Seymour Duncan Convertible 60W combo - AAAARRGGHH !

[Kid Squid]

Hi fellas,

Just came across this old post, and thought I'd reply, as the problem was down to the vias underneath the main filter caps.
After going around in circles for a while, I started from the beginning (again),  de-soldered the filter caps and checked continuity between the top and bottom tracks, and hey presto, one of the vias were missing !!

Was kicking myself...

Re-made the connection, and works a treat.
Sounds bloody lovely too.

Learned a load doing it too

Thanks to all the chaps for replying with help, it was really appreciated.

Kindest regards,
Steve

 

[tnjent98]

Hwyl lads,

First of all, I got to say, I am not by a long shot, a Guitar amp expert, so go easy on me...

Had a bit of spare time today (very rare !), and decided to have a look at that bloody SD convertable combo amp, and have another go.
this time with the speaker connected, instead of a load resistor.

voltages, Tubes and the O/P transformer seem ok.
Bias is a fixed negative voltage applied at point E, and is around 60 odd Volts

Plugged a guitar in, and -Well, it's loud as hell, but it seems to have turned into a 'ring modulator' of sorts :shock: and the problem seems to be coming from around the driver stage.

if fed with a 1KHz tone, from the instrument input - at the driver grid 1 - signal is clean, but the output on anode 1 is 'modulated.
After putting a scope on the amp , getting a real wierd waveform across the speaker, looking like the B+ is somehow, getting onto the output.

i have tried with the output valves removed,(2x EL34's) but the problem is still there ( modulated driver output on plate 1)

big version is here
http://www.diyguitarist.com/Schematics/60wSDC-Schematic.gif
Output drawing is here
http://www.diyguitarist.com/Schematics/SDC60-PowerSchem.pdf
It is a differential driver amp (ECC83 - 12AX7), with one side being fed off the fx return, and the other grid is fed off the feedback tap, off the O/P transformer (16 ohm output). now what i cant seem to grasp, is, why the diode is in the cathode circuit - what is it doing ? ( it is a CIL-41301 - a current regulating diode- I've never come across one of these before !)
voltage on the anode of the diode, is 1.88 V, whereas the cathode side , is around -50V.

CR5, CR12, JMP-1, JMP-2 are not fitted in this amp (240V operation)

I'm scratching my head a bit with this.....any help is really appreciated.
Steve :thumb:

Hi Gents!

I have one of these 60 watt convertible amps on the bench, I need the schematic, can some one send them or post them, the old link is dead.
thanks