Guitar Gear Forum

So....before i give any tips and tricks for this amp....and im speaking of the DSL40c, the 100h users may have the same problem.

so i buy all my amps used, when i bought this amp....hooked the scope up to make sure everything was testing fine....and i got one hell of a parasitic oscillation coming through.

so i took the chassis out of the cab and started doing more inspections around the PCB...and i noticed something odd.



i dont see the 47pf cap that usually goes across the PI stages plate resistors......either im blind....or its not on the board at all. most people think of this as a fizz cap and it does remove alittle fizz....but its actually in place to prevent oscillations from happening.

so after i placed a 47pf cap across the plate resistors of V4 ( the pi stage )....the parasitic oscillation went away....and i got a smooth sinewave from that point on.



if you can follow the picture above, and place a 47pf cap across the plates of V4....just like the picture shows.

the reason im talking about this, i bought my amp a good while ago....and i figured maybe someone cut it out doing some type of science experiment....until a friend of mine brought 2 of the dsl40c amps to me to do some touch up work....and he mentioned 1 amp was burning through V4 left and right....and the 2nd amp sounded out of place.

and ill be damned....both of these dsl40c's....which were bought brand new....had the exact same problem.

if you dont own a scope and cant test for oscillations, its still safe for you to place a 47pf cap across the plates of V4....just to be sure...and safe

EDIT: i forgot to mention, make sure the capacitor is 47pf with a min rated voltage of 500v

This is awesome, TTBull. I really appreciate the info you're posting.

Safe to assume V4 has same function (and need for cap) on the 100H?

And, that's a silver mica cap, right?

Please, continue with the tips and mods on this thread

yes i would say the 100h is the same way, being the dsl40c looks to be using the same PCB

and yes this is a silver mica capacitor.

make sure you test the resistance of the plate resistors over V4, these were all out of wack, 1 was 29k and the other was 73K.....should be 100k and 82k.

if the plate resistors are off, then you have something screwy taking place being when the pi stage isnt working right it can cause the plate resistors to drift like you wouldnt believe.

not to mention if you do happen to have that 47pf cap in the amp already, another 47pf cap will not hurt being most people replace this 47pf cap with 100pf to help reduce preamp fizz

...

...

...

Awesome stuff! Much appreciated!

Do you know if the Resistor / Capacitor numbers identified are same on the 100?

Interesting about the slope resistor. I have a 33k and 48k on my JCA, and the 33k really helps liven the feel of the amp, and give it a bit of Marshall flavor.

I may give that a try, but it does sound like it'll change the base tone of the amp a bit.

...

...

...

Problem #2

the 2x 330uf filter caps and the 2x 100uf filter caps coming right off the rectifier.....are only rated 315v to 350v....when the dsl40/100 fires up anywhere from 460v to 470v.



C78 and C79 are the 100uf filter caps in question
C86 and C87 are the 330uf filter caps in question

this is how the original DSL/TSL amps were like, filter caps rated at a low withstand voltage...granted they can handle it....until they cant and eventually go out due to over voltage.

it might be worth replacing these with the same value caps....but rated at 450v min...(if you do this make sure you match polarity of the capacitor to the PCB)



these filter caps crank out some heat as well.....most filter caps always get alittle warm....but these get hot...and heat the entire PCB up pretty fast

Aren't this caps in series to increase the effective voltage rating?

yes sir its based on the voltage doubling circuit.

so with that...voltage goes up, amperage goes down...and the caps can handle it for a fair amount time.


so this above is the jvm410, its based on the voltage doubling circuit as well, and it too can handle filter caps at 300v



back in the late 90's when the original JCM 2000 DSL/TSL style amps were being made, it was pretty hard to find a 330uf cap at 400v to 450v at that time...if you did find them, they were super huge and wouldnt fit in the DSL/TSL amps...the same thing can even be said about the 100uf filter caps at that time, it was difficult to find 100uf caps at 450v...they were alittle bigger at that time. so it really made sense at that time for marshall to use the caps at a lower rated voltage.

nowadays you can easily find 330uf and the 100uf caps with a rated voltage of 400 to 450v...and i think thats what its more about....back then it was just harder to find good filter caps because of those odd value's...now its easy

the original jcm's amps didnt have filter caps that lasted to long, if you played your guitar for a couple hours a day...the filter caps would last 5 years on average.

truthfully the Choke is a must have for this amp, works as a great isolation source between the filter caps in this amp.



so the real question is, is it worth replacing the filter caps for the amp? well i didnt for my amp, but i did buy replacement caps so if they ever go out...i can just replace them with stronger caps

so lets Review the changes that have been made thus far.

so after changing the slope resistance in the tonestack...this really helped tone wise...stopped the sharp...thin....shrill sounding mids and treble from coming through. made the overall amp sound...more like a marshall.

After doing the bass compression mod, changing the cathode resistor of v2b to increase the gain alittle...and dropping the voltage in the preamp....this made the biggest change.
*Bass is much tighter now, in fact the bass pot in the tonestack is fully useable at any volume range on channel 2, however the amp sounds thinner now from the bass compression (which is normal) so we need to thicken the amp up and give it some chunk.
*the gain increase...from the v2b cathode resistor change, and the voltage drop...is very awesome, its still very fizzy and we can take care of this alittle later...but the biggest thing i notice now on channel 2 is the beautiful sounding feedback starting to come through...this really makes the amp start to shine. after doing this it did make the amp alittle darker sounding, so we need to brighten it up alittle as well.

So we need to
1-kill the preamp fizz in channel 2
2-rebalance the slope resistor for the clean mode on channel 1....maybe even add a treble and bass pot for the clean mode as well.
3-brighten the amp up alittle
4-thicken the amp up alittle more


once these parts get here in the mail...ill continue on

TTBull wrote:yes sir its based on the voltage doubling circuit.

so with that...voltage goes up, amperage goes down...and the caps can handle it for a fair amount time.


so this above is the jvm410, its based on the voltage doubling circuit as well, and it too can handle filter caps at 300v

That looks like a standard full wave bridge rectifier feeding two series filter caps to increase the max voltage to 800VDC at 110µF to me. A doubler either has a center tapped transformer with the CT connected in between two series caps a la JCM 800 or a non CT transformer is used and the "bottom" two diodes in a full wave rectifier are swapped for caps. It's been drawn in a confusing way in that schematic.

yep your right

Whoa - what happened to all the postings?

TTBull - this was really great info, and very much appreciated.