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

StarGeezers · Post by **StarGeezers** » Mon 02/06/12 8:40 am

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'

Dustpuppy · Post by **Dustpuppy** » Tue 02/07/12 11:52 am

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.

leslie · Post by **leslie** » Tue 02/07/12 2:21 pm

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

katopan · Post by **katopan** » Tue 02/07/12 4:34 pm

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.....

GT350 · Post by **GT350** » Tue 02/07/12 10:33 pm

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.

Dustpuppy · Post by **Dustpuppy** » Wed 02/08/12 12:53 pm

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.

leslie · Post by **leslie** » Wed 02/08/12 1:25 pm

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.

StarGeezers · Post by **StarGeezers** » Thu 02/09/12 8:12 am

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

StarGeezers · Post by **StarGeezers** » Fri 02/17/12 8:21 am

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

StarGeezers · Post by **StarGeezers** » Sun 04/29/12 3:28 pm

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 ???

StarGeezers · Post by **StarGeezers** » Mon 05/14/12 5:56 am

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 ...

Dustpuppy · Post by **Dustpuppy** » Wed 07/25/12 10:24 pm

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.

StarGeezers · Post by **StarGeezers** » Tue 11/20/12 7:28 am

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 ....

Post by **zaphod_phil** » Wed 11/21/12 12:25 am

Does it have much resemblance to a Marshall 18W?

StarGeezers · Post by **StarGeezers** » Wed 11/21/12 6:49 am

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 ...

brewdude · Post by **brewdude** » Sun 11/25/12 9:46 pm

SG, I have been working on an amp (not an 18 watt) with a paralleled 12AX7 (LED biased) into a 5693 (set up a little different than the ab.min./01 pentode) into a paralleled 12AU7 Cathodyne PI feeding a pair of 6V6's. It also has a more complicated power supply with a choke right after the reservoir cap and a VVR controlling the PI and PA.

I realize this amp does not resemble an 18 watt, but as far as pushing the the 5693 with a preceding gain stage, I think it works pretty well. I have the 12AX7 paralleled with a shared 100k plate load and a shared red LED bypassed with a .1uF greenie providing 1.6 volts on the cathode. The supply voltage varies somewhat dependent upon VVR settings. I have played some with tone controls, but currently I am only using a switch to add a .001uF cap in series with the shared .01uF coupling cap. I have a 1M gain pot with a 120pF bright cap, no grid stopper at the 5693. There are no potential dividers other than the gain pot.

I can run the gain pot up full blast at any setting of the VVR or 1M drive pot between the 5693 and the parallel 12AU7 cathodyne PI. No squeals or oscillations and the 5693 can take all the gain the 1st stage has to offer and still sounds great and musical--at least to my ear. In fact, it doesn't really ever get too saturated.

StarGeezers · Post by **StarGeezers** » Tue 11/27/12 10:37 am

Wow Brew, That Must sound really NICE !!!

Guess in the end this is just another 18 watt amp with a Pentode gain stage ... which seems to work out very nicely .... Fender used them , Vox too, I dunno’ if Marshall ever did ? .... Curious ??? Wonder if there are any old Marshalls around with a pentode front end ???

Post by **zaphod_phil** » Fri 12/07/12 5:35 pm

I can't think of any authentic Marshall with a pentode gain stage. But a pentode front end sounds fantastic with a regular Marshall 18W type amp.

I realize this amp does not resemble an 18 watt, but as far as pushing the the 5693 with a preceding gain stage, I think it works pretty well. I have the 12AX7 paralleled with a shared 100k plate load and a shared red LED bypassed with a .1uF greenie providing 1.6 volts on the cathode. The supply voltage varies somewhat dependent upon VVR settings. I have played some with tone controls, but currently I am only using a switch to add a .001uF cap in series with the shared .01uF coupling cap. I have a 1M gain pot with a 120pF bright cap, no grid stopper at the 5693. There are no potential dividers other than the gain pot.

Then it's high time you built an EF86 Lite or 5693 Lite 18W!!! And let's please keep the discussions on-topic for this site.

brewdude · Post by **brewdude** » Sat 12/08/12 1:43 pm

zaphod_phil wrote:I can't think of any authentic Marshall with a pentode gain stage. But a pentode front end sounds fantastic with a regular Marshall 18W type amp.

I realize this amp does not resemble an 18 watt, but as far as pushing the the 5693 with a preceding gain stage, I think it works pretty well. I have the 12AX7 paralleled with a shared 100k plate load and a shared red LED bypassed with a .1uF greenie providing 1.6 volts on the cathode. The supply voltage varies somewhat dependent upon VVR settings. I have played some with tone controls, but currently I am only using a switch to add a .001uF cap in series with the shared .01uF coupling cap. I have a 1M gain pot with a 120pF bright cap, no grid stopper at the 5693. There are no potential dividers other than the gain pot.
Then it's high time you built an EF86 Lite or 5693 Lite 18W!!! And let's please keep the discussions on-topic for this site.

ZP, that is a pretty reasonable suggestion. I was only trying to add my observations with a triode preceding a pentode. I felt that I should qualify my experience with some design particulars.

StarGeezers · Post by **StarGeezers** » Wed 12/19/12 9:26 am

For those wanting to further discuss this rather “not exactly a Marshall amp “

... Please be so good as to join us here ... http://www.wattkins.com/node/17874"
Don’t want to clutter the newly refreshed 18W site up with any non Marshall 18 W babble ...

So long , and thanks for all the fish .....

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

Re: 6SJ7 Pentode 18 watt Lite...Formerly the ABSOLUTE Minimu