Ultra Linear power amp. What’s your opinion?

[Svempan]

I’m rather new on this forum and hope to get comments from the people here.

Ultra linear power amps in guitar amps seems to have a rather bad reputation. Could it be that that reputation is unfair? Maybe it depends what you’re after if you like it or not?

You here that have tried different things in amps. What’s your opinion about the differences in sound between ultra linear connections of the screen grids compared to the more conventional cap-choke-cap-filter-connection from the output tubes plate supply?

I mean in comparable circuits or in circuits that otherwise are the same.

Thank you,

Sven-Johan

[Darkbluemurder]

UL got a bad reputation when Fender introduced it in the silverface amps. This is unfair because at the same time they made many more changes which fouled up the sound of the amps. Yet the UL got the blame.

They also used it on the 75 model which indeed sounds a bit strange in stock form. I rather believe the culprit is that they use 500pf snubbers on every gain stage in that model plus a 0.01uF cap in parallel to the series resistor in the NFB circuit.

I believe Dr. Z uses UL in some of his amps. Noone seems to complain.

[Alexo]

I think Ritchie Blackmore used a 200 watt Marshall with a UL topology (Marshall Major?) It had a pretty good sound to me.

Really UL is a form of NFB - and even NFB has a bad rep among a lot of people who are just beginning to understand tube amps, but when it comes down to it, it seems like no single factor will determine if an amp sounds good or not, it has more to do with how a topology fits in with the overall voicing and design of the amp, rather than just the presence of a certain design technique itself.

[drz400]

I tired both on the same amp
UL is nice for HiFi, something about the way it overloads didnt appeal to me. just an opinion though.

[Bear]

Alessandro is big on UL and people don't seem to hate his stuff. It really seems to be all about those silverface Fenders.

Bear

[mhuss]

The big Sunns also used (Dynaco!) UL OTs.

--mark

[jaysg]

Alessandro is big on UL and people don't seem to hate his stuff. It really seems to be all about those silverface Fenders.

The SF Fenders with UL also have NFB hooked up. They're supposed to sound much better with it disconnected.

[Chris333]

I've tried UL in a couple builds (same OT) and I agree with drz400. Used a switch to go from pentode operation to UL, so it was pretty easy to compare the two sounds. Both amps sounded somewhat lifeless and dull in UL compared to pentode. Disconnecting the NFB helped, but pentode w/NFB always sounded better than UL with no NFB, and more so with UL + NFB. I think the power section, and maybe the whole amp, has to be designed around UL operation to sound good. The two amps I tried it in were not...

[skyboltone]

I tired both on the same amp
UL is nice for HiFi, something about the way it overloads didn't appeal to me. just an opinion though.

I've heard this a lot. Did you adjust for the change in plate output impedance between the two setups? That's what discouraged me. The chart for KT66's (designed for UL) indicate quite a difference in output impedance between Standard PP and UL. More has to change than just hooking up the taps.

I think.........No expert here.......frequently wrong.....YMMV

Anyway, Dogears is adamant that UL has no place in guitar amps. Unless that's changed in the last 6 months.

[klingo]

Hi!

my RT77 which is noNFB/ef86/vox PI/KT77 in a hifi OT UL according to MOV datasheet is sounding great. Our lead guitarist is playing it now and his mesa rectifier is waiting the RT77 to fail...
the KT77s were designed for UL operation, just need to find some real ones at an affordable price

I also tried a UL switch in my el84 18wlite.
first with 43% taps... a bit to much
then as i read el84 "prefers" 20/30% taps, i tried 20%.
it still robs you some highs(may be UL flatten the response??) so you have to turn tone control a bit but it's sounding good and will stay as it is.
it offer me two voices , wild break-up in pentode(ACDC) and more controllable one in UL(blues).

as Pentode mode has less damping factor than UL...speaker and baffle are an important part of the sound.

[Svempan]

Thanks for your answers .

I have this question on another tube amp forum as well (here in Sweden) and if I combine the answers here and there it seems as the meaning of my first question isn’t correct. Maybe you shouldn’t compare ultra linear and conventional pentode connected screen grids in “comparable circuits or in circuits that otherwise are the same” as I wrote. I guess that the whole power amp and the power supply have to be correctly and especially designed for the connection in question.

It seems that few people have tried amps for guitar with ultra linear power amp that really are designed for that, or am I wrong?

Didn’t the Sunn amps have the whole Dynaco Mark III circuit in their power amps?

Among other things I notice that the Dynaco Mark III has a rather high amount of feedback and it has a C-L-C filter before the output transformer.

My question is because I’m working on a guitar amp for myself that I want to have a clean sound. Especially the attack of the string I like to be clean. If it compresses a bit is just an advantage, but I don’t want any grainy distortion in this particular amp (I use other amps and pedals for that). I like the preamp in the amp and now I’m thinking of different solutions for the power amp. It now has two pentode connected 6550’s in conventional push-pull.

By yhe way: I own a Dynaco Mark III from which I could use the output transformer and choke if I want...

Take care,

Sven-Johan

[klingo]

clean & a bit compressed... if your voltage and OT turn ratio meet the 6550/kt88 spec you may try UL with cathode bias. i've not tried cathode biais in my RT77 (no room left in the little mig50 cab for the big power resistor) so it's just an idea...

pi filter C-R-C/C-L-C before the Output Transformer is what i have in my RT77.
there is diode rectifier/22uf solen/60 ohms in serie with a switchable 5H/80ohms choke/47uf JJ going to the OT.
anyway there a low 2ma increase in bias current in the resistor only position.(more B+ voltage with same bias voltage)
that said , there's not such a difference when you A/B the resistor/choke&resistor switch.
with choke&resistor there's no hum.

cheers

[drz400]

I tired both on the same amp
UL is nice for HiFi, something about the way it overloads didn't appeal to me. just an opinion though.

I've heard this a lot. Did you adjust for the change in plate output impedance between the two setups? That's what discouraged me. The chart for KT66's (designed for UL) indicate quite a difference in output impedance between Standard PP and UL. More has to change than just hooking up the taps.

I think.........No expert here.......frequently wrong.....YMMV

Anyway, Dogears is adamant that UL has no place in guitar amps. Unless that's changed in the last 6 months.

I have tried transformers from the same company designed for each purpose and not just switching taps. I have never heard a UL that I liked in fact even on my old Dynaco ST70 for Stereo I removed the UL wiring and found it warmed up the amp, sounded more pleasing. Then I did the Curcio mods and it sounded great. I'm going to a agree with Scott Lerner. But it also depends on where the taps are as to how much of an effect it has.
here is some info for you
http://web.telia.com/~u43200663/blocks/ulaudioamps.htm

[Mr. dB]

I have tried transformers from the same company designed for each purpose and not just switching taps. I have never heard a UL that I liked in fact even on my old Dynaco ST70 for Stereo I removed the UL wiring and found it warmed up the amp, sounded more pleasing. Then I did the Curcio mods and it sounded great. I'm going to a agree with Scott Lerner. But it also depends on where the taps are as to how much of an effect it has.
here is some info for you
http://web.telia.com/~u43200663/blocks/ulaudioamps.htm

I agree that even in hifi, I prefer the conventional pentode/tetrode connection with a screen grid supply, to ultralinear. I hear in ultralinear a certain problem with dynamics. There is a lot to be gained, though, in making the screen grid supply very stiff, or even regulating it. For hifi. For guitars, spongy supplies seem to do "what we like".

[Svempan]

Thanks for your answers.

The reason I’m asking is because my work with an amp supposed to play clean. Se my thread at the Dumble Discussion forum here https://tubeamparchive.com/viewtopic.php?t=2804

For clean sounds for guitar, one of the amps that have impressed me most the latest year or so, is a Marshall VBA400 bass amplifier. That amp has a separate supply for the screen grids with half the plate supply voltage. If you look at the Ampeg SVT (and maybe the Dumble Steel String Singer 150 and 300 Watt) they have a separate winding on the power transformer for the screen grid supply. 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? In that case it will probably be a separate transformer in my amp.

Comments?

Take care,

Sven-Johan