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Saturday, 14 June 2014

Meanwhile....

Cooking up a few things for you lovely people....

.... but while you're waiting , why not head along to my YouTube channel for some bits & bobs....

https://www.youtube.com/user/MrAndydoz/videos?view=0&flow=list&sort=dd

Sunday, 1 June 2014

Vox AC30 repairs

This delightful amp wandered in.

Apparently saved (some while ago) from landfill. The current owner had got it going, but after a fairly short time developed some faults and was put to one side.


 Good fairly unmolested condition.
De Banks EL84s. Never heard of them! Probably Chec or Russian. Logo looks a bit "Edicron" to me.

The driver's an Edicron ECC83. The rectifier a Pinnacle. The phase splitter is Mullard. Everything else is Brimar.
 NOT Celestion Blacks. Original through, and in great condition.
Under chassis of the output stage. Every single resistor in the cathode and screen is out of tolerance.
Electrolytic caps in the power supply and output stage reform 100%. Not so for the cathode by-pass caps in the pre-amp and tremolo circuit. New fitted. There's also a nasty Hunts 0.002 to go in the "Normal" channel.

Tremolo/Vibrato was not working. The oscillator was doing nothing, and the modulator was low in gain. The oscillator had an open circuit anode feed resistor. The modulator had a couple of resistors well out of tolerance.

First check in the output stage aren't too good. The phase splitter is way out of balance, the output is distorted and low and the anodes are starting to cherry-up.

3 out of the 4 resistors in the phase splitter were out of spec. All 4 changed. Bias checked and now at 46mA per tube, which is near enough. One side of the push pull section had obviously had a harder life then the other, with the phase splitter being so out of balance. This would explain the variances in the values of the cathode resistors in the output stage.

Sound is now bright, distortion is gone, and the power is there ... who needs 100s of watts?



Lovely. Now to be re-united with it's owner.