Soldano Jet City 20

working on a EL84 based combo 12 inch that sounds like a cat in a blender,

looks something like this>

too much gain, lousy reverb, output wave form looks hideous even at clean settings, got our work cut out on this one,

yes this is an offshore product selling under the Soldano name,

here is a before and after of the pc board after a weeks worth of R and D>

here is the preamp circuit, mods were reducing gain by increasing the cap/resistor combo to 3 meg/100 pf, kind of like Fender but different, Fender uses 10 pf but this amp has a treble rich speaker and cabinet,

we changed stage 1 plate R from 220 to 100 K, and bias resistors as well, (circled)

also, the voltage divider (R36/R33) was not layed out right so the pc board has an error in it, so we had to cut and splice to fix that,

we took out the afterthought cap spliced across R34 which was a stabilizing mod and the waveform got a lot better, that cap was causing hideous distortion on one side of the wave, next we adjusted a few coupling caps and tweaked the second divider,

also, two of the 12AX7a tubes (EH) measure 15 on the tube tester where we should have numbers around 8 or 9, where they got these supecharged tubes and how this happened is a mystery to me, never seen this before, so we reduced gain by swapping in some nice Sylvania 12AX7WA premium mil tubes,


DC volts blue  AC = green>

no we head for the power stage, reverb is mixed into a cathode follower driver and for some reason does not sound good, very weak and grainy, so we fed the return signal to the downstream side of the 3 meg like Fender and it sounds great!

put a 470K from grid to ground as the now open grid of V-3a was picking up 60 cycle hum as the grid was floating around like an antenna ,

a lot of companies are moving the tone stack further and further down stream, this ain't bad unless you are using cheap caps, the result is horrible distortions being created in the tone stack by increased AC voltages on inferior caps, from here it all gets amplified by the pwr amp and comes out the output transformer in all it's splendid glory,

luckily this is easy to fix with some good ol orange drops, have mercy the signal now actually resembles a sine wave,

the PI and pwr tube circuits were ok , however people have said that changing the 16 ohm spk to a Celestion 8 ohm is a tone change for the better.

here bis the stock reverb circuit,

we changed the tube to a 12AT7 and the rest of the circuit to a stock Fender Princeton  Reverb circuit,

old dude came in to pick up the amp so i know he will like the mods,

next one that comes in will take 2 hours instead of two weeks,