6SJ7 Pentode 18 watt Lite...Formerly the ABSOLUTE Minimum

[StarGeezers]

Better now ???

[Dustpuppy]

Holy smoke! What do we shorten that to? The 6P1WLFTABMIN??

Alas, a rose by any other name. Looks good Geez, although I like "The Kick Ass 18 Watt".

[StarGeezers]

That would be a Cool name , although I don't think the mods would have gone for that ... Tried to cover all bases, mention 18 watt and make ZP happy ... maybe I just should have called it ... Ralph !!!

[zaphod_phil]

Why not just call it the 6SJ7 Lite 18W aka "6SJ7 Lite".

[StarGeezers]

That makes sense !!! But then how would folks looking for the Absolute Minimum find it ??? I did post references to that name in a Gazillion other places and forums ... Figured the tag would make it "clearer" .... hopefully ... Just out there shaking the cyber trees for folks interested in the Marshall 18 watt amp , and how they sound so good in so many different configurations... All Cool stuff to me ... So I send them here... Hope that's OK huh ???

[katopan]

I've always been confused thinking I'd seen an AbMin with a triode pre. Now I realise I've been confusing it with the Minimalist from the download section.

[zaphod_phil]

But then how would folks looking for the Absolute Minimum find it ???

Just refer to both names for a while in any of your posts, like say, "6SJ7 Lite, the amp formerly known as AbMin" I'm sure the message will get through.

I did post references to that name in a Gazillion other places and forums ... Figured the tag would make it "clearer" .... hopefully ... Just out there shaking the cyber trees for folks interested in the Marshall 18 watt amp , and how they sound so good in so many different configurations... All Cool stuff to me ... So I send them here...

Awesome! Much appreciated!

I've always been confused thinking I'd seen an AbMin with a triode pre. Now I realise I've been confusing it with the Minimalist from the download section.

And that really highlights how the "Absolute Minimum" name is confusing for a lot of people.

[insanecopilot]

I still really dig my 6sj7 / 6v6 lite, I will probably build another one with el84's as the 6sj7 is a rather dark sounding tube. I actually prefer the 6sh7 which is a little brighter, and slight higher gain. I picked one of these at a surplus store loose in a bin so i thought i would try it. I think it was a good find.

Would a ppimv be worthwhile in one of these amps?

[StarGeezers]

OK, I can do that !!!

Insane , although I disliked the EL84s in SE , they really do sound Great in a PP amp ... So a "6SJ7 Lite" should sound really Good with them eh !!! I love my Baby Will with the 84s !!! I think you'd only need a MV if you didn't want to play it LOUD !!! This is really a Gig amp , and NOT a bedroom amp ... IMHO... Crank it Up ... Play it LOUD and Nasty ...
Fooling around with the 6SJ7 Cathode bypass caps and coupling cap really brightened up the tube...

Here's what i did ... from a previous post ... "the changes are : .015uf coupling cap (Russian PIO) 230pf ceramic across the V pot , and just the .68uf for the 6SJ7 cathode bypass cap ... " That worked out very well ...

[insanecopilot]

That looks like a good combination to me I really like the 6u8 / el84 lite2 too

[StarGeezers]

There's just something very Nice sounding about a Pentode in the preamp ... I don't know why , but it sure sounds Good ... Maybe ZP can explain it to us .. I'm sure it's something "technical"...

Insane , did you ever find any reliability issues with the 6U8 ???

[dotfret]

Duh - why do people focus on EF86 family when they start thinking about input designs?

OK, I'm trying to start a fire by rubbing two boy scouts together, SG and Phil.

As far as I'm concerned, if it has a pentode as valve number one, it is not an 18W - it might be a Voxer, or something else (like the fifty year old hifi amp I've got hiding in the shed).


But there are some interesting old trem circuits using triode pentodes like the Selmer Vanguard, and a trem circuit "like that" uses a VERY cheap valve, the 6BR8 . Since I've seen these as low as it gets lately, it might be nice to peruse a less temperamental trem circuit which minimalises cost and fuss.

[zaphod_phil]

Craig, nice of you to visit us again. FYI this site was always about tinkering with various flavours of the Marshall 18W design. That's why variants like the TMB 18W, the Lite II, the Vajra and the Marwatt were around even before I joined the 18W community as a green newbie. None of them are quite exactly the same as the classic Marshall 18W tremolo amp, yet all of them have the same PI and power stage, and also the Marshall 18W's underlying chime and grind. The DNA is clearly there. And yes, the Voxer is just an 18W Lite with an extra EF86 preamp channel. This site's home page clearly details what is and isn't covered here. HTH

There's just something very Nice sounding about a Pentode in the preamp ... I don't know why , but it sure sounds Good ... Maybe ZP can explain it to us .. I'm sure it's something "technical"...

Yes, it's really deeply technical, so I won't explain it, because it would be just too hard for you to understand.

OK, I give in... pentodes have a more non-linear characteristic than triodes, so that the amplified signal gets bent more (Germanium transistors also behave in a similar way). You can increase the effect by messing around with the screen resistor value (try 100k anode /220k screen) or by overdriving them - try a triode in front of the pentode.

That looks like a good combination to me I really like the 6u8 / el84 lite2 too

Yes you can build a really cool two-stage preamp with those, kind of like a pentode-superlite thing. The pentode side has less gain than EF86s and 6SJ7s, and the triode has a very low anode impedance, so it can easily drive a tone stack. FYI, the 6BR8 dotfret mentioned is exactly the same as the 6U8 (ECF82), but with a different pin base. Since these were designed as TV tubes, they're tough as old nails.

[Dustpuppy]

OK, I give in... pentodes have a more non-linear characteristic than triodes, so that the amplified signal gets bent more (Germanium transistors also behave in a similar way). You can increase the effect by messing around with the screen resistor value (try 100k anode /220k screen) or by overdriving them - try a triode in front of the pentode.

Would it work better to add a warm biased triode beforehand, or a cold biased triode after? I was thinking of trying a 12ax7 split by a 6SJ7. Seems like an inordinate amount of gain though...

[dotfret]

@ dustpuppy -You can get "something similar" in one convenient 9 pin package, actually.

Just before they closed the doors, Brimar invented the ECF804. This valve was used in some semi-military application (marine radio or radar or similar) and received the grand award of a CV number. In the US, they were sold under the H&K label and the European number.

In this miniature marvel of electron manipulation the triode is about half a 5751 and the pentode is ... oh, look it up, it is not very sharp and quite powerful - not intended for a TV set. The triode physically resembles some Brimar SQ 12AT7 equivalents I've got.

Am I rubbing the wrong sticks together?

[StarGeezers]

Dot, we will be on the alert for those tubes... Thanks !!! More to experiment with ...
The sole reason I used the 6SJ7 (actually the Red 5693) was because of all the whining and complaining about the EF86... (Run away, run away ) Since the Jones Octal One sounded so good with this tube , we just assumed it would be a a natural for something with a bit more growl... The normally marginal gain of the 6SJ7 in the Octal One , now fed into a LTP and power tubes (Mark Huss drawing ) produced quite dramatic results ... Much more gain , the ever important LOUD , and a reliable Pentode preamp with no worries of microphonic s or other such silliness... We've been gigging with these tubes for 2 years now , nary a hint of a problem ... The Red Specials are built like Tanks ... missile tubes supposedly... (must have been Old missiles ) ...

ZP, Thanks for that explanation !!! I sorta' understand it ...

[insanecopilot]

Insane , did you ever find any reliability issues with the 6U8 ???

Not with this build. It's been up and running for 2 months now and has plenty of gain with the ppimv, and the dual gain pot for both the triode and pentode. I have gotten rave reviews from all my jam mates



I had a lot of problems with the SE 6u8 / el34 amp and rebuilt it into something else, but that is a tale for another fourm.......

[StarGeezers]

Insane, Thanks .... I think I just had some bad luck with mine (6U8) ... one bad out the box ... one went South after a few hours, glowing Pink ... No troubles since then ... but I have been watching them closely now , and carry a spare tube in the gig bag ...just as a precaution ...
Never a hiccup with the 6SJ7s (5693 and even the regular ones ... ) 2 years + and counting ...

Please let us continue your 6U8/SE problem in a more "appropriate" location ... Pick a spot ... perhaps the O1 thread at Sewatt as there is much interest there in your 6SJ7 X2 preamp...

DP , I think the consensus was to put the triode first to drive the Pentode ...which retained more of the Pentode flavor "thing"... Check out David Jones other "octal" amps ... http://www.freewebs.com/jonesamps/

[Dustpuppy]

DP , I think the consensus was to put the triode first to drive the Pentode ...which retained more of the Pentode flavor "thing"... Check out David Jones other "octal" amps ... http://www.freewebs.com/jonesamps/

Well shoot! Looks like the Octalux is exactly what I had in mind. Is it still even possible to come up with an original amp design anymore?

[StarGeezers]

I don't think so !!! Somebody's done it all already ... probably many years ago too ...