Transformer search

[Mike McLane]

Looking to build a Robinette 763 Micro. The archetype 125B output xformer is reasonably small, but I wonder if there might be an even smaller alternative as the amp is only putting out 1W and the 125B is designed for much heavier duty. Any suggestions? Maybe something designed for a headphone amp?

 

[vigilante398]

My experience is that you're going to have trouble finding anything smaller than the 125B, which is unfortunate because they're a tad on the expensive side given their size.

I've done a headphone amp with a 125B and it was fine for guitar, but for bass I used a hi-fi transformer which is much bigger than a 125B so I can get the lower frequency response.

 

[Mike McLane]

Oh, you're no fun. . . . Not a biggie. It's still pretty small and not too pricey. Thx!

 

[Creach]

Looking to build a Robinette 763 Micro. The archetype 125B output xformer is reasonably small, but I wonder if there might be an even smaller alternative as the amp is only putting out 1W and the 125B is designed for much heavier duty. Any suggestions? Maybe something designed for a headphone amp?

Hello Mike McLane,

Not long time ago i ordered this one : https://primarywindings.com/product/amp-maker-4w-push-pull-output-transformer/

Wider bandwidth and cheaper than the Hammond, but cost may differ depending where you are, worth to take a look.

 

[Mike McLane]

Hmmm. . . interesting. Are you in Britain? It's almost identical in size to the Hammond 125B, but IS significantly cheaper. Wonder if I can get it in the US without getting clobbered on shipping? Thx.

 

[Tassieviking]

This is a small story that might inspire you to look at other types of transformers.

Cheap Output Transformers

100 Volt line transformers as output transformers in valve guitar amplifiers.

www.ozvalveamps.org

You might be able to find more info if you start there and then let google have a go at finding more.

 

[Mike McLane]

Thx! I followed the link, but unfortunately I'm not a very well educated electronics guy so I can't make a credible evaluation. Maybe Vig398 or someone else has thoughts. The 125B and the one Creach cited are about the same size (tad slimmer than 2" cubed) and are certainly small enough for a "mini amp" build, but anything even smaller would be cool.

 

[Tassieviking]

If you do find a smaller cheaper transformer I would love to know what it is, I also would love to make a tiny amp but I would like the pre-amp in one stomp box and the amp in another stompbox.
I would drive the power amp with the Sushi high voltage SMPS power supply, I already have the Bassdude and the Nobelium started so all I need is a power amp pedal to go with them and 1 watt sounds like a nice home practice amp.

 

[Mike McLane]

I discussed the current capacity of the Sushibox SMPS with Vig398. He's guessing 30mA. His off-the-cuff recollection of the current requirement of 12AT7 (phase inverter) = 5mA. . . 12AU7 (push/pull) = 20mA. That would need to be verified, but if that's the case the Sushibox SMPS is near the top end of its capacity for JUST the power amp stage so trying to push the whole enchilada with one SMPS may be a bridge too far. I was wondering about about putting the preamp & amp in the same box, give the power amp its own SMPS and "scab" off the upstream leg of one of the preamp's load resistors (should be full B+?) and jumper it to the HV output the amp's SMPS creating a parallel B+ feed. The 12AX7's appear to pull fairly little mA so being able to distribute the "surplus" capacity of the preamp SMPS to the amp would make for a happy world. Does that even make sense?

 

[Creach]

Hmmm. . . interesting. Are you in Britain? It's almost identical in size to the Hammond 125B, but IS significantly cheaper. Wonder if I can get it in the US without getting clobbered on shipping? Thx.

I'm in France, if i recall correctly the 125B go down to 150 Hz but the transformer from primary windings go to 85 Hz.

To me the 150 Hz was a no go, especially for a small amp you need thump.

 

[Mike McLane]

Understood. Makes good sense. I sent an email to Primary Windings about a US distributor or shipping charges from Britain. Thx for the recommendation.

 

[Tassieviking]

I discussed the current capacity of the Sushibox SMPS with Vig398. He's guessing 30mA.

I think that would be enough for just a small power amp at up to 1 watt, as I mentioned above there have been some success using PA 100V audio transformers as output transformers.
The PA transformers are usually very cheap and have a much better frequency range then normal output transformers as well, but I have no idea how to pick the right one for a certain amp.

I think the 125A transformer would fit in a 125B box or 1590BB2 box from Tayda, I am wondering if it would make sense to make the power amp section of the Firefly 3 by itself in a box with a 125A Transformer inside the box.
The Firefly uses just one 12AU7 tube in the power amp and two 12AX7 tubes in the preamp section, I would only need the power amp part if i use it with the Bassdude or Nobelium.

I am not sure if the 125B transformer would be very different then the 125A transformer when used with a low power output, the specs look the same to me apart from 125A=3watts and 125B=5watts.

The power amp part of the 763 Micro uses a 12AT7 and a 12AU7 tube so it would use a bit more then the Firefly amp.