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Poll: What's your KT150 bias set to per tube (ma?)

To: Owners with purpose-built KT150 based tube amps and the ability to measure bias for each output tube.

What are you biasing your KT150s to in your amps (45ma, 50ma, 55ma, 60ma, or more) per tube ?

Background:
Today I discovered I had been conservatively running bias a bit too low for my mono tube amps. Was a little thin. Went up +10ma per tube and wow. Hit the tube magic window with increased lower midrange presence and overtones coming in nicely, smooth detail. No loss of soundstage. Still within manufacturer spec for my amps. While I realize it can be circuit dependent for different amp designs, just curious what others have found to be a sweet spot for your KT150s in your amps? Feel free to post the name/brand of your amp and the bias setting if you are able to share. Please disregard if your amps are auto-bias or LED bias only. Thanks in advance for your replies.
decooney
tsushima1
1,165 posts
 
ARC REF 250SE’s...Master KT150’s currently biased at 65 mA 
decooney OP
2,109 posts
 
@tsushima1 Appreciate you sharing your first-hand experience here. After my KT150s were fully burned in, moving from 45 to 50, now 60ma has shown a nice change with my current mono amps. Have not tried 65ma yet. May try it next. I noticed a nice change in the sound of strings and piano keys as I started to approach 60ma. I do realize its different from one amp/circuit design to the next and yet its fun to compare notes in general terms, thanks again! :)
tsushima1
1,165 posts
 
@decooney
The ARC circuit it set up to run the KT’s a relatively easy life @ 65mA so you should have enough headroom in your amps
decooney OP
2,109 posts
 

Update: Posted this in 2021. Now in mid 2023, recently digging a little deeper into this, inside my amps, determined my two mono amps have a plate voltage of 600v per tube. This allowed me to calculate plate dissipation %.

I had been running the opt tubes at 45ma per tube, previously, past few years. They were definitely biased on the "very cool" side side of bias, nowhere near Class-A bias before for my KT120s or KT150s. My amp designer operator’s manual posted to run 40-60ma per tube, which is truly on the ultra-conservative side, apparently helping customers to spare output tubes and wanting to see their tubes last longer! The difference between 40ma per tube and 60ma per tube is notable.

BIG CAUTION: make sure your amplifier transformers/circuit can handle it if you try to bias up, not recommended unless you know plate voltage, your amps, transformers, capability - limits. Just showing how I figured it out for mine. Check with your amp designer or a qualified tech who knows tube amps and tubes, fwiw.

In conclusion - determined i could jump up a bit more, upwards to 60-70 ma per output tube (if I wanted to) without breaking a sweat with my plate voltage in my amps. Have been shooting for 70% PD. Disclaimer: yours may vary, this is just for reference.

Now running my KT120s at 60ma and KT150s 65ma, found no need to bias up hotter. Plenty of detail in lower 70% range. Sound does change, opens more, and produces a different sound for sure as I bias up past 65% plate voltage, and start inching closer into Class-A. Not sure if this helps anyone (gotta know plate voltage first) for your amps before this relates. Here it is for 600v plate voltage, KT150s, KT120s for a reference. AP has a nice calculator tool, here: https://www.amplifiedparts.com/tech-articles/tube-amplifier-bias-calculator

KT150 / 600pv:

Output Tube KT150
Max Dissipation 70 W
Plate Voltage 600 V
Recommended bias point for Class A/B amps 70%
Recommended bias point for Class A amps 90%
60% Bias Point 70% Bias Point 80% Bias Point 90% Bias Point 100% Bias Point 110% Bias Point
70.00 mA 81.67 mA 93.33 mA 105.00 mA 116.67 mA 128.33 mA

 

KT120 / 600pv:

Output Tube KT120
Max Dissipation 60 W
Plate Voltage 600 V
Recommended bias point for Class A/B amps 70%
Recommended bias point for Class A amps 90%
60% Bias Point 70% Bias Point 80% Bias Point 90% Bias Point 100% Bias Point 110% Bias Point
60.00 mA 70.00 mA 80.00 mA 90.00 mA 100.00 mA 110.00 mA

 

Use with caution, just for reference in how I approached it. Your amps, tubes, conditions may vary fwiw. Check with your manufacturer or capable local tech.

 

 

 

 

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