Marshall JCM800 Super bass clone

[blindboybenton]

Hi All

Ive got a old carlsbro PA100 amp i use as a bass head.. Which has been ok for the past few years. But im bored with it and ive wanted to build myself
a new bass head for a while now

Now while i can repair the amps etc. deciding on what i want to build etc. Is a little out of my comfort mode.

Im a Player of Fender and Gibson basses and my favorite bassist is Andy fraser from Free. He used marshall 100v valve heads. Ive alway loved his tone

So ive decided on a jcm800 Super bass 1992. Ive read up that its as close as you are going to the amp that marshall supplied in 1968+

http://www.drtube.com/schematics/marshall/1992u.gif This is the schematic. What do you think a good choice ? Whats your choice!

I did notice that the schematic has some other values on the resistors in ( ) can anyone help on that. The whole idea about doing this is the carlsbro is hard to work on
as you have to unsolder one half of the circuit board to get to the underneath, and the board has become brittle and tracks are becoming broken.

I will look forward to you feedback

BBB

[loudthud]

I'm not sure why the schematic shows the two different values. It probably indicates that some minor changes were made in production, possibly because of a change in the transformers. The most notable is the feedback resistor change from 47K to 100K. That will make a small but noticable change in the way the power amp distorts and the gain. The 820 to 1K is a similar thing for the preamp. The 22K to 27K just changes the range of adjustment of the bias. Overall these are not things to obsess over, they are things you might change after you get the amp built and tweek to taste. On bass amps I like to add a Resonance control and tweek the "Slope" resisitor.

[g1]

This probably won't help, but the 1959 schematic has the same resistor values shown in brackets, from what I can see.

[blindboybenton]

This probably won't help, but the 1959 schematic has the same resistor values shown in brackets, from what I can see.

Thanks.. i did see that. Was more looking for opinions of the amp. any changes or different amp ??.. I know you chaps over the pond seem to chat about them enough

[pdf64]

I think that the bracketed values were for amps intended for the USA market, which were fitted with 6550 power tubes.

It's a great circuit for a bass amp, can't go wrong with it really.
Pete

[Pedro Vecino]

For bass, better than 1992 standard is the version with sweep and slope in the eq. Itīs more complex to build, but much more flexible.
http://www.drtube.com/schematics/marshall/1992.gif

[g1]

I think that the bracketed values were for amps intended for the USA market, which were fitted with 6550 power tubes.

Here's a schematic from the same year, for the 1992 MKII with 6550's. Note that the 6550 designation at the output tubes is NOT in brackets. In the other drawing, the EL34 designation IS in brackets. So I'm guessing the bracketed numbers are for the EL34 version? http://www.drtube.com/schematics/marshall/1992mk2u.gif
Not sure what the V2 cathode resistor would have to do with output tube types though.

But this post title refers to the JCM800 version, which is actually this: http://www.drtube.com/schematics/marshall/1992.gif

[big_teee]

For bass, better than 1992 standard is the version with sweep and slope in the eq. Itīs more complex to build, but much more flexible.
http://www.drtube.com/schematics/marshall/1992.gif

It requires a lot more knob positions, best I remember.
I have an order this winter for a 100 watt bass amp, but was going to use the 1968 preamp instead of the fancier 1992 with Slope.
My guy justs want a good amp with good bottom.
Thoughts,
T

[blindboybenton]

Hi all

The amps is under way and looking good with new face plate. Before i start the wiring and populating the tag board. I want to fit a Line out. Some shows ive done Ive need to connect it to the pa. I don't want to muck about with a mic and stand anymore or go out and buy a DI Box....... Can anyone recm'd a good circuit ?

http://www.drtube.com/schematics/marshall/1992u.gif


BBB

[loudthud]

A DI out usually needs some kind of speaker emulation, simply some kind of circuit that shapes the frequency response in the same way a speaker does. From there either a balanced output made with opamps or a transformer. An external DI would be easier to swap out until you find something that sounds the way you want it to.

[mozwell]

i built an ampeg SVT2 pre, with a 400W solid state power amp, which i use for bass, and i love the sound.
It has a DI built in, so grab the circuit from there. The DI can be switched to be pre or post eq, and in pre eq, the signal is from the output of the first valve stage.
If anyone is interested, i have details of the pcb layout i used (to fit into an existing amp head i had) and the inductor i wound.