Hi folks,
As noted in my previous thread, my sIII is working but I did have a few bugs to work out. With Stephen's help I've worked out all the bugs now. He suggested I put something together noting the issues I had & how we fixed them.
First, after about 12 hours of build time my amp fired up on the first try...no smoke or electrocutions. I was pretty pleased about that, needless to say. The amp sounded awsome clean on both channels, but I did have a few issues.
1. I was getting a lot of hum when I jumped the channels
2. The amp was noisier (hiss) than I had expected when overdriven
3. The amp was strangely bright when over-driven, almost harsh
4. There was a strange twangy/buzzy artifact in the tone when over-driving the power stage.
5. The amp was squealing pretty badly. It was worse on the TMB channel, but happened when the normal channel was overdriven also.
Here's the things I did to correct all these problems.
- Re-soldered the input jacks. There was obviously a bad ground in there that was causing #1.
- Checked & re-checked (and checked again) all the ground points. I did find one bad ground (from the ground bus to the pre-amp chassis ground). This improved the noise issue (#2).
- Re-soldered the entire board (twice in fact). There must have been a bad connection in there somewhere. This further helped reduce the noise (#2). It also made the amp less bright/harsh (#3).
- Grounded the output jacks to the chassis transformer ground. This virtually eliminated the squealing (#5).
- Reversed & twisted the OT leads. This lowered the noise even more (#2) and reduced the harshness a tad (#3)
- Re-routed the shielded wire from the volume pot to V2 pin 2. This improved the noise level on the TMB channel (#2).
- Swapped the pre-amp tubes from 12AX7C's to new-stock Tung-Sols. This made a big difference to the noise level (#2), and warmed up the amp quite a lot (#3).
So, by this time the only issue I had left was the wierd twangy buzz when over-driving the power stage. Stephen felt this was a PI issue, and I felt it was either overloading the PI or output section. Here's a few things I tried to correct this.
- Re-soldered the PI & output stages again. Minimal if any difference.
- Swapped the OT leads (did help a bit along with benefits noted above).
- Tried several different tubes in the pre-amp, PI & output stages. No real difference.
- Tried different speakers/loads. Although this didn't eliminate the problem, I did discover that the amp seems to sound best using the 4-ohm tap. The 8-ohm tap sounds the worst of the three to my ears. My only cab is a 8-ohm 2x12. Figures!
- Tried a different recto tube. Didn't fix the problem, but I think the new EH tube works a little better than the NOS EFT I was originally using.
The problem was still there after all of this stuff. I was almost ready to start swapping components looking for a bad cap or resistor, but my picky nature stepped in first. I grabbed a pair of jumper leads and wired one end to the input of my other amp. I then jumped each input & output point of the signal chain trying to pinpoint exactly where the artifact was happening. Much to my surprise (chagrin?) the amp sounded perfect the whole way through! BUH?!?
I pulled the jumpers off and listened to the amp again and the noise was back. I was now convinced the problem was with the PI or output, so I popped the leads back on the output from the PI to the power tubes. The amp sounded fine again. I disconnected the positive jumper and tried it again, expecting the noise to return. It didn't.
So I pulled the other jumper lead (ground) off also and BAM...the strange sound was back and the noise level had increased noticably. So I did some experimenting. By running a lead from the PI output to any ground point (grounded to the chassis or externally didn't matter) the problem went away and the noise level dropped to pratically zero. The output to the power stage also decreased. Interesting.
My theory is that either the PI or output stage was overloading for some reason. There may still be a bad component or a bad ground in there somewhere that is the root cause, but I haven't been able to find anything. For now, I've wired this work-around permanently to the pre-amp chassis ground and the amp sounds fantastic. There is basically NO noise with the normal volume or TMB master volume dimed. Even with the TMB master volume dimed & the TMB volume on 8 the amp is still practically silent (except for just a bit of power hum).
Crazy stuff. If anybody can explain to my why grounding the PI output fixed this, I'm all ears!
Darrin
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