Problem with a JCM800 2204 homebrew - out of ideas :(

[hoelyguacamole]

hi all,

I have been trying to solve this problem for almost a month now with no luck whatsoever. If anyone has any ideas I would be extremely grateful for your input

THE PROBLEM:
The amp sounds perfect with the Master Volume below 2 (no hum at all even with single coil pickups, beautiful tone); as soon as it cracks 2 there is a terrible oscillation (buzzy,crackling distortion). Also, with guitar plugged in, but not striking notes, I can increase the Master Volume to about 8 or so and then there is a squealing oscillation.

THINGS I'VE ALREADY TRIED
Swapped Preamp and Power Tubes
Swapped OT primaries (when I did that it sqealed like mad at any volume)
Tested AND swapped out EVERY component on the board
Swapped out each 50/50 filter cap one at a time with a known good one
Swapped out the Output Transformer
Rewired the entire chassis (there is no wire within .5" of another)
Replaced both the Preamp and Master Volume Pots
Tried 10K resistors in place of the 5.6K grid stoppers
Tried about 4 different grounding schemes (even ran seperate grounds for everything to the PT bolt
Swapped out Input Jacks
Swapped out Speaker Jack

VOLTAGES:
V1:
pin1: 251
pin2: 0
pin3: 2.2
pin6: 306
pin7: 0
pin8: 3.3

V2:
pin1: 191
pin2: 0
pin3: 1.3
pin6: 348
pin7: 190
pin8: 191

V3:
pin1: 230
pin2: 30
pin3: 40
pin6: 220
pin7: 30
pin8: 40

Power Tubes:
V4:
pin1: 0
pin3: 422
pin4: 414
pin5: -38
pin6: 420
pin8: 0

V5:
pin1: 0
pin3: 422
pin4: 414
pin5: -38
pin6: 420
pin8: 0

Heater voltage is nice and steady at 6.5V
Bias is set steady at 34ma

My camera sucks so i apologise for the picture quality







The only things I haven't swapped out yet are the Power Transformer, Choke, and Tube Sockets.

I used the schematic from dreamtone.org, and I know the schematic is good because I had a friend do an identical build using it. I'll attach the PDF of the layout as well

If anyone could offer any insight I would really appreciate it - I'm completely out of ideas. This is also my first build, so I'm really pumped to get this thing working right. Sorry for the length of this post and thanks for reading!

[BBQLS1]

You wiring looks very messy. Some of your leads look excessively long. Had the same thing happen with my first build (Lite IIb) and this is a higher gain circuit. I finally figured out that when I pushed the wiring close to the chassis, the squeal went away.

Keep the leads as short as possible and as straight as possible without crossing other wires when possible. Don't cross the AC and signal wires and I try to keep from crossing the B+ with AC or signal. Keep the wires as close to the chassis as possible.

Look at some gut shots of Hiwatts or Harry Joyce amps. Then you'll get the idea.

[populartsl]

Hi

Have you tried a piece of aluminium sheet on the underside to shield the components.

Ben

[populartsl]

Also the black wire going to the input socket to the 68k resistor should be shielded if it isn't. I cant tell from pic. Also wire from the pre-amp volume to pin 2 should be shielded.

Ben

[hoelyguacamole]

I havent tried any aluminum shielding under the board yet - would aluminum foil work?

And yeah, both the input jack wire and the preamp pot wire to V1 is shielded

Lead dress is something that I've been somewhat confused by - The one thing that everyone says is to keep high voltage, and AC wires away from signal wires, but I've also heard that you should keep signal grounds away from them as well. With this layout I'm having trouble finding a way to run wiring following both of these tips. I do have all of my preamp singal wires away from all B+ and AC wires - it gets messy mostly around the filter caps and the PT ground though - can't relaly figure out a way to make them all happy. I have tried several lead dress changes though and routing the wiring many different ways, yet the problem stays exactly the same.

thanks much for the responses !

[pmurph1x]

Hi Hoelyguacamole. Welcome.
I'm also building a "2204" using the dreamtone layout. Mine has a 6v6 power section though. I'm just waiting on a choke to fire her up.

Anyway the only thing I notice is it doesnt look like you have used shielded wire for the High input. I think this is a must for this amp.

Paul

EDIT: OK you've shielded the input. Didnt see the previous post on time.

[jerrydyer]

Hi. I see a 500pf? cap on the v1. What does that do there. if its a plate res bypass cap to try to quench some squeeling it might be too close to the tube. What does the big brass resistor thats near what looks like a presence pot do?

[populartsl]

Aluminium foil will do for a quick try to see if it stops but be very careful. I would probably put some down then a piece of thin ply on top you dont want any of that foil to touch any of the components ouch!!!!!!!

I did a trainwreck and it squealed like mad until i plated underneath.

[hoelyguacamole]

the large brass resistor is just a failsafe in case the amp is turned on without a speaker load the jack will short to ground rather than be an open circuit

The capacitor is .68uF on V1 - I don't have a clue what it does, the layout showed it mounted there however, so i put it on

Thanks again for the help!

[briane]

yep, definetly an oscillation problem. suspect all components are fine.

I have dealt with a number of these by now, and every single one is unique and tricky to fix.

When a client wants an amp over 35 watts their price just went up, cause surely I will have this problem to deal with.

clean up the wiring. Not just short is best, but short is critical in any amp over 35 watts.

ground input to chassy as close to possible with jack. Or try running a small cap there. try 500 pf, but 250 pf may work.

Sheilded input cable to ground as close as possible to grid of tube. Or to ground with a small cap, Or to plate (will have HV on sheild), or grid to ground with small cap. Combine with 8.2 grid stopper or larger.

Same as above on volume to second triode grid.

Grid stoppers on power tube(s). 5.6k or 10k, but could go as high as 100k to 470k.

small cap vol pot return to ground.

heaters nowhere near any other signal wires. All signal wires clossing at 90 degrees.

non-polarized cap polarity. make sure foil side is towards plate.

thats enough for now. do all those, single and in combination. come back and tell me what worked. may take some time. Most oscillations can be fixed, none of them are easy, none are quick, and none are fun.

personally I would have started with a straightforward build rather than an advanced build for my 1st time, but to each his own. good luck.

[briane]

just realized probably not your first build. my apologies.

[Strat]

The thing that strikes me is the voltages on the plates of V1 (pins 1&6)

If you look at the schematic from V2 back you have 348V on V2b (pin 6, no load resistor) then you have 191V on V2a, the effect of the 100K plate resistor. If you follow back to V1 you have a 10K power resistor then each plate has 100K load resistors. Probably worth tracing that part of the circuit back from the 348V and check wiring and resistor values.

[paulster]

Your presence pot wire (orange wire from the resistor on the board) looks very close to the tone stack. That's often a contender for introducing squealing.

I can't see where they're run but the power tube grid wires could also be worth checking out. I prefer to run mine on top of the board, lifted off, so they end up pretty much against the baseplate when it's fitted.

It looks like yours could be running very close to the NFB wire and potentially injecting noise in there.

Actually, come to think of it, for your physical layout, that NFB resistor is really in the wrong place. You've got the NFB wire traversing half the amp to this resistor, and then doubling back to the presence pot.

You could maybe move this resistor directly to the presence pot and move the NFB wire to that. That'll cut down a lot of wire and move it well away from the power tube grids.

[hoelyguacamole]

just realized probably not your first build. my apologies.

No, you're exactly right, it is my first build hahaha - nothin like jumpin into the fire.

thanks much for the info - I'll try all of these points ASAP

[briane]

No, you're exactly right, it is my first build hahaha - nothin like jumpin into the fire.

Then your gonna learn a whole lot....A whole lot more than other first time builders, and maybe more than you wanted to know in the first place. ;-)

It took me many amps to come up with a methodical procedure for fighting oscillations. Its still trial and error, even though I make it sound like a science. You just have to keep at it, knowing it can be done.

I think the reason why we promote smaller amps (like the 18), is it is very difficult to get an oscillation with that little power.

When I have an oscillation thats what I do. One mod at a time, then check...make sure you discharge those caps inbetween....Ive been shocked many a time by forgetting. Plus on a high watt amp you ussually have bigger caps, which = bigger schock.

first things: try input ground to chassy near jack, grid stopper 8.2k, and sheilded input to ground (or plate). Then 500 pf cap grid to ground. That ussually solves my oscillation problems.

let us know how it goes.

[Phil_S]

Hi
Have you tried a piece of aluminium sheet on the underside to shield the components.
Ben

I second this, especially if you have the amp on top of a speaker cab. The magnets can be responsible for what you report. I typically smear glue on a wood plank and apply kitchen aluminum foil. The heavy duty kind is easier to work with, but the thickness isn't a factor. You need this between the speaker and the cab. An easy test it to simply put the head on the floor.

Lead dress is also a place to look. Use a chopstick to push the wires around (while the amp is "hot") and you might find a cure.

Most folks would not cut their teeth on this circuit. You will learn a lot before you get finished.

[joepopp]

Holy Guac:

I am a newbie amp builder too. Just throwing in a word of support to keep at it! I rewired my amp 3 times before I found that the hum I had was that the circuit didn't like Tung Sol in the V1.

Those red wires from the tranny look close to the tone stack as someone said here already. That fixed a bunch of my problems on my first wiring. I also had some improvement when I moved the ground from the IEC jack (Power cord input) off of the star ground. I just grounded it right below the IEC jack.

Here is a link to my 2204 (Ceriatone):
Joe Popp's JCM800 2204
Good luck and stay on it!!!

[Groundpath]

Have you pulled the NFB out to see if that was it?

[paulster]

Also, if you're getting the oscillation with no guitar plugged in and just the controls dimed, you could try this.

Pull each of the preamp tubes, one at a time, and fire the amp up. It can help to identify at which stage the noise is getting into it.

It can help to give you an area to focus on first.

[Bulatovic]

Holy Guac:

Here is a link to my 2204 (Ceriatone):
Joe Popp's JCM800 2204
Good luck and stay on it!!!

Is that just me or your electrolytic caps on the board are wired the wrong way as you have grounded the + side (or it should be like that? - don't see the point, only if -> was standing for + ???)

Cheerio