Ultra Linear power amp. What’s your opinion?

[roberto]

Marshall VBA400 bass amplifier has a separate supply for the screen grids with half the plate supply voltage. With that and the rather low values on screen grid resistors of these amps I suppose the screen grid supply is much more stiff than the conventional C-L-C filter from the plate supply. Maybe the VBA400 or SVT solution is better than a ultra linear connection of the screens?

I know it's an old thread, but I think it could be interesting anlo to debunk the mith of the UL as bad sounding on guitar/bass amps.
A stiffer solution for the screens is IMHO preferred in bass amps compared to guitar amps, to keep the curves stable even when pushing the amp hard.
With a tertiary winding for screens, and properly designed UL percentage, UL will also help to avoid screens to conduct, because they go down together with the plates, staying (most of the time) below them.

As for the UL, I've had guitar amps with switchable triode-pentode-UL, and the result was that all three were good sounding, with different characteristics, and all three needed to be optimized in NFB.
Old Fender UL 43% plus NFB were too much clean, and their output impedance was IMHO too low for a guitar amp, with too much feedback and not a great breakup.
A bit of negative current feedback could help, like it is done with SS amps to increase their output impedance and "tubify" them.

Anyone has done some experience in this field in the meantime?

[JD0x0]

"Wah TOO CLEAN, TOO STERILE, no POWER TUBE DISTORTION"


https://www.youtube.com/watch?v=w0GQTKHePzs


I'm pretty tired of the whole assumption that you need power tube clipping for an amp to sound 'good'.

Notice how besides the UL Fenders, no one complains about UL amps. SUNN amps are often UL and massively desirable to some players. Zappa used a Major 200W into an 18'' speaker cab for sub octave fuzz. Never heard complaints about the Major being UL.

I have some amps with a pentode/UL switch. It gets 50/50 use. The UL works way better for spanky funk or jazz stuff that should sound clean and punchy. There's definitely less harmonic content but it doesnt sound sterile or lacking, IMO, and it can still do the 'singing infinite harmonic feedback' quality. You can even keep NFB on the PI, if you dont go overboard with it and neuter the power amp too much.

[roberto]

Thanks for the link JD0x0,

that's perfect to debunk the myth of UL with musical instrument amplification.
And that's also the reason of my interest on it.
Well, something more powerful for bass and something way less powerful for guitar.

[didit]

Hello -

Magnatone M Series (eg, M10a) are fine sounding guitar amps, in their idiom. They are built around mid-sized UL transformers quite similar to a Dynaco z565. Hi voltage and relatively hi primary Z. Sparkly certainly, however not at all "sterile".

Best .. Ian

[roberto]

Thanks Ian,

UL has got a very bad reputation due to Fenders, where everything was just too much, but many design has proven that it can be very well implemented in guitar and bass amps, it is just needed to know what to do to optimize it for isntrument amplification.

[martin manning]

Here is a recent Dumble SSS build with UL power section: https://ampgarage.com/forum/viewtopic.p ... 83#p380883

[roberto]

Thanks for the link Martin!

[Colossal]

The Dr. Z Route 66 is a pair of KT66 running in Ultralinear.

[roberto]

Thanks Colossal, I think that one of the best ways to promote UL guitar amps, is to hide the fact that they are UL.

[pdf64]

...Old Fender UL 43% plus NFB were too much clean, and their output impedance was IMHO too low for a guitar amp, with too much feedback and not a great breakup....

It's worth noting that the late 70s Fender guitar amps OT primary taps were at about 12.5%.
I think that their main cause for inclusion was to reduce screen grid dissipation at high power outputs, rather than to increase linearity per se. Their plate curves were probably closer to regular pentode operation than they were to true UL.
The increased HT and power output of these amps probably necessitated this change. Note that these OT's also had switched OT secondary taps, built into the output jacks. I think that the main reason for this was to avoid the very low reflected load impedance that was likely when ext cabs were used in previous versions, as combined with the high HT, this might have resulted in excessive plate dissipation.
Regarding their tone, the bright cap on the master volume is best removed.
Reducing the NFB ratio may also be beneficial.
But I suspect that the main thing that hobbled their 'as heard' tone etc were the stock speakers, almost anything else helps them to sound better.

[roberto]

Thanks pdf64, I wasn't aware of that detail.

[mAx___]

Here's a good example of the ultra linear Marshall Major pushed to the limit and with a booster in front. It speaks for itself

[roberto]

LOUD and clear!

[JD0x0]

Old Fender UL 43% plus NFB were too much clean

FWIW I've had good success with Edcor's 40% screen tap in guitar applications. Still enough NFB on the PI to run an effective 'presence' circuit and I think the amp still has plenty of harmonic content if it's turned up to band volumes. The biggest difference is it doesnt 'bark' quite as much when you really slam the amp with percussive hard attack playing, but you can still get that 'swirly' harmonic behavior that makes it sound 'lively' and 'tube-y', unlike a bland solid state power amp with no desirable harmonic content.

[roberto]

Thanks JD0x0,

my idea is to get the presence-depth equalisation on the last preamp stage, then use UL and/or shunt feedback on the output tubes, without conventional NFB.
Just to have a reasonable damping while keeping the PI free of NFB. This to be used with low wattage amps EG pairs of EL84s or EL34s.