AX84 SEL KT88 build

[xtian]

Solved! I had connected OT primary to B+ instead of B+1. Much better! Still a little hum, but not much louder than the hum the actual PT is making from mechanical noise.

*big sigh of relief*

[xtian]

Now I need to reduce the gain in preamp stage 3. This is the stage that's bypassed by the Rhythm/Lead switch. I think there are two options. A) limit the volume entering the valve at the grid (pin 7), or B) reduce the gain of the valve itself by reducing the voltage differential between the grid and the cathode.

For option A, I'd just shunt more of the signal to ground by reducing the resistance at R17. Right? But I imagine this isn't optimal because I'm reducing my signal to noise ratio.

For option B, I'm not sure whether to increase or decrease R15. Advice appreciated.

[SteveXKR]

Looks very nice - did you paint that chassis or it looks powdercoated?

[xtian]

Looks very nice - did you paint that chassis or it looks powdercoated?

Powder coated by Hammond.

[xtian]

Now I need to reduce the gain in preamp stage 3.

Someone on AX84 said remove C10 to decrease gain in stage 3. How does that work?

[xtian]

I removed C10, and it's definitely better, but still too much gain. Suggestions?

[Structo]

It's a cathode bypass cap, it greatly increases gain of the circuit.

You can switch those in and out for a gain boost as well.

[ChrisM]

You can put a resistor in series with the bypass cap to limit the gain it is increasing, while retaining it's tonal effects.

[SteveXKR]

Any updates on this amp? Very curious to hear it in action!

[xtian]

Any updates on this amp? Very curious to hear it in action!

Just for you, Steve! Some audio recorded with the SEL, starting with clean, then snapping in the V1 cathode bypass cap, then adding the lead channel (stage 3 preamp). Input volume maxed except at end when it's turned down to 2/3. Master volume changes throughout to even out the overall volume (easy to rattle windows if I turn it up too loud).

There is some hi Q but low volume fizz in the distortion that I'm going to try to filter out with a conjunctive filter.

The Roy Bean tone stack is growing on me.

[moonbirdmusic]

Thanks much!!

[moonbirdmusic]

Bravo xtian!!

Think your layout works very well from the clips. Did you end up changing anything from the layout you first posted??

I looked over the thread again but did not see where you said which OT you are using? Mind sharing that?? thx again!

[xtian]

Did you end up changing anything from the layout you first posted? I looked over the thread again but did not see where you said which OT you are using? Mind sharing that?? thx again!

I did change some of the layout. You can see photos of the finished layout in my post from May 29 in this thread. If I did it again, I would do that trick where you wire the OT to a pair of headphones, and wire the PT to 120vac, and manually orient them so you don't hear any hum. I think that would be the only way to solve the faint hum I'm getting. But as you can hear from the recording, it's really not bad.

The OT is Hammond 125ESE, as spec'd in the schematic from AX84. It's a well-tested design, and as many people kindly pointed out, I was taking a risk by inventing my own layout. But I think of that as half the fun!

[SteveXKR]

Wow very nice! I think you've inspired me to build me one of these badboys!
And I agree part of the fun is figuring out your own layout etc.

[xtian]

I'm calling this one finished (for now…

I tried a number of remedies for the fizz, but the clear winner was adding a conjunctive filter (0.03uF and 4.6K 5W in series) across the OT primaries, with an SPST switch. Nice!

I also changed the power tube cathode resistor to 220R. Now the KT88 is running at 28.4 watts (67% of 42 watt max).

Finally, put dry transfer lettering on. Dig it!