Express build OT phasing question

[Matthews Guitars]

So I've got an OT for my Express build, which follows the Heyboer primary side color code. Blue center tap, green and purple to the power tubes.

Input signals from the PI are on blue and yellow wires to the power tubes.

To avoid having the amp start off as a howling screaming mess, which I know happens if the phasing is wrong, which is correct?

Green to the blue, or green to the yellow?

[Roe]

just try. instead of motorboating these tend to squeal a bit in the high frequencies instead

[sluckey]

I never trim the OT plate leads until I am absolutely sure the phase is correct.

[Matthews Guitars]

Oh, I'll try, but I figured someone here has to have built an Express using the build guide available here, and the same color codes and transformers, and thus has figured out which phasing works best. Other than that, I've just got to run some shielded cable to the input jack and then it'll be ready for first light.

My variations on the theme: To save chassis space I used two multisection capacitors (3 sections each) in place of the stack of 3 40s and 3 20s.

[pdf64]

Just use sig gen and dual channel scope to ascertain the relative phasing of the primary and secondary.
Once that is known it's easy peasy to get it right first time.

[Roe]

Oh, I'll try, but I figured someone here has to have built an Express using the build guide available here, and the same color codes and transformers, and thus has figured out which phasing works best. Other than that, I've just got to run some shielded cable to the input jack and then it'll be ready for first light.

My variations on the theme: To save chassis space I used two multisection capacitors (3 sections each) in place of the stack of 3 40s and 3 20s.

even if they had, heyboer is know for sometimes changing the colors. After a few incidents, I measure each transformer before I connect it

[martin manning]

Just use sig gen and dual channel scope to ascertain the relative phasing of the primary and secondary.
Once that is known it's easy peasy to get it right first time.

I also check new OT's for correct turns ratio on all secondary leads and primary lead phase. Pretty sure the OP is so equipped, but only low voltage AC, heater voltage e.g., and a multimeter are required to make these measurements.
https://ampgarage.com/forum/viewtopic.p ... 94#p463994

[Matthews Guitars]

Assembly complete. Going to wait for this evening after dinner to final review all my work and then slowly variac it up.

[Matthews Guitars]

That went surprisingly smoothly considering it's literally the first all new scratch built amp I ever made.

The closest to this I've done before was a literal ground up restoration/rebuild of a '69 Marshall 100 watt Plexi. I got it as a total basket case,
there was ONE original part remaining on the tag board. I literally built it like Marshall does, every bolt, nut, screw, washer, component, and solder connection was made by me.

Now for this Express....first I had no audio and found that bias was crazy high at 250 volts. Turns out I'd failed to ground the bias circuit. Adding the ground wire brought things to normal and suddenly I could hear sounds coming from the speaker, with the power tubes not in full cutoff.

Phasing, as luck would have it, was good. I only had to shorten the leads.

The amp is NOT a screaming howling mess but I have some pots to change. And put in some new Switchcraft speaker jacks.

I had intended to use Marshall style Cliff jacks all throughout the amp but since all the jacks are chassis grounded, I realized that sticking with Switchcraft jacks is the best policy.

The amp is quite bright at the moment, but since I know I have some less than wonderful pots in it, I'll replace those first before looking for other problems. Fix what you know needs fixing,
before chasing other problems which may go away while doing the other repairs. This principle has saved me a lot of headaches over the years.

This went pretty smoothly for a first build.

Next up: A replica of a Marshall 2061X 20 watt Lead & Bass. I have an original 2061 and it's my current favorite amp overall. What a beautiful expressive tone machine!

[statorvane]

I have an original 2061 and it's my current favorite amp overall. What a beautiful expressive tone machine!

+1. I built a clone. It is my favorite push-pull amplifier. A real rocker.

[Colossal]

I have an original 2061 and it's my current favorite amp overall. What a beautiful expressive tone machine!

+1. I built a clone. It is my favorite push-pull amplifier. A real rocker.

I apologize for the thread derailment, but what about the 2061 do you guys like? Have you compared them to a 1974?

Thanks

[Matthews Guitars]

Well, for starters, it's nothing short of a mini Plexi in its tonal range!

And what can you say about an amp that lets me keep entire CHORDS in sustained harmonic feedback, and not just single notes?
That behavior is absolutely remarkable!

I've never had an amp other than the 2061 (and 2061x) that told me so much about what I'm playing into it. It's not just a box that amplifies,
it's a partner that has intimate sonic conversations with me.

[Colossal]

Well, for starters, it's nothing short of a mini Plexi in its tonal range!

And what can you say about an amp that lets me keep entire CHORDS in sustained harmonic feedback, and not just single notes?
That behavior is absolutely remarkable!

I've never had an amp other than the 2061 (and 2061x) that told me so much about what I'm playing into it. It's not just a box that amplifies,
it's a partner that has intimate sonic conversations with me.

Well, that's pretty high praise! I've heard some decent demos of them, but haven't played (or built) one. Lots of experience with the 1974 which can be fantastic amps. Some interesting features in the amp.

[Matthews Guitars]

Back to the Express build, I realize there has to still be some tweaking to be done.

I use a Marshall Power Brake as my attenuator. It has a fan in it I've learned to use this as a means of detecting ultrasonic oscillation in an amplifier that can't be heard.
Set the Power Brake to a high degree of attenuation. Turn up the amp's knobs and at some point, if there's a problem, and oscillation occurs, the fan will start spinning
even if you hear no sound. This is a simple visual indication of an oscillation problem.

[jrdamien]

Back to the Express build, I realize there has to still be some tweaking to be done.

I use a Marshall Power Brake as my attenuator. It has a fan in it I've learned to use this as a means of detecting ultrasonic oscillation in an amplifier that can't be heard.
Set the Power Brake to a high degree of attenuation. Turn up the amp's knobs and at some point, if there's a problem, and oscillation occurs, the fan will start spinning
even if you hear no sound. This is a simple visual indication of an oscillation problem.

From here, it’s all about dialing in those settings, right? Maybe a little tube swapping, or maybe tweaking the gain or tone stack can help calm things down.