Suggestions for two extra triodes?

[dwinstonwood]

So, I've been thinking about this AC15 project I build a while back:
https://el34world.com/Forum/index.php?topic=26947.0

And, there's room on the chassis to add another 12AX7. I'm not really interested in tremolo or reverb. What would you use two extra triodes for? An extra paralleled gain stage, etc?

Thanks for any comments. Here're the schematic and layout:

[sluckey]

Look at my Dual Lite amp.

[SILVERGUN]

Maybe a second pentode?

[dwinstonwood]

Thanks SILVERGUN, how would that work exactly?

sluckey, I think that might be the way to go. With my one input is there a way to join the two preamps before my PI and use a 3-way switch?

Thanks.

[SILVERGUN]

Thanks SILVERGUN, how would that work exactly?

Like any other cascaded gain stage. You could switch it in and out with a DPDT.

[SILVERGUN]

I like sluckey's suggestion better.
His is a proven design, I was just spitballin'.

[dwinstonwood]

Like any other cascaded gain stage. You could switch it in and out with a DPDT.

Thanks. I guess two EF86's would have a combined gain factor a lot higher than a 12AX7. 
In a small 6G2 combo cabinet that could spell EF86 trouble.

[sluckey]

With my one input is there a way to join the two preamps before my PI and use a 3-way switch?

I would mix channels like this. If you want a single input and switch to select either channel or both channels look at my Phoenix schematic.

[PRR]

....I guess two EF86's would have a combined gain factor a lot higher than a 12AX7....

Single pentode, gain of 100, maybe 200 with extremely light loading.

Both sections of 12AX7, gain of 50 each, 50*50 is 2,500. Over ten times more.

Why do you want gain? You want distortion. Gain is not the only path.

[SILVERGUN]

Why do you want gain? You want distortion. Gain is not the only path.

But it is a cool one. It helps make feedback.

I was going for the rare and elusive TONE. So, not really worried about milking max. gain. I would've trudged through the idea that just because the new tube will sit in the middle of the other two physically, it doesn't mean your signal path has to. I backed out quickly knowing that I couldn't back it up with design, it would have to be trial and error for me and I'm not setup for that experiment at this time.

Certainly, you have all heard about that one amp that one guy had that has a cascading pentode preamp and it was "the absolute best TONE he had ever heard."    He sold it and wished he never did. C'mon, I know you know the one.

[tubenit]

Certainly, you have all heard about that one amp that one guy had that has a cascading pentode preamp and it was "the absolute best TONE he had ever heard."    He sold it and wished he never did. C'mon, I know you know the one.

Wasn't me, but I did experiment and build this HoSo56-OD at one point. Kept it like that for a few weeks or a few months or something like that and then completley changed it.  There was not much contrast between the clean and OD.  And the OD did not have the sustain that some of my other amps have had. It did have a very smooth OD tone though.

With respect, Tubenit

[SILVERGUN]

Wasn't me, but I did experiment

This is the most important thing that Tubenit has taught me. Just experiment. Not all experiments make sense but they all teach you something.

I still like sluckey's idea because it will be easy to implement and it makes sense without requiring experimentation.
Plus, with your limited space you need limited scope.

[pdf64]

Maybe use one of the triodes as a CF or DCCF to remove the loading from the pentode.

[HotBluePlates]

... AC15 project ... there's room on the chassis to add another 12AX7. ... What would you use ...?

Maybe a second pentode?

... Why do you want gain? You want distortion. Gain is not the only path.

Look hard at the AC10 schematic hosted here.  An AC10 is very much like the AC15, but without the Vibrato circuit (and exactly how the AC15 was before it got that circuit).

The AC10 has trem, which you can turn off with the footswitch.  Turn off the trem but keep the Amplitude control high, and the EF86 distorts more readily.  IMO, it's key to getting the best sounds from the AC10.

Personally, I would add that Amplitude pot, even if not the whole trem circuit.  At the end of the day, it reduces the screen voltage on the EF86, and makes that tube quicker to distort.  You might want to give it a listen.

[dwinstonwood]

Hey, thanks everyone for all the great suggestions! I need to think through them for a bit. I like sluckey's idea of two distinct preamps.

This amp was originally conceived as a hybrid of the AC15 EF86 channel and the PI of the AA964 Princeton, which worked out really well (I added a 470K grid stopper to the PI recommended by Merlin to address Cathodyne blocking distortion).

Maybe I could simply add the AA964 preamp/tone stack with a switch and I'd have a Princeton with cathode biased EL84's as well as an AC15, and maybe both at the same time.

With a separate volume control for each preamp I might get some interesting sounds mixing them together.

https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_aa964_schematic.pdf

[tubeswell]

FX loop ideas

[SILVERGUN]

This was a "happy accident design" that I did on a 5E3.
The Dude was very happy with the addition of the 2nd option.

I'm posting it so you can see the simplified switching for adding a cascaded stage with a CF. (just food for thought)

[dwinstonwood]

Thanks tubeswell and SILVERGUN! There are so many ways to go with this, I still haven't decided.

But, here's a new schematic grafting together the AC15 with the AA964 using sluckey's Phoenix preamp switch. There are more than likely errors.
 

[dwinstonwood]

Thanks again for all the ideas and suggestions everyone. But, in the end I'm just going to convert the cathodyne PI to a LTP like the original. This will get rid of the extra gain stage after the tone selector. The gain from that extra 12AX7 triode may have been why I wanted to change things in the first place. I know we usually want more gain, but I might have been losing some of the EF86 sound underneath that extra triode. At any rate, it will be "closer" to an actual AC15. And, it won't cost me a penny to fix.

[SILVERGUN]

Well, that sounds like a perfectly reasonable idea.

Let us know what you'll want to try next. 

[dwinstonwood]

Let us know what you'll want to try next. 

Maybe a few different speakers? And then there's the screen, plate and cathode resistor rabbit hole for the EF86. 

Really, I'm just looking for ways to keep the soldering iron active. I did finally replace the 8 Ohm OT in my 6V6 Plexi build with a 4/8/16 secondary version and selector switch.

[dwinstonwood]

I merged my drawings into one PDF:

[Edited to fix errors. Disregard previous version.]

This is the fixed version:

[dwinstonwood]

I thought I could do these mods without buying any parts, but I ended up ordering some stuff to do it right.

I've been playing around with PSUD2 again. I should have pretty low ripple at the first B+ node.

[dwinstonwood]

I've finally soldered in all of the parts for this project. I need to go over everything to catch any errors, and then I'll fire it up.
I've attached my latest files. The layout shows the under-board connections.

[edited to update PDF]

[sluckey]

Nice work and pretty layout. I like your style. And I really hate to say this, but your tone switch layout is not right.

[dwinstonwood]

Thanks Steve! I think I enjoy making the drawings almost as much as the actual build. 

Ah! Yep, I need to have one leg of each capacitor connected to the input side of the switch. Dang!  I'll fix that.
I had it right with the original layout as shown in this image.
The actual switch in the amp is wired correctly. I've taken capacitance readings and it's working.

[bmccowan]

Nice! I built a similar AC-15 lite in a Hammond AO-39 chassis following Sluckey's documentation. Then, since I can;t leave well enough alone, I rewired it for a 5879 in V1 and 6973s for the power tubes. Both were good. I think Dirty Girl amps used to make a 5879 > 6973 amp before he closed shop and moved to building custom black powder rifles. I forget his name, but his work look great.

[sluckey]

Thanks Steve! I think I enjoy making the drawings almost as much as the actual build. 

Me too! I like the whole process, but I always spend more time in Visio than I do at the workbench.

[dwinstonwood]

Thanks bmccowan! For the next amp I build, I want to use some different tube types. I know you like the 6BM8's. Then, there's the sluckey L'il Smoky with a 6AQ5 and a 5654. I have a couple of matched pairs of GE JAN 5654W's I bought for a hifi tube buffer/preamp. It would be cool to build a 7-pin amp. Maybe a 7-pin Musing 40...

Steve, I definitely spend more time in DIYLC than I do soldering. 

[bmccowan]

Hear ya - this hobby tends to grab you and send you off exploring. One of my current curiosities is the 6CY7 tube. Good for guitar? http://diyaudioprojects.com/Tubes/6CY7-SET-Amplifier/  The builder of this project raves about this tube for low power audio.

[PRR]

As a TV tube, 6CY7 is good for hi-fi. It give lowish THD, good speaker damping, and it wide-bands the OT. Very low gain.

But hey. It may float your ice-fishing hut.

[bmccowan]

Thanks PRR - I will save those for a hi-fi project.

[dwinstonwood]

This build was a winding journey that ended up pretty close to how it started, a single-channel AC15 using sluckey's documents. But, it was a lot of fun, and the 6G2 chassis and cabinet worked great. I consider it done. Thanks for all the input and help!

Here are the drawings I finally ended up with. A very basic amp with a wide tone range.