bigger transformer? why?

[pietro_moog]

hi guys.
i was wondering, was difference does it makes a bigger OT?
i read somewhere that a 5e3 is around 13 Watt but does have a 20W OT. why?
and the original 5F2a was made with a bigger OT than the 5F1, and so on..
bigger OT SE and Bigger OT PP make the same effect?

[Jazz P Bass]

You noticed that too, huh?
Maybe Fender used what they had on hand.
Some amps I do not understand.
Fender Hotrod Deluxe output transformers are little.
Compare it to an old Bassman.
I got a 50 watt Marshall replacement output transformer for a 50watt Traynor.
It was huge compared to the original Traynor (gee, I wonder wht it failed?)
Go figure.

[Chuck H]

When you consider that guitar amps have a "limited" frequency range (misnomer) the spec for the OT requirements goes way down. But... If you take into account that tubes are more proficient at pulses of peak power than transistors, and it's not uncommon to see a 100 watt tube amp putting out prolonged pulses of 250 watts, then the bigger OT starts to make sense. In contrast to your analogy about the 5E3 having an over rated OT, from what I've read original 5E3 OT's are hard to come by because they are known to fail under over drive. So...

As far as I can tell, and as gutiar amps go, you do want an OT that will limit the frequency response as compared to a hi fi product, BUT you also need a unit robust enough to handle extremes and stresses that are encountered with over driven tubes. The fact that these are tube amps, and tubes behave differently than other amplification devices, is the whole point. I've seen very little documentation that covers what tubes do when they're operated to gross extremes as guitar amps often do.

[pietro_moog]

so you're saying that OT is partially responsible for distortion. smaller transformer-squashed sound. is that right?
that could explain why my 5f2a screams while my ac4 doesn't (which is 4w and i used a 10w OT in).
cool.
i always thought that overdrive in small amps comes from powertubes pushed hard.
between my amps there's a big difference, very big: my 5f2a starts screaming a 1/4 of the gain but my ac4 doesn't till full gain, and it's just a small crunchie.
so upgrading OT increase headroom a lot?

[Chuck H]

so you're saying that OT is partially responsible for distortion. smaller transformer-squashed sound. is that right?
that could explain why my 5f2a screams while my ac4 doesn't (which is 4w and i used a 10w OT in).
cool.

Nope. No one said that. Any clipping you hear is the tubes themselves. But smaller OT's will saturate and roll off certain frequencies sooner than larger ones. This is a generalization but this effect does play a roll in the tone of certain amps.

i always thought that overdrive in small amps comes from powertubes pushed hard.
between my amps there's a big difference, very big: my 5f2a starts screaming a 1/4 of the gain but my ac4 doesn't till full gain, and it's just a small crunchie.
so upgrading OT increase headroom a lot?

Again, clipping is the drive voltage and bias condition of the tubes. But it is true that a large OT will generally have a broader frequency response, and hang onto it as power increases longer than a small OT. So in this regard it's more about efficiency than clipping. For that matter it's similar with the PT in an amp. A larger one will provide a steady supply of current while a smaller one will "sag". A compressing effect that, while softening attack can also dull dynamics.

Try to remember that "distortion" is any non linear effect on the waveform, not just overdrive. Transformers can and do contribute to distortions but not clipping. Well, a sagging power supply WILL generally clip sooner than a stiff one. But in general clipping and gain are not affected by the iron on the amp. What the iron contributes is it's effect on the envelope of attack (PT) and the ability or inability to handle or recreate frequencies (OT). This AND the OT is there in the first place to mate the tubes to a speaker. How a specific OT does this can also affect EQ and dynamics.

So it's just not as simple as big vs. small. It will depend a lot on other circuit conditions and what a certain trandformer (PT or OT) brings to the table may be great in one circuit but not in another. Even if both circuits use the same tubes for the same watts.

[guitarmike2107]

Can you give a direct link to the schematic you used, the problem is you lack gain to overdrive the power valve, i.e. not a problem with the OT, we cant tell you how to increase gain if we dont have the schematic.. well we can but it may not be the best advice

[pietro_moog]

yeah, you're right.
this was just a thread i opened to learn something. and i did.
i have another one on the build your amp section.
i'll post the schematics there.