Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jasonvilla on May 26, 2020, 04:42:39 pm
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Hi there, I have a home brew 2204 that Ive been working on because it sounded a bit fizzy, otherwise is seems to function just fine (I had a post on a different forum about the fizz). On my hunt for the fizz I noticed I got -21vdc after second gain stage coupling cap with the volume all the way up AND a 1khz sine wave running through it. The negative voltage goes down as I lower the preamp voltage and it read 0 if I turn off the sine wave generator or turn the pre amp volume off. Can someone please explain what's going on and if/how I need to fix this. Thank you!
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if/how
I'd go for a better meter and double check
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What's the voltage level of that 1Khz sine wave?
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This is the sine wave measured at the input jack, 260mV
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You're way overdriving the amp!
That's 260mVRMS which is 735mVPP. Lower the input level to about 100mVRMS. Any better?
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Seems like it does the same thing, only now Im reading -11.1 on my meter.
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Well, don't do that. :icon_biggrin:
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lol what!?
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By the time your test signal reaches the second triode grid it is so large that it causes the second grid to draw current during the positive half cycle. The grid current creates a negative voltage to appear on the grid.
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Thank you Sluckey. So seeing a negative voltage, referenced to ground( on the side of the coupling cap I wasn't expecting to see dc voltage), is fine?
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Thank you Sluckey. So seeing a negative voltage, referenced to ground( on the side of the coupling cap I wasn't expecting to see dc voltage), is fine?
No, it's not fine. That grid has zero volts on it when it is being operated as expected.
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Does your amp EXACTLY match that schematic?
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Pretty close, a few changes based on what I had. The power tubes are JJ6v6s. I added that 500ohm resistor to lower the screens. Other than that, I have 2 resistors and a cap after the first dropping resistor to elevate the heaters to 60v, but it's currently bypassed.
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If there are no other preamp tweaks to note you should post your voltages...
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Take another look at your schematic.
Either the 500 ohm resistor in the power rail is mis-drawn or it is out of position.
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Voltages taken with no signal and volumes turned to 0, TMB at 50%.
I forgot to erase the line that connects the input jack to the first .22 cap on the schematic. I only have one input.
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I believe thats where the 500ohm resistor is in the amp.
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Voltages
Nice job.
So, compared to the original schematic your preamp voltages are a little low.
On V2a you have 1V on the cathode. So, with no signal the cathode is 1V positive in relation to the grid.
When the grid voltage (approaches and then) exceeds the cathode voltage you get grid current.
Drive it harder with a big signal, more grid current....which you saw in your original experiment.
The chance of this happening is increased by the lower (than spec'd) plate voltage because the anode is less capable of drawing electrons away from the grid at lower voltages.
One bandaid solution would be to add a grid stopper.
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I believe thats where the 500ohm resistor is in the amp.
There should be an additional cap node between that 500R and 4.7K dropper
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...seeing a negative voltage, ...., is fine?
It is fine. If you want to sound like crap. Or cut-out.
What *most* loud players do is crank-up to get "small" bias shift on the loudest notes.
Watch the meter or 'scope while you really play. At some point in the overdrive profile that grid will go negative and the sound probably changes substantially. On a small tube if you beat it to -20V of bias, the sound probably cuts-out, then fades back when the beating stops.
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Thanks guys
I moved the 500ohm resistor to in-between the choke and the second cap (where I should have put it in the first place).
I switched the sine wave for a guitar and was able to see -12 volts on the v1a grid with the preamp turned all the way up and big open E chords with humbuckers. I could see the voltage go negative with the volume pot around 1.5.
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BTW I was going off of Ceriatone's voltage chart when swapping out my dropping resistors. They seem lower than mine!
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BTW I was going off of Ceriatone's voltage chart when swapping out my dropping resistors. They seem lower than mine!
Yeah, your voltages arent "the problem".
I just chose to point out that they were lower than what was listed on the original schematic posted.
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Haha oh. Any idea what is causing my problem? Should I up my voltages? Thank you
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Haha oh. Any idea what is causing my problem? Should I up my voltages? Thank you
Try adding a 100K grid stopper resistor right at the grid pin on the socket.
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Hi there, I have a home brew 2204 that Ive been working on because it sounded a bit fizzy, otherwise is seems to function just fine (I had a post on a different forum about the fizz). On my hunt for the fizz I noticed I got -21vdc after second gain stage coupling cap with the volume all the way up AND a 1khz sine wave running through it. The negative voltage goes down as I lower the preamp voltage and it read 0 if I turn off the sine wave generator or turn the pre amp volume off. Can someone please explain what's going on and if/how I need to fix this. Thank you!
That’s perfectly normal, overdrive any single ended stage and the same thing will happen.
It’s due to RC coupling and the g-k diode. Overdrive the grid and the diode will chop off the top of the wave. All that’s left is the bottom of the wave, that’s the negative voltage you’re measuring.
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:/ haha alright now Im confused. You say it's normal, Sluckey says its not fine. PRR say it's fine, if I want it to sound like crap.
Did I screw up somewhere, or is this just what that stage of a 2204 does?
Also, you're suggesting I put a 100k gird stopper on V1a, correct? Not V2a
Thanks for the help you guys :)
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Also, you're suggesting I put a 100k gird stopper on V1a, correct? Not V2a
No, it would go onto V2a and help to reduce grid current right where it's happening.
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Also, you're suggesting I put a 100k gird stopper on V1a, correct? Not V2a
No, it would go onto V2a and help to reduce grid current right where it's happening.
Oops, sorry, I wrote that backwards. Thank you
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As far as the rest of the confusion goes, there are plenty of guys playing amps with grid current. And there are plenty of fizzy sounding amps.
We are here to try to identify your issue and inform you to the best of our ability. Grid current doesn't sound good.
Does it happen, yes. Should you avoid it, yes, if you don't like the way it sounds. If you are trying to tweak your amp to function optimally then we can help you do that.
If you are a purest who believes that a masterpiece of an amp should not be tampered with then you are not alone.
One solution would be to turn down the preamp volume pot. I'm pretty sure that's what sluckey meant by "don't do that".
There are other solutions that would require modifying your amp. It all depends on what you want out of it and how far you are willing to bend the original design.
My grid stopper idea is a tiny little bend that may or may not mitigate fizz.
Before going any further I would confirm that all of my resistor values are correct. Make sure that you haven't made a mistake and then you can go forward.
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THANK YOU SILVERGUN! That is exactly what I wanted to hear.
I originally posted (on a different forum) about this new amp sounding a bit fizzy. It worked fine, but I didn't quite like the distortion. I have a few other 2204 ish amps i've built over the years, but not an actual Marshall 2204 to compare it to. Right around this time I got my first scope. Always wanted one. I've never even touched one before! I've been having lots of fun playing with it. Seeing is way easier for me. Im definitely a visual person! Reading voltages and looking at them on the scope WITH a signal applied led me here - asking about the negative dc on the grid.
Im completely open to bend the original design! I just wasn't sure if I have done something to cause this issue. All resistors appear to be correct... I guess I could pull a leg of the 10k cathode resistor and measure it.
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I'm thinking I will lower my second dropping resistor from 10k to 4.7k and add a 100k grid stopper to V2a.
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I was most concerned with the 470K:470K voltage divider right before V2a. If you have correct values there then you have the design pretty much right where it was intended. (except the slightly lowered voltages)
Raising the volatges and installing the grid stopper makes sense.
Since you are willing to bend the design, and you're looking for more overdrive, there are bigger changes that you can make to get it there without having to nail the grid of V2a. Let us know how it goes.
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Thank you, I'll report back after I make those changes.
Here is the amp, and the board. The 470k voltage divider is at the bottom.
Thanks! :)
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I'm thinking I will lower my second dropping resistor from 10k to 4.7k
Ohm's law tells me you can go even lower with that value if you'd like to get closer to the listed voltages on the original schematic.
Not suggesting you have to, just sayin'
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I'm assuming I understand your layout and there is an extra resistor here that I don't quite understand. :dontknow:
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That is the 33k tone slope resistor. I just didn't have the wire on it when I took that picture.
I think I originally settled on 2.7k, 4.7k, 10k as my dropping resistors, before changing it back to what I have now.
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Alrighty, I changed the dropping resistors from 6.8k, 10k, 10k TO 2.2k, 5.6k, 10k. I also still have that 500ohm resistor in after the choke.
My B+ is now: 390, 384, 358, 334, and 323. V1 is now 289 and 245. They're still a bit low, but mostly after the 2.2k resistor.
I also added the 100k grid resistor. With the sine wave playing I was getting -13v after the second coupling, and now im getting -10v. So its getting better...
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Ok, you can set aside the B+ adjustments...you are close enough for rock and roll.
2 things you can try to reduce the onset of grid current:
-I personally would see what happens when I up'd the cathode resistor on V2a. (maybe 1.5K, 1.8K or 2.2K)
-You can also increase the value of that grid stopper (220K up to 470K) and see what happens.
Just understand that I have no allegiance to the JCM800 and I'm just trying to get you to see cause and effect.
There's one bombshell move in the holster but I want you to get past your focus on the grid current.
I was walked through a similar situation a long time ago and learned a valuable lesson along the way.
In no way shape or form am I the only person who you should listen to.
My goal is to be helpful and not push my opinions on you.
I am in treatment for tweakers anonymous.
Where is the preamp gain control set during your measurements?
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Lol Thank you Silvergun, I really appreciate the help.
I left the dropping resistors alone and changed the cathode resistor from 820R to 1.5k and changed the grid stopper from 100k to 470k.
Now the most I see is -5.3v at that spot, this is with the pre amp on 10.
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I would say you're in a better place now.
Plug your guitar in and set the preamp volume to where it sounds best. Probably not on 10.
Report back on how the amp sounds to you now and where you would like to go from here.
If not on 10 doesn't yield you enough overdrive we can source it elsewhere.
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....-5.3v at that spot...
Follow along to the speaker, I figure 3Vrms at that spot implies 1,000V at the speaker. Which is about 125,000 Watts. Which is a couple thousand times more than a couple EL34 can do.
The bias shift IS "normal" when you OVERload a stage. Most audio designers avoid this. Here, you *may* want it.
And when you don't, Turn It Down. There most be some reason the factory spent all those pennies for pots.
Can you *play* the amp at these settings and levels? Not just 'scope it, but play it?
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I would say you're in a better place now.
Plug your guitar in and set the preamp volume to where it sounds best. Probably not on 10.
Report back on how the amp sounds to you now and where you would like to go from here.
If not on 10 doesn't yield you enough overdrive we can source it elsewhere.
Well if definitely sounds different. I think it sounds better, but I feel like it has lost something. The distortion sounds different past about 7 on the preamp volume. Id say the amp sounds louder too.
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....-5.3v at that spot...
Follow along to the speaker, I figure 3Vrms at that spot implies 1,000V at the speaker. Which is about 125,000 Watts. Which is a couple thousand times more than a couple EL34 can do.
The bias shift IS "normal" when you OVERload a stage. Most audio designers avoid this. Here, you *may* want it.
And when you don't, Turn It Down. There most be some reason the factory spent all those pennies for pots.
Can you *play* the amp at these settings and levels? Not just 'scope it, but play it?
But I like playing it with the preamp on 10! :D Or at least around there... Turning down the volume on my guitar makes the amp clean up pretty well.
Im not sure I know what you mean about playing at those levels.
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...I ... changed the cathode resistor from 820R to 1.5k ...
I think that may not be a good idea, as it will mess up the DCCF operating point, ie the pn 1 and hence pin 8 cathode Vdc will rise.
This is a well proven circuit, a bad sound is more likely due to a fault than a design issue. So I'm not sure that mods should be a first resort.
Grid conduction is a perfectly normal mechanism of tube amp overdrive, for non master volume p-p amps it is by far the principal one, ie the 'other' p-p tube takes over before cut off. I don't see why correlation should be drawn between grid conduction per se and a bad tone?
Are you sure that the amp is stable? A fizzy, cut off sound can be caused by instability, as well as extreme bias shift.
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Well if definitely sounds different. I think it sounds better, but I feel like it has lost something. The distortion sounds different past about 7 on the preamp volume. Id say the amp sounds louder too.
Well, that's because it is slightly different. Remember what pdf64 told you about the top half of the wave being chopped off. Now it's doing that less.
Plus, by adding the 470K grid stopper resistor you will lose some highs. It is common to then run a cap across that 470K to allow some highs to bypass it (like you see in other spots). A good value would be like 100p up to 500p depending on what sounds good to you.
Don't let your ears fool you. Sometimes different is better and sometimes it's not. None of what I've had you try is so far out of whack that it would be a huge concern. Although there are millions of JCM800 phanatics that probably think I'm leading you down a dark path. Everything is reversible.
You are developing a skill set. If you like something, keep it. If you don't, don't. If you have questions, please ask them.
I agree with pdf64's point that there are other possible sources of fizz and bad tone, but since you've mentioned having other Marshall amps I'm assuming that we really aren't troubleshooting a major issue. Once you get to a point of acceptance as to where you are now we can help you make informed decisions on how to get the amp where YOU want it. Without being there we have no way to gauge how much fizz or what you are actually hearing.
Everything affects everything and one of the best techniques you can use is to make small incremental changes one at a time and then decide whether to keep or not..
So far all we have done is reduced your 3rd stage grid current. That ain't crazy. If you'd like to run another experiment, try a different 12AX7 in there and rerun your measurements.
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Oh boy. Well... :w2:
I can always put the cathode resistor back to 820 and leave the grid leak.
What do you mean by stable? The amp seams to operate normally, I just think the distortion sounds a bit fizzy. I know thats almost meaningless over a forum. I wish there was a good way to record it.
On past builds Ive put a .68 cap across that 820R and added a few snubbing caps to mitigate the fizz, but this amp sounds different. I guess it could be the lower voltages, OR something that Ive missed or messed up. It also has smaller transformers and a smaller choke, etc.
These are all new tubes, and Ive also tried old ones.
On a side note, wouldn't you guys expect to see more than ~390v from a Hammond 272FX 300-0-300 PT?
https://www.mouser.co.uk/datasheet/2/177/200-1390014.pdf
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On a side note, wouldn't you guys expect to see more than ~390v from a Hammond 272FX 300-0-300 PT?
https://www.mouser.co.uk/datasheet/2/177/200-1390014.pdf
That PT will put out 424V unloaded. The heavier you load it, the more it will sag.
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‘Stable’ means that (almost) whatever conditions of input, load, control settings are applied, the amplifier exhibits no tendency to oscillate.
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Do your other amps have 6V6 power tubes?
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‘Stable’ means that (almost) whatever conditions of input, load, control settings are applied, the amplifier exhibits no tendency to oscillate.
Got it, thank you. I can turn the preamp and master to 10 and it doesn't explode. It's a bit noisy all the way turned up, but it's a pretty small chassis for this circuit and the chassis is sitting on the ground.
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Do your other amps have 6V6 power tubes?
Yea pretty much haha. I think they all have JJ6v6s these days. One of the Marshall - ish amps I built has standard 50 watt iron in it, and I had el34s in there for a while. But it was just too damn loud. I don't love the sound of 6v6s being over driven, but you can get a bit of break up from them without my ears bleeding.
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This is the first time Ive done anything like this so be nice :)
Its hard to hear, and describe, but theres like some fizz when I play with the pre amp turned to ten. I just dont like how the distortion sounds, but I think it cleans up well with the guitar volume. This is with the 1.5k cathode resistor* Pre amp on 10, Master on 3. TMB on 8/5/8. P on 7. It was quite loud. Also, the cab doesnt have the chassis in it so the back is pretty open
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Do I see 2 or 4 sound absorbers, 2 on the wall, maybe 2 more on the floor, right side?
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Do I see 2 or 4 sound absorbers, 2 on the wall, maybe 2 more on the floor, right side?
Haha yea those are two DIY acoustic panels I made from mineral wool ( I have two on the other side of the room ) And the black thing on the bottom right is a DIY subwoofer. Actually, Ive built all my home theater speakers... they're a bit overkill :dontknow:
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You did a good job with the video and that is how I would expect it to sound.
Unfortunately, I don't have anything more hi tech than some headphones and I was unable to detect the issue.
It sounds like you have a good running amp and almost sounds as good as that YZ250.
Does the fizz go away if you play with the preamp gain down around 6-7?
I would poke around more with the scope and see if you can see where the problem happens.
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You did a good job with the video and that is how I would expect it to sound.
Unfortunately, I don't have anything more hi tech than some headphones and I was unable to detect the issue.
It sounds like you have a good running amp and almost sounds as good as that YZ250.
Does the fizz go away if you play with the preamp gain down around 6-7?
I would poke around more with the scope and see if you can see where the problem happens.
Dang, I was hoping that would help. I really need to find someone with an actual JCM800 to compare it to! Maybe this is what it's supposed to sound like :dontknow:
The scratchiness, or fizz, is almost gone with the pre on 5.
I like the scope idea! How do I do that? :)
And damn, you caught me off guard with that YZ250 comment! I restored that a few years back, over the course of a year or two. Heres a before and after, just for fun :D
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I like to keep things on the lighter side and I also like to show you that I'm paying attention.
In this case I'm guilty of leading you off topic so I apologize. That is a nice bike and project.
I would argue that it is relevant because it shows competence but since we're not in court it doesn't matter.
You can use the same sine wave input and just check every grid and plate throughout the amp to see what your distortion looks like. Make changes to your controls an see the results. You could then compare those results to what you see in your other amps. This would be a long process but might help you understand what you are hearing. You could take pictures of the scope traces at similar settings for reference.
Keep talking because it all helps. I think you'd agree that this stuff is cool to learn. We all started somewhere.
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Thanks again for your help :)
The scope is a lot of fun, but I dont know what Im looking for. I know what the sine wave looks like and what happens when it clips, but I dont know what that sounds like. There is a math function on the scope that I can look at harmonics, which is cool to see even/odd order harmonics, but Im not sure how helpful that is in debugging the sound I hear. :w2:
I took some pictures of the scope at different spots on the amp. Im not even sure if Im testing in appropriate spots. Do I need to test on the plate side? I assume my wave would just be elevated, right? :dontknow: haha, again Im not sure!
I made a map of my test points :) Everything is to ground except across the PI. I kept the same exact settings I used in the video.
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2,3
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4,5
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6
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I don't have much time today, but I can follow up later. You're doing fine. Great job with the map.
My suggestion was that you could take and log some measurements on this amp and then do the same thing with the same conditions on one of your other amps that doesn't have this issue.
By comparing the 2 waveforms you could possibly narrow down where your issue is coming from.
You might be hearing the effect of that spikey squared off wave at test point 4. See how it starts there and then stays that shape throughout?
If you've still got it on the bench add a test point at the grid of the cathode follower/plate of V2a.
Keep documenting like you're doing and it might become apparent to you.
Since you have a Ch2 input you could add a second test lead and overlay the grid signal vs. the plate signal. This can be useful for visualization.
Don't worry so much that you don't have a lot of experience with the scope. The only way to learn is to do it. It's the best way to see your sound.
:thumbsup:
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since you're tinkering, like SG pointed out, the overshoot spikes can sound bad. Measuring at the TS can be problematic. A quick test, put another 470pF in parallel with the treble cap and see what the results are at point 4
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I suggest to also monitor the waveform at the DCCF, V2 pin8. As with the amp controls set up as they are, that’s the final clipping point. Whereas the waveform at ‘4’ will be sensitive to the exact settings of the treble and mid controls.
Also I’m wary of paying much attention to waveforms within a feedback loop (ie point 5); they can be confusing, look weird, as they’re a composite of the actual signal and the feedback generated error correction signal. And obviously the presence setting will also affect that.
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I had a couple minutes and think I spotted an error.
It appears that the pic labeled #4 location does not match the mapped location for #4.
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SG I've been reading through this thread and wonder why you would ask about 6V6 equipped amps?
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SG I've been reading through this thread and wonder why you would ask about 6V6 equipped amps?
Maybe you missed this...
https://el34world.com/Forum/index.php?topic=26041.msg283529#msg283529
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SG I've been reading through this thread and wonder why you would ask about 6V6 equipped amps?
He mentioned having 6V6s in this amp but didn't say what the other 2204'ish amps had in there. IMO the 6V6 would be more likely to have a ratty sounding distortion than the EL34. Also, at some point I will recommend that he make a couple other changes. He already dropped the NFB resistor to 56K but it might have to go lower.
I'd also switch out those 220K grid leaks for 100K. sluckey handled all of this on the Plexi 6V6.
We're just not there yet and I didn't want to overwhelm him, but if he would have said the other amps have EL34s I would have pointed to that as a possible reason for what he is hearing.
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I don't have much time today, but I can follow up later. You're doing fine. Great job with the map.
My suggestion was that you could take and log some measurements on this amp and then do the same thing with the same conditions on one of your other amps that doesn't have this issue.
By comparing the 2 waveforms you could possibly narrow down where your issue is coming from.
You might be hearing the effect of that spikey squared off wave at test point 4. See how it starts there and then stays that shape throughout?
If you've still got it on the bench add a test point at the grid of the cathode follower/plate of V2a.
Keep documenting like you're doing and it might become apparent to you.
Since you have a Ch2 input you could add a second test lead and overlay the grid signal vs. the plate signal. This can be useful for visualization.
Don't worry so much that you don't have a lot of experience with the scope. The only way to learn is to do it. It's the best way to see your sound.
:thumbsup:
Thanks again for your help, for everyones help. I really appreciate it.
I understood your suggestion but I dont have an exact copy of this amp to compare it to. Anything close would be something that I built. I have a fender super sonic 22 that I could poke around at, but it doesn't have a CF and its set up a bit wonky compared to these "simpler" circuits. It's been very educational getting used to a scope and visualizing things. I havent taken the second lead out of the bag yet! haha Thanks again, this is the cathode follower/plate of V2a.
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since you're tinkering, like SG pointed out, the overshoot spikes can sound bad. Measuring at the TS can be problematic. A quick test, put another 470pF in parallel with the treble cap and see what the results are at point 4
I noticed that the tone controls really changes what the wave form looks like, which is pretty cool to see :)
This is TP4 alone and with a 500pf cap along the 500pf treble cap.
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I suggest to also monitor the waveform at the DCCF, V2 pin8. As with the amp controls set up as they are, that’s the final clipping point. Whereas the waveform at ‘4’ will be sensitive to the exact settings of the treble and mid controls.
Also I’m wary of paying much attention to waveforms within a feedback loop (ie point 5); they can be confusing, look weird, as they’re a composite of the actual signal and the feedback generated error correction signal. And obviously the presence setting will also affect that.
Thank you, I wasn't sure where to test from, but that makes sense. I was just a little hesitant to start measuring high voltage DC. This is V2 pin 8
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I had a couple minutes and think I spotted an error.
It appears that the pic labeled #4 location does not match the mapped location for #4.
I dont think it's wrong? It's the outside lug of the MV, coming from the treble pot.
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SG I've been reading through this thread and wonder why you would ask about 6V6 equipped amps?
He mentioned having 6V6s in this amp but didn't say what the other 2204'ish amps had in there. IMO the 6V6 would be more likely to have a ratty sounding distortion than the EL34. Also, at some point I will recommend that he make a couple other changes. He already dropped the NFB resistor to 56K but it might have to go lower.
I'd also switch out those 220K grid leaks for 100K. sluckey handled all of this on the Plexi 6V6.
We're just not there yet and I didn't want to overwhelm him, but if he would have said the other amps have EL34s I would have pointed to that as a possible reason for what he is hearing.
I actually started with 100k grid leaks. Ive tried 100k, 150k, and 220k (which is what is in there now). Actually, sluckey's 6v6 Plexi is the only example of a 6v6 Marshall that uses 100k grid leaks, that ive seen. Everything else Ive seen has 220ks in that spot. Even Rob Robinette's 6V6 JCM800 - which this amp basically is. I've tried different values in the NFB as well, I did all this stuff before starting to post on here - ran out of ideas :) And the reason I havent been focusing on the power amp section is because I hear what Im hearing at low levels and I didnt think the power amp wouldn't come in to play. BUT I could definitely be wrong about that. I have thought about running EL34s at this lower B+ to see how that sounded, but I wanted to address the pre amp first and not get off topic.
...Now I want to try EL34s in there :icon_biggrin:
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I had a couple minutes and think I spotted an error.
It appears that the pic labeled #4 location does not match the mapped location for #4.
I dont think it's wrong? It's the outside lug of the MV, coming from the treble pot.
Ok, I couldn't see the pot in the pic. It looked like it was attached post PI.
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SG I've been reading through this thread and wonder why you would ask about 6V6 equipped amps?
He mentioned having 6V6s in this amp but didn't say what the other 2204'ish amps had in there. IMO the 6V6 would be more likely to have a ratty sounding distortion than the EL34. Also, at some point I will recommend that he make a couple other changes. He already dropped the NFB resistor to 56K but it might have to go lower.
I'd also switch out those 220K grid leaks for 100K. sluckey handled all of this on the Plexi 6V6.
We're just not there yet and I didn't want to overwhelm him, but if he would have said the other amps have EL34s I would have pointed to that as a possible reason for what he is hearing.
I actually started with 100k grid leaks. Ive tried 100k, 150k, and 220k (which is what is in there now). Actually, sluckey's 6v6 Plexi is the only example of a 6v6 Marshall that uses 100k grid leaks, that ive seen. Everything else Ive seen has 220ks in that spot. Even Rob Robinette's 6V6 JCM800 - which this amp basically is. I've tried different values in the NFB as well, I did all this stuff before starting to post on here - ran out of ideas :) And the reason I havent been focusing on the power amp section is because I hear what Im hearing at low levels and I didnt think the power amp wouldn't come in to play. BUT I could definitely be wrong about that. I have thought about running EL34s at this lower B+ to see how that sounded, but I wanted to address the pre amp first and not get off topic.
...Now I want to try EL34s in there :icon_biggrin:
Good stuff. That devil horn signal ain't pretty.
Really good communication across the board.
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I'm on the run today and the pics are too small on my phone. I can't keep up.
I'll catch up reading later but maybe the others will chime in with some opinions.
If you have time, see if you can dial out some of the spike in the tonestack and then plug back in and see how it sounds. See if dialing out the spike makes the fizz go away.
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If you're still tinkering;
monitor TP 5, adjust your input signal until the sine wave just starts flat-toping, then measure your input signal n post the #'s
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Good stuff. That devil horn signal ain't pretty.
Oh man, but thats soooo anti rock n roll!!!! :D
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I'm on the run today and the pics are too small on my phone. I can't keep up.
I'll catch up reading later but maybe the others will chime in with some opinions.
If you have time, see if you can dial out some of the spike in the tonestack and then plug back in and see how it sounds. See if dialing out the spike makes the fizz go away.
Alrighty, at TP4 I was able to make a square wave (no more spike) by adjusting the TMB like so:4/7/1. Pre amp on 10.
I made a video of what this sounds like (it sounds like poo) But you can hear the sizzle/fizz crap a bit better on this recording .
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If you're still tinkering;
monitor TP 5, adjust your input signal until the sine wave just starts flat-toping, then measure your input signal n post the #'s
Forever tinkering!!
Im not sure if I did this right. I kept the same TMB settings as the test before. Left the same 136mv sine wave going in to the amp, but adjusted the pre and master volume to get the largest signal i could get that was just starting to flatten. Should I do this differently? Thank you
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Yup, I could hear the sizzle/fizz now. It seems to be external noise, not due to too much drive. I would try a new tube in V1 if you haven't already.
Pic 1 - Curious about this cap. I don't think it was on your schematic. If it is where I think it is clip it out and report back.
Pic 2 - I looked over your board and there is some missing solder around one of the black mica cap leads. Can you confirm a solid solder connection there?
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While you're in there check this one too
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Yup, I could hear the sizzle/fizz now. It seems to be external noise, not due to too much drive. I would try a new tube in V1 if you haven't already.
Pic 1 - Curious about this cap. I don't think it was on your schematic. If it is where I think it is clip it out and report back.
Pic 2 - I looked over your board and there is some missing solder around one of the black mica cap leads. Can you confirm a solid solder connection there?
Ive tried 3 tubes in V1 but Ill try again since we've changed some things around.
Pic 1 - nah, thats a 100pF cap that wasn't in there when I made the schematic. At first I had no cap, then a 68pF, then no cap, and now theres a 100 in there. It doesn't seem to do much.
Pic 2 - theres a butt load of solder in there! It's hard to fill up those big turrets with only one small lead going inside. Ive done the tap test through out the board listening for cold solder joints and just not I put the output of that on the scope and watched as I moved that cap around and didnt see anything.
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While you're in there check this one too
Took me a second to find that one! That was cap I had put in there to check that location, it's not in there anymore. I have the Sozo back in that spot. So that pic is now old :)
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Pic 1 - nah, thats a 100pF cap that wasn't in there when I made the schematic. At first I had no cap, then a 68pF, then no cap, and now theres a 100 in there. It doesn't seem to do much.
Where does it connect? It seems to come off of V1A plate, but to where?
Why?
Did you try it without it?.....I mean to see if it makes a difference with the fizz
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Pic 1 - nah, thats a 100pF cap that wasn't in there when I made the schematic. At first I had no cap, then a 68pF, then no cap, and now theres a 100 in there. It doesn't seem to do much.
Where does it connect? It seems to come off of V1A plate, but to where?
Why?
Did you try it without it?
Its just a snubber cap from pin 6 to 8 - one of the many things ive tried to get rid of the noise. I can take it back out.
Yea I built it without it, and have had it in and out a few times - trying various things.
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Its just a snubber cap from pin 6 to 8 - one of the many things ive tried to get rid of the noise. I can take it back out.
Yea I built it without it, and have had it in and out a few times - trying various things.
Ok, I needed to make sure it wasn't a different mod that I have seen where they go from plate to grid. When placed there if the cap shorts you wind up with B+ on your strings.
Take it out or leave it. I was worried it might have been associated with the fizz.
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Not sure what else to try. I think ill replace the pre volume pot and maybe a few of those black micas.
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Not sure what else to try. I think ill replace the pre volume pot and maybe a few of those black micas.
The caps, maybe...the pot, not. You could try strapping a 1M resistor across the outside lugs of the pot and see if that helps. And you could always try adding a grid stopper to V1b.
But, like pdf64 mentioned before, these are mods to what should be a working circuit.
There was another point of interest to me from the pics. The solder joint on the negative side of the preamp filter cap looks possibly cold. I would reflow that joint to be sure. Don't forget to use a heatsink on the lead.
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Forever tinkering!!
the scope shows ~40Vac rms for clean drive to the 6V6.
that setup should give you nice chime-y clean sound.
It seems to be external noise
If there is something sneaking in, your scope "should" see it, should be quick, sharp spikes, intermittent somewhere "on" the sine wave. Using a chopstick tapping n wiggling you should be able to get it to happen. quick twist knobs also
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Randall Aiken mentioned recently that after numerous early failures, he’s never going to use silver mica caps ever again. And he will have been buying kosher brands from bonded parts wholesalers; it’s not like he would be sourcing stuff off ebay.
When you’re making high end amps for touring pros, a $1 cap can cause an urgent warranty claim costing $hundreds to get fixed.
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While the iron is hot, here is a couple items to address.
The ground connection on this (I'm assuming presence) pot doesn't look all that great. I would try to get a more secure path to ground.
The other thing is that it is bad form to just float resistors off of tube socket pins. That higher wattage resistor is heavier than needed and would eventually snap off at the 90 degree bend.
It looks like there are a couple other instances of this throughout the amp.
A common trick is to use the socket hold down screw and attach a little terminal strip to give you a mounting point.
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silver mica caps
+1
gave up on them a few years back, fail rate was way up compared to other caps
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I will try swapping out the 4 silver micas on the board for ceramic caps (all I have) to see if it makes a difference.
I will reflow the areas SGUN mentioned as well. Im not sure where I would put them now, but in future builds ill make sure not to hang resistors off of tube sockets. Thank you for the advice.
I just spent a good ten minutes looking at the scope, bending the leads on the silver micas, tapping every solder joint, and I didnt see anything move :(
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Looking and thinking more at the pic above.
You definitely want to create some distance between the orange and yellow grid wires and the green and black wires that are going to the speaker out jack.
This is the pic that made me question the ground on the back of that presence pot.
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I didnt see anything move :(
then you have no problem :icon_biggrin:
EDIT:
try this;
set your sig-source to 880hz, set all your amp knobs to 5, play through speaker
If it's clean, play with knobs,
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Well, I think it might have the silver micas ...I think the attic fan above my living room is causing noise as well. I have been playing with more NFB and I think it sounds better with more. Still working on that.
I switched out the 4 micas for cheap ceramics AND I took out the 1.5k cathode resistor and put back the 820R. Interestingly TP4 shows -4.5v instead of the -5.5 it saw with the 1.5k. The 470k grid stopper is still in there because its hard to get to :)
I know it looks like the wires are on top of each other/touch but they're not. It just looks like it in the photos. They're as far apart as I can make them!
I have been reading through old threads and It seems like I could try out the EL34s and keep the 6.6k OT I have right now. And it should be ok with this lower than normal B+... Is that correct? I have pins 1 and 8 tied to ground*
Thanks again everyone!
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The negative Vdc appearing at tp4 with heavy preamp overdrive is a total non issue in my view, so its magnitude is somewhat irrelevant.
If without a big grid stopper, the next stage’s bias is shifting excessively, such that sound is blocking out, then by all means reintroduce sufficient grid stoppering to avoid that.
Maybe the Vdc shift at tp4 is a good indicator of that, but the Vdc at the actual grid would seem a better point to measure/monitor it?
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The negative Vdc appearing at tp4 with heavy preamp overdrive is a total non issue in my view, so its magnitude is somewhat irrelevant.
If without a big grid stopper, the next stage’s bias is shifting excessively, such that sound is blocking out, then by all means reintroduce sufficient grid stoppering to avoid that.
Maybe the Vdc shift at tp4 is a good indicator of that, but the Vdc at the actual grid would seem a better point to measure/monitor it?
I’ll measure it at the tube socket tonight :)
I’m now wondering if I should reduce the power tube grid leaks to 100k and/or reduce the nfb resistor and/or change the PI cathode resistor from 470 to 820...
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Where are you with the fizzy tone?
I’m not sure that either of those changes would help much with that.
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The reason I was concerned about the distance between the secondary wires and the grid wires was I was picturing possible feedback getting into the grids. I've never experienced this and I'm not sure it would even sound like that but it was just a hunch.
The other point was the presence pot ground and I'm only repeating this because it is the ground point for the PI and I didn't like how that solder blob was sitting up off the pot. Again, not sure if it would cause your issue but just trying to clean up issues that should get resolved.
You had mentioned that switching out the micas might have done the trick. Have you confirmed?
Here's a quick something to try. Disconnect the NFB wire.
Did the fizz get louder?....softer?....same?
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the attic fan above my living room is causing noise as well.
did you figure out how to stop that while toubleshooting
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Here's an example of how to secure those grid wires/resistors
https://www.tubesandmore.com/products/terminal-strip-2-lug-2nd-lug-common-horizontal
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Well... Sadly, I take it back. I thought it was the micas. I thought I was done. I mounted the presence cap/resistor AND the power tube grid stoppers to a turret strip. I reduced the NFB resistor to 33k. I assumed I was good, so I took out the V2 470k grid stopper. ...But I tested it again tonight (attic fan turns off at night) and its still fizzy. Doesnt sound all that great. Not as good as I think it sound. But hey, not having heard an actual Marshall, maybe this is just how it sounds. I don't know. Im about to give up on this thing. It's very discouraging!
Before I did anything - I measured, with a sine wave applied, at the V2 tube socket.
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still curious what it "sounds" like when you make that 1/2 square wave a nice sine wave going into to the PA tubes.
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You've done a great job of trying to get this resolved. Don't give up.
You and shooter should watch some videos of how this amp should sound.
The wave will have to go square at some point, it's just that fizz that's riding on top that is the problem.
https://www.youtube.com/watch?v=3fziJpBigt8 (https://www.youtube.com/watch?v=3fziJpBigt8)
EDIT - I watched this video and the preamp gain control never moved off of 7. Go back to what PRR said earlier in the thread.
On 7 there is enough gain here to satisfy most high gain guys. jasonvilla doesn't strike me as an over the top gain guy. Feels like we are missing something. :dontknow:
I searched for a scope video of a JCM800 and came up with this.
Right around 4:20 you'll see that same devils horn wave form, so there goes that theory.
I hope this gives you at least some point of reference:
https://www.youtube.com/watch?v=-8qjq8hlQRc (https://www.youtube.com/watch?v=-8qjq8hlQRc)
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You and shooter should watch some videos
:l2:
Not sure either of us are up for a night on the couch :icon_biggrin:
I simply want to know if the amp sounds good clean, or there's a problem in the build
why Marshall "designed" devil horns is not my problem :laugh:
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Well... Sadly, I take it back. I thought it was the micas. I thought I was done. I mounted the presence cap/resistor AND the power tube grid stoppers to a turret strip. I reduced the NFB resistor to 33k. I assumed I was good, so I took out the V2 470k grid stopper. ...But I tested it again tonight (attic fan turns off at night) and its still fizzy. Doesnt sound all that great. Not as good as I think it sound. But hey, not having heard an actual Marshall, maybe this is just how it sounds. I don't know. Im about to give up on this thing. It's very discouraging!
Before I did anything - I measured, with a sine wave applied, at the V2 tube socket.
Those grid stoppers on the power tubes look to be quite a high power type? 1/2W is plenty; each to their own, but I'm happy to leave 1/2W types dangling.
Are those V2 socket measurements at pin 2?
Have you tried lifting the V2 pin3 cathode bypass cap yet? That will give a somewhat smoother overdrive.
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Well... Sadly, I take it back. I thought it was the micas. I thought I was done. I mounted the presence cap/resistor AND the power tube grid stoppers to a turret strip. I reduced the NFB resistor to 33k. I assumed I was good, so I took out the V2 470k grid stopper. ...But I tested it again tonight (attic fan turns off at night) and its still fizzy. Doesnt sound all that great. Not as good as I think it sound. But hey, not having heard an actual Marshall, maybe this is just how it sounds. I don't know. Im about to give up on this thing. It's very discouraging!
Before I did anything - I measured, with a sine wave applied, at the V2 tube socket.
Those grid stoppers on the power tubes look to be quite a high power type? 1/2W is plenty; each to their own, but I'm happy to leave 1/2W types dangling.
Are those V2 socket measurements at pin 2?
Have you tried lifting the V2 pin3 cathode bypass cap yet? That will give a somewhat smoother overdrive.
I probably have 2/3 watt resistors in that spot so I didnt have to buy more than one wattage and could use them for B+ dropping resistors. Or it's what I had on hand :dontknow:
Yup, that last pictures of the DMM and scope are taken right from the tube socket BUT when the 470k was still installed.
Oh I never had a bypass cap on V2. Just the standard .68 on V1
The amp circuit should pretty much a stock 2204 right now - with the exception of the NFB resistor and the B+ dropping resistors.
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You and shooter should watch some videos
:l2:
Not sure either of us are up for a night on the couch :icon_biggrin:
I simply want to know if the amp sounds good clean, or there's a problem in the build
why Marshall "designed" devil horns is not my problem :laugh:
But I have a nice home theater! lol
I think the amp sounds pretty good with the pre turned low and the master turned up. Which is why I think the problem is in the pre amp somewhere... but idk!
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You've done a great job of trying to get this resolved. Don't give up.
You and shooter should watch some videos of how this amp should sound.
The wave will have to go square at some point, it's just that fizz that's riding on top that is the problem.
https://www.youtube.com/watch?v=3fziJpBigt8 (https://www.youtube.com/watch?v=3fziJpBigt8)
EDIT - I watched this video and the preamp gain control never moved off of 7. Go back to what PRR said earlier in the thread.
On 7 there is enough gain here to satisfy most high gain guys. jasonvilla doesn't strike me as an over the top gain guy. Feels like we are missing something. :dontknow:
I searched for a scope video of a JCM800 and came up with this.
Right around 4:20 you'll see that same devils horn wave form, so there goes that theory.
I hope this gives you at least some point of reference:
https://www.youtube.com/watch?v=-8qjq8hlQRc (https://www.youtube.com/watch?v=-8qjq8hlQRc)
Thank you, I wish I had a better result to go with the effort. I appreciate all the help.
His JCM800 sounds pretty cool but its going into a reactive load and a pretty popular Ownhammer IR.
Oh man that second video is REALLY cool! Thanks for that link. I sure wish he made more videos. Too bad its a 1959 and not a 2204! :( But its still really cool to see his signal through the various stages... My plan was to make a Plexi next anyways :) If I ever get this sorted out...
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still curious what it "sounds" like when you make that 1/2 square wave a nice sine wave going into to the PA tubes.
Sorry, are you saying I should put a sine wave through the amp, but use the speaker instead of the dummy load?
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you answered it, my way of "asking" if it plays good clean :laugh:
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...I think the amp sounds pretty good with the pre turned low and the master turned up. Which is why I think the problem is in the pre amp somewhere... but idk!
If you prefer the sound with a grid stopper on V2 pin2, then I'd refit one. 470k may roll off some high end, if it's a bit dull then try a lower value.
Before doing so, it may be worth checking that with high gain settings, there aren't any parasitic (ie signal dependent) oscillations occurring.
Monitor the waveform at V2 pin8, whilst applying a wide range of level and freq signal variation at the input.
Looking for this kinda thing
https://encrypted-tbn0.gstatic.com/images?q=tbn%3AANd9GcSYMPa3erOe4iT9vN51CtFtmQK5jfiuh0RBawjJC4BuC7gaY3hc&usqp=CAU
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...I think the amp sounds pretty good with the pre turned low and the master turned up. Which is why I think the problem is in the pre amp somewhere... but idk!
If you prefer the sound with a grid stopper on V2 pin2, then I'd refit one. 470k may roll off some high end, if it's a bit dull then try a lower value.
Before doing so, it may be worth checking that with high gain settings, there aren't any parasitic (ie signal dependent) oscillations occurring.
Monitor the waveform at V2 pin8, whilst applying a wide range of level and freq signal variation at the input.
Looking for this kinda thing
https://encrypted-tbn0.gstatic.com/images?q=tbn%3AANd9GcSYMPa3erOe4iT9vN51CtFtmQK5jfiuh0RBawjJC4BuC7gaY3hc&usqp=CAU
Thank you for this. I tried for a while but didnt see anything like the example in the picture.
I can get the wave to dance around a bit with a square wave, but I just started reading about square wave testing and I dont know what im doing yet :)
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I finished this amp a while ago but forgot to follow up. I ended up giving it to a friend of mine, and he seems to be enjoying it! :)
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.
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And a quick demo I made for IG. No making fun of me :P