Getting a head of myself - Marshall 2204 JCM800 inspired build

[xtian]

Have you had a JCM800 before? I have some advice if you are new to them.

[kuDo]

No, I have never had one of these, apart from an anniversary JCM1H. A little one watter.
I really liked it until I noticed it didn’t have a effects loop for my looper pedal.

Any advice is welcome.

[xtian]

These amps are meant to cut thru the mix, and this can make them uncomfortable in the highs and mids for some (including me!). They're often run with Treble and Presence controls at zero. I suggest you make it very easy to remove the 470p treble peaker cap (between the Low input and Volume control) and the 1n bright cap on the Volume control. You'll probably want to try the amp without these.

[kuDo]

Okay, great. I will keep that in mind. I think I red something about those on the website of Rob Robinette.
He made shure what he was thinking of those caps

Have you made one of these yourself?

[xtian]

Have you made one of these yourself?

No, but I had an '83 2203 for years. Finally sold because no gigs, and no possibility my AC/DC tribute was going to get back together...

[kuDo]

That’s a shame. That was the even more brutal 100 watt version if I am not mistaken.
Do you still have something comparable sound wise to these? Or have you moved on to something else.

[xtian]

The only high gain stuff I have is a couple of ODS builds, and my Helix for emulation.

[kuDo]

The only high gain stuff I have is a couple of ODS builds, and my Helix for emulation.

Aah, an ODS! Maybe one day...

[thetragichero]

i like a 2.2nf cap for the treble peaker. 2.2nf/220k might also be nice if just the 2.2nf is too much low mids. both these will have more signal passing. but the beauty of rolling your own is tweaking to your tastes

[pdf64]

Provided there’s plenty of over voltage capability with the rectifier, I don’t see the point of fusing both HT winding phases?
A single HT fuse in the CT return to 0V seems plenty for this design to me.
If you’re determined to stick with it, then put the bias feed to the hot side of the fuse; it would be bad for that fuse to blow and take out the bias supply, whilst leaving the other phase supplying the HT.

[kuDo]

i like a 2.2nf cap for the treble peaker. 2.2nf/220k might also be nice if just the 2.2nf is too much low mids. both these will have more signal passing. but the beauty of rolling your own is tweaking to your tastes

It is indeed cool to tweak your amp to your own liking. In my case it will be getting a valve effectsloop.
The sound tweaking will come if I don’t like the sound. That is when it works of course.

[kuDo]

Provided there’s plenty of over voltage capability with the rectifier, I don’t see the point of fusing both HT winding phases?
A single HT fuse in the CT return to 0V seems plenty for this design to me.
If you’re determined to stick with it, then put the bias feed to the hot side of the fuse; it would be bad for that fuse to blow and take out the bias supply, whilst leaving the other phase supplying the HT.

I have chosen to place two fuses, even though the diodes are rated to handle enough power.

In de unlikely scenario that two opposing diodes short out, a fuse in the CT return will not protect the transformer. The two diodes would short circuit the winding and it would burn out.

I will however think about your remark about the bias supply. Thanks!

[romberg]

After a lot of back and forth, I now came up with a layout that is a lot more satisfying.

Looks good! The board for the loop is a good idea!

There still seem to be a couple of long leads, but I thing that will be unavoidable.

There always seems to be a few long runs you can't avoid. Only two jump out at me on your new layout. I think both can be fixed by using shielded cable for the runs. Specifically, I'd use shielded cable for the run to the top of the master volume and on the run from the center wiper of the volume pot to the grid. You may have been planning on doing this already.

Mike

[romberg]

Provided there’s plenty of over voltage capability with the rectifier, I don’t see the point of fusing both HT winding phases?
A single HT fuse in the CT return to 0V seems plenty for this design to me.
If you’re determined to stick with it, then put the bias feed to the hot side of the fuse; it would be bad for that fuse to blow and take out the bias supply, whilst leaving the other phase supplying the HT.

I have chosen to place two fuses, even though the diodes are rated to handle enough power.

In de unlikely scenario that two opposing diodes short out, a fuse in the CT return will not protect the transformer. The two diodes would short circuit the winding and it would burn out.

I will however think about your remark about the bias supply. Thanks!

Merlin makes a pretty good case for using two fuses on the ac side of the secondaries on his site here:

http://www.valvewizard.co.uk/fuses.html

Having one leg of the rectifier short out does seem like it would send current through the secondary and bypass any sort of fuse on the center tap. I'm using this fusing arrangement on all my new build now.

Mike

[kuDo]

Provided there’s plenty of over voltage capability with the rectifier, I don’t see the point of fusing both HT winding phases?
A single HT fuse in the CT return to 0V seems plenty for this design to me.
If you’re determined to stick with it, then put the bias feed to the hot side of the fuse; it would be bad for that fuse to blow and take out the bias supply, whilst leaving the other phase supplying the HT.

I have chosen to place two fuses, even though the diodes are rated to handle enough power.

In de unlikely scenario that two opposing diodes short out, a fuse in the CT return will not protect the transformer. The two diodes would short circuit the winding and it would burn out.

I will however think about your remark about the bias supply. Thanks!

I explained it wrong here. I was thinking of a normal bridge Rectifier, but the Marshall design uses a two fase rectifier setup with double diodes. So in the event that all diodes are shorted out the fuses will save the transformer.