18Watt.com

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... (Run away, run away )

FYI the 6BR7 and the EF83 are also great 9-pin pentodes, which have low microphonics.

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.

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.

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 ... 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 ?

Did I mention these amps were built for < 100USD ???

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.

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.....

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 ...

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.

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...

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.

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/

NICE build DP !!!!!!!

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?

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

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

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.

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.

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 ...

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.

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

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