6G15 Reverb Unit - orientation of PT & OT

[sookwinder]

OK.. my homebrew reverb unit project is off and running.... have some vintage NOS 6K6GT valves and the transformers arrived yesterday.

As I started to get my head around how I am intending to layout the circuit ... (it will be different to the normal Fender Reverb Unit in as much as it will be in essentially a cab similar to a "Vibro Champ Head") .... I noticed in many of the photos I have seen on the interwebz that the orientation of the PT and the OT are identical.

Yet it was my understanding when designing/building amplifiers that to reduce potential hum issues the PT must be aligned 90 degrees from the OT. Tbhis is written in basically every design book I have read and can be seen in all Fender (and other) amps.

But as you can see in this example below, it appears that the original Fender units and home brew projects have the OT and the PT orientated the same way .
Even the kits from Weber and Triode appear to have them oriented in the same direction

I do not understand ... can any one shed any light on this for me. Surely the two transformers should be 90 degrees apart ...

[PorkyPrimeCut]

I built mine using the Triode kit which, as you've stated, has them side by side.

I'd not heard about the 90 degree rule but, for the record, I've never had any issues with hum at all. It's deadly silent in that respect.

[øøøøøøø]

I think they've probably just placed the OT in the null of the PT's magnetic field and then rather far away. For this application, should be fine. 90 degree rotation might be a little better. I'd probably just roll with it, unless I was designing the chassis myself. In that case, I'd do some experimentation on my prototype. Get a jazzmaster (because they pick up hum really severely), mount the PT, and turn the PT on. Plug the JM in to another amp and use the pickup to find out where the hum coming off the PT is at its worst, and verify where the null is. Place the OT in the null and rotate it 90 degrees. The null will usually be where the bobbin is... i.e. on the side that isn't "all core." In other words, an EI core power transformer should usually be mounted like the one above.

Remember also that the hum picked up decreases with the inverse of the square of the distance... meaning that it gets way quieter really fast as you move further away. So it can get negligible really fast. Also, the choke will radiate a field, too, so orient the null of that with care as well.

[sookwinder]

Brad .. yep that is what I was thinking.... I
am building this reverb unit into a Champ chassis, so I have plenty of space to have the OT way from the PT and 90 degrees away.

[Mil]

Theoretically 90 degrees is the best but often in practice due to various factors the angle with the lowest noise is not exactly 90 degrees. The only way to find out is to use an oscilloscope and try different positions. Move them apart as far as practical.

[duncanjames]

Are you planning on using a full size reverb pan like used with a 6G15? If I recall the 4AB3C1 pans are roughly 16.5", where Champ and Vibrochamp chassis are about 15.5". The only way you could really fit one in a head cab is if you cut a hole on each side of the head cabinet that it could slide into.

[sookwinder]

no ... using a small tank....
have used them on other builds and am impressed with them
a 3 spring short (with long decay) is like a 2 spring long.

[46346]

MOD 3-spring?

[sookwinder]

when I was on business in korea last year I picked up a whole lot of long and short, 2 spring , 3 spring, tanks directly from Acutronics (Belton) in fact I ordered them as a special order and had them delivered to the Hilton where I was staying !!

(had issues going through the x-ray machines at Incheon airport as the idiots there wanted to know if they were part of a bomb ! (even though I had an eport licence thingy piece of papaer for the 16 tanks)

I do not know whether it is the reverb circuit I use in my amp builds or what, but I have had no issues with the acustronics tanks ... the reverb I have with them is on par with my 66 PR.

This 6G15 obviously is essentially the Fender design (rather than my own), although I have/will add to the design modern fix ups (actually old fix ups ) like grid stoppers and a far better layout .... so it will be interesting to see how the acutroniucs tank (a short one) works with the Fender reverb unit circuit.

I did connect a short acutronics tank to my Re-issue reverb unit and it sounded far far better than the stock long tank that is in there.... mind you have have no idea what the OEM tank is that Fender used in these reissues from 2005.

[Mil]

Are you going to use Fender's OT? Do all tanks you bought in Korea have the same in/out resistances (Fender spec again)? Just curious.

[sookwinder]

yes the two and three spring variants, long and short are all spec'd for fender circuits.
At the same time I brought back some other tanks for Jimboyogi (James) who uses a different reverb circuit than the fender style ie. no reverb tranny.

[Mil]

yes the two and three spring variants, long and short are all spec'd for fender circuits.
At the same time I brought back some other tanks for Jimboyogi (James) who uses a different reverb circuit than the fender style ie. no reverb tranny.

Nice. I know someone who mismatched the tank and OT
I'm planning to build my own design with a little self splitting push-pull 12AU7 driver.