Marshall JCM800 100watt super bass clone

[blindboybenton]

Hi Pete

Yes it does it with bass/treble/presents turned down. V1/2 are not using the screened cans. I have some cans here. I will fit them later. keep the idea's coming pete. You gave me some info last time i made a clone amp that fixed it and its still working well

Bassman1965
Update the still does it with 2 cans over V1-V2

[blindboybenton]

Hello Bassman
On the bit on the layout. see if you can find a picture of a marshall jcm800 super bass or lead and see if the layout on your tag board matches. Im guessing you have checked the components and wiring are correct.??

BBB

[big_teee]

The Amp and Schematic posted is probably the JMP 1992 Super Bass.
The JCM800 1992 super bass is a different critter.
http://music-electronics-forum.com/a...80308-1992.pdf

[blindboybenton]

Hi all

Ok seems like i was miss informed. Anyway the one i built is to the schematic posted. I wanted a super bass, i thought it was a jcm 800 super bass
bassman1965. Still need some help !! Anybody

[blindboybenton]

Hi all

well my hair is all out all over the floor. We testing last night and the amp when crazy without out a signal if the volumes were turned up 1/2 way and you used the probe from the scope or DMM. Pictures of the scope while going crazy in pre post

Is there anyone who can help any idea

Bassman

[pdf64]

Some photos of the build may be helpful.
The key point to bear in mind is that there's excessive coupling occurring between an output and an input alongside sufficient gain, resulting in oscillation.
Maybe it's due to lead dress, poor layout, too many cascading stages sharing a ground wire, a B+ decoupling cap with excessive ESR etc. Or a combination of things.
Verify that the easy stuff is eliminated first, eg bad B+ cap.
Then analyse the schematic and layout; consider each grid circuit and then how any subsequent area of the circuit that has at least 2 polarity inversions and plenty of gain may couple to it. Identify the worst offenders and consider what mitigation is feasible.
Does a ~22k grid stopper on every pre-amp tube grid socket terminal help?
Pete

[blindboybenton]

Thanks pete. I still aint got hold of a camara yet. I did have one but it bust under warranty, brought it from comet !! Ive checked against a picture of the same marshall layout of components look good. Lead dress all been kept short and following the same route to the valves. " sharing the ground wire. Ive got V1-V2 cathodes sharing the same all others connected.
Ive seem to come to a point where i want to rip it all out and start again. It started out looking good and has had this fault from the beginning. But now it looks like a dogs dinner.
I brought some tag board which is poor. just don't take alot of soldering and desoldering before the tags drop off.

I just wanted to get it working so i could then rip it out and start again

Bassman

[big_teee]

Here' so good info on the 1992, with alternate grounding.
You might pick something up from it.
1992 Layouts
Good Luck,
T

[blindboybenton]

Thanks all for your help

Amp is now gutted and starting out from scratch. Going to use the recmd layout. Carlsbro chassis hadn't any holes cut for can caps. Going to do that this time

Question ........... is it worth putting in the master vol mod on this bass amp

Bassman

[big_teee]

Thanks all for your help

Amp is now gutted and starting out from scratch. Going to use the recmd layout. Carlsbro chassis hadn't any holes cut for can caps. Going to do that this time

Question ........... is it worth putting in the master vol mod on this bass amp

Bassman

If you are starting over I would not do the 68k inputs on the board with loose wire.
I would use shielded wire like on most higher gain amps.
Also I would not have multiple preamp grounds.
That is the source of most hum and noise.
Don't ground to the back of the pots, like the old layouts.
On marshall type builds I modify the heater wiring, and make it a tighter twist, with no preamp wires cutting over the heater wiring.
Especially on V1.
The Master Volume is great if you are playing at a lower level.
If you play wide open all the time, there is no benefit!
T

[blindboybenton]

Thanks terry.

I had put the 68k on the jacks, but didnt use screened cable but will this time. I will run all the pre amps to the same ground point. About the only thing left in it is the mains tx with heater wiring still in place. I only use this amp to gig which is running up high. There is a spare hole on the front panel due to me moving stuff. I had stuck a pot in it as a dummy. Maybe ill have a master vol. The old chassis used a circuit board and axial caps so there is no holes in the chassis. Im going to get a hole punch and i think i can cut them hopefully in the same places as the marshall.. This type its going to work........... Hopefully

bassman 1965

[big_teee]

Thanks terry.

I had put the 68k on the jacks, but didnt use screened cable but will this time. I will run all the pre amps to the same ground point. About the only thing left in it is the mains tx with heater wiring still in place. I only use this amp to gig which is running up high. There is a spare hole on the front panel due to me moving stuff. I had stuck a pot in it as a dummy. Maybe ill have a master vol. The old chassis used a circuit board and axial caps so there is no holes in the chassis. Im going to get a hole punch and i think i can cut them hopefully in the same places as the marshall.. This type its going to work........... Hopefully

bassman 1965

Since you have the extra hole on the front I would go ahead and use it for MV.
I put it on the end past the presence control when I build it in a amp.
I like the Lar/Mar PPI-MV.
When the PPI-MV is turned up full it is like it the amp is stock just with the 220k resistors.
If it still goes crazy again on high, you might try some lower gain tubes, like AT7s and maybe disconnect the Cathode bypass Cap just for testing.
That is a pretty simple preamp circuit, you should be able to fix it.
Good Luck,
T

[blindboybenton]

Hi Terry

Got a question for you. i decided that the old amp looks rough. so i decided to buy a new chassis pre cut and drilled and the only thing that was not new was the mains tx and choke. If i used that it would spoil the look
Ive got a company over here that makes tx's to make me a tx and choke. So he can avoid a problems he wants me to confirm the ac output voltages, Ive searched the net and cant seem to find anything. do you have any info ? I know there is some voltage reading on the schematic, just would like to get them right and make an improvement. Would you make any changes

Bassman 1965