12XXX Build 05/2010

[vanhalen5150]

Call in sick........

[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.

I was measuring pin 2 instead of pin 3. So, now in Hv, I'm getting a reading of 462 and 390 in Lv on pin three of the power tubes. I should bias the hottest tube (which is V3) to 37mv?

I'm still confused about the Night Owl instructions that I've been following. They don't seem to work with the pot on the 3-way switch when I try to take the measurements. Where dose he get 41 and 28 from? Here is the page:

[vh junkie]

Sounds like he has the switch thing backwards.

For 41 and 28, his voltages are different then yours. Watch your voltages in the first few hours of break in, things may change. For your 462 and 390 do the following:

Start with switch pot in the middle of its range.

1) Let the amp warm up for a couple of minutesin HI v, both volumes on 0.
2) Use board pot, bias hottest tube to -37mvdc(-.037vdc) at pin 1.
3) Switch to LOv, wait a minute.
4) Adjust switch pot to bias hottest tube to -44mvdc(-.044vdc) at pin 1.

Check the voltages often, reapply the formula as necessary.

Check the bias frequently and readjust always using those 4 steps in order. Don't assume your hottest tube will always be the hottest. Look out for redplating!

[Strat78]

Ahhhhhhh

[vanhalen5150]

This running yet?

[vh junkie]

Ahhhhhhh

Certainly sounds good! To late to crank it last night... look out this morning!

Now just gotta:
1) check the plate voltage periodically.
2) Watch the bias on all four tubes. They are bound to morph, with a new one become the hottest.
3) Once you start to beat on it, you may blow fuses. I think George provides fast blow fuses. I (and others) had to switch to slo blows.
4) Keep the amp situated so you can see the tubes. If your gonna get redplating, it will probably happen in the first 10 minutes. Listen for the Mmmwwaaahhh sound!

[Strat78]

Have not been able to play it above a whisper because of the baby, but it sounds really crisp and musical at low volumes. It makes very little noise at this point even when the volume is way up. I'll watch for the red plating when I really get a chance to crank it on Sunday. Thanks everybody for the help and patience! Can't wait to make some clips. Oh, I think my old 50W marshall would be fun to open up now for a long over due biasing. Can I add those little resisters between pin 1 and 8 and ground of the power sockets just like the 100w?

[vh junkie]

Glad you got it going

How old is your 50w? Point to point or PCB? Might not want to add the resistors if it an older collectors piece. There are other ways to do the biasing without adding the resistors.

[Strat78]

It's a late 73 so it has the pcb board. Yeah, I really don't want to add those resistors either. Who sells a good biasing kit that plugs into the tube socket, or are there other ways to check the bias from the inside?

[vanhalen5150]

It's a late 73 so it has the pcb board. Yeah, I really don't want to add those resistors either. Who sells a good biasing kit that plugs into the tube socket, or are there other ways to check the bias from the inside?

I have this one. Its $50 but so is an amp tech per hour.
http://amprepairparts.com/biasplug.htm" onclick="window.open(this.href);return false;

Webber has them as well with the meter attached and reads all the tubes at once. If you get the one for 4 tubes they say you can still use it for the 50 watter as well.
https://taweber.powweb.com/biasrite/br_page.htm" onclick="window.open(this.href);return false;

Do a short clip.

[vh junkie]

Those links, something called bias-rite maybe? I have not used any of them.

There are ways to do it that I can't remember right now. One way is embedded in this link:
http://www.aikenamps.com/Biasing.html

The third bullet in the "Plate/cathode current method" section is the beginning of the process, but I can't remember how figure the idle plate current helps us.

EDIT: I found one of my old posts that describes this:
http://forum.metroamp.com/viewtopic.php?f=28&t=23950
The first post is the correct formula for a 50w, in the third post I figured out where I went wrong and corrected it for a 100w amp

[Strat78]

The plate voltage drifted from 464 to 454 after 15 minutes of playing, so I adjusted the bias to -38. I want to put the chassis in the box already but I guess I should continue to monitor it for a couple of weeks until it settles down. It sounds pretty sick at this point, here is a short clip (volume on 8, attenuated about 50%):
http://soundclick.com/share?songid=9337212" onclick="window.open(this.href);return false;

[vh junkie]

It sounds pretty sick at this point, here is a short clip (volume on 8, attenuated about 50%):
http://soundclick.com/share?songid=9337212" onclick="window.open(this.href);return false;

Nice!

Is this HIv or LOv mode?

Whaddayagot that you can slave it into?

[Strat78]

That was HIv. Is it too early to start hitting the front end with a little boost like a ge-10 or 6-band? Those plates in the tubes were just starting to get a little glow at one end, is this red plating? If I lower the voltage to 90 or 110w should I re-bias the tubes too? Hey, I'll slave it into my Valve Junior!

[vanhalen5150]

Sounds just like mine! There is more gain to be had past 8! My tubes glow more at one end as well. Thats the one drawing a little more current I think. It will drift a bit each time you play. After about 5 sessions mine stopped. It will noticable start to sound better as well. Not sure if this is break in or what. GE10 sounds awesome in front. Try it for a short time at least. Try jumpering both channels for some extra meat without the boost as well.
Much quieter in the box. I made a top from some thin car body aluminum over a panel to sit on top when its out of the chasis. Worked well.
Slave into the Junior!