YMI5150 Early '68 12xxx build journal

[YMI5150?]

I started my build thread in the general build section and am taking the advice of others to move here. Its taken far less time and effort than I thought it would. Thanks to those who have supported me so far!

I'm up to the point of wiring the main turret board. Reference here:

http://forum.metroamp.com/viewtopic.php?f=28&t=31826" onclick="window.open(this.href);return false;

for build progress to this point.

I need a quick course in biasing the power tubes and discharging the circuit. any help would be appreciated.

Cheers!

[Vegard]

Build looks good so far : ) In the end of the 100w instructions there is written lots about how to bias. Have you read it?

[YMI5150?]

Yeah, I read through it last night.

Made some good progress. Please review for errors. the main board is all wired and ready for components.

Things to do aside from the board are:

low voltage bias pot (can someone describe the wiring on that? cant tell which of the 4 posts are used from pics)
resistor on V2
Orange and green taps
final decision on NFB tap (currently on 16ohm but 4 is tempting by description)
right two board mounts (missing from kit)

see anything else?

After that I'll check continuity throughout the circuit and start voltage checks.
Getting close...

[vanhalen5150]

I was missing the board mounts as well. Mix ups with the other 100w kit I think.

As for the switch: single middle/bottom pot lug on the top switch lug right, top left pot lug on middle switch lug. The pot used in the Nightowl thread is a little dfferent than mine.

Pot lug you see not used is left bare. Just tuck under a bit. You'll see mine is exposed a bit.

http://forum.metroamp.com/viewtopic.php ... a&start=60" onclick="window.open(this.href);return false;

[Strat78]

Here is a good shot:

[vanhalen5150]

Yup, What Strat78 did is the same, just different ends of the pot. I would do it his way for cleaness. Learning curve gets better with those who re-think the past fella's!!!!!

Dont forget to add the rasberry jelly to the lugs.....

[YMI5150?]

not much progress last night/this am. wired orange and green leads, added resistor to V2, added cap to presence pot. question - does purple lead from presence pot go to negative feedback resistor (see pic)? want to verify before proceding. hard to tell from pics.

still need to add pot to bypass switch. a bit confusing but I'll go with it. seems its bridging (with resistance from pot) top and middle pins and I'm not sure why it would do that but that's okay...

will start adding jelly to contacts to verify continuity before and after populating the board.

[vanhalen5150]

Purple is yes.

[vh junkie]

question - does purple lead from presence pot go to negative feedback resistor (see pic)? want to verify before proceding.

Yes, but push the purple wire down on the chassis. Kind of so its just running under the edge of the board. Keep it away from any other wires. I it must cross another wire, do so at right angles. This guy is a prime inducer of oscillation. Don't want him anywhere near the wires connected to the other
front panel pots.

still need to add pot to bypass switch. a bit confusing but I'll go with it. seems its bridging (with resistance from pot) top and middle pins and I'm not sure why it would do that but that's okay...

The pot is shorted out(removed) when your in HI V mode. It's extra resistance is in play in LO V mode. Dial in the bias to around -35v(using the board pot) in HI V mode, then use the switch pot(ONLY!) to dial bias to around -43v in LO V mode.

[YMI5150?]

more progress. presence circuit complete, bias pot soldered onto 3 way switch (please verify for me). NFB tap moved to 4 ohm.

V1 and V2 components added to board. Please verify values. I see Strat78s board used some of the brown bodied resistors. Not sure if it matters. All components so far have good continuity and DC resistance measurements from controls to tube pins.

Thanks for your sets of eyes.

I'm really starting to focus on how I'm going to manage the volume. will likely buy the PPIMV (LarMar) tomorrow and add it next week. won't be diming it before then anyway. will also add some sort of attenuator box. no slaving any time soon. funds are low so I've got to come up with a reasonable solution for now. gonna be weird playing a completely dry signal for a while...

[vanhalen5150]

The brown resistors are just carbon jobs. So you used blue and yellow wire from the tranny correct? The top blue wires come from the 178 on the tranny?
Any noise problems latter you may want to check the heater wires between V2 and V3. They are running a bit loose there in parallel. May want to add a few turns there.

[YMI5150?]

Yes, wasn't totally clear what everyone meant by "tranny" but I'm assuming its the PT. blue 178 wires go to top two posts on 3 way switch (see below).

Any tone difference with carbon resistors? should I swap them out? Now's the time...

good eye on the heater leads. they've got 3 turns minimum per George but could be tighter. will keep an ear for noise there.

presence wire and NFB wire have plenty of air around them. tough to do with NFB near power tube sockets.

[vanhalen5150]

Some say there is a differnce, but I wouldnt worry about that for now.

[YMI5150?]

How about Sozo cap orientation? I see you were told to change orientation, George's build has orientation, but the instructions and another build I referenced did not. again, now's the time...

[vanhalen5150]

How about Sozo cap orientation? I see you were told to change orientation, George's build has orientation, but the instructions and another build I referenced did not. again, now's the time...

Your orientation is fine. There has been many a debate over the orientation. Some say one way towards the tube sockets, others say it does not matter. After a bit of research and talking to a few "big dog" amp builders I came to the conclusion it does not matter. Just remember how they originally went in and dont reverse them after the break in forming period.
Some of the builds George did not have a one way orientation.
I left all mine as they were. Amp sounds good.