AX84 SEL KT88 build

[Torquil]

Do you find that the rhythm/lead switch is noisy or not? I mean, do you hear a noise from the speaker when flipping the switch?

I'm thinking about a similar feature in one of my amps, hence my interest.

Thanks!

[xtian]

Nope. It does not pop.

[Torquil]

Nope. It does not pop.

Thanks xtian, that's very helpful! My amp has a cathode follower in front of the PI stage. I'm going to switch between two separate preamp channels. This means that it might be worth a try to keep the CF common to both channels, and have the switch just in front of it.

I theorized that it might cause a sudden change in CF grid voltage (and a pop) since the plate voltages of the stages preceeding the CF would be different.

Anyway, great job on our amp! I have built an AX84 kit that turned out very nice, although it is now somewhat modified, e.g. with hotter preamp tube bias.

[xtian]

The SEL had too much gain at first--I had to reduce preamp gain!

Which AX84 did you build?

[xtian]

Follow up: Recorded a bit with a drummer. This is my Viking Deluxe played thru my SEL thru a 2x12 cab with Eminence speakers, with an EV N/D468 on the cone.

[xtian]

Added Hammond cage and black bottom cover.

[breakfastbuddy]

you said
The SEL had too much gain at first--I had to reduce preamp gain!
after this sound clip i think you have reduced to much

[xtian]

you said
The SEL had too much gain at first--I had to reduce preamp gain!
after this sound clip i think you have reduced to much

In the sound clip with drums I dialed in a nice classic rock vibe. There's still plenty of gain left on the dial!

[ChrisM]

What OT and PT are you using? Specs?

Thanks...

[xtian]

What OT and PT are you using? Specs?

As spec'd on schematic: Hammond 270EX and 125ESE.

[bigbeck]

It looks like you only have 4 power supply filter caps. Thats not enough for an SE amp with 2 preamp tubes. It looks like you're missing a 100 to 200 ohm resistor and a 47uf cap following the diodes.

If you eliminate that filter node, you will get some hum. I know this first hand because I tried it. SE amps need more filtering than push pull.

Another fairly common hum problem in SE is not separating the preamp and power amp grounds. Preamp grounds should be connected directly to the common grounding point. They should not be connected to the power amp grounds.

In your bank of filter caps you have 2 caps that are preamp caps,right? The ground terminals of those two caps should not be connected to the power tube caps - it should go directly to the star ground. Just that one simple mistake can cause, Same with the power tube cathode cap - don't connect any preamp grounds to it.

[xtian]

Hey, bigbeck. I appreciate your careful look at my SEL!

First, there are only four 47uF caps in the power supply of the AX84 schematic, so I don't understand when you say, "Thats not enough for an SE amp with 2 preamp tubes." I'm using a proven schematic, so I didn't consider adding additional filtering.

However, your point about star grounding is well taken. There are several black wires running to the negative side of the tag strip that you cannot see. The negative ends of the power capacitors are not all tied together. Furthermore, the photo I posted earlier is wrong, because the brown wire, which leads to the power tube, was incorrectly connected to B+ rather than B+1.

So I opened her up to have a close look. It turns out my grounding scheme could be improved--I have the negative ends of the first cap grounded separately to the star ground, and the next THREE caps tied together and then jumped to the star ground.

I'm going to resolder this stuff, and I'll let you know if it helps the hum!

[xtian]

I fixed the power section grounding scheme, and I think it improved the hum a small amount. I can still hear it, though. Probably an additional filter cap would help, but it's not worth the bother. New photos attached. Thanks for commenting!

[toadpipe]

Hey folks, sorry if this is a non-sequitur but I just hours ago finished my very first AX84-SEL, and I used xtian's layout.

I LOVE this layout, having the power supply and the tone circuit separate from the main board is wonderful.

I have one main problem though. My Rhythm/Lead switch is only giving me rhythm, no lead. The center of the switch is connected to R12 with R12 being no longer connected to the junction between R14 and pin 6 of V3. The Lead end is connected at the junction of R14 and pin 6 of R14 and the the Rhythm is connected to the junction of R18, C11 and pin 1 of V4.

Other than that, I went off the reservation with my tone circuit, and it sucks. Great part is, I can just drop in a new one. On that last bit, as the great Blind Willie Johnson said, "nobody's fault but mine."


Again, much thanks xtian! I love this layout!

[sepulchre]

Your connections all sound right. That's how I did it on one of mine. On the other the connection points are the same but they go to a relay instead of a switch so I can use a footswitch - works great.

You might try taking the switch out of the circuit and reattach R12 to R14/Pin6 to make sure that stage is working. (I'd also disconnect from R18/C11/P1 just to be safe.) If it works okay like that then check out the switch itself. If not then you'll have to see what's going on with V3b.