AX84 SEL KT88 build

[SteveXKR]

Looks nice so far! Hope you'll record some samples of that tonestack in action

[xtian]

Here is the Roy Bean tone stack wired to the pots. (BTW, see that big hole in the chassis? Yes--I FUed the chassis by flipping my drilling guide. Now I have to wait for a new chassis to arrive next week. Aahh!)

[img:1024:683]http://www.monkeymatic.com/pub/sel/sel5.jpg[/img]

And here is the adjustable bias, with questions:

[img:1024:683]http://www.monkeymatic.com/pub/sel/sel6.jpg[/img]

The red gator clip you see is at the point that connects to the cathode (pin 8) of the KT88. The AX83 SEL schematic calls for 430ohms of resistance, which is the 5W resistor you see there. The adjustable bias mod adds that 1ohm resistor and the trim pot, all in series.

Why is the 1ohm resistor rated only at 1/2 watt? I think because the resistance is so low, it converts very little current into heat. Right?

What about that trim pot? I can't tell how it's rated. Here is the spec sheet: http://www.piher-nacesa.com/pdf/13-PTC10v03.pdf. Is it going to burn up? It's probably the wrong choice here anyway, because 430 ohms is optimal (according to the schematic) for the KT88, and I just want the ability to add a little more resistance if I want to run the KT88 a little cooler. How much is a little extra resistance?

Can someone point to a better trim pot for this application? Something that would work well if I replaced the 430ohm 5W resistor with 220ohms.

[Structo]

There is very little current or voltage on the cathode so 1/2 watt is fine and the trimmer pot is fine as well.

[xtian]

Tube data sheets list the 6550 as equivalent to the KT88. What difference in tone or distortion might one expect between these two? Is the 6550 a drop-in replacement in my SEL schematic?

[Jana]

According to your schematic there is 71ma. current flowing through the cathode. That is 2.2 watts on the 430 ohm resistor. I would not use that little trimmer.

[Colossal]

According to your schematic there is 71ma. current flowing through the cathode. That is 2.2 watts on the 430 ohm resistor. I would not use that little trimmer.

Yeah, FWIW the AX84 adjustable cathode bias mod for SE amps is a fixed resistor (such as 100-200R) in series with a mil-spec 1k 2W high quality pot. The fixed resistor could be a precision 2 or 3W metal or carbon film so the total wattage handling is well greater than 2W with the pot in series.

[xtian]

According to your schematic there is 71ma. current flowing through the cathode. That is 2.2 watts on the 430 ohm resistor. I would not use that little trimmer.

Thanks for your observation. What would you use, if you wanted variable bias?

And if you would be so kind, please say how you figured the current!

[Structo]

Sorry my bad but I recall discussing this in regards to other amps and it was said it was low, maybe those tubes draw a lot more juice.

I thought this was a cathode biased amp?

So why do you have a fixed bias?

[xtian]

So why do you have a fixed bias?

I'm following the AX84 design. Details here: http://www.ax84.com/sel.html

[Jana]

"...how you figured the current!"

Ohm's law. The schematic says there is 30.5 volts at the cathode. The resistor is 430 ohms. E/R = I.

[xtian]

The schematic says there is 30.5 volts at the cathode. The resistor is 430 ohms. E/R = I.

Thanks, I needed that. *smacks forehead*

I'm perplexed by the connection marked "A" in a square. This indicates that one leg of the 6.3vac heater circuit is connected to the power tube cathode. Why?

[Colossal]

I thought this was a cathode biased amp?

So why do you have a fixed bias?

Tom,

The SEL design <i>is</i> cathode biased but the mod with a trimmer (or ideally a 1k 2W pot) in series with a lower cathode resistor value is just to allow the bias to be fine tuned to one's tastes and a chosen dissipation. So, it's still cathode bias but gives the appearance of being fixed at a cursory glance.

[xtian]

I'm perplexed by the connection marked "A" in a square. This indicates that one leg of the 6.3vac heater circuit is connected to the power tube cathode. Why?

Can anyone 'shplain this, please?

[Jana]

It's a way to elevate the heaters to reduce hum.

[xtian]

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?