KT66 vs. bias

Skrawboy · Post by **Skrawboy** » Tue May 13, 2008 2:13 pm

hey guys.

I just finished my SLP upgrade on my super bass amp, and was looking forward to put in KT66 tubes, butt the amp is way to hott.
The lowest VDC i can turn the bias trim pot down to to is -46.. And i read that you should get it down to at least -60 and preferably -80 VDC..
What to do???

Roe · Post by **Roe** » Tue May 13, 2008 3:49 pm

you can always change the bias resistor for more range

Flames1950 · Post by **Flames1950** » Tue May 13, 2008 4:07 pm

While I would think you definitely need more than -46 volts, you really need a way to measure the current that the tube is drawing at idle. Otherwise you're shooting in the dark.

JimiJames · Post by **JimiJames** » Tue May 13, 2008 6:03 pm

Roe wrote:you can always change the bias resistor for more range

I just temporarily replaced a tube (EL34)and this was the case because it was on the hot side.
I had a 33K ohm which I had to change out to a 27K ohm to get me in range with the bias trimmer pot. I'll be changing it again if necessary after doing the math ofcourse...
My new quartet comes next week...

Skrawboy · Post by **Skrawboy** » Wed May 14, 2008 9:55 am

Ok. I forgot to tell you that i'm a beginner at the art ao ampbuilding... So as far as i know, the Metro pdf-file is my holy bible.

The bias resistor, is that the 47K thatg runs from the bias pot?? And if so, should i use a larger or smaller one, 27K, 33K, 56K??

And for you Flames1950. I bought some Valve Art KT66 from Metro, and with each tube there was a little note with the mA (80, 81, 82, 74) written on it. Is that the current??

JimiJames · Post by **JimiJames** » Wed May 14, 2008 1:25 pm

YES, the bias resistor is that the 47K that runs from the bias pot.
(Keep in mind this is just a starting point...)

Well , once your tubes are in and doing the math, take your reading with your meter properly set and see if you are in range.
To raise that number you'll need a lower value resistor. This is what happened in my case.

I had a B+ reading of 385 and needed a reading of .045
My amp was showing high 30's with the bias pot maxed out.
I set my bias pot to 12 O'clock and replaced the 33K with a 27K.
Flipped the amp on, took a reading, dialed it in and presto !
I had room to play if I wanted a hotter/colder sweet spot ...

Skrawboy · Post by **Skrawboy** » Wed May 14, 2008 3:12 pm

Thank a lot guys, i'll give it a try