12XXX Build 05/2010

[vanhalen5150]

I didnt like the way I saw other circuit board bias pots done so I did mine this way. For the middle bottom tab I made a "U" shape with the bus wire to go underneath and link to the 2 lugs on each side. One top tab goes directly on the lead(rt.) and I made a small bridge to link the the other resistor on the left. I later trimmed that bridge, looks much better than in this pic.

[YMI5150?]

you guys biasing with a bias probe or just a multimeter?

[vanhalen5150]

Multimeter.

[Vegard]

I use probe since i didnt install the 1ohm resistors

[Strat78]

15K?

The learning curve is awesome, especially the soldering skills! Yes, it's 15k. I did not see anybody bundle the wires from the power board, so maybe that was not such a good idea. Thanks for the closeup shots of that biasing pot, that helped allot! With the SDM grounding method, why is the bass pot excluded from the buss wire?

[vanhalen5150]

15K?

The learning curve is awesome, especially the soldering skills! Yes, it's 15k. I did not see anybody bundle the wires from the power board, so maybe that was not such a good idea. Thanks for the closeup shots of that biasing pot, that helped allot! With the SDM grounding method, why is the bass pot excluded from the buss wire?

Sometimes you need some room in between wires. Im not sure myself, but if you have some noise to trouble shoot later your going to end up separating them anyway to try moving stuff around. I used a wooden kabob skewer. I think the bass pot is done that way because it will be grounded to the chasis anyways and this just gives it another path to ground rather than following the others. Spreading the grounds out over the chasis for the '68 I think is a key upgrade. I did the standard but am considering the Larry ground style at some point. Having to many grounds in the same area seems to be what causes ground loop noise problems.

I would consider before you power up for the first time to check every single soldering joint for electrical continuety. Some testers have this feature. It will save you a headache later.

[Strat78]

What is the setting for continuity on the meter, is it the horseshoe symbol? On at least one of the my solder joints the wire is suspended by the solder and not touching the ringlet (it's in a hard place to reach otherwise I would just re-solder it), will this conduct ok? I also un-bundled the wires coming from the power board. I'll power this thing up when my daughter goes down for her nap. When I do power up, should I have a speaker jack plugged into my Marshall power break? Fun!

[YMI5150?]

horseshoe is ohm meter and would work for testing continuity. it shoudl read OL for open loop before you test. then we you check continuity, it'll give you a value (or beep if there's an audio setting on your meter). plain wire woudl be on the order of a few tenths of an ohm. suspended connections should be fine, its all conductive. air gaps are bad if there are any. will increase current required to jump and generate heat. possibly blow fuses...
I believe you have to put a load on the amp when you have your tubes in and you're ready to energize the entire circuit.
good luck!

[vanhalen5150]

If you dont have a load on the amp you will wreck your output tranny. Set the brake to load, for testing. Dont have a guitar plugged in either.

[Strat78]

If you dont have a load on the amp you will wreck your output tranny. Set the brake to load, for testing. Dont have a guitar plugged in either.

The power break does not have a load setting but I guess turning the dial all the way to the minimum setting will do the same thing? I powered it up and took some initial readings on pin 2 and 7 of V4 to V7 and got 3.44VAC (same on the pre amp sockets) so I guess I'm in the ball park there? The bias readings were between -49.7 and -34. Now I need to draw up your chart and start taking all those readings before installing the power tubes. Your right, this is getting exciting, especially that initial subtle crystalline sound of the power flooding all those virgin caps and resistors. It's cool putting a face on all this stuff that has been discussed here for so many years (It all just flew over my head until now, sorta).

[vanhalen5150]

Plug a small jack cord into the input of the Brake and measure the resistance at the other end of the cord without a speaker in the input. The line out tap to the speaker should be irrelevant because its just a tap.

I'll be interested to see if the 12000 kits sound identical...

[vanhalen5150]

I would just put in the tubes and bias from there. What was your reading at V1 pin 1? Every thing seems to be ok at this point.
As with me, you may have a tube thats drawing a bit more current. Power down and change that tube with the least -valued tube. Which really is a more positive reading. If the values follow the tube just bias that tube to about -40mv.
Definately do up the chart....it maps out the whole amp geography voltage wise to really see what direction you need to go in.
You bias readings are not too high to put the tubes in really because they are not going to stay like that for long. Set the bias pots in the middle position. You'll have to bias with the tubes in again anyways and it will change for a week or so just due to break in time. With the way yours is wired, remember the stanby switch in off in the middle position. With the chasis upside down and open facing up...with the switch down/towards the bench is your 120v(use circuit board bias pot). With the switch in the up position you are now at the 95v....use switch bias pot.
What fun!

[Strat78]

With the way yours is wired, remember the stanby switch in off in the middle position.

Really!? I had it switched down towards the bench thinking that was standby. I'll start the chart readings with it in the middle position tonight.

[Strat78]

Ok, here is what I got, but I only got these readings with the switch pointing down towards the table. How do they look?

[Roe]

in general, they look good but a little high. what's your wall voltage?