Iron orientation guidance

[ampgeek]

I am building an all octal 5F6A with a one tube reverb circuit for a friend and request guidance on tranny orientation.

I have poked around a bit for super reverb, twin reverb, etc...layouts and pictures but none match perfectly with my selected iron. Largly due to my use of a stand-up PT.

I threw them down in the attached picture using the simple "orient at 90 degree" philosophy (reverb transformer at 45 degrees) but am not convinced it is right. I recognize that the rev tranny should go much closer to the chassis edge (e.g., nearer to the tank in-out RCA jacks).

Input and suggestions would be greatly appreciated!

TIA
Dave O.

[labb]

From our friend over at AX84 (FAQ section)....

What is the headphone trick?
This is the best way to find the quietest place to put a reverb, output, driver, or any other signal transformer to get the least hum. CAUTION: THIS INVOLVES WORKING ON THE AMP WITH WALL VOLTAGE PRESENT. WALL VOLTAGE CAN BE FATAL.
First set up the power transformer. If it's not installed yet, all the better, but if it is, this will still help. If the PT is not installed, wire up the primaries to a power cord, but do not wire up any secondaries-- tape the ends of the secondaries. If the PT is installed, fine, just make sure there is no circuit on any of the secondary leads (pull all tubes, and disconnect leads and tape them as necessary).

Now set up the signal transformer (output, reverb, whatever). Hook one set of OT secondaries (your choice, but I usually go from ground to the highest impedance tap, don't worry about impedance matching here!) to a set of headphones. Tape the primary leads.

Put the headphones on, plug in the power cord, and start moving the signal transformer around. Try it all over the chassis, and also rotate it. While you might expect the least hum with the transformers as far from each other as possible and at right angles (and that may be the case for you), it might come at an odd angle and/or position. Sometimes the least hum occurs when the transformers are side by side. When you find the quietest spot, use a permanent marker to mark where the mounting holes should be.

If you have more than one signal transformer, repeat this for each one. When you're finished, unplug the PT. That's it!

[rbryerton]

Hey Dave, perhaps take a look at how Marshall did it on the Plexi and later heads. I've built a 1987 and a 2204, and the transformers are all in a line: power, output, then choke; all 90 degrees rotated from the last.

Although the 1987 was a laydown power trans, the 2204 I built used all stand up models, and I just oriented them 90 degrees from the last, about 1 inch between them. Sounds good to me...

I'm sure someone will also suggest the headphone trick; I just follow the classic designs for these "problems". Someone already figured it out

Edit: labb beats me to the post *and* suggests the headphone trick...lol

[xtian]

I read a cool tip about this. Cap all the output leads from the PT (so nothing shorts or zaps anything) and connect the 120vac mains to wall power. Connect headphones to the output of the OT. Now listen to the headphones while you orient the PT and OT. You will hear the hum induced in the OT by the PT when you get them too close or oriented in sympathy! Wish I had done this with my SEL.

I'm typing from memory, which may be faulty or incomplete, so don't try this until you find the original instructions.

[EDIT: Wow! Two responses while I was typing!]

[Jana]

Spin the PT 90 degrees and you're good to go.

[M Fowler]

This is how I did my 60w Rocket Reverb, 40W Rocket Reverb, and 80w Rocket Reverb tranny orientation.

[ampgeek]

Thanks everyone! Really appreciate the input and guidance.

The headphone gig sounds like something new and fun to play round with to expand my knowledge base.

With regard to the signal iron (e.g., OT and RT), should the secondary "ground" actually go to an earth ground? Presumably, the same ground as the power supply. And then one of the headphone leads to the same ground?

Thanks as always!

Dave O.

[Structo]

No!

You wire the headphones to the ground and 16 ohm tap only on the OT.

The headphones pickup the inducted hum from the PT.
Be sure to cap off or tape all wire ends not being used on the PT.

[ampgeek]

Got it! Thanks Structo. Much appreciated.

How cool a technique is that?!?! Works like a champ. Much more sensitive than I would have ever believed. Thanks for turning me on to that technique mates.

Demonstrates quite clearly that PT/OT laminations on center and at 90 degrees is hum free. Well...at least below my hearing threshold.

Interesting...with the choke in-between, you cant get them close enough to each other to induce hum. But....move it out of the way and hum is just barely discernable.

How would current flowing through the choke effect the coupling (if at all)?

Thanks as always,
Dave O.