Marshall JCM800 100watt super bass clone

[blindboybenton]

Hi All

Ive built my second ever clone amp. Its all together and working and i think its about 95%

Ive used a old 100watt chassis and fitted a new JCM800 output tx. The amp has been built using new components. But it seems to have an issue at high output

testing with a dummy 8ohm load and a signal gen. The sine wave is perfect up until about half way then you can see some sort of distortion
then if you turn it up much more the sine wave goes all over the place and the output tx starts to make noises. When this happens the 480v stays stable and drops about 5 v

Play the bass though it sounds great no issues with the sound



I don't really know what is causing this.. need some help there
The amp has been built using the correct values of components. ( V2 using 1k on cathode and 47k on feedback )
Tubes are good with biasing at about 36ma per tube.
Heaters at 6.7v ac
V1 Cathode 1.49v
V1 Anodes 173v
V2 Cathode 1.55v and 140v
V3 cathode 37v
V3 Anode 227v and 220v
V4/5/6/7 Anode 487v and at 1k 5w 480v

There is a little too much hum it think. if you turn bass pot ( 1m pot ) too full on it reduces to an acceptable amount. If you back it of it goes up Can't see any ripple if i add a extra 100uf 500v cap makes no difference.. I'm going to check later on tomorrow for earth and solder joints etc. just mentioned about the hum incase its linked to the main fault




Can anyone help

bassman1965

[Jazz P Bass]

Is there a feedback loop?
If so, you may want to try reversing the output transformer primary winding, blue & brown.

Or if you want to test it:
Run a signal into the amp while monitoring the output.
Measure the output amplitude.
Now disconnect the feedback wire & remeasure the output.
If it goes up, the OPT primary is wired correct.
If it goes down then the OPT primary is wired for positive feedback & the wires need to be reversed.

[blindboybenton]

Hi Jazz bass

Yes there is a feedback loop Connected too the 8 ohm using a 47k.I did the check as you said , The feedback was disconnected and the output went up. I never knew that the primary connections to the tubes would make a difference. Learn everyday

Bassman

[Jazz P Bass]

So that wasn't the problem.
Does the amp still act up with the negative feedback removed?

Can you monitor the output tube grids & see if they go crazy when the output does?
You may have to remove the negative feedback.
If they do not , then the problem lies in the output finals.
If it was oscillating I would expect to see that on the grids.

Of coarse it may be a faulty EL34.

[blindboybenton]

Hi Jazz

The tubes are good have tried others and the same.. Its the same with neg feedback connected too 4/8/16ohm or disconnected. Using a dummy load and the sig gen if you turn it up say just over half way the sine wave starts to distort ( see the pic in first post ) the if you go any higher the amp goes crazy and op tx goes noisy. When it is going crazy the signal on the scope is unreadable. Using the bias meter i noticed when the amp goes crazy the bias shots up from 35ma too around 150ma per tube. With a dmm on the neg voltage. jumps about up about 5 v. Biasing circuit is exact

scope reading from grid 2 just before it goes crazy

Are the voltages too high. I did notice marshall using 1.5k 5watt resistor on the grids in other models. Ive used 1k 5w. changing them to 1.5k would this help.?

Many thanks for the help

Bassman

[Jazz P Bass]

Scope the previous stage.

[blindboybenton]

Hi Jazz bass

Just about to go out to a show.. You say scope the previous stage ? do you mean the same but on the other pair of El34?
will check later tom

many thanks again

Bassman

[Jazz P Bass]

The tube stage before the phase inverter.
You need to find out if the signal itself is unstable or the power amp.
If the signal at the grid of the EL34's is unstable, to me, the next step would be to check the circuit feeding the PI tube.

[blindboybenton]

Hi Jazz bass

If i remove the output tubes. and scope the output from the 0.1uf decoup caps its
this is taken at about at just about full power with no signs of the amp going crazy
I tried a used set of Ruby 6L6GC in it this morning biased ok and still the same

I connected up a speaker cab this morning to it. sounds really good. and seems to be more forgiving with the bass and speaker instead of the signal gen and dummy load,But still temperamental !!

Bassman

[Jazz P Bass]

Not too sure what we are looking at.
Is that the output tube grids?
Sure would be nice to have the proper schematic.
http://www.drtube.com/schematics/marshall/1992mk2u.gif
http://www.drtube.com/schematics/marshall/1992.gif

[blindboybenton]

Hi again

I did put a schematic link up on first post. but it seem to gone.Ive reattached it, My version is the same i think of the Mk2 version you attached, But i have 220k bias resistors and that version has 82k.. The above picture is output from the PI valve after the 0.1uf decoup caps with the EL34 removed

bassman

[blindboybenton]

Hi all

well im having another look at this Marshall JCM800 1992 Super bass clone

http://www.chambonino.com/carlsbro/carl100par.html This is the donor that i used the Mains tx and choke from

Just a recap on the details. Built using a 100watt carlsbro chassis with a new marshall output tx. The amp has been built using new components. and all to the spec
The 1k 5watt resistors on v4/5/6/7 have been changed to 1.5k. Feedback resistor is 47k and V2 cathode resistor marked as 820k is 1k
I did notice that the carlsbro mains tx didnt have a CT on the HT rails. But the Marshall one did. Could this cause any problems. See both schematics
Voltages without a signal with all valves in
V1 Pin 1-6 = 192v
V1 Pin 3-8 = 1.3v
V2 Pin 1 = 178v Pin 6 = 336v
V2 Pin 3 = 1.7v Pin 8 = 178v
V3 Both cathodes at 34v
V3 Pin 1 = 230v pin 6 = 223v
EL34 478v and 490v
Biasing is good at about 30 ma per tube
Have tried new valves matched quad set of JJ's El34
Pre amp tubes all been swapped out Currently using Sovtek 12AX7 LPS
Heaters are at 6.7v ac
No ripple can be seen on the dc rails

The Fault

All seems to be working ok. sounds great though the bass. But as soon as you get about 3/4 and play a note it goes crazy and out tx makes noises
If you probe about with your meter or scope and touch the V1 pin 1 or Pin 6 it triggers the amp into going crazy. If you touch the jack lead at high volume
with the speaker attached it. Seem to go into motorboating. All caps are new and JJ's and correct values. Have added extra caps to all filter cap stages and it makes no differents
Earthing has been check
If you remove the EL34 tubes the signal at the 0.1uf De coupling caps at full output is



This is when it goes crazy with a signal gen and dummy load

Is their anyone out there that can help. Please read the prev posts and there is more info on the help ive had. I aint got a good camera only a mobile, if you want more pics of the scope, please ask
Please help im running out of idea's and pulling my hair out
Bassman

[pdf64]

Almost certainly just oscillation - not a big deal!
Overall layout and lead dress are the topics to investigate.
Could you take photos of the overall chassis and then 2 or 3 photos of it closer in, to get all areas in good detail.

The global feedback resistor in the scheme is 1k; to change it to 47k seems a big increase?

Pete

[blindboybenton]

Hi PDF64

Many thanks for the reply. I dont have a decent camera at preset. will try to borrow. The schematic has some values in ( ) The feedback resistor is 47k. Ive fitted 100k and still the same
I think it has a typo mistake on V2 K1 has 820K and ( 1K) think it should of been 820R ? Yes
In the mean while before i can get the camera. anything i can look for ? I think the layout is good. Its had a lot of work on it trying to find this fault. which was on there from the min i turned it on

Do the voltages look right to you ?

Thanks for your help

Bassman

[pdf64]

Sorry I was getting mixed up with the donor amp schematic.
Yes, V2B cathode resistor should be 820 ohms.
Your Vdc seem fine.

Does it go crazy if the presence / treble are turned down?

Are there screening cans on V1 / V2?

Pete