Transformer Swap

[bradicusmaximus]

I'm trying to figure out the best way to go about this and figured I would ask you guys...

I am swapping out the stock power transformer in my Fender TRRI chassis. The new transformer is a different size and doesn't line up with the stock holes. The new one is smaller.

What do you find is the best method to mount the new transformer?

My thought is to get a metal plate, bolt it on to the chassis, and drill mounting holes for the new PT. This isn't something I've had to do before, so I figured I'd see if there are any words of wisdom or easier ways to accomplish this.

Thanks!

[pops]

Sounds like a plan, but i would use at least one of the original mounting holes for the transformer.

[Stevem]

x2

[Phil_S]

Here's another way, though there's nothing wrong with using a patch plate. You can get L shaped .75" x .75" aluminum bar stock at the home improvement store. Cut a couple of pieces to size and drill holes for wires if needed. Bolt your PT to the bar stock. Bolt the bar stock to the chassis.

Aluminum is easy to cut and contour, yet stiff enough to give a very stable mount.

[sluckey]

I am swapping out the stock power transformer in my Fender TRRI chassis. The new transformer is a different size and doesn't line up with the stock holes. The new one is smaller.

This does not address your issue with mounting the new transformer, but may be worth mentioning...

The TRRI uses a different power transformer than any of the other Twin Reverb amps. The HT winding (two wires only) has no center tap and uses a bridge rectifier. There is a completely separate bias winding (two wires).

If you try to use a replacement transformer that uses a CT on the HT winding and a conventional full wave rectifier, you will get a big surprise. Your B+ will likely be more than 900V if the filter caps last long enough to measure it.

AFAIK, Mercury Magnetics is the only company that makes a drop in replacement that actually fits and has the correct windings. Most all other TR replacements are meant to be used with a conventional full wave rectifier.

I would not drill any holes until I was sure of the electrical properties of that replacement transformer. If you are aware of the differences and are maybe converting a TRRI to an older TR version, then this info probably doesn't apply. But if you are repairing a TRRI, it's worth verifying.

[lord preset]

Tube Amp Doctor also offers the correct non-center tapped TRRI replacement PT. But if the OP already has the intended replacement and it it center tapped, it is possible to use it with FWB if one half of the secondary is used ( i.e. connect 330-60-0 only) and the center tap and second 330 tap are left unconnected.

[sluckey]

But if the OP already has the intended replacement and it it center tapped, it is possible to use it with FWB if one half of the secondary is used

Yes, but you only have half the current capacity. And you still have to figure out how to get the bias voltage.

[lord preset]

But if the OP already has the intended replacement and it it center tapped, it is possible to use it with FWB if one half of the secondary is used

Yes, but you only have half the current capacity. And you still have to figure out how to get the bias voltage.

A while back I ordered a transformer from Heyboer for a voltage doubling application and I misspecified it to have a center tap when it was not supposed to have one. Heyboer said that current rating of the secondary would not be reduced by only using one side. It worked for me and the amp didn't overheat or melt so I think they were correct at least for their transformers if not others. At the time I also found a scheme for getting bias voltage from the non CT secondary too but I cant find my notes on that. Bummer since I will need to do the same thing on an upcoming build.

But, yes it would be much easier to just have the correct replacement PT

[sluckey]

I didn't realize that about the current.

Page 6 of this pdf shows the bias circuit to use with a FWB. Marshall and Ampeg used the exact same circuit in a few amps.

http://home.comcast.net/~seluckey/amps/ ... apbook.pdf

[lord preset]

I didn't realize that about the current.

Page 6 of this pdf shows the bias circuit to use with a FWB. Marshall and Ampeg used the exact same circuit in a few amps.

http://home.comcast.net/~seluckey/amps/ ... apbook.pdf

Excellent. That was what i was looking for. btw, are the values of rhe electrolytic caps critical? Are 25uf/100 or 50uf/100 ok?

[sluckey]

Marshall used a two pole filter, 10µF--15K--10µF. The ecaps are not critical. I'd use whatever I had on hand. 25µF is what I had on hand to build that little test circuit.

You don't want to get crazy with the filter values though. Just remember, the bigger the cap, the longer it takes to charge. That means big caps will delay the bias voltage rising to full value. You definitely want the bias voltage to be stable before B+ comes up and the tubes warm up.

Here's one Marshall circuit. You have to look carefully. The bias circuit doesn't just jump out at you.

http://el34world.com/charts/Schematics/ ... xx_50w.pdf

[bradicusmaximus]

Here's another way, though there's nothing wrong with using a patch plate. You can get L shaped .75" x .75" aluminum bar stock at the home improvement store. Cut a couple of pieces to size and drill holes for wires if needed. Bolt your PT to the bar stock. Bolt the bar stock to the chassis.

Aluminum is easy to cut and contour, yet stiff enough to give a very stable mount.

That is a great idea - thanks!

[bradicusmaximus]

I am swapping out the stock power transformer in my Fender TRRI chassis. The new transformer is a different size and doesn't line up with the stock holes. The new one is smaller.

This does not address your issue with mounting the new transformer, but may be worth mentioning...

The TRRI uses a different power transformer than any of the other Twin Reverb amps. The HT winding (two wires only) has no center tap and uses a bridge rectifier. There is a completely separate bias winding (two wires).

If you try to use a replacement transformer that uses a CT on the HT winding and a conventional full wave rectifier, you will get a big surprise. Your B+ will likely be more than 900V if the filter caps last long enough to measure it.

AFAIK, Mercury Magnetics is the only company that makes a drop in replacement that actually fits and has the correct windings. Most all other TR replacements are meant to be used with a conventional full wave rectifier.

I would not drill any holes until I was sure of the electrical properties of that replacement transformer. If you are aware of the differences and are maybe converting a TRRI to an older TR version, then this info probably doesn't apply. But if you are repairing a TRRI, it's worth verifying.

Thanks for the advice. Luckily, this isn't a repair on a TRRI, I'm just repurposing the chassis for a different project.

[JMFahey]

But if the OP already has the intended replacement and it it center tapped, it is possible to use it with FWB if one half of the secondary is used

Yes, but you only have half the current capacity. .

A while back I ordered a transformer from Heyboer for a voltage doubling application and I misspecified it to have a center tap when it was not supposed to have one. Heyboer said that current rating of the secondary would not be reduced by only using one side.

That was a trick answer.

Should be read as:
"if you have a transformer with a center tapped secondary which has, say, 200mA current capability and you use half winding, it will still have 200mA current capacity" which is true but misleading.

True because the full winding is wound end to end with same diameter/section wire, so end to end or half of it will have same current carrying capacity.

Misleading because they didn't tell you that if they had designed the transformer with a half voltage secondary (same turns as the other one has up to the center tap) now they woild have twice the window area available (since you won't need the other half winding) now you can use twice the section (1.41 times the diameter) for a doubled current capability.

That's why it's never recommended to turn a "full wave" 2 diode rectified supply transformer into a full wave, bridge rectified one using half the winding and taping out the extra unused one, what sluckey said.

[lord preset]

Yes, but you only have half the current capacity. .

A while back I ordered a transformer from Heyboer for a voltage doubling application and I misspecified it to have a center tap when it was not supposed to have one. Heyboer said that current rating of the secondary would not be reduced by only using one side.

That was a trick answer.

Should be read as:
"if you have a transformer with a center tapped secondary which has, say, 200mA current capability and you use half winding, it will still have 200mA current capacity" which is true but misleading.

True because the full winding is wound end to end with same diameter/section wire, so end to end or half of it will have same current carrying capacity.

Misleading because they didn't tell you that if they had designed the transformer with a half voltage secondary (same turns as the other one has up to the center tap) now they woild have twice the window area available (since you won't need the other half winding) now you can use twice the section (1.41 times the diameter) for a doubled current capability.

That's why it's never recommended to turn a "full wave" 2 diode rectified supply transformer into a full wave, bridge rectified one using half the winding and taping out the extra unused one, what sluckey said.

That sounds right. Because I mispecified it, the PT was bigger, heavier and more expensive than it needed to be for my intended application. Wasteful yes, but OTOH I had already paid for it and didnt want to order another.