Vlavecaster variant: The JCM caster

[qtws]

hi all,

Hi all,

relative noob here, but inspired by my successful valvecaster build, I built a twin valve preamp based on a JCM800 (2203) and using 12AU7 valves at 12V. Just got it to work tonight, and have spent the last 3 h pretending I'm Jimmy Page  , firing it into a Marshall Class5 combo.

It sounds very similar to the 1978 JCM800 I owned 20 years ago - quite sharp, great for rhythm, but could do with a boost for really overdriven solos.

I will be tweaking it further, with (possibly) volume control between stages and a marshall tone stack (just to play with it), but I think that the basic circuit could be labelled "some" with a third valve being labelled "more" - need to get another valve ASAP !

It cleans up beautifully with guitar vol control.

I'd attach the schematic, but I don't know how - how do I post a pic? I really want to share this one....I can't find a "how to"... but I have a jpeg ready to share.

Cheers,

Nigel

[J0K3RX]

Nigel,

You have to use a host to post pics like (www.dropbox.com) or some other site...Then put a link in your post to the pic/s   Unfortunately you can't attach pics/files in this forum, at least not that I know of.  

[Kesh]

You have to find somewhere to host your jpg, like photobucket or something, then grab the pics url and put it in img tags, which you get by pressing third button from left, bottom row, when you are posting. above all the smileys.

would be very interested to see it

[gcme93]

For my images, I use this website, http://postimage.org/index.php?code=5225ea76

1. Rescale to the 640 x 480

2. When it's uploaded copy the "direct link"

3. Click on the picture frame icon above the  smiley, and insert this direct link between the two squared bracket bits.

Should make sense when you do it

[scuzzphut]

Imgur is super-duper easy-peasy 

http://imgur.com/

[pinkjimiphoton]

i would reccomend AGAINST resizing the image, as if using a jpg or whatever, that will seriously degrade the image if you try to re-expand it,
and every time it's re-saved as a jpg it will be worse quality because of the compression algorhythms involved.

kinda like re-encoding an mp3, the trying to decode it. lost data is GONE.

my advice? photobucket. free, stupid easy, unlimited storage.

you'll want the direct "link" to paste on the page.

if ya get stuck, just post the link, one of us will help make it visible for ya.

look forward to seeing your project!! sounds fun!! any chance of a clip?

[gcme93]

Fair enough on the resizing! I only resize for pretty pictures so they fit on the page, not with technical gut shots I suppose.

[mremic01]

This is exciting. I liked the Valvecaster, but not enough that I'd put it on my board. A JCM800-implementation of it sounds like something that could be a staple though.

As far as presenting the layout and schematic, I'm a big fan of the way tonepad does it. Everything's in one nice PDF file, at the right size, and kept to a single page (or two if that's just not enough space). The builds where people post all the info in threads as separate jpgs or gifs tend to be a hassle to work with and often need resizing or tracing over in inkscape. General Guitar Gadget's project files are also kind of annoyingly set up as separate PDFs for a layout and a schematic and BOM. Don't get me wrong. I'm grateful to those that make the info available, but tonepad really has the presentation down. If you've got to use jpgs hosted on imgur or photobucket, I do think it's important to avoid resizing it.

[qtws]

Hi all,

well, there seems to be some interest in my 'knowingly incompetent' efforts 

Hopefully this link to the schematic/BoM one page pdf will work ok:  https://www.dropbox.com/s/ipwywxpozd79ysn/JCMcaster.pdf

I have this built on breadboard at present as I'm waiting on some perfboard to arrive, so there is no layout as yet.

The basic circuit is taken from study of the 2203 model 100W head.  V1a copies the "hi gain" input, and I have cascaded it into V1b, modified in that the 1k cathode bypass resistor has a 680nf cap across it.  This alone gives a pleasing volume boost and a bit of crunch.

Next is a voltage divider with a 470 pf 'Marshall emphasis' cap across one of the 470k resistors, followed by stage 2 which is more or less a duplication of stage 1, except that V2a&b cathode bypass resistors are both 1k, again with 680nf bypass caps on both.

22nf Coupling caps between stages, and a 470k/470pf Marshall emphasis resistor/cap between valve stages. 1M pulldown resistors on input and outputs.  And that's it.

I'm sure there are lots of ways in which the basic circuit can be modified for more/better/different etc, but I thought I'd better get this posted before starting to experiment!

[Renegadrian]

quick suggestion: a cap (1-10µ) at the output - I got some problems with a couple of original ckts of mine trying them with 2 different amps - solved with that cap.

Also, I can see a quite resemblance with the 2203 only at the first gain stage (apart that plate resistor) and out to the second (470k/470p couple) - no preamp volume, no cathode emitter on the 4th stage, no tone stack...

Let's say the jcm was a good starting point to your experiments!

[Kesh]

Any reason no grid leak resistors on V2? Where do the grids bias themselves. Am curious. Do you have Vgk readings?

[mremic01]

That's a nice looking doc, and the schematic is very easy to read. Now I need to decide whether I want to attempt a layout myself or wait for someone else to come up with on. Either way, I'm planning on putting one of these together.

[qtws]

Many thanks for the interest and kind comments.  Did I say "knowingly incompetent"....? How very prophetic of me...   The schematic as posted is incorrect in the places that folk have queried....

On the breadboard, there IS a 470k grid leak resistor on V2b pin 7 to ground. AND a 22nf cap on the output of the circuit.   This is what comes from building on a small bit of breadboard and then trying to transfer that to a drawing in powerpoint in the early hours....Oh, and I've just spotted a 1M from pin 2 of V2a to ground - a remnant of getting the first stage to work by itself.

Its not really intended to be a straight copy of the 2203 circuit - as Renegadrian points out, the first gain stage is kinda similar, but the second gain stage actually has a more 1959 flavour, if you like, with the 1k cathode resistor/680nf cap, albeit on both triodes.  I did have a 10k cathode resistor on V1b as per the 2203 master volume preamp schematic, but changed it to the 1k/680nf as per v2a of the 1959 mkI preamp circuit for a bit more gain.  Then I repeated this 1k/680nf for the cathodes of the second gain stage (on the premise that if one is good, then another two is better? :-) 

I initially used 220k plate resistors based upon valvecaster experimentation, but I have gone back to 100k all round - less farty/blarty on low E string, but still gives a "half stack" sort of whump in the low end.  I might try to tune that out a bit by varying the cathode bypass resistors/caps, though: its still a bit much, however I can dime my class5 on low power now.   I have put a 1M volume pot between valve stages: plenty of control, but some experimentation with where exactly in the circuit the volume control pleases me most is in order.  Once I'm happy, I will attempt a Marshall cathode follower TMB tone stack connected to V2b or V1b.

At lower volume, the JCMcaster is fairly quiet, but with the class5 turned up, theres a lot of radio frequency whoosh in the sound. Granted, its sitting out in the breeze on breadboard, so not unexpected.
Switch, PSU jack, in and out 1/4" jacks, vol pot and the tone pots are all mounted in a 75mm x 130mm x 30mm plastic dryliner box (99p from local discount store, easy to drill!), with 150mm wires running from this to the breadboard thus allowing switching/volume etc. without disturbing the breadboard.

I started this post over 3 h ago, then decided to swap out the plate resistors and stick in the vol pot and play some guitar to see what the effect of the changes was.....I'm liking the noise this makes a lot: the hardest part is putting the guitar down in order to make changes.

[Renegadrian]

thx for sharing your schem! Maybe the name is a bit misleading, but who cares after all!?  
Main thing is that thing works good!!!

I have a couple folders in my HD regarding my studies on marshalls (jcm on top) and fenders, tried some things in the past...gotta share something as soon as things are settled and working...