Bias Truble

butinox · Post by **butinox** » Sat Feb 11, 2006 7:22 am

Finished the building stage of my JTM-45 yesterday. And Started testing.
Got some descent voltages compared to the chart, a little below the chart on most of the readings because i used the 240v tap, and i've measured 230v in my socket, so thats not unusual. Without the power tubes i had about 420v on pins 3,4 and 6. And around -42v on pin 5.
So I poped the tubes in, and hooked up my multimeter.
I had my Hot plate set on load, and i also used the line out and ran it into my computer, so that I could hear any wierd noises if something was wrong..
When i hit the standby switch, i got a pretty wierd oscillating noise, and about 140mV on pin 8

How could that happend.. little confused about the matching of the tubes also, the boxes says, 3.6 / 3.3 but the notes inside the box says on one tube 80ma
and the other 50ma..

Flames1950 · Post by **Flames1950** » Sat Feb 11, 2006 7:40 am

What output tranny aare you using? If you've got the Heyboer, make sure the brown wire goes to the power tube pin 3 closest to the preamp tubes, and the blue wire goes to the power tube pin 3 closest to the rectifier tube and filter cap can. If you've got the MM, you may want to try switching the wires to pin 3 as well, what you're seeing is typical of having the leads swapped around.

A real quick way to tell (before you go to the trouble of swapping the wires to pin 3 on each power tube socket) is to simply lift one end of your 27K feedback resistor (the one with the purple wire from the impedance selector going to it) and if your troubles disappear you need to swap the wires from the output tranny to the pin 3's. Then you will be able to bias properly after the wire swap and hooking the feedback resistor back up.

Dunno about those tube markings....50mA is too hot from the sound of your plate voltage readings, and 80mA would be a meltdown.......so they don't sound relevant......

butinox · Post by **butinox** » Sat Feb 11, 2006 10:27 am

I've got the MM Transformer.

I took out the feedback resistor and powerd up. And then i got a more normal 35 mA and a high frecvency noise at the monent i flipped the standby switch.

I tried once more putting it back in, and same again I got this extremely loud 400hz noise.
I looked at the power tubes and, I saw a sort of green-ish light

So Ill just try to switch the primary of the Output Transformer,
allthough I've wired it after the transformer code.
Red wire to pin 3 on the right socket, and black to pin 3 on the left.

Flames1950 · Post by **Flames1950** » Sat Feb 11, 2006 12:30 pm

You should be good to go after swapping those leads!

butinox · Post by **butinox** » Sat Feb 11, 2006 1:01 pm

Well.. the loud noise is gone.. but now it's dead quiet hehe.
biased to 39mA.
Heard a scratching noise, but otherwise nothing.
I'll duble check the preamp, and everything..
I know the tubes work because i've tried them out in my other home built amp. (50w marshall type).

butinox · Post by **butinox** » Sat Feb 11, 2006 1:11 pm

Haha.. forgot to connect earth to the input jacks

no wonder..

butinox · Post by **butinox** » Sat Feb 11, 2006 2:14 pm

It's up and running.
And I have to say.. .HOLY S**T Thats good tone

Best tone I've ever heard

:D

Flames1950 · Post by **Flames1950** » Sat Feb 11, 2006 3:00 pm

Oh yeah baby.........

BashCoder · Post by **BashCoder** » Sat Feb 11, 2006 7:12 pm

Congratulations!

Mama says that whenever a new JTM-45 fires up, an angel gets it's wings...

butinox · Post by **butinox** » Sun Feb 12, 2006 7:34 am

Here's some pic's

butinox · Post by **butinox** » Sun Feb 12, 2006 8:07 am

Kinda worried about one tube there. When i first hit the strings on the guitar i can see one tube light up from inside the tube. You can see it on the first picture here.
The other tube doesn't do that..

When i hold a chord the volume seems to get lower, and I also can see the bulbs on my Hot Plate is bearly glowing, compared to my 50w Dan Amp that I built.

Also when i turn it on, it makes a noise and the indicator glows a little less, like it draws a heck of a current when I flip the standby..

shakti · Post by **shakti** » Sun Feb 12, 2006 11:13 am

Sorry, cant' help you with the tech trouble, I am a newbie with this... but I can say: congrats on a nice amp! I thought I'd be first in Norway, but apparently not...was that a complete Metroamp kit? Which Sozos are you using, the standard or the premium?

butinox · Post by **butinox** » Sun Feb 12, 2006 11:30 am

Yep thats the metro kit.
Exept the head and 2x12 below that I built myself, you know.. even designed the circuit board my self hehe.
But I have to say the JTM kit sounds a lot better than what I built.
well not so strange anyway, I just used parts from other stuff I had around.
And it looks like Hiroshima inside

but I got it working after a lot of trubleshooting.

The sozo's are stuff from George.. don't know if it's premium or not.. but it sounds nice..
I've also got some allan bradly resistors in the pre-amp, a BC 330µF just for the looks, Mercury Magnetics O.T., Tung sol preamp tubes and 6v indicator, since the kit amber lamp is 120v.. would have lit up very bright for a little while though.. hehe

butinox · Post by **butinox** » Sun Feb 12, 2006 11:34 am

Sound exactly like Deep Purple - Woman from Tokyo

Also some Dokken.. like "Burning like a flame"

shakti · Post by **shakti** » Sun Feb 12, 2006 11:36 am

Ouch, does that mean I have to specify a different indicator lamp? I just ordered a kit from George...really love the look of that amber lamp, though!