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

Getting bias right on KT150 output tubes sure makes a difference. Wow.  

Seems so. Just heard from my amp designer/builder. My KT150 amps sounded a little dry at 40ma per tube. 50ma showing real hope. Op manual shows 40-60 for my model. Asked if I could go up to 55 or 60ma. Designer shared "60ma is still well below the maximum rating of the KT150 tube". Still well within the safe zone it seems. An easy and fun experiment.

In my case I could go even higher on bias +5-10ma more, and remain in spec. First time I experimented with wider adjustment sweeps since amps were new. Seems I originally underestimated KT150s ability to sound more lush and "tubey". It is possible, and discovered it a bit later with my amps after months of burn-in of all tubes, upgraded coupling caps, and more. Felt kinda stupid once I found the proper bias setting window many months months later, like "ah, there it is!", thus realizing KT150s can do it too if I simply adjusted bias to the sweet spot, duh...

Now I have no desire to go back to KT88s, KT77s, or EL34s (or back to my former parallel strapped triode based amps) any more. Was not expecting this realization to actually happen with my particular KT150 mono amps, and it just did. Ultra conservative low-biasing is no longer an option, the higher bias is well worth it now :)

@oldhvymec - If you could limit the bass duty on the valves, I think you’d really be surprised.

An unexpected added surprise was the lower register midrange and upper bass range that just came through biasing up on the KT150s. I usually run custom 12" Scanspeak active subs for 50hz below in my setup. Now after having biased up the main mono tube amps, immediately had to turn the sub down, off, almost don’t need it. oldhvymec, i know you know the "sound", its reminiscent tone and sound closer to running two V12R mono blocks, yet I’m running only (4) KT150s instead of (12 or 24) EL34s across two mono V12Rs. Using Mullard Blackburn mid 60s dual-triode 12AU7, 12AT7/CV4024s in front of the KT150s to shape tone. Also worth noting, Mundorf Silver-Gold EVO coupling caps (same as my former Cary amps had) now in my current KT150 mono amps. This too brought the KT150s to another level with nice tone and texture, probably closer to what you might know from a V12R with these same caps paired in Cary amps and GL KT88s, a tad more transparent than EL34s.

@oldhvymec,   "All the rage", hard to say.   Fit for purpose, yes, most definitely.   Took several steps to get to where I wanted it all to land. It took a lot to best my highly modified parallel strapped triode V12R amp, like yours. Part of this had to do with my speakers, and more options to run different speakers later on. Not sure I will, but knowing I can.

Now running slightly modified Quicksilver Mono 120 amps with KT150 output tubes for footprint consolidation and reduction of heat purposes. Tried KT120s in them too, nah. KT150s better all around in these amps. Capable up to 120wpc, yet I bet I rarely go over 2 watts max ever. I use these amps for mostly low volume listening on larger custom designed 2.5-way air motion (electrostatic) transformer speakers. Set in a fairly well ventilated 14’x19’ room. I don’t have to turn it up to hear everything now, it’s just "there". Much less transformer heat, less amp noise, tubes run cooler. Good power and dynamics resulted. Less complex point to point circuit and wiring too. Easier to upgrade or repair these down the road, all key factors for me. A reduction from 12 to 4 output tubes, yes! Another benefit. And, the 12Au7 and 12At7 small tubes are easy to find.

Goal was to run brand new mono amps with a larger / modern / robust output tubes and select dual triode input/driver tubes on the amp’s input stage, and feed it with a nice 6SN7 based tube preamplifier which I already owned and liked a lot. Wanted slightly smaller footprint Monos, vs. super large 87-100lb stereo amps, another +. Still heavy yet I can move them easier and they take up a little less rack space in open racks, more air.

This initial setup was followed by some more rolling of 6SN7 tubes in the preamp and the small signal tubes on the mono amps to gain back a more familiar tone, texture, and presentation. And, of course typical obsessive compulsive round(s) of interconnect swaps (all over again) finally settling back on all OCC continuous copper single-ended RCA interconnects. Final tune and cherry on the sundae was adding the familiar EVO Mundorf Silver-Gold caps same as I ran in my prior Cary amplifiers. At this point, closing in on the "the" sound I wanted showed a glimpse of hope, then a 200-300hr roller coaster ride with caps settling. After all of this, back to biasing output tubes again recently and, wallah!

@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! :)

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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