AX84 SEL KT88 build

[xtian]

I'm building an AX84 SEL (singled ended lead) using a KT88 power tube. I'm implementing the Rhythm/Lead switch (bypasses 3rd stage preamp for Rhythm) and the Roy Bean Judge/Bear ("rainbow") tone stack, and probably variable cathode bias. Modified schematic is attached.

I have completed a drawing of the mechanical layout, attached. I'm using a 12x10x2 aluminum Hammond box. I try to keep the signal path and wiring short, transformers perpendicular, PT away from input. I try also to please my eye, because this build will remain naked (no cabinet)--so, for example, the big ol' KT88 is centered on width, and PT is centered on depth.

Please take a look and warn me of any obvious blunders or harmful artistic choices. Thanks!

[passfan]

What do the hounds think over at AX84 ? Those guys over there do their layouts to minimize noise. To change it is to invite noise. You'll know when you're done. That's a smaller layout on the turretboard which means you'll be crowding it a bit. I built a SEL with a 10 " board and had noise issues, yours looks to be around 8 or less, good luck.

[John_P_WI]

I don't think I would switch the signal with B+ on it that way to add subtract a gain stage.

Just switch at the grid of the 4rd tube stage. Take your green line from the grid of the 3rd stage, arond the tube as drawn. Then select either the output of the 3rd stage OR the green bypass around the 3rd stage. Of course the 3rd stage would have a dc blocking cap. / attenuation before the switch, either selecting the bypassed 3rd stage or the 3rd stage output. No B+ switching.

John

[passfan]

I don't think I would switch the signal with B+ on it that way to add subtract a gain stage.

Just switch at the grid of the 4rd tube stage. Take your green line from the grid of the 3rd stage, arond the tube as drawn. Then select either the output of the 3rd stage OR the green bypass around the 3rd stage. Of course the 3rd stage would have a dc blocking cap. / attenuation before the switch, either selecting the bypassed 3rd stage or the 3rd stage output. No B+ switching.

John

I think he meant to take it after C11 and not before. It seems I remember that mod over at AX84 and it works well

[xtian]

I think he meant to take it after C11 and not before. It seems I remember that mod over at AX84 and it works well

This is from the SEL Options document (see image). It clearly shows taking the bypass route before the coupling cap. This makes sense if you observe how the cathode follower receives it's input from preamp stage 3 (no coupling cap). If you remove preamp stage 3 from the circuit, shouldn't the cathode follower get it's input from stage 2 in the same manner (that is, no coupling cap)?

On the other end of the switch, I can't understand why the author does not show resistors R12 and R13 and diode D3. Did he mean I should abandon them? This doesn't make sense. EDIT: Now I see that the option was written for the P1 amp, not the SEL, and the SEL is supposed to have those additional components.

[John_P_WI]

Xtian,

My bad, In my hurry I did not notice you were feeding a buffer. Sorry. That being said, make sure you use a high quality switch to switch the DC.

John

[passfan]

It's been a while since I wired this mod and I'm too lazy to search over there It just looked funny the way you drew it. The SEL circuit is a screamer and you'll like it. You can get it to go chimey by maxing out your gains and your MV and rolling down on your guitar. Hope it works out for you.

[KellyBass]

Looks great! I've thought about building one of these...It's on my list of things to build.

I've got the perfect edcor PT for this, as well as a 20watt doberman single ended OT. I like your idea for the gain stage switch. It would be real cool if this was on the floor!

Let us know how it's coming along!

[xtian]

Let us know how it's coming along!

Will do. Last parts order arriving today from Hoffman. Will be building this weekend.

[jhaas]

I just powered up one of these SEL's for the first time a couple nights ago. I haven't spent a lot of time with it yet.

The reason the Lead/Rhythm schematic switch doesn't include the resistor and diode is because an older version of the SEL schematic didn't have those as part of the design, and when the schematic was updated, the "option sheet" wasn't updated to reflect it.

I had the L/R switch wired in, but during debugging I was having issues and wired around the switch to "get back to basics" until the issue was resolve. It turned out to be a bad solder joint that was leaving my MV and Gain pots ungrounded. Squeal City! Now that that is fixed, I hope to get the L/R switch back in the circuit tonight.

My layout doesn't conform to the posted one, only because I was converting another build. That other build was also a big-bottle SE (EL34 in my case), so I only had to rip out the preamp, and make the SEL preamp fit those turrets. I'm also using a choke to feed the plates, which is different that the normal SEL power supply, and a relay to control the Lead/Rhythm switch.

[jhaas]

I've got the perfect edcor PT for this, as well as a 20watt doberman single ended OT. I like your idea for the gain stage switch. It would be real cool if this was on the floor!

Let us know how it's coming along!

I have an Edcor XPWR106 (275-0-275, 125mA) PT in mine, with an AllenAmps TO11C OT.

[SteveXKR]

In the original stack drawing ( http://www.freewebs.com/balijukka/roybeanstack.htm ) near the top left I see an 'R3' with a 36K value I don't see in your implementation? Not sure if that was left out intentional but figured I'd mention it

[xtian]

In the original stack drawing ( http://www.freewebs.com/balijukka/roybeanstack.htm ) near the top left I see an 'R3' with a 36K value I don't see in your implementation? Not sure if that was left out intentional but figured I'd mention it

Yes, that's to limit the input signal. I don't need that in my circuit. Thanks for taking a close look!!

[xtian]

Today I built the SEL circuit and the Roy Bean tone stack. I love this part!

I made one mistake. Resistor leads are too short to span the whole board! So I had to lengthen them in some places. You'll see my creative solutions in the photos.

[img683]http://www.monkeymatic.com/pub/sel/sel0.jpg[/img]
[img683]http://www.monkeymatic.com/pub/sel/sel1.jpg[/img]
[img683]http://www.monkeymatic.com/pub/sel/sel2.jpg[/img]
[img683]http://www.monkeymatic.com/pub/sel/sel3.jpg[/img]

Now I see why it's called a tone "stack"--those components are stacked up! Roy Bean tone stack sub-board:

[img682]http://www.monkeymatic.com/pub/sel/sel4.jpg[/img]
[img::]http://www.monkeymatic.com/pub/sel/sel5.jpg[/img]

[moonbirdmusic]

Really like the layout you are trying -- hope it works well!