DIY 2204 negative DC voltage on grid of preamp tube

[pdf64]

Where are you with the fizzy tone?
I’m not sure that either of those changes would help much with that.

[SILVERGUN]

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?

[shooter]

the attic fan above my living room is causing noise as well.

did you figure out how to stop that while toubleshooting

[SILVERGUN]

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

[jasonvilla]

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.

[shooter]

still curious what it "sounds" like when you make that 1/2 square wave a nice sine wave going into to the PA tubes.

[SILVERGUN]

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

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. 

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

[shooter]

You and shooter should watch some videos

 

Not sure either of us are up for a night on the couch 

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 

[pdf64]

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.

[jasonvilla]

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 
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.

[jasonvilla]

You and shooter should watch some videos

 

Not sure either of us are up for a night on the couch 

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 

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!

[jasonvilla]

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

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. 

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

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...

[jasonvilla]

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?

[shooter]

you answered it, my way of "asking" if it plays good clean 

[pdf64]

...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

[jasonvilla]

...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 :)

[jasonvilla]

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! :)

[jasonvilla]

.

[jasonvilla]

And a quick demo I made for IG. No making fun of me :P