Here's A Good Rebuild Story

flemingmras · Post by **flemingmras** » Sun Jun 12, 2005 12:28 am

Here's a funny story for you guys.

For those of you who remember when the first plexi I built (aka Spinal Tap #2) suffered the drink spill into it 2 weeks after it was built, this is the same amp I'm referring to.

Anyway, ever since I replaced the blown output tranny in it, it's been running great with no problems and this head has been my main head ever since, so the Sozos are well broken in. Since I did that major repair to this head, I've done absolutely no internal work to it other than rebiasing when I retube it. However, it was not one of my best builds as I did it in a hurry, and on top of this all I had shielded in the amp was from the input jacks to the inputs of V1. On top of this, my grounding job wasn't the best in the world, so the amp has always had a hum about it, which I never worried about...that is until I built my band's BIG PA system and I started having to mike up my amp, which amplified the hum and I was like "OK THAT'S gotta be fixed."

So I go and I completely rip the guts out of the amp. I unsoldered every component from the board and tightened up the component leads to look more professional, then also tightened up the chassis wiring. This time I shielded everything from the input jacks to V2's input, on the pot side of the board I connected the shields to ground on the pot end of the shielded wires while on the tube side I connected the shield to the plates of their respective tubes, a cool little trick I learned years ago. All the grounded shields were connected together at the same exact spot on the ground bus.

I then ran a piece of buss wire from where the cathode resistors/caps for V1 connect to V2's cathode resistor/cap. Then a single wire carried that connection from V1's cathode resistors/caps to the exact same spot on the ground buss where the shields were connected, along with the input jack grounds as well. I then ran a wire from there over to the ground lug on the preamp filter cap underneath the board.

Now this amp also had another intermittent problem that I never fixed. Every now and again when I'd hit the Standby switch, out of nowhere the amp would start loudly buzzing and crackling. Well this would go away once I cycled the standby switch a few times, so I assumed that this problem was the switch. So the standby switch was one of the things I ended up swapping in this teardown/rebuild. However, as I was reconnecting the tubes to the board, I think I found the EXACT problem and it was not the switch.

As it turns out, when I originally built the amp, I found that where V2's plate resistor solders to pin 1 was never soldered!!!The connection running from pin 1 to pin 7 was soldered on pin 7, the other end of the resistor was soldered to pin 6, but was never soldered to pin 1!!! Now how in the FUCK this amp has been working for over a year like that I have no idea. However, once I saw that, my intermittent buzz/crackle problem made perfect sense! I made sure to triple check the fact that that connection got soldered this time so hopefully that problem is gone for good.

I tested the amp out yesterday, and to my amazement...I set my Power Brake on the 7th click from full off(no sound), so I'm only using about 12dB of attenuation on the rhythm setting, and at this volume the amp's hum was pretty apparent. Now I can't even tell the damn thing is on anymore!!! This thing is now super quiet and I have a gig tomorrow so I'll be paying attention tomorrow as well as the next few gigs to see if that intermittent problem shows up again.

How I could've been so stupid that I left that connection unsoldered is beyond me, but that just goes to show that even the best of us make stupid mistakes like that.

Jon

Billy Batz · Post by **Billy Batz** » Sun Jun 12, 2005 6:46 am

Hey man. I had a problem with the sound dipping in and out of my first Marshall clone. Tried everything especially tubes. Was like that for months and drove me nuts. Here I never soldered the turret which held two leads of the diodes in the bridge. They were tight against the turret but still. Not soldered. How I didnt lose the PT is anyones guess. I was lucky.

Eoin · Post by **Eoin** » Sun Jun 12, 2005 8:44 am

What kind of shielded wire do you use, Jon?

I tell myself I'm very methodical, but I'm pretty sure I'll have all these mistakes and more to report as soon as I get building. Which is, eh... any day now.

flemingmras · Post by **flemingmras** » Sun Jun 12, 2005 12:18 pm

I use the stuff that George carries, which is just 24 gauge Belden microphone cable. What's cool about that stuff is that the shield wire is just a single stranded wire, but the twin conductors along with the shield are foil wrapped, which works WAY better than a braid I think.

There's people that will swear up and down that "Oh...don't use super small gauge shielded wire cause it sounds like crap" and shit like that. However, honestly this stuff will not degrade your tone. Mic cable has to be good enough for all microphones, meaning if it's good enough for vocal reproduction, it's definitely good enough for guitar.

On the board end of the cable the shield is not connected to anything, so the foil and the shield are cut back, then I use a piece of heat shrink tubing on the very end so that it doesn't short to anything. However, on the pot side the shield is grounded whereas the cables on the tube side the shield is connected to the plate of the tube stage that the cable is feeding, which is sort of an "active shield" trick I learned years ago that works like a charm.

Also, I shielded EVERYTHING from the amp's input to the input of V2. This includes the connection from both of V1's output coupling caps to the normal/bright volume controls, as well as from the volume controls to the mixer resistors, and finally from the mixer resistors to V2's grid at pin 2.

Hope this helps.

Jon

Eoin · Post by **Eoin** » Sun Jun 12, 2005 9:14 pm

Thanks for the tips!

Do you use the same shielded wire going to the plates or just in the lower voltage places?

flemingmras · Post by **flemingmras** » Mon Jun 13, 2005 3:25 pm

Yeah, what you do is on the end of the cable that goes to the board the shield is lifted(not connected to anything). Then on the end of the cable that connects to the grids of the preamp tubes, the shield on that end gets connected to the plate. On V1, there is a shield wire running from one set of 68K input resistors to pin 2, and the shield on that cable goes to pin 1. Then there is another running from the other set of 68K input resistors to pin 7, and the shield on that one goes to pin 6 of V1.

Then for V2, there is a shielded cable running from the 470K mixer resistors to V2 pin 2. The shield on that cable goes to pin 1 of V2.

Jon

dynaman · Post by **dynaman** » Mon Jun 13, 2005 5:02 pm

What's the advantage of tying the shields to the plates instead of ground?

myker · Post by **myker** » Tue Jun 14, 2005 1:16 am

arent dc voltages at ac ground potential? or something like that..
mike

flemingmras · Post by **flemingmras** » Tue Jun 14, 2005 1:23 am

myker wrote:arent dc voltages at ac ground potential? or something like that..
mike

EXACTLY!

Jon

Guest · Post by **Guest** » Fri Jul 08, 2005 1:42 am

Thanks for the tips. My '76 100w SL has the same standby switch crackling problem. I've replaced the switches, but it came back after a couple of months. My buddy who built it thinks it could be the indicator lamp. Has anyone ever heard of such a thing?

Pat