jcm800 clone with strange power oscilation

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12afael

Well-known member
Joined
Aug 6, 2004
Messages
1,330
Location
Helsinki
Hi I have built a JCM800 clone with an original OT, four EL34, 100W and I have an issue with the presence control, if I put the control on a low value it make some loud noises. It`s worse if I turn up the master volume control. it don`t happen if I cut the Feedback wire. so I suspect is a weird problem of the power amp and the NFB, maybe too much NFB.

here is a sample of the noise.
www.soundclick.com/fireland

I`m using this layout
http://www.dreamtone.org/pdf/NEW2204LayoutSchemBomDreamTone.pdf


at this point it`s clear it is oscillanting, I reduced the high frequency responce of the PI and it work a little better. I also reduced the feedback resistor to 50k but it didn`t help. I`m using 0.1uf caps between the pi and the power tubes could this noise be blocking distorcion?

someone had any similar problem or have any idea of what more I must check? I just have an old rca 75KHz one channel oscilloscope that just can read AC so I can`t see all what is happend in this beast.

I try to keep guitar items elsewhere but this case is killing me.:mad:

cheers
Rafael
 
Your output transformer may be wired out of phase giving positive instead of negative feedback. Try reversing the wires leading to the anodes.
 
The most obvious cause is the good old bad solder joint, bad pot, bad cap, bad tube. I know you are good, but nobody is perfect.

Have you tried reversing the plate OT leads? If they are wrong now (positive feedback) and you swap, that is a complete cure. But if they are right now, and you swap, it could go to FULL-power howl as soon as it warms up, or you touch a string. (Lurkers: the 5F6-A and family have so little feedback, and an output stage biased so cold, that it can be mis-wired for positive feedback yet be "stable" at idle.)

> I just have an old rca 75KHz one channel oscilloscope that just can read AC

This amp is unlikely to oscillate higher than 75KC, and any major DC clue can be found with your voltmeter (do any voltages go way-whack when it blatts?). 'Scope the speaker output. 'Scope an EL34 grid. 'Scope the top of the Presence pot. Guitar notes will be poorly synced waves. Supersonic oscillation will be "fuzzy".

The "Presence" is a 4.7K fixed resistor and a 22K pot (with 0.1uFd cap). I would not expect this knob to do much of anything until turned up past "7". If it does, something is not wired right or some part is bad.

> I`m using this layout

That's a two-EL34 layout, you say you have four. Where did you fit the extras? I see V4 Plate lead awful close to V3 plate lead; if you jammed the extra EL34 in this area you may have EL34 plate wire spraying all over V3 which drives the EL34s.... round and round it goes.

Try 270pFd at C18.

Try changing 5.6K at EL34 grids to 22K. That rolls a little supersonic, and also resists grid-blocking (perhaps "too well" for rock guitar, which should de-bias a bit in overdrive).

Huh. No electrolytic caps in the main signal path. I wuz wondering if you could have one backward, but you can't.
 
This sound sample you put up sounds to me like something is arcing. Check El34 sockets (remove tubes) for traces of arcing, take a good look at tube bases too. Look for thin black lines between pins and/or black powdery deposits.
 
I had some similar "howling" in an SE5a build and it was a single strand of
wire the was bridging across one of the pins on the EL84 - sounded awful
and took a while with a mag glass to find !
Since then I always "twist and tin" all such wires.

MM.
 
yes I swap the primary of the OT before it had a sort of inestability and the presence didn`t work well. anyway the noise was present too.

the presence work like you say PRR.

I add the extras el34 one on the right and one in the left. I thought about the plates of v3 and v4 very close so completely reversed the push pull stage so if the two plates were in phase now no, but no help at all.

I`m using 1.2n at c18 that help but doesn`t solve the problem completely.

I also change the caps between pi and power tubes from 0.1uF to 0.022uF (stock marshall) and even 0.01uF without luck.

I tested the amp at complete darkness and I wasn`t be able to see any spark in the power tubes and there is not seem to be any black line on the sockets.

Also I increase the feedback resistor to 200k.

I realized that at extreme volume noise appears even without feedback.

I will check with the scope again later.

Rafael
 
finally I found the problem. I had exchanged one wire that goes to the plate and the b+ on the primary of the OT. :oops:

now the amp rages as a lion :grin:

Thank for the help
Rafael
 

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