Two output transformers instead of one - How to connect?

[lacrebob]

I understand from Internet info that the Marshall JTM 45/100 prototypes used two 50-watt transformers.

I can't find a schematic that shows how they were connected or what their impedances were.

If you wanted to substitute a single output transformer with a primary impedance of 5K with two smaller output transformers, would you use transformers with 5K primary impedance and connect them in parallel?

Or, would you use two transformers with 10K primary impedance and connect them in series?

[Structo]

Not sure about the impedance issues but I believe they were wired discretely, in other words one OT fed one speaker cab and the other another speaker cab.
So each pair of power tubes had it's own OT.

[martin manning]

A Metroamp forum thread with what you want is here:
http://forum.metroamp.com/viewtopic.php ... 2b1e372d57

[Kagliostro]

Look also to this tread

http://www.el34world.com/Forum/index.php?topic=8471.0

Hope this can be of some interest

Kagliostro

[Kagliostro]

The SE images

Kagliostro

[lacrebob]

any reason the secondaries couldn't be independent from each other so long as both transformers are feeding speaker(s) with matched impedance?

[Firestorm]

Not sure what you mean by "independent" secondaries. There's either parallel or series. Unless you deliberately wire them in series, the secondaries will automatically be parallel because there is a ground connection on each OT secondary.

[lacrebob]

All of the schematic show two transformers connected to a single speaker jack. Can you connect each transformer to its own speaker jack?


For example, with a parallel 6v6 single-ended amp, could you connect 6v6#1 to single-ended output transformer #1 which connects to speaker #1, and also connect 6v6#2 to output transformer #2 which connects to speaker #2?

[Firestorm]

Then you'd have two monoblocks. Tube A on transformer A to speaker A; tube B on transformer B to speaker B. That's the way stereo amps are made, except they feed the tubes from separate channels. You can feed both transformers from the same B+ node and you can split the driver output to feed both tubes. In fact, you could have two different transformers and use two different tube types that require different impedances.

Of course Marshall wouldn't have done it that way. I'm sure they would have wired the jacks in their standard parallel configuration so you could plug in all four of your stacks.

[Jana]

The original intent of the OP was to replace one larger transformer with two smaller ones. No inherent problems with that provided the phase of the transformers is observed.

Now this thread is morphing into all kinds of what-ifs. Think these through--two transformers from the same power section with separate speaker jacks, whether it be one set of tubes or separate, well, not a good idea. Yes, stereo amps do this (with separate drive/pre-amp sections) but with a stereo it is a given that there will be speakers for each section. Are you ALWAYS going to have speakers for each section? If not, you're in trouble. Two 10K transformers wired in parallel will only give the 5K load if each transformer has the appropriate speaker load. Leave one of them unloaded and you are back to a 10K load with one transformer susceptible to flyback.

All these what-ifs can be done, just make sure to write a mil-spec check list for operation to go with the amp.

It's a mono amp, keep it mono from front to back. Keep it simple: that's my motto.

[Firestorm]

Keep it simple: that's my motto.

There's a thought! Marshall did it because they had to (briefly). As soon as they could source high power iron, they went right back to one OT.

[Andy Le Blanc]

Hey... aren't the cores of those opt supposed to be inductively coupled?

Here's the Crowhurst twin coupled.

[Luthierwnc]

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