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

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.

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

Ok, I finished the amp tonight. At first it was extremely clean and chimey at all volumes and the gain had no effect. I rechecked the wiring and found it was incorrect. I fixed that and it was louder with a little breakup with the gain. I had a 330uF voltage divider after the first 6c4 so I removed that and gave it a 2k cathode resistor. This thing rocks now. It is very chimey with the 5654 but lacks some of the 18 watt punch with the 6c4 PI. I'd like to find a hi MU 7 pin to put in there. the tone has a little bit of a muffled sound like the mids are a little low. I'll continue to tweak. All in all I'd call this a success. Definitely a different amp from the 18 pentode.

Here are some crappy iphone pics of the finished product.

http://s924.photobucket.com/albums/ad89 ... 20pentode/

StarGeezers · Post by **StarGeezers** » Tue 01/25/11 7:54 am

NICE build DP !!!!!!!

insanecopilot · Post by **insanecopilot** » Tue 01/25/11 12:19 pm

Nice job dp! I have a couple of 6c4's laying aroud that i wondered about, but i thought they were like 1/2 of a 12au7. Do you have a schematic?

Dustpuppy · Post by **Dustpuppy** » Tue 01/25/11 1:21 pm

Here you go bud, I'm pretty sure this is the as built.

http://i924.photobucket.com/albums/ad89 ... e/7pin.png

insanecopilot · Post by **insanecopilot** » Tue 01/25/11 8:59 pm

It's a very interesting design. Very original

I would like to hear it after you get it tweaked. The 18watt guru's might want you to move it to Watkins or something though. I have two Hammond ao29's i was planning to get to soon that have a couple of these 6c4's in them. That's why your design piqued my curiosity. good job though.

Post by **zaphod_phil** » Tue 01/25/11 10:16 pm

Yes, looking at that particular schematic, almost everything appears to have been changed from the 18W design. So maybe our companion site ppwatt.com is where it will be most at home.

StarGeezers · Post by **StarGeezers** » Wed 01/26/11 9:54 am

That is er, ... different ... I think it should have a proper home of it's own too ... but since Wattkins is currently discussing the 6AQ5 , it would be of Some interest in the Tone per Buck Club thread

... and much appreciated , unlike .... well , lets not get into that ...

dotfret · Post by **dotfret** » Mon 01/31/11 10:50 am

insanecopilot wrote:I have a couple of 6c4's laying aroud that i wondered about, but i thought they were like 1/2 of a 12au7.

I think 1/2 of an ecc804 is a more accurate description.

StarGeezers · Post by **StarGeezers** » Thu 02/03/11 6:49 am

Dot , always on the lookout for good usable tubes to make amps out of ...

StarGeezers · Post by **StarGeezers** » Mon 02/07/11 5:57 am

Just FYI, we found out the Russian 6Z8 is a replacement for the 6SJ7 ... just in case you have trouble finding the 6SJ7 ...

Dustpuppy · Post by **Dustpuppy** » Mon 02/07/11 9:44 am

I've seen the 6x8 and 6j8 advertised as 6sj7 equivalents. Anyone have any experience with them?

StarGeezers · Post by **StarGeezers** » Tue 02/08/11 8:36 am

DP, I'm going to order some and check them out !!! Cheap enough eh !!!

Dustpuppy · Post by **Dustpuppy** » Tue 02/08/11 1:31 pm

Cool, can't wait to see a full report on them!

insanecopilot · Post by **insanecopilot** » Tue 02/08/11 10:39 pm

Don't know about those but i have tried the 6sh7 and 6sg7 which i got at a surplus for cheap. the 6sh7 is brighter with slightly more gain than the 6sj7 without rewiring anything.

StarGeezers · Post by **StarGeezers** » Wed 02/09/11 7:03 am

Insane , Thanks ... yet another tube to try ...

Thinking now about putting a 6N2P in front of the 6SJ7 for even more gain ...

This could get Crazy ....

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