My 'Wreck Express, dubbed "Sugar", is up and running. I had to let it sit for a week while I was away with the family on vacation, but now I am back, and I am trying to diagnose a buzz in the distortion.
here is a rundown of what I have:
Stock build using JJ 6V6's with 3 slight mods - a split load after the second stage (to control the volume a bit, split between 47K + 56K); preamp grounds all run with individual wires to a single star ground point very close to the input jack; I used 3" wide garolyte for the power board, so I modified the layout -slightly- (angled the 220K bias resistor instead of coming straight across). All the rest is layed out as close as I could make it based on the Fran pics. I even took the time to twist & route the heaters as close to the pictures as possible.
I have tried to chopstick the leads, but most of them are very short to begin with, so not much wiggle room exists.
I had it on the scope today with a sine wave on the input. Signal looks good all the way through the first two stages. Once it hits the PI, something interesting happens - one side clips slightly different than the other - they both clip the top of the wave with some overshoot, but one side clips the bottom with an overshoot, while the other side produces more of a "hook" - looks like the bottom half of a backward uppercase "D". Not sure if this is normal, perhaps due to the NFB? I tried several PI tubes with no significant differences in this signature.
At the power tubes, the signal heading to the OT has the typical overshoot clipping signature, but I see a little bit of crossover distortion ledge in the middle of the wave once the gain is turned up. Unfortunately, I have the bias turned as far cold as possible, and it is just a bit hot for the 6V6 - 23mA @ 398V, 80% (measured with a homemade bias head with parts from Doug Hoffman's site) - so I'll replace the 220K bias resistor with a 180K, to give me a little more room to manuever.
Could the crossover issue be the source of the buzz? Anything else I can check?
Any help is appreciated.
Help with some Sugar Buzz
Moderators: pompeiisneaks, Colossal
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MooreCowbelle
- Posts: 81
- Joined: Mon Mar 20, 2006 5:03 am
- Location: Tacoma, WA
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MooreCowbelle
- Posts: 81
- Joined: Mon Mar 20, 2006 5:03 am
- Location: Tacoma, WA
Re: Help with some Sugar Buzz
Wow, no love here. Oh well. Luckily I have "others" helping me, so no worries. 
Re: Help with some Sugar Buzz
I've got love, just not answers 
I don't think the problem is bias but if you put a pair of EL34's in there, you could probably get the bias in range.
I think we have the same problem and I chased it for awhile and I've been on a break.
I don't think the problem is bias but if you put a pair of EL34's in there, you could probably get the bias in range.
I think we have the same problem and I chased it for awhile and I've been on a break.
Re: Help with some Sugar Buzz
MC...the funny part is the "others" are the only ones on this site!MooreCowbelle wrote:Wow, no love here. Oh well. Luckily I have "others" helping me, so no worries.
Allynmey