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Old 3rd June 2012, 12:56 AM   #21
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Don't move the ground for the input jacks to the star, that will increase the hum.

Does the oscillation stop when you turn down any of the controls? Does it stop if you remove any of the preamp tubes? If the controls have little or no effect, try disconnecting the 4 ohm tap from the turret board to see if that stops the oscillation.
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Old 4th June 2012, 01:40 PM   #22
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Now get this:
Squeel? Gone!
Rattling? Gone! (one exception though, explaining it later)
Hum? None to very tolerable!
The amp is running like a charm. Despite its cute size it has a montrous, aggressive sound!

What happened? Besides smaller changes...
1) I exhanged wirewound resistors on powertubes (grid stopper?) with metal film resistors. I'm not 100% sure, but I think they caused the "gain squeel".
2) On loudthud's advice I was turning every knob to find a relation to hum. By turning PRESENCE all the way to the right, this means running it over the cap exclusively, removed the rattle COMPLETELY!

So I need your help here once more: why do I get a rattle, when presence runs over the resistors instead of the cap? That's the last piece in the puzzle that prevents my amp from fully functioning.
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Old 4th June 2012, 06:27 PM   #23
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In all the technical process I forget what my goal was actually. I wanted an amp, which was loud, provided massive amounts of gain and was versatile. My 8 Watts tube hadn't enough punch for band and wouldn't stand out on a concert.

So I decided to rip out the guts of my old Marshall DFX 15 Watts and insert into it the beauty of vales. My decision fell on the JCM 800 because I played it in a music store and loved it

Now I'm listening to my OWN amp, that's an amazing feeling. But what about the sound? Speaking musically: It's rough, harsh, aggressive, dirty. There is no sweet side. Speaking technically: you have to turn gain all the way up to get a decent result. There's a lack of sustain and "cream" when on cleaner levels. I was thinking: is it possible to change the amp layout for a bluesy touch on lower gain?

Another thing about low gain: You're more likely to hear thin crackling, let's say unpleasant distortion when strumming hard / chording. It's very subtle, but it's there. Maybe it's normal, or maybe it's related to the "Presence" issue??

Now it's the moment to say thanks. You guys really helped me out and I don't know what I should have done without you!! I'm sharing a very short audio demo and pics of the assembled amplifier.

Feel free to have a look / listen. For the audio track, please rename "Memo.asc" to "Memo.m4a".

Please leave some comments on this post.

photo.JPG

photo-1.JPG
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File Type: asc Memo.asc (303.6 KB, 5 views)

Last edited by Cypheron; 4th June 2012 at 06:39 PM.
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Old 4th June 2012, 10:48 PM   #24
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Sounds alright!
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Old 5th June 2012, 04:01 AM   #25
gootee is offline gootee  United States
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Nice! I like it!
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Old 5th June 2012, 08:50 PM   #26
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Thanks. Does anyone have an idea how i could resolve the buzz issue on presence? At least give me a hint where start looking :P im trying to exchange the cap, because the problem only occurs on those positions.

Last edited by Cypheron; 5th June 2012 at 09:17 PM.
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Old 6th June 2012, 12:03 AM   #27
gootee is offline gootee  United States
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Quote:
Originally Posted by Cypheron View Post
Thanks. Does anyone have an idea how i could resolve the buzz issue on presence? At least give me a hint where start looking :P im trying to exchange the cap, because the problem only occurs on those positions.
I think you said the opposite, before:

Quote:
2) On loudthud's advice I was turning every knob to find a relation to hum. By turning PRESENCE all the way to the right, this means running it over the cap exclusively, removed the rattle COMPLETELY!

So I need your help here once more: why do I get a rattle, when presence runs over the resistors instead of the cap? That's the last piece in the puzzle that prevents my amp from fully functioning.
It looks like you're saying that when the Presence pot is set to put a very low series resistance between the 0.1uF cap and ground, it doesn't "rattle".

If you have a scope, you could look at the signal there. Maybe there's an oscillation or instability from somewhere else and the cap is shunting it to ground, a that point.

Is the correct DC voltage present at that point (test point #11 on phase splitter schematic), i.e. 23V or 24V?

Are your pots' bodies grounded?

Actually, the wiring diagram shows the pot upstream from the cap but the schematic shows the pot downstream from the cap. It shouldn't matter, I guess.

Tom
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Old 6th June 2012, 04:46 PM   #28
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Hey tom, you're a very cautious reader. I actually was testing with the amp upside down on the first post, and confused left and right.

The issue is not present on resistor position!

I remember measuring the cap voltage and it displayed about 24V.

Pot bodies are NOT grounded. If its necessary, ill ground them with a straight wire.

What do you mean by "a point"? (the paragraph about shuting oscillation to ground)
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Old 7th June 2012, 12:27 AM   #29
gootee is offline gootee  United States
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Quote:
Originally Posted by Cypheron View Post
Hey tom, you're a very cautious reader. I actually was testing with the amp upside down on the first post, and confused left and right.

The issue is not present on resistor position!

I remember measuring the cap voltage and it displayed about 24V.

Pot bodies are NOT grounded. If its necessary, ill ground them with a straight wire.

What do you mean by "a point"? (the paragraph about shuting oscillation to ground)
I meant to write "at that point", just meaning that maybe the cap was draining some nasties to ground (or attenuating them while acting as part of a low-pass filter) before they got to the next stage, or something. It was just a wild guess.

But the situation is actually the reverse, anyway, such that if the cap is basically cut out of the circuit, by turning the Presence pot so that a large resistance is between the cap and ground (22k), then the buzzing is gone. Correct?

So maybe there's something wrong with the cap, or even the pot. Or maybe the circuit becomes unstable (in the feedback control system sense of instability), when the series resistance is too low (for other readers: the pot is wired as a rheostat, in series with a cap to ground).

I'm not much of a tube guy, and it seems like there are parts of the schematic missing, but, in many other types of amplifiers, having too much capcitance to ground at an inverting input, i.e. without enough resistance between the input and the cap, is pretty-much guaranteed to cause oscillation. Are you sure that the pot is the right value, etc? What is that control supposed to do, anyway?

Since it would be so easy, you could first try grounding the body of that pot, temporarily, to see what happens. I would probably also try replacing the cap, and then the pot if there is still a problem. (But I guess that the probability seems low that any of those actions will change anything.)

Maybe it would be even easier to first temporarily connect another similar-valued cap in parallel with the original one, to see if the problem gets worse. If it does, that might give a clue.

One more idea: How does that cap's ground go back to the star ground? If it's not too much trouble, try unhooking it from wherever it goes now and running a separate wire from it, back to the main star ground.

By the way, is your ground buss made with a large-diameter wire, or a large cross-sectional area?

Edit: Have you tried moving any wires around? If you have any long-ish wires lying about, try routing them differently. (It's difficult for me to even look, without shuddering, at a wiring diagram where they apparently paid no attention to minimizing enclosed loop areas (aka antennas), not to even mention using a buss-type ground.) If you have a good scope or an AC Volts meter setting (AC mV would be better), it would be interesting to try to measure the AC voltage between the "star ground" and the other ground points, while sound is being amplified. If using a meter, you might need to verify that it can measure for frequencies above 120 Hz or so; at least 1 kHz would probably be necessary, so it could catch music-frequency voltages. But you'd still miss any high-frequency stuff that might be there. Anyway, you would also want to try measuring with the leads where they would be for a real measurement, but with the leades shorted together, just to see how what the "ambient" level might be, when measuring.

If there are appreciable voltages (which might still be tiny) measured between the star ground and certain other ground points, then you might want to report back. Those "ground bounce" voltages can create a form of feedback, since they will effectively sum with the voltage at the non-ground end of a resistor that's connected from, say, a stage's grid input to ground.

Cheers,

Tom

Last edited by gootee; 7th June 2012 at 12:54 AM.
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Old 7th June 2012, 01:19 AM   #30
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That's a really cool little killer amp, now will the speaker hold up?
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