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.
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.
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.
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.
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.
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.
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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 lookingim trying to exchange the cap, because the problem only occurs on those positions.
I think you said the opposite, before:
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
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)
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
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Hey tom,
Here's what I did today.
1) Exchaning pot - no effect.
2) Exchanging cap - no effect.
3) Adding cap of same value in parallel - no increase/decrease in buzz.
4) Grounding pot case - no effect.
5) Grounding cap / resistor to star ground - no effect.
6) Shorting out the pot, thus connecting cap to resistor at pot end - no buzz, of course.
7) Measure AC between cap ground and star ground on amplification - nothing.
8) Measure resistance between several ground points and star ground - 0,4 Ohms max.
9) Moving wires around - nothing found.
Puh, that's it. Problem seems to be caused somewhere else. So what basically happens is: when i increase resistance between resistor and cap, i get a buzz. I have about 13,86V DC voltage drop on the cap/resistor, 10.6mV on the pots left and right leg. Turning the pot doesn't change any of this, which I - as an non-expert - find weird.
The pot is supposed to raise certain frequencies, so that the guitar stands out more. As far as I can tell. That's why I don't think we're facing an high oscillation issue here, because max presence, where the high frequencies should be boosted most, the amp runs just fine.
I am so clueless right now.
Here's what I did today.
1) Exchaning pot - no effect.
2) Exchanging cap - no effect.
3) Adding cap of same value in parallel - no increase/decrease in buzz.
4) Grounding pot case - no effect.
5) Grounding cap / resistor to star ground - no effect.
6) Shorting out the pot, thus connecting cap to resistor at pot end - no buzz, of course.
7) Measure AC between cap ground and star ground on amplification - nothing.
8) Measure resistance between several ground points and star ground - 0,4 Ohms max.
9) Moving wires around - nothing found.
Puh, that's it. Problem seems to be caused somewhere else. So what basically happens is: when i increase resistance between resistor and cap, i get a buzz. I have about 13,86V DC voltage drop on the cap/resistor, 10.6mV on the pots left and right leg. Turning the pot doesn't change any of this, which I - as an non-expert - find weird.
The pot is supposed to raise certain frequencies, so that the guitar stands out more. As far as I can tell. That's why I don't think we're facing an high oscillation issue here, because max presence, where the high frequencies should be boosted most, the amp runs just fine.
I am so clueless right now.
Here are some insights: the more I turn gain up and the more I turn presence to left, the more buzz I get while playing. The interesting thing is, slowly decreasing presence (going left) first leads to buzz on low notes, and going further left, higher notes keep adding until (on max left) every note has a tendency to buzz.
The buzzing seems to have a frequency. It sounds like buzz-normal-buzz-normal (killswitch effect) with a frequency depending on how hard i strumm. Playing harder results in a higher frequency. Sometimes a high pitched - i think - oscillation gets stuck, but stops randomly if playing further notes.
So I have to take back what I said. There is obviously still some oscillation going on. But hell, I'm out of ideas what to change next.
The buzzing seems to have a frequency. It sounds like buzz-normal-buzz-normal (killswitch effect) with a frequency depending on how hard i strumm. Playing harder results in a higher frequency. Sometimes a high pitched - i think - oscillation gets stuck, but stops randomly if playing further notes.
So I have to take back what I said. There is obviously still some oscillation going on. But hell, I'm out of ideas what to change next.
Im so f@()?.! Happy right now. It's incredible! loudthud, you saved my life man! So simple and now the amp runs perfectly! I dont know why getting rid of the 4 ohm tap was the key, but it works.
Can i leave it unwired? Anyway, I want to thank everyone helping me on my project. I couldnt have done this without you! Im glad i found a great forum with awesome people on the first run.
Can i leave it unwired? Anyway, I want to thank everyone helping me on my project. I couldnt have done this without you! Im glad i found a great forum with awesome people on the first run.
It's hard to tell exactly what's going on without an oscilloscope, but it's safe to say that the power amplifier has stability issues. If you leave the 4 ohm tap disconnected, the Presence control won't work.
It could be that the phase is just wrong, but that usually causes an audible squeeling. In that case you would reverse the phase either at the output tube plates, the output tube grids, or the phase inverter plates.
More likely it is just a layout or lead dress issue. Without a clear picture of the phase inverter and power tubes, I can only guess. In general you need to shorten the wires in that area and try to keep things away from the wires that go to the plates (pin3) of the output tubes. Twist the wires that go to pin 3s together if they run under the turret board until they get closer to the tubes.
If those things do not solve the problem, try increasing the 100K resistor that is connected to the 4 ohms tap. Try 150K, 220K, or 330K. This will decrease the effect of the Presence control. Another thing to try is simply adding a small resistor in seres with the cap that connects to the Presence control. Try 470 ohms or maybe 1K.
It could be that the phase is just wrong, but that usually causes an audible squeeling. In that case you would reverse the phase either at the output tube plates, the output tube grids, or the phase inverter plates.
More likely it is just a layout or lead dress issue. Without a clear picture of the phase inverter and power tubes, I can only guess. In general you need to shorten the wires in that area and try to keep things away from the wires that go to the plates (pin3) of the output tubes. Twist the wires that go to pin 3s together if they run under the turret board until they get closer to the tubes.
If those things do not solve the problem, try increasing the 100K resistor that is connected to the 4 ohms tap. Try 150K, 220K, or 330K. This will decrease the effect of the Presence control. Another thing to try is simply adding a small resistor in seres with the cap that connects to the Presence control. Try 470 ohms or maybe 1K.
Oh man, I'm a bit disappointed. I actually thought the issue was solved. So back on the workbench I did following things:
1) Seperate OT Primare and Secondary. Both running in 90° angles on the side corners of the chassis. The Secondary goes over pots to output jacks next to Stand-By switch. Primary wires are twisted together (red and white) and run along the other side of the chassis. They drop down to Pin 3's from above.
2) Shortened resistor legs on power tube sockets. The 1K, 5Watt resistor is wirewound, because I couldn't get any metalfilm's with that power. It's fine right?
3) Shortened 4Ohm tap. It goes along with all the other Secodary wires. On height of the 100K resistor, it dives below the board and connects on the other side.
So the 4Ohm tap wire is now one of the very few wires running under the borad, ähh board. It's incredible how clean my wiring is becoming.
But! I can still hear the buzzing. Noooo! I know that increasing the 100k resistors value would limit the effect. Same is true for adding a resistor in series with the cap. But that's not what I want.
I want to trace down the problem and get rid of it, at the root. I don't want compromises. I hope you understand that.
I'll take some pictures tomorrow, in the daylight. Because, it's not easy to follow any progress or give suggestions, if you don't see what is actually going on.
Gonna stick around in this topic a little longer that I thought, but that's fine!
1) Seperate OT Primare and Secondary. Both running in 90° angles on the side corners of the chassis. The Secondary goes over pots to output jacks next to Stand-By switch. Primary wires are twisted together (red and white) and run along the other side of the chassis. They drop down to Pin 3's from above.
2) Shortened resistor legs on power tube sockets. The 1K, 5Watt resistor is wirewound, because I couldn't get any metalfilm's with that power. It's fine right?
3) Shortened 4Ohm tap. It goes along with all the other Secodary wires. On height of the 100K resistor, it dives below the board and connects on the other side.
So the 4Ohm tap wire is now one of the very few wires running under the borad, ähh board
. It's incredible how clean my wiring is becoming.But! I can still hear the buzzing. Noooo! I know that increasing the 100k resistors value would limit the effect. Same is true for adding a resistor in series with the cap. But that's not what I want.
I want to trace down the problem and get rid of it, at the root. I don't want compromises. I hope you understand that.
I'll take some pictures tomorrow, in the daylight. Because, it's not easy to follow any progress or give suggestions, if you don't see what is actually going on.
Gonna stick around in this topic a little longer that I thought, but that's fine!
Ok guys, I put in some efford taking pictures and commenting them. I think that'll help.
Right section:
Left section:
Power section:
Preamp section:
Filter Caps 1+2:
OT section:
PT section:
If, for some reason, the images won't maximize properly, here is a link to the gallery:
Gallery
Right section:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Left section:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Power section:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Preamp section:
An externally hosted image should be here but it was not working when we last tested it.
Filter Caps 1+2:
An externally hosted image should be here but it was not working when we last tested it.
OT section:
An externally hosted image should be here but it was not working when we last tested it.
PT section:
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
If, for some reason, the images won't maximize properly, here is a link to the gallery:
Gallery
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@Loudthud: I figured that buzzing frequency is dependend on master level! Turning master up raises buzz frequency (the killswitch effect), turning it down reduces it. On very low master levels I don't get any buzz. If a high pitch oscillation gets stuck, lowering master doesn't help - same is true for EQ controls. It stops when turning presence to the other extreme, playing further notes, or after random events.
So your statement about that evil sounding parasitic feedback may be true. Maybe someone can give me advice on this after looking to the pictures.
So your statement about that evil sounding parasitic feedback may be true. Maybe someone can give me advice on this after looking to the pictures.
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