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

StarGeezers · Post by **StarGeezers** » Tue 12/14/10 9:00 am

Before .... I think

... following the coupling cap , probably wire it right to the V pot input eh ... ???

StarGeezers · Post by **StarGeezers** » Fri 12/17/10 7:12 pm

Just got in my .0047uf coupling caps .... Anxious to try it out for even more brights ...

StarGeezers · Post by **StarGeezers** » Mon 12/20/10 6:13 pm

Now although we've been tweaking the coupling cap on the 6SJ7 to lessen the bass, a Quick conversation with Davent has given us another alternative ... Tweaking the bypass cap on the 6SJ7 Cathode, and the screen cap ...

He explains it better than I could ...

QUOTE: " My experience with playing around was, bigger cap- bigger fuller bass, smaller cap thinner with less bass. I ended up using 33uF to up the bass but i had dropped the screen cap from 47n to 15n in order to brighten things up, which had in turn decreased the low end response. Becomes a game of cat & mouse looking for the balance. (Also dropped the screen resistor from 2M2 to 1M2, i'm sure this is not the end of the story but for now... we're good!) " ...
Thanks Davent .... more ideas to play with

StarGeezers · Post by **StarGeezers** » Tue 12/21/10 12:01 pm

So without changing too many things at once ,played it first to listen , then I merely snipped off the 2.2uf cathode cap on the 6SJ7, leaving the Jedi cap (.68uf )

Mischief managed !!!! Much better now , more balanced and not woofy' ...Thinking I'm going to try it out tonight at band practice and see how it holds up with the group ...

So now the changes are : .015uf coupling cap (Russian PIO) 230pf ceramic across the V pot , and just the .68uf for the 6SJ7 cathode bypass cap ...

Ps, and this was into my sealed 12" cab ... so an open back cab will be even brighter...

Dustpuppy · Post by **Dustpuppy** » Tue 12/21/10 2:13 pm

So on the V pot, are you running the bright cap from 1 to 2 or 1 to 3?

StarGeezers · Post by **StarGeezers** » Tue 12/21/10 6:01 pm

1 and 2 ... I hope that was right ...

StarGeezers · Post by **StarGeezers** » Wed 12/22/10 8:41 am

Played it with the band !!! Had to turn it down

Too LOUD at first , but sounded much more balanced now ... Good Bass, mids and highs , not too much of either... Really Happy with it now ...

Guess the next step is to make some faceplates to cover up the Swiss cheese Brown Note experimental chassis , which is , er, somewhat UGLY as it is now ...

hotair · Post by **hotair** » Wed 12/22/10 10:09 pm

Just a quick question from left field. Would there be much of an effect in using a 1M volume control on the input of one of the parallel input triodes? Is there enough of a difference with one biased hot and the other cold or hitting the input hard with one distorting and the other running more or less clean?

StarGeezers · Post by **StarGeezers** » Thu 12/23/10 6:56 am

Parallel input triodes ???

The only triodes on this amp is the 12AX7 PI, which should probably be treated equally ... The preamp tube is a Pentode , which gives it the Sweet Tone ...

hotair · Post by **hotair** » Thu 12/23/10 8:04 pm

See, that is what I get for not reading the whole thread.

the first page and assumed it was on the 18 Watt Lite.

StarGeezers · Post by **StarGeezers** » Fri 12/24/10 7:20 am

Well it's sorta' like a an 18 watt ...

That particular Pentode in the preamp gives it a Nice Sweet Tone ,and seemed less problematic than the EF86 (cheaper too)... This amp started out as the Jones Octal One , a Champ-like SE amp (that we're not allowed to talk about here , not being a Marshall and all

) .. The preamp of that married to the PI and PA of the Lite ... It was just an experiment but sounded So GOOD , we kept it ... that and it's LOUD ...

Been thinking of a 6L6 version with bigger iron , so it will be even LOUDER ..

(not that it needs it ... through my 4-12 cab this one easily overpowers the Band , and I start getting those dirty looks ...

)

Dustpuppy · Post by **Dustpuppy** » Mon 12/27/10 2:19 pm

Hey Geez, I just got my 42 watt OT to run this with a quad of 6V6's. I have high hopes for this one. I have to admit thought, so far it sounds best with a pair of EL34's.

Has anyone tried using those O-rings on the 6sj7 to help with the microphonics? I can't decide if it sounds reasonable or is just a gimmick.

Post by **zaphod_phil** » Mon 12/27/10 3:07 pm

In my experience, the O rings do help reduce microphonics. They won't necessarily cure a badly microphonic tube. You need to place the rings over where the mica spacers are inside the tube. Mounting the tube socket off the chassis, using rubber grommets above and below, can also be a good idea with pentode tubes such as 6SJ7s or EF86s. You could also try EF83s or 6BR7s, as they're supposed to have low microphonics.

StarGeezers · Post by **StarGeezers** » Mon 12/27/10 6:11 pm

Dust , you have to go with whats sounds best ...

Quad of 6V6s has to be LOUD eh ???

ODD, I've been using the 6SJ7 for a while and never had any microphonics ...

But I must confess , I CHEATED

and have been using the 5693 Red Specials , and I do use a thick rubber isolation pad between my amp-head and cabs all the time ... Don't know if that makes any difference ...

Now , being just a "builder" I always follow what ZP says !!! So next time . I'll try the rubber socket mounting .. just for the "ounce of prevention"...

Just rubber grommets and float the socket eh ? What about the bolt through the grommets ..??? Rubber tubing around that to isolate the bolt too .... maybe silicon tubing ???

StarGeezers · Post by **StarGeezers** » Sun 01/09/11 7:27 am

Update .... Loaned my Ab Min to our recording engineer , to play the violin through

... He really loved the sound of the Octal One amp , with the 6SJ7 up front ... just needed something LOUDER to gig with... Waiting for the report of how it went at his gig ...

Dustpuppy · Post by **Dustpuppy** » Thu 01/13/11 5:28 pm

I've been using my EL34 AbMin with the band for a couple of weeks. We had to get the bass player a bigger amp so I could turn it up past 3 o'clock. Getting very good reviews from those that have heard it.

I still haven't started the 4 6v6 version, I got side tracked by converting a 2204 into a Wreck Express. I think this thing is gonna scream!

Who originally started the AbMin project?

StarGeezers · Post by **StarGeezers** » Fri 01/14/11 12:28 pm

DP ... some Fool over at Wattkins ...

First there was the Jones Octal One SE amp which had a sweet TONE to it , then an arranged marriage between the O1 preamp and the 18 W power amp .. An attempt to dispel the myth that Pentodes were trouble in the preamp (like the problematic EF86)..
We've Never had any trouble ... GLAD you like the sound.. I do too !!!

ZP had his hands on this too, a fact for which we are eternally grateful for his guidance and expertise !!!

Post by **zaphod_phil** » Fri 01/14/11 2:47 pm

I did?

Better disinfect your amp then - but not while it's on.

While we're on the subject, here's a comment I just posted on another thread, about the name....

BTW I've never understood how an 18W with an octal pentode preamp gets called "Absolute Minimum". There's nothing absolute minimum about it - just basically an EF86 Lite with 6SJ7 instead of the EF86. The Minimalist 18W and the Lite III are more minimal than the AbMin. Can I request all concerned that we call it something less inaccurate, like 6SJ7 Lite?

Dustpuppy · Post by **Dustpuppy** » Sat 01/15/11 2:08 am

I have to be honest, since I've been using the AbMin preamp in all sorts of non minimum circuits, I started calling it the "Pentode 18 Watt" in my personal documentation...

StarGeezers · Post by **StarGeezers** » Sat 01/15/11 7:48 am

ZP, Yo!! I'm Cool with that !!! Call it anything you like!!! ... The name was derived , because it was the Absolute minimum amount of technical skill that your humble narrator had on hand at the time

... so the amp was a cut and paste job ... easy for me to understand and experiment with at the time ...and with your help reviewing the values , success was assured...

(and by the way Big Thanks for the assistance !!! )

Nothing original about it at all , more accurate name might be "'Dumb arse builds an amp " But I got Lucky , and it sounds/works Super ... for me ... so I thought I'd share the Happy experience ...

DP , Pentode 18Watt is Cool !!!

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