Asteroid unpleasing distortion

[arjepsen]

It seems to me, that the spike is actually the guitar signal being amplified. Looking at it - it repeats going all the way from positive to negative. However, all parts of it going from negative to positive sort of pauses in the middle. Thus the downgoing signal has a larger "movement" all at once than the upgoing signal.
Does that make sense?
I still wonder why the guitar signal looks like that. Again it looks the same on a different guitar.

I have several different set of powertubes that I've tried. These 6L6's are the ones that seem to emphasize the bad sounds the least.
I have set the bias so one is at 32mA the other at 37.
I've tried several different tubes in the PI - both balanced and "regular".

The cathode resistor is there, underneath the cap. I've tried to run the grounding scheme following guidelines in Merlins books.
The resistor and cap is grounded right at the filtercap negative. From this point it leads to the the general star ground connected to the chassis. The input jack is also grounded at the general star ground.

Here's a mockup of the layout I made - hope it's understandable:

layout.pdf

[romberg]

It seems to me, that the spike is actually the guitar signal being amplified. Looking at it - it repeats going all the way from positive to negative.

You input signal follows a pattern down/up/down...down/up/down... The first down is not displayed because your scope triggers on the upward swing. But the next two are the repeating waveform.

But after stage 1 your signal is not an amplified version of this which reversed would be up/down/up...up/down/up. Instead your scope is showing up/down/up/down...up/down/up/down. There is an extra positive peak. This distortion is then amplified and very visible on the output. Exactly why it is appearing on stage ones output, I am unsure.

But I'm also unsure that your modification on the stage two plate is okay. Stage 1 and two share a power node (B+5). So, perhaps this is the problem. You have added two coupling caps and a 2.2m anti pop resistor to the gradual fast switch. At first blush this seems a good idea to keep high voltage off the mini toggle switch. But I wonder if it is causing unintended feedback?

The caps are connected across the 82k plate resistor at all times. This plate resistor will have a dc and ac signal on it. The caps will block the DC but the ac signal will be passed by both caps. And an ac signal will then exist across your 2.2m resistor mounted on the switch. one side of this is floating. I wonder if some of this ac signal is being injected into the circuit in a way that is causing havoc. I'd try defeating (totally) that part of the circuit and see if things clean up. Either take out the 2.2m resistor or use the schematics version of the switch.

Mike

[arjepsen]

You input signal follows a pattern down/up/down...down/up/down... The first down is not displayed because your scope triggers on the upward swing. But the next two are the repeating waveform.

It's quite possible I'm wrong, and please tell me if I am, but what I'm thinking is this:
Techniacally you could look at the input signal as a normal sine wave: up/down.
However, there's a "pause" when it's going down. You could write it as: ½ down/1 up/ ½ down - "pause" - ½ down / 1 up / ½ down.
If you look at the amplified traces, they show a slight up, everytime there's a ½ down, and a bigger spike down when there's a 1 up.
So the amplified traces could be written as: slight up, spike down, slight up - pause - etc...
Again, I'm not sure about this at all, please correct me if I'm wrong!

But I'm also unsure that your modification on the stage two plate is okay. Stage 1 and two share a power node (B+5). So, perhaps this is the problem. You have added two coupling caps and a 2.2m anti pop resistor to the gradual fast switch. At first blush this seems a good idea to keep high voltage off the mini toggle switch. But I wonder if it is causing unintended feedback?

The caps are connected across the 82k plate resistor at all times. This plate resistor will have a dc and ac signal on it. The caps will block the DC but the ac signal will be passed by both caps. And an ac signal will then exist across your 2.2m resistor mounted on the switch. one side of this is floating. I wonder if some of this ac signal is being injected into the circuit in a way that is causing havoc. I'd try defeating (totally) that part of the circuit and see if things clean up. Either take out the 2.2m resistor or use the schematics version of the switch.

Sorry if I didn't mention this - I've taken out one of those .1 uF caps. Right now there's only one in there, from the junction between the 82k and the 22k resistors. Like it's constantly in gradual mode. The scope traces are taken with only the one cap in there.

[Reeltarded]

I cry. I cry induction.

When I am developing an idea like a new layout or a pile of inductors tacked to a pot with resistors all over it.. I might be super unsuccessful putting it in an existing amp.. but that is a development step and I learn something, then I prepare an amp for the similar idea based on what I learned and test that, and then it evolves, and it evolves, and most of my ideas end up as working additions that operate just like they did when I imagned them, only I had to imagine them somewhat differently.. this is why when it first sounded like a parasite problem with PPIMV I said I would usually never install one until I have a completed working amp..

Deviations from all known working layouts or designs are a breeding ground for frustrations and nightmare about electrons doing bad things.

So.. in summary, I would have built an Asteroid.. then I would have started reducing it from the input end until I had a working prototype PTP Asteroid.. then I would have added circuit functions, then I would have installed a PPIMV.

(and all along the way I would have been pinging Dave just to make him hate me when he asks me what the voltages are and I answer "awesome!") lol

Laugh. This is funny. It's a 10 year old story of how to develop a method of development, subtitled DIY Is Easy * after while.

Your amp is breaking my reality. hehe

[romberg]

Techniacally you could look at the input signal as a normal sine wave: up/down.
However, there's a "pause" when it's going down.
...

Except your guitar signal (I assume measured at the grid of V1?) is not a sin wave. So, we can just accept it is what it is which is a funky signal generator . Regardless of what is going on we know it is not what should be going on. Even on a fully cranked express, the first stage never distorts. With a normal guitar signal, the output of stage one should be an amplified mirror image of what is on the grid. What you have going on is far from that. I've attached a modified image of one of your scope shots where I circle the extra "peak" comming out of your stage one and included an arrow on the input that shows no sign of this peak at all. Does your guitar put out a similar signal when connected directly to a scope probe?

So, something is not right here on stage one. If this were mine, I'd start at the front end here and get it working before worrying about what is going on later in the PI/power section. Stage 1 and 2 are pretty much clean amplified images of the input even on an express which has no voltage dividers at all knocking down the signal. So, I really expect that they should be on this circuit as well.

The funny bit is that the signal does look clean with sine waves. What happens with sine waves if you varry the frequency say between 200 and 5k hz? I'd crank all of the tone stack up full for this sorta test. Does anything surprising happen? I'd expect "normal" to be just the signal to vary a bit because of the tone stack. But if it starts cutting out or oscillating then maybe you can capture where.

Sorry if I didn't mention this - I've taken out one of those .1 uF caps. Right now there's only one in there, from the junction between the 82k and the 22k resistors. Like it's constantly in gradual mode. The scope traces are taken with only the one cap in there.

You did mention it and I forgot. Sorry. I was just hunting for possible reasons for why stage one seems to not just amplifying. There are not many parts involved here. What are the voltages on the pins of V1? I wonder if something like the cathode resistor or bypass cap is bad and throwing off the bias? It has got to be something simple. Do the voltages look reasonable on the plate grid and cathode?

Of course the tone stack is involved. So, you might try lifting the ground on the mid pot to bypass it. Or just bypass the whole thing with another coupling cap until stage one is sorted out.

[arjepsen]

ok, got around to it again today.
I've disconnected the whole tonestack from the first stage, so now it's just 33k gridstopper, 100k plate, 2k7+.68uF on cathode.
I soldered a 100nf cap to the plate, and in the following shot, yellow is the grid, and light blue is after the 100nf cap from the plate.
Again, this is plucking the a string capturing using the scope.

DS1Z_QuickPrint11.png

[tictac]

One thing I would do is separate the the power amp filter cap grounds from the preamp grounds... The PI ground is also an area of concern, the presence control should be grounded in the same location and with the preamp grounds preferably. Daisy chaining the grounds like that instead of using a ground buss can invite noise issues if you have grounds in the wrong place. Merlin explains this better than I....

TT

[M Fowler]

The V1 & V2 & PI are all grounded to the same place on the Asteroid Layout.

The B+3 has a 10k dropping string resistor into a diode onto B+2 and B+3 into a 1k/5w screen node into 1k/5w screen resistors. Is the diode in the circuit correctly?

I have never encountered such noise issues on either a clone or with my own layout design.

Hard to figure out what is going on.

Mark

[tictac]

FYI the asteroid layout that was shared a few years back on this forum has four grounding points:

1) the input jack + V1 (chassis near input jack)
2) tone stack + V2 (chassis near tone stack)
3) presence + speaker ground + preamp filter (chassis near presence control)
4) filter caps + PT sec. CT + filament CT + power tube grounds + bias test point grounds (rear chassis between power tube and rectifier socket)

TT

[romberg]

Your bonus bump is still there. And I think Mark has possibly identified the suspect part. That diode is an unusual part in that location for a guitar amplifier. And I think that even if installed as per the schematic, it is causing extra ripple on the power supply rail for stage one and two.

My reasoning is that a diode will not conduct until the voltage across it exceeds it's forward voltage threshold. So, when the grid of stage one goes positive the current in the valve decreases and the power supply cap for this node charges up. Eventually the power supply cap reaches a voltage high enough that this diode stops conducting because the voltage across it is less than it's forward voltage. Which is fine for now because not much current is flowing in the valve. But then the grid goes back to the bias voltage and the valve then starts to conduct and drain the cap. But the cap will not start charging until a voltage differential that exceeds the forward voltage exists. Then the diode switches on and BING a spike of current flows to charge the depleted cap. Thus the diode is injecting a signal onto the rail.

If you have probes that are safe to connect to your rail you can probably see an ugly triangle type wave/spike this diode is adding to your DC. Do, this only if you are sure your probes and scope can handle 350ish volts. Or use a coupling cap to measure just the AC on the line.

This may be part of the "mojo" or tone of this circuit. I don't know. But I would try bypassing/eliminating that diode and see if it makes that spike after the signal swings from negative to positive go away. I bet it will.

Mike

[arjepsen]

Yeah, actually I have wondered about why that diode was put in there.
Anyways, my probes can't handle that much voltage, but I can try using caps.
Maybe just try shorting it with a short wire, and see.
Although, I do wonder why noone else would seem to have these same problems then.

[Reeltarded]

We don't care! Delete that biotch and let's hear it!

Mark's questioning if it's installed backwards.. I do believe.

[arjepsen]

We don't care! Delete that biotch and let's hear it!

Mark's questioning if it's installed backwards.. I do believe.

Lol
I shall henceforth delete her, and see if it intensifies her beauty....
I do believe it's installed correctly - you can see it on the first gutshot I linked, back on page 1.

[arjepsen]

unfortunately, that didn't solve anything. The scope still shows the same. I reconnected the tonestack to check the sound, and the buzz is still there.
Here's the scoping - I put the two waves together for close comparison;

DS1Z_QuickPrint1.png

[Reeltarded]

uhhg.. anyone have photoshop.. maybe we could fix it with image editing..

Is there any possible way a single strand of shield is touching positive.. or a preamp tube is loopy.. or you are near a gamma emission source.. or someone has a horse tranquilizer I could mix with liquor?