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

StarGeezers · Post by **StarGeezers** » Tue 01/18/11 6:33 am

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 · Post by **dotfret** » Tue 01/18/11 11:19 am

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.

Post by **zaphod_phil** » Tue 01/18/11 11:50 am

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 · Post by **Dustpuppy** » Tue 01/18/11 2:10 pm

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 · Post by **dotfret** » Tue 01/18/11 4:37 pm

@ 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 · Post by **StarGeezers** » Tue 01/18/11 7:37 pm

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 · Post by **insanecopilot** » Tue 01/18/11 10:48 pm

StarGeezers wrote:
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 · Post by **StarGeezers** » Wed 01/19/11 7:24 am

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 · Post by **Dustpuppy** » Wed 01/19/11 10:35 am

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 · Post by **StarGeezers** » Wed 01/19/11 11:27 am

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

Post by **zaphod_phil** » Wed 01/19/11 11:59 am

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...

Just use a lower gain triode-pentode tube for both sections, like the 6U8/ECF82, 6BR8, etc.

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 )

FYI the 6BR7 and the EF83 are also great 9-pin pentodes, which have low microphonics.

Dustpuppy · Post by **Dustpuppy** » Wed 01/19/11 3:56 pm

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.

That sounds very intriguing except, I don't seem to be able to wrap my melon head around the concept of biasing a sharp cutoff pentode. I've read Merlin's book, I've read his online treatise, but I seem to be missing something fundamental.

The anode on the ECF804 has a much lower anode max than the 6sj7 so I'd have to plot a load line to make work.

Post by **zaphod_phil** » Wed 01/19/11 5:03 pm

Probably not worth the effort when there are so many other TV triode-pentodes freely available, with nice, low, usable gain levels. Apart from the ones already mentioned, there's also the 6AN8, 6BL8, 6GH8/6GH8A.

Generally, designing pentode preamp stages for guitar amps is a matter of trying some different component values to see what you like. In guitar amps you're also trying to maximise their nonlinearity, unlike in hi-fi, so you can end up using some pretty oddball component values.

StarGeezers · Post by **StarGeezers** » Wed 01/19/11 9:09 pm

The 6U8 does make a rather Nice sounding preamp ... See: Pushy Croaker /Wattkins/ Tone per Buck Club /Amp building... or just go here : http://www.wattkins.com/node/15993 They Do sound Very Nice ...

All worked out/tweaked already for you ... I built the Dog Sled , which was just a 6U8>6N2P>quad of 6P1Ps, and sounded very 18watt in character... which is why I don't hesitate to mention it here... I think ?

Did I mention these amps were built for < 100USD ???

Dustpuppy · Post by **Dustpuppy** » Thu 01/20/11 5:49 pm

Generally, designing pentode preamp stages for guitar amps is a matter of trying some different component values to see what you like. In guitar amps you're also trying to maximise their nonlinearity, unlike in hi-fi, so you can end up using some pretty oddball component values.

Well, I have a bunch of 7 pin tubes lying around that I hadn't found a place for. I'm going to try doing a 6sj7 18-like amp using a 6c4 for the input pre, a 5654 for the pentode pre, 2 6c4's for the PI and a pair of 6aq5's for the power end. I'll call it the 18 Watt Sept. 7 tubes with the recto (6x4) and all 7 pin.

katopan · Post by **katopan** » Thu 01/20/11 9:41 pm

Man.... now you guys are making me wonder if I should just change out the EF86 preamp I have in my 18W for a 6U8. Bought a couple to put in another amp which I can't find the time to build, so they're just sitting there. Same valve socket, change the pentode/triode switch (which I never use) out for an interstage volume, and I'd be good to go. Gone off the EF86 stage quite a bit lately anyway. Mmmm.....

StarGeezers · Post by **StarGeezers** » Fri 01/21/11 8:41 am

DP, go for it Dude !!!!

Kato , Might give that a try ... the 6U8 does work and sound rather Nice as already proven in many amps ...

Dustpuppy · Post by **Dustpuppy** » Fri 01/21/11 4:14 pm

Got the board done last night. 2"x12" and managed to get the filter caps on too. I think tonight it's chassis drilling time. I'll try to get a photo up.

StarGeezers · Post by **StarGeezers** » Sat 01/22/11 11:20 am

Now here's an interesting idea ... having a triode in front of the 6SJ7 with a TMB between the two ...

(Not Absolutely Minimum , but I guess we're not bothered with that any more

)

Referring to the words of our Fearless Leader ZP ... " You can also place the triode and a TMB tone stack in front of the pentode, since the triode has a very low anode impedance suitable for directly driving low-impedance tone stacks.This also helps overdrive the pentode more, so you can enjoy more of those nice rich chimey harmonics. "

Very interesting , and a bit more gain to play with !!!!!!!

DP, anxious to see the pic...

Dustpuppy · Post by **Dustpuppy** » Mon 01/24/11 4:11 pm

I've got all but the IEC wired on this new 7 pin amp. I used 6c4's for the triodes so I'm not sure what the PI is going to do. I'm not a fan of the tmb tone stacks so, I used my favorite, the tweed princeton single knob. Once I finish wiring it up, I'll post some gut shots.

Donations

Links

Welcome

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