JCM 18w power section

[samuraijackson]

Hello,
first post here! I'm going to build my first 18w, I'm a marshall lover so this will be a JCM preamp into a EL84 power amp. It's a tube town kit and here is the schematic:

https://www.tube-town.net/cms/userfiles ... -schem.pdf

I noticed that the EL84 grid resistors (100k) are way higher than the usual 2.7k marshall uses in the 18w amps. Coupling caps is also 100nf instead of .022.
Any reasons for this change? Do you think this is a good build or should I change something?

Thank you for your attention, please consider that I'm a newbie in amp building, but I already built a 5f1 which I love and works great.

Regards,
SJ

[crgfrench]

That is a fixed-bias amp with a buffered tone stack. Not going to sound like a true 18 Watter at all. Your build choice will depend on what kind of sound you want from the amp.

[samuraijackson]

hi and thanks for your reaply
I'm looking for a low watt plexi/jcm kind of sound, it doesnt matter if it doesn't sound like a proper 18w.

[geoff 1965]

have a browse through the "downloads" section of the forum,the schematics/layouts are tried & tested and there is a good variation of 18W's to choose from.the 18W plexi sounds like what you are looking for but define "low watt" because the 18 watters are loud if you are wanting room level volumes! consider using VVR or a post inverter volume.

[samuraijackson]

Thanks for your reply Geoff, yes 18w is still plenty loud (I had a Tiny Terror amp for several years) but still less deafening than a 100w Super Lead! I hope to build my dream amp with a JCM frontend into a 18w power amp, I don't have gigs at the moment but I suppose it will be plenty loud for small clubs as well. Also if I get it right fixed bias amps have more clean headroom than cathode bias so this is a plus. The Soldano Atomic 16 is fixed bias as well but still if the schematic I have is right the power tube grid block resistors are still way smaller than 100k. I'll take a look at the download section now.

[geoff 1965]

You can get cleaner headroom with the cathode biased el84’s as well,B+ and grid leaks come into it also the post phase inverter volume.food for thought!

[JMPGuitars]

If you want the benefit of cathode bias, along with the feel of fixed bias, then you can use a high cap value like 2200µF. That's what I typically build with.

[samuraijackson]

Is that the cathode bypass? Sorry, as I said I'm a newbie when it comes to amp build. Also, do you guys prefer cathode bias over fixed bias for specific reasons? I'm going to start a thread build as soon as I get the shipping in.

Meanwhile I'll have some wood working going on!

[Bieworm]

Is that the cathode bypass? Sorry, as I said I'm a newbie when it comes to amp build. Also, do you guys prefer cathode bias over fixed bias for specific reasons? I'm going to start a thread build as soon as I get the shipping in.

Meanwhile I'll have some wood working going on!

+1 on the cathode bias!!! Overdrive transition is smooooooth

[katopan]

TubeTown have been around a long time so are going to know their stuff. The bigger coupling caps and bigger grid stopper resistors is an interesting choice. The bigger caps are not adding any more bottom end (22nF is already a roll off below guitar) but usually change the bias shift with clipping and result in harsher distortion than 22nF. But then using 100K resistors is normally a trick to smooth out the distortion more. I'm not sure why they'd do that instead of using the "normal" values in an 18 Watt. But maybe it was their preference in a fixed bias version.

A JCM800 preamp into the standard 18 Watt PI, power section and power supply rail is a proven success. I've heard clips of that sort of build from this forum long ago and it was unreal. Personally I think that'd be the way to go, but you'd probably be happy with either that or the fixed bias version. Comes down to what you like in your amps. As the others said the cathode biased EL84s is part of the magic and it does work well with the JCM800 preamp up front. You loose some of the feel and dynamics going to fixed bias and such a stiff power supply rail (which is going to make a big difference too).

[samuraijackson]

Thanks for your reply Katopan, really the answer I was looking for. With that filtering and voltage values can I still go with a cathode biased power amp? I now wonder why so much filtering?

[samuraijackson]

Hi guys,
just a little update, I built the amp. It took almost 30 hours but the building went smooth. I messed up the input jacks but it was an easy fix. I ended up with the fixed bias circuit, I love this amp, sounds exactly as I expected.



Cheers

[Bieworm]

Hi guys,
just a little update, I built the amp. It took almost 30 hours but the building went smooth. I messed up the input jacks but it was an easy fix. I ended up with the fixed bias circuit, I love this amp, sounds exactly as I expected.



Cheers

Very nice. I recognize the silicon wires. I use them too from tube town.

[samuraijackson]

Very nice. I recognize the silicon wires. I use them too from tube town.

Yes, very easy to peel!

[Bieworm]

Very nice. I recognize the silicon wires. I use them too from tube town.

Yes, very easy to peel!

And very heat resisting!

[katopan]

Nice work, glad to hear that it sounds good.

[ledvedder]

Are you in the US? If so, how long did it take for the kit to be delivered? I was looking at their PX18 kit. I want to build an 18 watt plexi style amp.

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