Building for EL34 AND 6L6GC, transformers choice

[Hardtailed]

The final kicker on the amp I'm planning will be to have a pair of EL34 and a pair of 6L6GC, with the option of running all 6L6GC.
I loved what that did on my Mark IV.

I'm trying to stick with "off-the-shelf" replacement transformers, cause I'm broke and they're cheap, and mostly I can literally get them off the shelf at the local surplus store!

At first, I wanted to use a Plexi 100W set.
But I'm afraid this could melt 6L6s because of a screen voltage that's a bit too high.
What I learned here is that, since 6L6 don't draw much screen current, the resistor doesn't drop much voltage, therefore they would be mostly likely idling at about 480V on the screens (max is 450V for 6L6GC).

So I figured a PT for a Fender Twin Reverb would do the trick, with about 450V on the plates. Plus, with 450mA current capability (vs 300mA for Marshall PTs), I should be able to get a huge low-end by virtue of the power supply being up to the task.
Only problem is the filament winding is not enough to run 4 EL34s if I so wished. Not a big deal though.
Actually, it's not even enough for 2 EL34s, but I'll have an extra 4A transformer for the preamp tubes anyway (no off the shelf PT is up to the task, and the extra filament PT was 12$)

Sounds good so far?
To be clear, I'm building a 3 channel metal machine. I notice most Mesa run on about 450V, so I guess this should be good.

Now, about the output transformer, this might sound stupid, but I'm worried that the Fender Twin OT is too small.
If I look at the Hammond specs for example, the 100W Marshall is 8lbs, while the 100W Fender is 5.5lbs.
Another problem is the Fender only has a 4ohm taps, but Magnetic Components makes a multi-tap version.
The reason the weight bothers me is that, since this is to be a metal machine, I need the biggest low-end possible, and usually more iron = more low-end.
So based on that, the Marshall one appears the best option, but I'm wondering about the primary impedance...
The Marshall one is 1.7k, while the Fender is 2k

If the Marshall with EL34s sounds good at 495V, it should sound good at 450V too (what I mean is, when you lower voltage, you'd lower the output impedance too, not the other way around).
I've drawn the loadlines (though I'm no expert) and if anything, at 450V it crosses the dissipation limit less.

As for the 6L6GCs, I read they usually like a higher load, though this usually means even more than 2K anyway. Drawing the load lines, I notice it would stay in class A territory for a narrower portion.

To be fair, I'd rather use the Marshall one since I can then buy everything from the same vendor and save on shipping, not to mention they have it in stock at the local store (the Hammond one that is, I used the 1750N in my Plexi clone and love it)
And the heavier weight makes me feel more confident.
But if 15% different in impedance makes an audible impact for the worse, then I need to reconsider.

Any thoughts?

Many thanks!

[wizard333]

More metal does not = more low end.

Having done quite a few OT swaps, I'd warn you to not believe all the hype you hear.

You can hurt your amps ability to produce low end by severely under-sizing the OT, but just changing the OT won't give you more low end than you had, assuming your stock OT wasn't the limiting factor there.

What a bigger OT will give you, maybe, is more ability to stay defined and produce low frequencies accurately as your volume approaches "ok I'm leaving the room, this is too GD loud". Depending on the amp, the quality of the stock OT, and the OT you are replacing it with, its more than possible you wont hear much difference at all, at least until your volume is way up there.

I've built modded Fender DRs with a slightly larger MM OT that will make the hair on your arms move with no problem at all, and that OT is no where near as big as a Twin, etc. It moves way more low end than a lot of larger amps. That has nothing to do with the size of the 2x6v6 OT, it has a lot to do with the circuit I put in it.

There are things you can do in the circuit that will limit or boost low end that make a much larger difference than an OT swap, again assuming your stock OT wasn't just pure crap. Most are not.

I don't think you'd go wrong with a multi-tap BF Twin OT at all. I've installed a few and they are great sounding OTs. Listen to a Twin some time with the bass dimed and tell me if you think it lacks low end! You could also use the Marshall OT if you preferred. Neither is going to limit your low end reproduction at all. Many companies building "high gain" amps limit the size of their OTs in order to try to get OT saturation. Mesa comes to mind; their OTs are generally under-sized.

[tedmich]

in my opinion, Antek PTs are the bargain of the century, if you don't mind a "non traditional" (ie better) toroid tranny perched on your head. For OT, Edcor is the real bargain. You can go with an incrementally lower OT primary imp. to PUSH the tubes harder for more PA crunch. I think high gain amp makers use undersized iron cause its cheaper and 99% of their users couldn't find their OT with both hands and a map...IMHO