Size of Output vs. Power Transformer?

[Jerry2013]

I have been considering having my Traynor Bassmaster early 70's tube amp modified to try and get a Plexi Hot rod 'brown'/ Van Halen 1 sound.
The transformers are huge on this Traynor amp. I read where it could be a good thing to have a extra large output transformer, for that punchy SRV type sound, so thats already there.
I am also considering replacing the big Hammond Power tranny with a smaller Marshall sized /spec one so it can get the tube saturation of an old Marshall as well as reducing or tightening the bass ( this Traynor was originally a Bassman clone of the Fender Bassman circuit) .
Is there a mandatory size / other (?) relationship of the output to power transformer, or can the two be similar size?
I am not sure if I'm making any sense with this inquiry , but hopefully so.

[David Root]

A 50W Marshall OT lamination stack is 3 1/8 x 3 3/4 x 1 1/2, whereas a BF Bassman OT is the same but only 1 1/4 thick.

So the Marshall weighs maybe 1/2lb more than the Bassman, ~5 lb vs ~4 1/2 lb. This surprised me as I had always thought Fenders were a tad heavier than Marshalls.
I'm basing this on a Merren Audio clone of a 784-139 and a '64 Bassman OT, both of which I just measured and weighed.

I'm assuming your Traynor "Bassman" is a 50W (whereas VH's Marshal was 100W, which makes a difference all by itself) so this is an apples and oranges kind of situation. In this case if the Traynor Bassman OT is similar size/weight to a BF Bassman OT, it will be a tad smaller than a 50W Marshall OT.

If your Traynor "Bassman" is in fact a 100W amp, then similar differences hold up, with a SF Twin OT at ~6 lb and a M-C Marshall at ~8 lb.

Either way my guess is the Traynor iron is a bit bigger than either Fender or Marshall iron. I have a '70 Traynor Voicemaster 50W PA amp on my bench right now and the OT is enormous, 3 x 3 1/2 x 2 stack, so I'm sure you'll have something similar if not identical. The PT stack is 2 1/4 x 3 1/8 x 3 1/2, so a tad bigger than the OT. This is pretty normal.
This would bode well for "that SRV sound", but that's not "the VH brown sound" so I think you need to make up your mind here which way you want to go. Either way I don't think the Traynor iron is a negative.

What WAS a bit different in VH's 100W was the filter cap values, lowish on plates & screens but a whopping 100uF on the phase inverter. Loose power tubes but tight PI.

So personally I wouldn't mess with the Traynor iron, it's pretty solid stuff, but by all means play with the filter caps.

[R.G.]

Is there a mandatory size / other (?) relationship of the output to power transformer, or can the two be similar size?

There is no direct correlation between the sizes of the PT and OT.

This is an outgrowth of the physics of transformers (which I'll try not to drag you through very much ) and the fact that they are designed with different objectives.

The physics are the same, of course: the inductance of the transformer primary is largely determined by the lowest frequency to be transformed and how intense the magnetic field inside the iron must be. The number of turns and the wire size is set by both the inductance and how much current has to flow, so as to keep the temperature down to some limit and let enough current flow to the load.

What's different in this case is the frequency and the type of use. The PT always has AC power line frequency as an input, so it's going to be 50 or 60Hz, all the time. The resulting size then is determined by how big the wires must be to let the current through and get the heat out, and that winds up determining in a complex thermodynamical way the size of the core and so on.

The OT has to carry "audio". Audio is from 20kHz? 15kHz? 10kHz down to 100Hz? 82Hz? 42Hz? 20Hz? depending on how you define the necessary bandwidth.

For the same lowest frequency, a transformer handling the same power at 20Hz will be three times the mass of one handling only 60Hz. Back in the day of 25Hz power transmission, PTs were very large and heavy indeed. Switching power supplies get to use tiny transformers because the internal frequency on the transformers are 100kHz to 1MHz.

So if you define your OT as handling equal power down to line frequency, they'll wind up similar size.

... if the power is continuous, and both are working in what amounts to full push-pull.

The class of amplifier operation (A, AB, etc.) and the ended-ness (single ended vs push pull) determine how much of the internal magnetic field ability is used in an OT. In particular, a single ended OT only gets to use half the magnetic field "swing" as a P-P, and so the size for a given power balloons up hugely.

The net is - there is no particular correlation of the size of a PT versus the OT in a guitar amp. Well, there is, but it's complex and subtle as they are designed with different objectives and limitations.

[Gaz]

Great post, R.G.

[JMFahey]

I am also considering replacing the big Hammond Power tranny with a smaller Marshall sized /spec one so it can get .....

Please don't.

Don't know where you got that harebrained idea from, but it's akin to turning a nervous race car into a family sedan by mixing water with gasoline or something similar.

There' s 1000000 things you can do to sound like SRV, last but not least practicing a lot.