182/jpeg/32x32/3/1509321600/0/2/sDSC_3831.jpg/ELz8wYQFGAwgAigC/K5cIldGk2uCR0uFgZduULDYNXgvllRH0MlujAL3QzYA?dl=0&size=1280x960&size_mode=3[/IMGDEAD]
[IMGDEAD]https://photos-3.dropbox.com/t/2/AAA7baTVXdPYFtjKZEOfAP4bg7z7PDwEosVJsexGt7YqIQ/12/621785182/jpeg/32x32/3/1509321600/0/2/sDSC_3832.jpg/ELz8wYQFGAwgAigC/K5cIldGk2uCR0uFgZduULDYNXgvllRH0MlujAL3QzYA?dl=0&size=1280x960&size_mode=3[/IMGDEAD]
[IMGDEAD]https://photos-3.dropbox.com/t/2/AAA7baTVXdPYFtjKZEOfAP4bg7z7PDwEosVJsexGt7YqIQ/12/621785182/jpeg/32x32/3/1509321600/0/2/sDSC_3833.jpg/ELz8wYQFGAwgAigC/K5cIldGk2uCR0uFgZduULDYNXgvllRH0MlujAL3QzYA?dl=0&size=1280x960&size_mode=3[/IMGDEAD]


Circuit Diagram found recently on the Net, accurate and represents the unit I have here:
[IMGDEAD]https://photos-5.dropbox.com/t/2/AAAtQJth4BAYpylmQrgJDTGrJZMn_mWKeyvR5DIR9TTv3A/12/621785182/jpeg/32x32/3/1509321600/0/2/Ex-C%26S%20Talking%20Book%20Mk1%20-%20AC%20Existing.jpg/ELz8wYQFGAwgAigC/xwACK_DLhxhvj7CWMsAsbrKNTltl-3K0Uq-Te4fH5u8?dl=0&size=1280x960&size_mode=3[/IMGDEAD]


It is missing the input transformer and the tape unit drive motor.
What is there currently: a mylar 0.1uF capacitor on the input to the first stage magnetic pickup preamp, this would do I expect for a Phono input?


What I would propose to do is:
  • To go through the circuit and replace the aged electrolytics, the small Mullard Mustards look to be in good condition.
  • Mount the unit within a replacement housing to smarten it up and add a bit more protection/electrical safety.
  • Add a switch to the HT Line after the choke for warm-up purposes.
  • Add an additional input to the second stage preamp, probably at the intersection of C7 and the 1M RV1 to function as a line-level/CD Input.
Any thoughts/recommendations? It's been a few years since I've repaired tube amps.


Alec.
 
The gain structure is set up for the mV. signal level a tape head produces. A single 12AX7/ECC83 section is enough, when a CDP is the signal source.

The B+ supply already starts "softly", as the EZ80 is indirectly heated. No switch is necessary.

If music bandwidth is to be had, the O/P trafo requires upgrading. Also, NFB of some kind is needed in order for the damping factor to be satisfactory.