12XXX Build 05/2010

[vanhalen5150]

Your wall voltage is probably running a little higher, but thats ok. The main thing is you have a good consistancy across the table. Also with the tranny's you will see differences from one amp to another.

Are these with all the tubes in? With El34's you'll need to bias to about -35mv with those readings taken from pin 3. Bias your power tubes at pin 1 and plug in!

I suggest using your power brake as well. This amp has no off other than the standby. It has room volume on 0. You will get some extra noise with it out of the head box. Let it sit for about an hour as well with guitar plugged in and volume on about 7 with the brake fairly low. Still should be able to hear a hiss from the speakers. Unplug, power down, drain caps and recheck the bias. You'll notice it will have changed a bit.

Just think how many of these things rolled of the assembly line having only been biased once and perhaps never biased for years or in fact ever again! Probably the reason every old Marshall sounded very different, even with identical parts. Variances in components yes. Keep in mind there was not sites like this with the knowledge freely available, and the ability to create your own sound. How many amp techs were around? Who ever rebiased after tube changes? Certainly not your average Joe.

You now have some fantastic knowledge of the best tone ever heard! Welcome to the club.

[Strat78]

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

I just checked the wall and it is 121volts. Should I do something about the high numbers or should I proceed with installing the power tubes and biasing? Wow, considering all this work is for the guitar, I'm more interested in this build than actually playing!

[vanhalen5150]

121v is ok

[vh junkie]

Geez! Take a few days off to finish "spring" projects, and all sorts o' 12xxx are being birthed!

Should I do something about the high numbers or should I proceed with installing the power tubes and biasing?

The tubes will "drop" some of the voltage. I think it says this in the 100w instructions. Just be sure you have a load(speaker) connected before you fire it up. The power tubes complete the output circuit connection loop. So you really don't need the load until then, but best to just leave it connected.

Make sure both volumes are down before you fire it up, and until the bias voltages get sorted out. You can find where that was done with vanhalen5159 earlier in this thread.

[Strat78]

First off, I am still not sure which is standby, lo and high voltage on the 3-way switch. I've been getting the readings with the switch towards the table (chassis upside down) and nothing in the other positions. I started biasing using the 100w instructions. I found that V6 had the hottest tube so I biased that socket to .035mv. As instructed I went back and checked B+ on pin 3 of V6 and it had dropped to 331 from the original 488. The instructions said to apply the formula again (17.5 divided by 331 = .052!!!!!), and readjust the bias accordingly. Night Owl's instructions completely lost me at step 17 part "i" and the 100w does not take into account the 3-way switch. Am I really supposed to bias V6 to .052? I can't get the pot on the 3-way switch to effect anything at this point.

[vh junkie]

Let's do a reality check here.

Two of the three center lugs on the standby switch are hooked by blue wires(under the board) to the two nearest diode pins right?

If so, set your meter to VAC. Stick one probe into the chassis (ground). Push the standby switch down (towards table) HI V should measure 170ish on each of those pins. Turn standby off (center position). Push the standby switch up (away from table) LO V should measure 140ish.

Yes?

[vanhalen5150]

As your oriented with the switch down towards the table you are at regular voltage. Middle is standby. Up in the low voltage setting from what I understand. Im wired the same as yours. Its quite noticable when your playing.

[vanhalen5150]

Let's do a reality check here.

Two of the three center lugs on the standby switch are hooked by blue wires(under the board) to the two nearest diode pins right?

If so, set your meter to VAC. Stick one probe into the chassis (ground). Push the standby switch down (towards table) HI V should measure 170ish on each of those pins. Turn standby off (center position). Push the standby switch up (away from table) LO V should measure 140ish.

Yes?

Yes thats correct.

[vanhalen5150]

Strat78....So you dont get any readings with the switch in the "up" or low Voltage position?

[vh junkie]

Let's do a reality check here.

Two of the three center lugs on the standby switch are hooked by blue wires(under the board) to the two nearest diode pins right?

If so, set your meter to VAC. Stick one probe into the chassis (ground). Push the standby switch down (towards table) HI V should measure 170ish on each of those pins. Turn standby off (center position). Push the standby switch up (away from table) LO V should measure 140ish.

Yes?

Yes thats correct.

So you have verified that this is true for your 12xxx? Let's see what strat78 gets... these are the same voltages you should see on the transformer lugs (outside lugs HIv, inside lugs LOv). It bugs me that he doesn't get any voltage with the switch up. Let's hope he doesn't have a bad PTx.

[Strat78]

Ok, I got 177 with the switch down to the table and 152 with it in the upper, standby in the middle Guess I was doing all my readings in high voltage.

[vanhalen5150]

Ok, I got 177 with the switch down to the table and 152 with it in the upper, standby in the middle Guess I was doing all my readings in high voltage.

Man, I was doing the same shit with mine.....intially that 3 switch and the 2 bias pots can be confusing at first.

[vanhalen5150]

Let's do a reality check here.

Two of the three center lugs on the standby switch are hooked by blue wires(under the board) to the two nearest diode pins right?

If so, set your meter to VAC. Stick one probe into the chassis (ground). Push the standby switch down (towards table) HI V should measure 170ish on each of those pins. Turn standby off (center position). Push the standby switch up (away from table) LO V should measure 140ish.

Yes?

Yes thats correct.

So you have verified that this is true for your 12xxx? Let's see what strat78 gets... these are the same voltages you should see on the transformer lugs (outside lugs HIv, inside lugs LOv). It bugs me that he doesn't get any voltage with the switch up. Let's hope he doesn't have a bad PTx.

Yes I did that awhile back but it still will depend on the wall voltage.

[vh junkie]

Ok, I got 177 with the switch down to the table and 152 with it in the upper, standby in the middle Guess I was doing all my readings in high voltage.

OK, that sounds good. Voltages being read before the diodes are AC... everything that we measure from here on out will be DC. Let's put the switch into HI v mode (down), and take another look at pin3 of the power tubes should be close to 500vdc in HI v mode (and closer to 400v in LO v mode). Give the amp a minute to charge those caps before taking the readings.

If the pin 3 voltages are not at 500v, check the voltages on the positive end of each of the large capacitors on the power board and report back.

[Strat78]

Ok, I got 177 with the switch down to the table and 152 with it in the upper, standby in the middle Guess I was doing all my readings in high voltage.

OK, that sounds good. Voltages being read before the diodes are AC... everything that we measure from here on out will be DC. Let's put the switch into HI v mode (down), and take another look at pin3 of the power tubes should be close to 500vdc in HI v mode (and closer to 400v in LO v mode). Give the amp a minute to charge those caps before taking the readings.

If the pin 3 voltages are not at 500v, check the voltages on the positive end of each of the large capacitors on the power board and report back.

Thanks, I'm off to work so I'll get these readings tonight.