18Watt.com

Recent changes to the Ab Min , include a .0047uf coupling cap , a 10-20uf cathode cap on the 6SJ7( bigger cap more gain ) , and a .015uf cap on the 6SJ7 screen .. All producing a much tighter Bass and a generous amount of “sparkle” ... Getting it tuned in to best fit playing with the band ...

Very happy with the Tone now, more chimey' and jangly' ...


Ps. After a lengthy time we discovered we had the Wrong Cathode cap on the 6SJ7 22uf , instead of the 2.2uf ... with the 22uf it was a fire breather... with the 2.2 it’s pretty tame ... (Clean) Choose what you like ...

Hi SG. Been a long time. How does the .015 on the 6sj7 screen change the tone compared to the .047? My 6sj7 6v6 lite is a little dark sounding compared to the 6u8 one. But it just sits these days as there are others i play. Might have to go back in.

Hmmmm, I personally like the somewhat darker tone of a 6SJ7. I've never liked amps that are too toppy, and IMO any amp with EL84s in the power stage is already a bit on the bright side. "fire breather" also sounds like a great idea to me!

ZP, I much appreciate your help getting the PI in this amp sorted ... The amp as originally built was a bit too dark for my needs (leads) , with the 6V6s .. and the 22uf cathode bypass cap that was mistakenly installed on the 6SJ7 (instead of the 2.2uf on the schematic ), gave it a shed load of Gain .. on 3 Useable and quite good sounding over time , but I just couldn’t help keep tweaking on it to get it where it sounded just Right with the band .. It’s not overly Bright , like my Dog Sled, but now has enough sparkle to get right in the sonic territory I want to be with the band (used it at practice last night ... perfect !!! ) And that nice pentode chime to make the sound , for lack of a better word ... Pretty to the ear !!!
Insane , I think the screen cap change was most responsible for the extra bit of sparkle , but I can’t say for sure , because I changed the coupling cap at the same time .. My Dumb !!! But Hey , it works Great !!!

Well, that was some time ago ... More time playing with the amp still leaves me with the desire for even more brightness , born of the newer harder Rock selections we’ve been asked to play ...
Just wondering if the 6SJ7 is inherently “dark” ??? It does really work Fine for our “Oldies” gigs , and some Jazz selections , but lacks the “cut” for some of the more “aggressive" music ...

You guys have any idea what would happen manipulating the values on the PI caps ??? There seems to be something blocking the “brightness” ... can’t be in the 6SJ7 after all those changes ... Schmaybe' ??? I dunno'

I think I posted this over at wattkins but, I changed the grid stoppers to 1m and the bias to 470r with a 10k tail and this amp really came alive. Before all I was getting to the power tubes was pi distortion but now, the pentode sparkle is really coming through. Nice tight bass and shimmering high end. I'm really getting into these amps again.

Guys, anyone tried 6au6 (EF94) or 6au6A pentode in pre. ?

I was just recently given an old reel to reel tape player that has two old Aussie made AWA brand 6AU6s and two 6AQ5s. First thought I had was wondering what the 6AU6s would be like in my 18W in place of the EF86 I curently have in my pentode channel.....

Dustpuppy wrote:I think I posted this over at wattkins but, I changed the grid stoppers to 1m and the bias to 470r with a 10k tail and this amp really came alive. Before all I was getting to the power tubes was pi distortion but now, the pentode sparkle is really coming through. Nice tight bass and shimmering high end. I'm really getting into these amps again.

I think there really is something to the PI grid resistors value in this situation. (I think this what DP is referencing here) I changed mine from 470K to 1M and I do think it helped "open up" the tone. My theory is that the output impedance of the pentode is so high that tone gets strangled by the input impedance of the PI that is nearly equal if using 470K grid resistors.

katopan, in looking over the datasheets, they look like they have potential.
To date my favorite sharp cutoff pentode has been the 5654 7-pin tube.

katopan wrote:I was just recently given an old reel to reel tape player that has two old Aussie made AWA brand 6AU6s and two 6AQ5s. First thought I had was wondering what the 6AU6s would be like in my 18W in place of the EF86 I curently have in my pentode channel.....

I had it in the tremolo in one of my amps, one of the best tremolo sounds so far ( Cooder flavour) but that's of course different use. But I red on the net somewhere by some ampbuilder mentioning great usage of tweaked 6au6 instead of ef86 ... could be a ticket.

So just change the grid Rs , the tail R , and the Bias R ... Sounds like a Good plan !!! Have to try it ... THANKS !!!!

Just FYI, Interesting discussion going on at Wattkins about the 6SJ7/5693 and the Ab Min .. manipulation of the component values , cause and effect ... some Math involved... http://www.wattkins.com/node/18147

I know I was on about a lack of brightness , sparkle and chime , so I made a couple cap changes ...Tweed TC 500pf silver mica to a 500pf ceramic ... sorta backwards eh ? And the cap on the PI tail was a cheapo film cap , changed that , and now it’s Very Bright and Sparkly ...
Seems my poor parts selection was choking the highs and not the design ... Now it’s totally gone from a dark smoky Jazz or rhythm amp to a Voxy sounding screamer ... Tweed TC and guitar’s TC manages this , but it can get BRIGHT ... and LOUD ...

I may have gone too far ???

Extensive testing With the Band , has indicated the last changes were Not too bright at all , and now I have not only the Pentode sweet Tone , but the “cut” I needed to be heard over the 6 other players.... a little bright for “bedroom” playing , Perfect for gig use ... Been a couple weeks now since the last mods , others have played on it too and Like it ... so it will stay this way for a while ... Happy at last .... for now ...

SG, I'm glad you found a sound you like!
I've been having a lot of fun playing through a hybrid of the abmin with half a 12ax7 in front of a 5654 pentode and 6aq5 power tubes.

DP , after months of testing/playing , we’re still Very Happy with the amp ...
Something NICE about the 6SJ7 /5693...
Can you tell us More about your amp .... Please ....

Does it have much resemblance to a Marshall 18W?

Having never owned a Marshall 18 watt , I can only say .... Maybe ???? The extensive changes have made it much Brighter/ tighter than the original amp presented ...
We’ve had other Higher powered Marshalls , and they sounded Good , but don’t /didn’t have the “sweetness” of Tone (Cleans ) that this one has ... JMHO
We recently had a Marshall 30watt amp (EL84s) in for repair ( overheating and rectifier block dropped out ) and once fixed , it sounded really nice , especially overdriven ... can’t say as we compared it though .... just glad to get it done and out of the shop , cause’ it was a PIA to work on and it bit me twice, despite the usual cautions ... grrrrr!!!

I did compare it to my new 5E3 “Special , which is very Fender’y (naturally) , and it’s sorta similar in tone , but again , the Cleans are sweeter with the 6SJ7 ... Just something Nice about that tube ... Playing distorted (more so lately) both amps sound really Good ...

Just as a point of interest , last week we had avery OLD Princeton (Brownie ) come in for repairs , and that one had a really SWEET clean sound ( and Great OD tone too) ... for some reason which I’ve yet to figure out ...

ZP, we really appreciate all the help getting this sorted ... THANKS !!!

Hoping to get more info soon on DPs amp with the tube in front of the pentode ... Thinking that should be really Nice , with the added gain ...