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

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

Moderators: zaphod_phil, Daviedawg, Graydon, CurtissRobin, colossal

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zaphod_phil
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Post by zaphod_phil »

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... 8O (Run away, run away )
FYI the 6BR7 and the EF83 are also great 9-pin pentodes, which have low microphonics.
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Dustpuppy
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Post by Dustpuppy »

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. :blush:
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Post by zaphod_phil »

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.
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Post by StarGeezers »

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 ... :wink: 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 ? :roll:

Did I mention these amps were built for < 100USD ??? 8O
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Post by Dustpuppy »

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.
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Post by katopan »

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.....
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Post by StarGeezers »

DP, go for it Dude !!!! :wink:

Kato , Might give that a try ... the 6U8 does work and sound rather Nice as already proven in many amps ... :thumbsup:
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Post by Dustpuppy »

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.
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Post by StarGeezers »

Now here's an interesting idea ... having a triode in front of the 6SJ7 with a TMB between the two ... 8O (Not Absolutely Minimum , but I guess we're not bothered with that any more :roll: )

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 !!!!!!! 8O


DP, anxious to see the pic... :wink:
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Post by Dustpuppy »

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.
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Post by Dustpuppy »

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/
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Post by StarGeezers »

NICE build DP !!!!!!! :thumbsup: 8)
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insanecopilot
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Post by insanecopilot »

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?
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Post by Dustpuppy »

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

http://i924.photobucket.com/albums/ad89 ... e/7pin.png
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Post by insanecopilot »

It's a very interesting design. Very original :thumbsup:

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.
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zaphod_phil
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Post by zaphod_phil »

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. :)
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Post by StarGeezers »

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 8) ... and much appreciated , unlike .... well , lets not get into that ... :roll: :lol: :lol: :lol:
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Post by dotfret »

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.
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Post by StarGeezers »

Dot , always on the lookout for good usable tubes to make amps out of ... :wink:
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Post by StarGeezers »

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