Ultralinear tube amps

[Axtman]

Is the major difference between an ultralinear and non-ultralinear tube amp is that the UL has taps off the transformer to the screens?

Could one just remove the UL taps from the screens to create a lower powered non-ultralinear amplifier?

[Helmholtz]

UL operation doesn't really increase output power. High power comes from high B+.

[loudthud]

I built a 4x6550 amp with a UL-Pentode switch. Didn't really like the sound in UL mode, so I never used it. Waste of time and a good switch.

[mhuss]

UL is (arguably) better for hi-fi, not so much for guitar amps. You cannot just remove the screen connections, however; you'd need to come up with a screen voltage supply (and add individual screen limiting resistors). The other alternative is resistors & connect to anode for 'psuedo-triode' mode, which is something I never really liked the sound of.

[Helmholtz]

^^^That!

As high power UL amps often use B+ at or above the screen voltage limit, it might be necessary to reduce screen supply voltage with pentode/tetrode mode.
This will somewhat reduce available output power. In fact, screen voltage can be used to vary/reduce output power.

[Axtman]

My plan is to build a tube bass amp. I thought I might put a switch to disconnect the screen taps in case I want to play guitar through it.

[Helmholtz]

I thought I might put a switch to disconnect the screen taps in case I want to play guitar through it.

You will want/need to provide an additional screen supply node to connect the screens to in "guitar mode".
This serves 2 purposes: lower screen voltage and improved screen supply filtering.
Also - as mentioned - screen stoppers are highly recommended.

[pdf64]

My plan is to build a tube bass amp...

Note that every UL amp with a decent rep has a pi filtered HT feeding the OT; that requires a large, heavy, high current choke. Hence the ubiquity of pentode mode amps

[SoulFetish]

If these so-called Output Transformers were so "linear", then how come you never see 'em used with power Triodes?

[Helmholtz]

If these so-called Output Transformers were so "linear", then how come you never see 'em used with power Triodes?

What do you mean? Which OTs?

[eschertron]

If these so-called Output Transformers were so "linear", then how come you never see 'em used with power Triodes?

Pretty sure screen taps are irrelevant to triodes?

edit: oh, wait! Triodes are inherently linear, so no 'ultralinear' OTs needed. Pentodes, however, need all the linearity they can get... if that's your bag

[pdf64]

If these so-called Output Transformers were so "linear", then how come you never see 'em used with power Triodes?

Not sure if you’re being ironic or serious?

[Leo_Gnardo]

If these so-called Output Transformers were so "linear", then how come you never see 'em used with power Triodes?

If you did employ an "ultralinear" OT with triodes, what would you do with the UL taps? I'm sure some boffins would manage to put them to a purpose. As it's been stated, power triodes tend to be linear without any prompting. Also, they're very rarely found in MI gear. I did have a customer who wanted to make an amp with 300B but I managed to dissuade him. When he found what the tubes cost, he definitely dropped the idea altogether and got happy with his Vibrolux Reverb.

The observation was made here a couple years ago, that "ultralinear" OT's such as you find in late 70's large Fender amps, are not really ultralinear. (Helmholtz was that you?) The ratio isn't right. So - they're OT's with primary taps. As are the OT's found in Mesa's amps that feature "simulclass" operation. So.. what we have then is quasi-ultralinear. "Something sorta like but not quite." With a label like that, they might appeal to some sector of hi fi hounds & guitarists who like spouting scientific sounding names for their gear.

[Helmholtz]

[QUOTE=Leo_Gnardo;n919550]

The observation was made here a couple years ago, that "ultralinear" OT's such as you find in late 70's large Fender amps, are not really ultralinear. (Helmholtz was that you?) The ratio isn't right. So - they're OT's with primary taps. /[QUOTE]

No, not me. Maybe pdf64?
In this case I tend to be less dogmatic.

There might be some misconception.
The primary taps for the screens don't make the OT more linear than a standard OT. Rather the whole circuit is termed "ultralinear".
Connecting the screens to primary taps forces the screen signal voltages to follow the plate signals, meaning that part of the plate signal is fed to the screens.
As a screen is a grid, it acts as an inverting input. Thus we get negative screen feedback, resulting in a linearized transfer characteristic.
UL mode can be seen as a cross between triode and pentode operation.

[J M Fahey]

There seems to be some confusion caused by the name.

It is not that some transformers are more linear than others as, in having lower distortion, itīs not "better transformers" here, but "better circuit operation".

In a crude way: somebody was very bored, there was nothing good on TV, so he started experimenting with pentode outputs.

* screens connected to +B (OT center tap?) : amp worked but had some distortion.
Just pulling a number out of the blue, say 8%.

* now he connected screens to plates, "triode mode" ... distortion was much reduced.
Not real triodes, but very near. Could we say, again out of the blue, 3%?
Definitely an improvement.
Maybe he called it "Linear mode" , just not to use the word "triode" in the description.

* the experiment might have stopped there, what else?, but he then proceeded to get an OT with different taps, intermediate between full pentode and full triode, and repeated the experiment, just for kicks, probably expecting a smooth variation from "pentode" to "triode",what else?

To his surprise, he found that at some intermediate values, he had *even less* distortion than with full triode mode
Say ... 1.5%?
Further experimentation found that 43% taps were best and significant power was NOT lost as in triode strapping.
Now all he needed was a new name for this new connection; and being more linear than former linear (triode) mode, Ultralinear was it.