AX84 SEL KT88 build

[ChrisM]

I noticed the schematic does not have 100K resistors from the heater wires to ground, as I see in many other designs. Is this an alternative method of reducing hum?

100R not 100K.

Yes you will need to add that (it's called an artificial centre tap). If your PT has a CT use that. If it does not have one then create one with two 100R resistors. 1/2W or 1W is good.

[Structo]

That letter A box is linking to the filament wiring.
When you connect the center tap of the heaters to the cathode resistor, it elevates the voltage for less noise.
It is still using the 6v but it sits higher above zero.

[xtian]

Chris: right, 100R. But my PT does have a center tap. So I don't need the 100R resistors, right?

Tom: The filament wiring is 6.3 vac, which means I can imagine it like a sine wave, centered at 0 volts, with peaks at + and - 3.15. Right?

And you're saying connecting to the cathode adds its DC voltage, basically raising the sine wave 30.5 vdc on the y axis? So it's still a sine wave (alternating current) but it's peaks are at 33.65 and 27.35.

[Structo]

Right no need for the 100R but sometimes the center tap is not centered well so you get more hum because if the imbalance.
So if you do get hum you can try the 100R trick or just try the center tap at the cathode of the power tube.

No, since it is peak to peak, the 30v or whatever you have at the cathode becomes the 0.
So you still get the normal peak to peak heater voltage, it is just sitting higher.
Hey, I don't pretend to know all this stuff, and quite frankly I have never tried that trick but it is supposed to lower the 60Hz hum if you do that.

[xtian]

I finished my SEL build, and it works! Haven't played much, because of a loud 60Hz hum and evidently high voltages. Please lend your eyes, and look over my gut shots.

Should I add those 100R heater resistors after all? Also, Hammond suggests using the Black primary lead for lower voltates (instead of the grey one). Any other suggestions?

My voltages are
B, B+1, B+2, B+3 = 383, 383, 373, 350
valve 1, pins 1,3,6,8 = 216, 1.43, 255, 2.11
valve 2, pins 6, 8 = 215, 1.32
valve 3, pins 3, 8 = 377, 30.8

[img683]http://www.monkeymatic.com/pub/sel/sel7.jpg[/img]
[img683]http://www.monkeymatic.com/pub/sel/sel8.jpg[/img]
[img683]http://www.monkeymatic.com/pub/sel/sel9.jpg[/img]
[img683]http://www.monkeymatic.com/pub/sel/sel10.jpg[/img]
[img683]http://www.monkeymatic.com/pub/sel/sel11.jpg[/img]
[img683]http://www.monkeymatic.com/pub/sel/sel12.jpg[/img]
[img683]http://www.monkeymatic.com/pub/sel/sel13.jpg[/img]

[Colossal]

I finished my SEL build, and it works! Haven't played much, because of a loud 60Hz hum and evidently high voltages. Please lend your eyes, and look over my gut shots.

Should I add those 100R heater resistors after all? Also, Hammond suggests using the Black primary lead for lower voltates (instead of the grey one). Any other suggestions?

Congratulations on your new build dude! SE designs can be notorious for buzz and hum and more care must be taken to eliminate opportunities for it to occur because there is no phase canceling in the power stage as occurs in push-pull.

If your heaters are not ground referenced you may want to add the artificial center tap via a couple of well matched precision 100R. You can also elevate them to DC by connecting the cathode on the power tube to the junction of your filament artificial center tap and let the heaters reference ground via the cathode.

Additionally, consider your lead dress. I might be inclined to chopstick around, moving various wires around on your preamp tubes and see where the 60Hz hum might be getting induced. Check that any B+ wiring is well separated from nearby AC or, if they must cross, that they do so at 90 degrees. You also might want to unbolt your OT and rotate it and see if it is interacting with the field from the PT. Typically, PT and OT are oriented such that the laminations are perpendicular to one another and barring that, well separated. I'm not being critical of your build, just trying to throw out the typical suggestions that start when validating a new build and dealing with annoying hum. Try one thing at a time and make a note it it helps so that your approach is methodical and you aren't introducing more variability or forgetting what ground you've already covered.

[EDIT] p.s. I'm interested to see what you think of the Judge Roy Bean and His Brown Bear tone stack. Seems like there is a lot of potential there. I've been eyeballing the UberSEL schematic with great interest (scratching chin...)

[Jana]

I'm a little confused and a little concerned--in the fourth picture, the one of the power supply filter bank, the two diodes are connecting to the red PT secondary leads, right? It looks like you used the lugs on the terminal strips that are also the mounting lugs. Are they connected to the chassis? It looks like you may have insulated them with grommets, but still, that makes the screws on the top side of the chassis live with HV AC.

[xtian]

It looks like you may have insulated them with grommets, but still, that makes the screws on the top side of the chassis live with HV AC.

Thanks for looking carefully! I epoxied the standoffs to the chassis, and made sure they were electrically insulated.

[xtian]

If your heaters are not ground referenced you may want to add the artificial center tap via a couple of well matched precision 100R. You can also elevate them to DC by connecting the cathode on the power tube to the junction of your filament artificial center tap and let the heaters reference ground via the cathode.

Thanks for the great comments. Notice that the power tube's cathode is already connected to one leg of the 6.3vac pair (might be hard to see, but pins 7 and 8 are connected). Considering this, can I still add the 100R pair between the heater runs and ground?

EDIT: Yeah, that made it far worse. Perhaps I should disconnect the heater from the power tube cathode?

[Cygnus X1]

Looks good.

1 question-why did you use a 10uf for C4?
Should be a 47uf like the rest.

[xtian]

1 question-why did you use a 10uf for C4?
Should be a 47uf like the rest.

C4 is indeed 47uF, just like C1-C3 in the power section. They're all the same value (and physical size and shape). What are you looking at?

[Cygnus X1]

1 question-why did you use a 10uf for C4?
Should be a 47uf like the rest.

C4 is indeed 47uF, just like C1-C3 in the power section. They're all the same value (and physical size and shape). What are you looking at?

OK you're right.
You changed the layout a bit.

I don't know just why they did it, but C4 is shown to be located on the preamp board a good distance away from the rest of the filter caps.
The 10uf is shown directly adjacent to C1, C2, and C3.

So the values are correct, but the layout might be part of the issue.

[xtian]

In order to diagnose the hum, I disconnected the output tube pin 5 from the circuit, and then grounded pin 5. See schematic below. Essentially, I believe I have removed the entire preamp section, tone controls, everything except the power amp, and fed it silence.

[img:794:477]http://www.monkeymatic.com/pub/sel/test_schem1.gif[/img]

This would seem to reduce the suspects for the 60-cycle hum. I cannot rotate the OT or PT, because all the leads are trimmed to fit. What's your next best guess?

More clues for you: I have large sheets of copper sheeting for insulating cavities of guitar electronics. Placing this around and between the OT/PT/tubes on the top of the chassis made no difference in the hum. Also, moving heater wires around inside makes no diff. I can't seem to influence the hum at all.

[Structo]

I think you misunderstand the heater voltage center tap.

If your transformer does not have a center tap for the heater winding, you need to make an artificial center tap by soldering a 100R resistor to each side.
This can be done anywhere in the chain but should be ideally near the cathode bias resistor.

So one 100R to each side, (green wires) then twist and solder the other ends, and either solder that to the hot side of the cathode resistor or solder a wire to it then to the cathode resistor.

If you only connect one side to the cathode resistor you are creating a huge imbalance and the hum will get way worse.

[xtian]

I repeated my experiment in light of your clear explanation, but could not improve the hum. As you suggest, each leg of the 6.3 vac pair connects via a 100 ohm resisor--this time to the hot side of the cathode resistor (R9 in the schematic). Please note that the schematic calls for only one leg of the 6.3 vac line to be connected to the cathode resistor…why? As you pointed out, it should be balanced, right?

Anyway, out of curiosity I removed the artificial center tap (the two 100Rs) and the direct connection between cathode and 6.3vac, and I observed a loud buzzing. Connecting the 100Rs effectively silences that, and the amp sounds quite good, except for the 60Hz hum.

Other ideas?