UL amps less powerful?

[Axtman]

An amp friend remarked the other day that tube amps with ultra-linear transformers are less powerful than their comparable designs without UL transformer taps.

I thought it was the other way around. Fender's UL amps claim to eke out a few more what's than the non-UL amps. For example, a Fender Twin claims to put out 100 watts, while the UL Twin claims to put out 135 watts.

[g1]

As far as the Twins go, I think there are other differences aside from the OT that need to be considered. It's a way we identify them by name, but it's not like they are identical except for the screen supply/OT.

[Helmholtz]

By principle UL operation should lower power output as the screen NFB increases plate saturation voltage.
But as the UL Twin uses 20% taps (as opposed to "real" UL with 43% taps) the effect will be small.
I think the higher output of the UL Twin comes from increased B+ and a stronger PT having less sag.

[pdf64]

I recall the Fender OTs Ive checked had screen grid taps at about 12% voltage.

[Helmholtz]

I recall the Fender OTs Ive checked had screen grid taps at about 12% voltage.

I might be wrong about the 20% taps with the UL Twin. Don't recall where I read it.
According to Kuehnel 20% taps give max. power, while 43% taps give minimum distortion, % numbers referring to turns.

[Axtman]

Other UL/Non-UL Fender amps that I can think of are

• Fender Bassman Ten. Early versions are 50 watts, and later versions are 75 watts.
• Fender PA100 and PA135 (100 watts and 135 watts respectively).
• Fender Bassman 50 and Bassman 70.
• Fender Bassman 100 and Bassman 135.

[Helmholtz]

The higher B+ at full output, the more power.
Roughly 10% higher B+ gives 20% more power.

[g1]

I think all those examples were attempts to increase power and fidelity. So power output was boosted and screen grid taps were used. But the increased power was not caused by the use of screen taps.

[Chuck H]

I think all those examples were attempts to increase power and fidelity. So power output was boosted and screen grid taps were used. But the increased power was not caused by the use of screen taps.

Yes. This ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The UL Twin sort of put the idea that UL means more power into the heads of the lay person. "ULTRA LINEAR" Sounds big, right? Like big, clean, solid power. The reality is that "ultra linear" (and there were a lot of people back in the day that hated that term) was never intended to make more power, per se'. It was a way to make more power at a low target distortion level for hi fi. And it works well for that. But it really has no place in guitar amplification (IMHO). I remember learning about this and being confused. Because I had only studied guitar amplifier models and amp basics. I absolutely did think that ultra linear was implemented to achieve maximum power. Learning I was wrong and what it really meant some years ago was a reality check to be sure.

[tedmich]

FYI some refs on OTs and UL winding

[Helmholtz]

As a reminder, the maximum possible clean output of a class AB PP amp is given by:

Pout = 2* ( (B+) - Vsat)²/Raa = 2*(Vsat - B+)²/Raa,
where B+ is measured at full output and Vsat is the voltage drop between plate and cathode at max. plate current.
The formula assumes an ideal OT.
B+ minus Vsat is the available plate voltage swing.
Vsat typically is between 50V and 100V.
It is interesting to note that the power output doesn't explicitely depend on tube type, screen voltage or other circuit details like NFB (UL is local NFB).
Such differences may somewhat change Vsat, though.

[J M Fahey]

That´s the proper *Engineer* way to design a Power Amp although you also NEED to include available current swing.

Tubes are *strongly* current limited so lowering Raa does NOT automatically increase power and it may very well REDUCE it.

Tubes are not transistors [tm]

Biasing to a given % of maximum power dissipation, with no other data thrown in, is IGNORANCE.

In fact, over biasing tubes actually REDUCES current swing, which oscillates between MAX and IDLE.

The higher Idle is set, the less swing is available.

Consider this: almost all Factory amps are accused by Gurus of being biased cold.

Maybe because Factories hire Engineers and not Gurus?

[Helmholtz]

The formula I showed above always works ( I often verified ).
It automatically takes care of current limiting by the tube by increased Vsat.
For instance, a steeper loadline above the knee results in a significantly increased Vsat, thus limiting output.
The correct Vsat is found from the intersection point of the loadline and the Vgk = 0 plate curve (or even better by scoping the plate voltage in the real amp).
I like to use this online calculator: https://www.vtadiy.com/loadline-calc...ge-calculator/

[Chuck H]

FWIW biasing hot for guitar amps as a trend usuallly equates to tone. Not all amps are better off for guitar tone for being biased hot. This gets ignored by "gurus" that have seen hot bias work as a tonal gimmick. Guitar amps aren't reference amps. They are signal processors! The same rules don't always apply. The important thing is to stay within safe operating parameters for the tubes and circuits!!! This is where the guru's get it wrong more often than not.

There absolutely are amps that do the hot bias thing so well that players can't live without it and will feed their amps powers tubes like meals. Witness most Matchless circuits. But the old BF Fender designs, which also sound great, don't benefit from this practice at all. Yet some players and even techs think that a hotter bias is a tonal benefit for these amps. They don't know why they're wrong and there's almost no explaining it to them in my own experience.