Jcm 800 Build Problems

[theholyweed]

Hey guys, Iv just finished my first 800 build based on the Ceriatone 2016 layout, Im having some issues though, Although the "low" channel works and sounds pretty sweet the "high" channel deffo isnt sounding right, its not distorting enough and sounds very weak an clipped also when using it the master volume seems to bottom out halfway and makes a noise like the pots dirty (if that makes sense) Iv checked my wiring a million times and cant get to the bottom of it, my input jack wiring seems ok....total head scratcher!

[g1]

Please post a schematic (not layout) for whichever JCM800 model this is.

[Chuck H]

also when using it the master volume seems to bottom out halfway and makes a noise like the pots dirty (if that makes sense)

First, I'm not sure I understand this. Is the master pot only able to be physically turned halfway? Otherwise, please explain "the master volume seems to bottom out halfway". Since you said the low jack sounds alright, can we assume that the master is working fine when the low jack is plugged into?

A schematic as g1 asked for would be very helpful. In the meantime, can you tell us if this is the 800 type circuit that cascades the channels when plugged into the "hi" input jack, but not in the "lo" input jack?

[theholyweed]

[theholyweed]

V1
1. 292
2. 0
3. 3.4
4. H
5. H
6. 243
7. 0
9 2.2
9. H

[theholyweed]

V2
1. 200
2. 0
3. 1.2
4. H
5. H
6. 342
7. 200
8. 201
9. H

[theholyweed]

V3
1. 259
2. 26
3. 43
4. H
5. H
6. 253
7. 27
8. 43
9. H

[theholyweed]

most of my voltages seem all right with a ceriatone 2204 voltage chart I found online but just all a bit high overall, I dont know if I need to play with any of the 10k 2w voltage dropping resistors to lower the voltages in the preamp section, do these look right to you guys?

[Sowhat]

It's not any of this stuff by your description, it's the dreaded faulty wiring or maybe more exactly the mysteriously hidden "cold solder" that is at fault. It can't really be anything beyond the input jack wiring and components right there at the input jacks. Those Cliff jacks while having cool circuits can be a bit tricky to solder correctly aside from being flimsy plastic junk that breaks and cracks apart at the slightest touch... not really, but I like steel frame jacks better.

[mozz]

This model?

[theholyweed]

thats the one. I believe the high channel gets fed into half of V1 and then mixed with the low signal, this seems to be where the problem lies, when the preamp volume is raised the distortion cuts in an out and sounds very clipped, but the low channel can be heard fine mixed in with the strange clipped high channel..I have spent hours rechecking everything over an over again and cant seem to find the issue, I know im clearly being dippet and missing something!

[g1]

The high doesn't get mixed with the low, it becomes cascaded. The wiring of the 2 jacks is critical. The drawing of the layout is a bit awkward, so if the actual schematic doesn't make sense to you, post some good pics of your input jack wiring.

[Chuck H]

I'll make this easy, and hard at the same time, but hopefully enlightening.

First, the problem is isolated to the input jack wiring and possibly the circuits on pins 6-7-8 of the first preamp tube. But we should focus on the jack right now. On that note... It's a good idea to NOT mod, change, upgrade, etc. any part of the amp NOT associated with the problem before the problem is solved. It can complicate analysis if anything goes wrong. So now is not the time to modify the voltage supply.

g1 is absolutely right. The Ceriatone layout drawing is utterly ambiguous about the cliff jack contacts. Study the SCHEMATIC supplied by mozz. The pointy arrows are the short contacts on the cliff jacks and the drawn tip and sleeve are the long arms on the cliff jacks. The long arms contact the jack when a jack is plugged in and are simultaneously disconnected from the short contacts. Re-read this until you understand it. Draw it out if you must. Then LOOK at the cliff jack construction. Examine it until what you've just read makes sense. Again, you can draw it out to help understanding. Once you have done this it will be much easier to see any mistakes.

[Pedro Vecino]

Layout around the input jacks is fine but can be confused because they are not oriented to the same side. Ovals on gain potentiometer and input jack indicate that coaxial cable must be used towards the first tube grids with the shield conected only in those places. Without it the most probable thing is that an oscillation occurs using the "high" input jack.
Make sure that the ground screw to chassis is perfectly tight and that it has all lines assigned.

[theholyweed]

So I triple checked all my wiring and it seems fine, I dont think its an issue with my input jack wiring because when using the high channel it sounds fine if all the eq controls are turned right down, its only when you start turning up any of the eq's that the distortion starts cutting in an out and sounding really gated...its strange because the eq seems fine, albiet a bit squeely when maxed out on the low channel...