....and tell me if you have any ideas what the $%#@ is wrong ! Seriously, i just finished the copy of my Marshall style 50 watter and theres a problem i can't figure out. I did make one mistake i made due to a mistake on my schematic that i blindly followed even tho had i thought about it instead of just blindly following the schematic i'd have realized. ARGGG! In any case i put the resistors from the AC supply after the diodes in my dual bias supply and 2 of the 4 caps went south. I don't know if this could have screwed anything else up, but the caps caused hum and replacing 2 of them fixed that. Then this problem, so thinking maybe the other 2 caps were bad i replaced all 4 and still this issue exists. i've gone over and over it, especially looking for bad grounds but all seems good. Below is a link to a mp3. I played a chord with the volume down on the guitar for a cleaner tone, then full gain, then at the end i roll the volume up and down so you can see how when the signal is louder it's bad, then as i roll down it clears up. It's a buzzy staticy sound you've all heard before, as have I. But i can't put my finger on the most likely causes, so i'm hoping some of you might be able to tell what the likeliest cause is. thanks.
It feels tube related only because it's ok when the guitar level is downwhile the amp stays the same. So it's inthe amplification of the signal. Check the other caps around V1. Andtry swapping out for a known good tube.
The tubes are fine, as i swapped them all aout to make sure that wasn't it. I've done what you listed except checking the caps. The only caps that were bad were where the mistake was made....the electro's in the bias supply. But those are all swapped now and diodes all read perfectly. I did check for DC on the .022's and .1's, but i'm really not sure how much is acceptable and where. At the PI the .1 on the unused side shows around 25v on both sides, and i'm not sure thats right considering one side is a grid. But not being a tech i'm not sure if thats ok there....don't understand theory well enough to figure that out. .
No, same room i always play the old one and never a issue like this. But check this out, it may be a clue. The amp is similar to a JCM 800 single channel except that the second stage is not cold bias. V1 has a 2.2k/.68uf on A and a 1.5k/1uf on B. If i lift V1's cap nothing changes as far as the staticy noise. If i lift the .68uf on B it clears up probably 80% ! However, that doesn't mean the cap is the issue because it still does it,just a lot less. What it tells me is the extra gain the cap supplies is causing it to get as bad as it does. Like you said....it's the gain. But why not if i cut gain by lifting V1A's cap? So i swapped out the coupling caps going to the 2nd and 3rd stages and that wasn't it. Checked the resistors, looked all the connections with a magnifier. couldn't find any thing. This is a tricky one ! Not fun when i'm dying to hear my new amp.
Yes, but as the volume fades it lessens and stop as it fade past a certain point, albeit a VERY low point. Anyways, bias is at about 70% which for 475v is about 36ma and it's at 38. Actually, i don't think 36 takes screens into account, but in any case it's no where near far enough off for this. I could drop my other one way lower than this w/o this kind of thing.
Doh ! got it....i had the board unscrewed and lifted at a 45 degree angle so i could have access to the underside while i troubleshoot it. Of course to do that i had to have the pots out of the chassis and hanging in the air. Well, i had not idea that if the pots aren't grounded this could be the result. But i realized when i touched a pot shaft the noise subsided and sure enough that was it! Can someone explain this to me? i mean, you have all these wires going to the pots and they aren't shielded....why do the pots have to be grounded? I realize the pot's casings act as a shield for the area of signal that passes thru the trace, but why is that any more sensitive than all the wires outside the pots? I don't get that at all ! But what the heck....the amp sounds great now !
No, i have a solid core bare ground wire that runs the length of the tagboard on the pot side of the board, but not on the pots. The pot lugs that are grounded go right to that as do all the parts on the board. There is a main bolt near the PT that the cans are grounded to, then the tagboard wire also goes to that at the PSU end of the board. It's a real quiet setup. I've used it in 4 amps and they have all been very quiet. But it's just so bizzare how this went. The amp is dead quiet now and none of that staticy garbage.
On a side note, i tried the magnetic components marshall OT in my old amp that this one is a clone of, (not as layout goes, just circuit) and didn't care much for it. Well, i'm hearing what i heard from it in this new amp too, so i think i may be sticking my heyboer in it from the old amp. Or possibly a marstran if i feel i want to spend that much more. Opinions on those would be nice by the way, hint hint.
Seriously, i just finished the copy of my Marshall style 50 watter and theres a problem i can't figure out. I did make one mistake i made due to a mistake on my schematic that i blindly followed even tho had i thought about it instead of just blindly following the schematic i'd have realized. ARGGG! In any case i put the resistors from the AC supply after the diodes in my dual bias supply and 2 of the 4 caps went south. I don't know if this could have screwed anything else up, but the caps caused hum and replacing 2 of them fixed that. Then this problem, so thinking maybe the other 2 caps were bad i replaced all 4 and still this issue exists. i've gone over and over it, especially looking for bad grounds but all seems good. Below is a link to a mp3. I played a chord with the volume down on the guitar for a cleaner tone, then full gain, then at the end i roll the volume up and down so you can see how when the signal is louder it's bad, then as i roll down it clears up. It's a buzzy staticy sound you've all heard before, as have I. But i can't put my finger on the most likely causes, so i'm hoping some of you might be able to tell what the likeliest cause is. thanks. 
