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Hey guys, I just got a pair of Gold Lion KT 66 tubes (They're HUGE!)and I am trying to get the bias within the 20 -40 mA range. It seems that full clockwise on the adjust pot gives me a minimum of 54 mA.Is there something I can do to rectify this?

Hmm. Is the bias voltage sweep ok with el84's and bias switch in the other position?
Stew

It was ok with the Nos 6V6's I had in there.

Does the bias voltage ( on the grid) actually change when you flip the el84/Kt66 switch?

I didn't try that . I will and get back to you . Thanx Stoo

No Prob! Glad to help! Going to be signing off here in a bit. When you put that switch into the Kt66 position it puts a resistor (R44) across the dropping resistor R43 thereby reducing the value of the dropping resistor so the bias voltage goes up which lowers the idle current. If that's not happening I'd check your wiring around that part of the circuit. If those Gold Lions are a little different than normal 66s you may have to lower the value of R44 from 620K to ?? ..maybe 560k/470K.

FYI we have tested RI Gold Lion KT66s in Triwatt amps without any problems. Also the bias pot sweep should be more around the 30 to 50mA range.


EL84??

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Great tones come in small glass jars!

Yeah ,the bias range with the KT66 is 54-70 and fairly well matched on each side.
the nos 6V6 is 10 -18 and reasonably matched so maybe I need to change the 820k dropping resistor.


As for the EL84 thing, I think Stoo was channeling an 18watt circuit when he typed that!

Well I did say I was turning in!!Lol! I meant 6v6s
54-70? are you refering to the range of idle current (measured across the 1 ohmers) or the actual grid bias voltage?
on the schematic that resistor is 620K not 820K!! If you have an 820K. that might explain your prob.
Stew

Ok talk about red faced! First, it is ,in fact, a 620k. I think for all the time I have had this amp, I have been running the 6V6 set on the KT66 bias. I flipped the switch and tried the 66's on the other setting and got 25 to 40 mV. I wrote the 6V6 / Kt66 notes on the chassis on the inside as the layout shows and on the tube side of the chassis wrote it the opposite way!Doh!Tried both sets of tubes in their proper switch positions and am able to set them correctly . Just goes ta show ya! I will post my opinions on the new tubes when I get out to my next noisy practice!Thanks for checking in on me guys!

I did exactly the same thing...and was equally red-faced due to a similar biasing issue. Keeps us honest...

Kirb

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SG + Brit tube head = Nirvana

Good to know other people are directionally challenged besides me!

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I'm just glad I didn't screw up my 6V6 set!

40 to 42mA is around the current you're looking for with KT66s in a Triwatt. The KT66 bias setting should be able to take you close to 50mA max on the bias pot. If not, you may need to tweak resistor values to be able ti get that. OTOH you could argue that 40mA is enough current and just leave it the way it is.

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Great tones come in small glass jars!

I'll try it at 41.5! Thanks Zaph!

BTW the KT 66 tubes sound fantastic in my amp -biger and fatter than I expected money well spent so far.Can't wait to rehearse loudly!!