My official JTM45 kit build thread. Pics & CLIPS ADDED

mushmouth · Post by **mushmouth** » Thu Aug 30, 2007 6:53 pm

Can anyone explain the bias adjustment kit to me? What does it do? When do you install it?

The instructions don't talk about it at all. They just have a separate diagram of how it's hooked up.

EDIT TO ADD--A question about my latest earth-shattering dilemma:

Ok. This is a bummer. I meant to get to the electronics place in town today to pick up a missing 82k I need for the bias phase, and totally forgot to go. What would you other whackos do? Would you wait until tomorrow to go get the 82k, or would you strap a 27k and a 56k together and get to work?

Or do I use a 68k instead and hook up the bias adjustment kit? I've also noticed there are two parts in there, one says 25K415M on the side. The other says 25k651M on the side. I'm lost here... Anyone?

mushmouth · Post by **mushmouth** » Thu Aug 30, 2007 8:52 pm

Spoke with George. Bias adjust kit is now installed. Time to finish testing. Report coming in a bit...

Pic:

mushmouth · Post by **mushmouth** » Thu Aug 30, 2007 10:52 pm

Ok, I've started the 3rd phase of testing.

I've installed the bias resistor, and when I check pin 5 on the output tubes, I get -36V on each. It says you're supposed to get -45.

Is that a result of the bias kit being on there? Is that number acceptable?

EDIT: Raised the bias adjust and got both pin 5's to read -45V. HOWEVER, when I read pins 3, 4, and 6, I'm not getting the 440 I should be. I'm getting like 1 volt. *scratching head* Black probe is still grounded to the chassis... Wondering if that's wrong on this step...

EDIT 2: SOLVED. The standby had to be in the "ON" position to measure the voltage across pins 3, 4 and 6 of the output tubes. Then I'm getting a nice 436V reading on all of them. BEAUTY.

mushmouth · Post by **mushmouth** » Fri Aug 31, 2007 1:34 am

Score... Readings on pin 8 of both output tubes are 39.4 mV. Boooooo Yaaa!!!!

Now to take all the pin readings...

Pin 1
v1: 190.8
v2: 162.6
v3: 213.7

Pin 2
v4: 3.322AC
v5: 3.338AC
v6: 394.2

Pin 3
v1: 1.526
V2: 1.056
V3: 39.7
v4: 386
V5: 386

Pin 4
v1: 3.300AC
v2: 3.306AC
v3: 3.306
v4: 382
v5: 382
v6: 316AC

Pin 5
v1: 3.315AC
v2: 3.326AC
v3: 3.330AC
v4: -44.9
v5: -44.9

Pin 6
v1: 168.3
v2: 264.4
v3: 191.2
v4: 363.6
v5: 363
v6: 317.8AC

Pin 7
v2: 149.1
V4: 3.307AC
v5: 3.328AC

Pin 8:
v1: 1.387
v2: 148
v3: 36.32
v6: 356.1 in DC mode (in AC mode, I'm getting like 3.0, meter moving lots in AC mode)

Pin 9:
v1: 3.268AC
v2: 3.272AC
v3: 3.281AC

HOLY SHIT, IT'S ALIVE!!!!!!! ALIVE!!!!!! MUAAHHHHHH HA HA HA HA HAAAAAAAA!!!!!

Everyone's asleep so I had to hotplate it and have it practically off, so I have no idea what it REALLY sounds like until tomorrow, but it already sounded like a JTM45, so this is going to rule. I'll record some sound clips tomorrow before I start doing any break-in, then post some post-break-in clips in a couple of weeks for comparison.

It figures I got a zap AFTER tha process was done. I saw a small scratch on the faceplate while I was playing, reached over to feel it, and my middle finder went inside the chassis and touched the standby switch inside quickly. OUCH, JESUS!!!! That'll learn me...

George, take a look at those readings and see if anything is out of line, but it appears all are within an acceptable range. I can't believe I pulled this off.

SO, it IS true that all you need to know is basic soldering skills to put this amp together. I took my time, paid attention, and I'll be damned... it worked.

Thanks to everyone who gave advice throughout the process. It was appreciated more than you know.

toner · Post by **toner** » Fri Aug 31, 2007 1:46 am

EDIT: I just saw your last post. So B+ is now 386 VDC? Substitute that for 436 below... Congrats.

It looks like you're almost done! With a B+ of 436, your bias should be roughly between 35 and 40mV (read across the 1 ohm resistor on pins 1 & 8 ). Set the bias so the highest reading tube is where you want it.

formula:
max tube watts (25 for KT66) * desired % / B+

examples:
60%: 25 * .6 / 436 = 34.4mV
65%: 25 * .65 / 436 = 37.3mV
70%: 25 * .7 / 436 = 40.1mV

The B+ (pin 3) may change a little as you adjust the bias trimmer so you may want to double check it and adjust your formula. I usually prefer a bias between 65% and 70% (at least with EL34's).

mushmouth · Post by **mushmouth** » Fri Aug 31, 2007 2:35 am

toner wrote:EDIT: I just saw your last post. So B+ is now 386 VDC? Substitute that for 436 below... Congrats.

It looks like you're almost done! With a B+ of 436, your bias should be roughly between 35 and 40mV (read across the 1 ohm resistor on pins 1 & 8 ). Set the bias so the highest reading tube is where you want it.

formula:
max tube watts (25 for KT66) * desired % / B+

examples:
60%: 25 * .6 / 436 = 34.4mV
65%: 25 * .65 / 436 = 37.3mV
70%: 25 * .7 / 436 = 40.1mV

The B+ (pin 3) may change a little as you adjust the bias trimmer so you may want to double check it and adjust your formula. I usually prefer a bias between 65% and 70% (at least with EL34's).

Ok. I'm a little confused. I put one probe on an output socket pin 1 and the other probe on pin 8 to get the bias reading? I ground at the chassis and probe pin 3 to get the B+, I then adjust the bias adjustment to find the happy medium, so that bias (1 & 8 ) reads between 35-40, and the B+ reads as close to 436 as possible?

Wait, duh, 1&8 have a resistor across them. so you ground at chassis and touch pin 8 to get the bias, and pin 3 gives B+.

marT · Post by **marT** » Fri Aug 31, 2007 2:49 am

Yeah ground to chassis to get those values.

jimapr · Post by **jimapr** » Fri Aug 31, 2007 3:24 am

Okay, I will open a new Thread

Very clean and well ordered work

One little question:

Did you connect pin 1 and 8 with the 1 Ohm resistor and then to ground. Or did you connect pin 1 and 8 direcly and than above a 1 Ohm resistor to ground?

(Sorry for my bad english, I am learning

)

toner · Post by **toner** » Fri Aug 31, 2007 7:09 am

mushmouth wrote:Ok. I'm a little confused. I put one probe on an output socket pin 1 and the other probe on pin 8 to get the bias reading? I ground at the chassis and probe pin 3 to get the B+, I then adjust the bias adjustment to find the happy medium, so that bias (1 & 8 ) reads between 35-40, and the B+ reads as close to 436 as possible?

Wait, duh, 1&8 have a resistor across them. so you ground at chassis and touch pin 8 to get the bias, and pin 3 gives B+.

Just ignore the 436 VDC. I wrote that because you mentioned it was the B+ in an earlier post but it must have changed when you adjusted the bias trimmer.

Both readings are referenced to ground. Pin 1 is tied to ground. That's why I said to read across the resistor. Sorry about that.

New 386 B+ examples:
60%: 25 * .6 / 386 = 38.9mV
65%: 25 * .65 / 386 = 42.1mV
70%: 25 * .7 / 386 = 45.3mV

Your 39.4mV bias is a little on the cold side but still well within range. I'll look forward to some sound clips!

mushmouth · Post by **mushmouth** » Fri Aug 31, 2007 2:06 pm

UH OH... PROBLEM...

I hope I didn't screw anything up. I was working on the bias, and while taking a reading off pin 3 of v5, the probe tip slipped and arched on pin 2. There was a little spark when it tapped it. That blew out the 1/2A fuse in the flat holder. I shut it off, replaced the fuse, turned it back on and took readings on the V5 pins 3 and 8 to make sure I didn't screw anything up and they measured where they were before.

Then I went back to bias work, replacing the 68k resistor with a smaller 47k to try to get my b+ and bias numbers in line.

I installed the 47k resistor, turned it back on, waited, flipped the standby and **POP**, the 1/2A fuse blew again. Now I'm worried.

What might I have done when that probe on pin 3 arched pin 2 for a split second? Damn damn damn... I thought I'd be spending a 3 day weekend playing.

Running to get more fuses. Ideas?

toner · Post by **toner** » Fri Aug 31, 2007 3:49 pm

mushmouth wrote:...I installed the 47k resistor, turned it back on, waited, flipped the standby and **POP**, the 1/2A fuse blew again. Now I'm worried.

What might I have done when that probe on pin 3 arched pin 2 for a split second?...

I'm guessing that you either have a burn between pins 2 & 3 or the fuse blew because the bias was too hot after swapping resistors.

Here's what I would suggest doing:

With the power off, check for a burn mark between pin 2 & 3 on the V5 tube socket. Then check for continuity between them. There shouldn't be any.

If it looks okay, install the fuse and power up on standby. Before switching standby to "On", check pin 5 and adjust the bias pot so you get the largest negative voltage. Check pins 2 & 7 for proper AC voltage.

Then switch standby to "On" and see if the fuse holds. If so, you're probably okay. If it blows again, then hopefully George or someone else can help you out.

Wallace · Post by **Wallace** » Fri Aug 31, 2007 3:59 pm

I did that on V2 when testing mine - it only took the preamp tube out.
Hope you're ok.

Wallace.

mushmouth · Post by **mushmouth** » Fri Aug 31, 2007 8:36 pm

So far so good, doesn't look like I fried anything. George had me test some stuff. It only popped fuses (popped 2 more after that) with the hotplate hooked up. I eventually pulled the power tubes out and put them back in. So far it hasn't popped another fuse. George is concerned the rectifier tube might be dropping the b+ too far, so we may test that, HOWEVER

Just discovered a new problem... MY SECOND SET OF INPUTS (inputs 2) are not functioning. No sound. When I turn up that 2nd volume control, i DO get hiss from the vol 2 knob, but no input is getting to it. Any ideas of things I could look at, or things that would cause that?

mushmouth · Post by **mushmouth** » Fri Aug 31, 2007 10:25 pm

I lifted the board to make sure I wired the top green wire from that set to the 2nd hole, and the lower green wire to the 3rd hole. That is how it is wired. Rules that out...

Edit to add: I just found gourmet chocolate chunk cookie dough in the freezer. It doesn't solve my input 2 problem, but I'm going to bake them and eat 10 or 12 of them while I wait for a response.

mushmouth · Post by **mushmouth** » Sat Sep 01, 2007 12:20 am

Still trying to solve the non-working inputs 2 problem. I had a pm with bluze who suggested testing the following points for continuity:

input jack terminal (top green wire terminal) to the 68k resistors, then checking back side of 68k resistors to the v1 tube socket pin 2. I tried this and had continuity at all those points. SO does that mean it's probably a BAD INPUT JACK itself? Because of the way they're wired, if one didn't work, could it render BOTH COMPLETELY non-working?

Here's a pic of the continuity test paths that checked out fine:

*note: this pic I used for this illustration is from before installing the bias adjust kit...