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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 11 Jul 2010, 02:41 
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Cold Macaroni wrote:
I'm almost finished working up my schematic. One thing I don't have from your new schematic is the decoupling caps and extra resistor coming from the B+. I'll add those and try to post my schematic tonight (tonight where I'm at anyway). I couldn't get the 5u4gb to jive with the current I want/need for the KT120s, so I finally decided to just use two rectifiers and wire it essentially as monoblocks (planning on one chassis). Not sure if that means I'll need two separate PTs or if I can wire them up in parallel from one big PT... I don't see why I couldn't stick with one big PT as long as the voltage was under control and I had enough current for both sets of heaters.

For start-up and testing purposes I'll use the 300R cathode resistor, but is there any reason I can't use a ground referenced rheostat in its place and a ammeter down the road to play with the bias current?

I'll begin with the 6n1p for the driver stage, but if everything else pans out alright, I might experiment with the 6SN7, anything odd to look out for when setting them up?

You must try the 6SN7 driver stage - you must!!

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PostPosted: 11 Jul 2010, 06:42 
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You are aware I have some listening notes, regarding these KT120 tubes, on the blog. From another site I found these listening notes. Oddly they agree very close to my notes:

"Listening impressions ...
1. In general, the Tung-Sol KT120's seem to have a more "tubey" sound in this amp than the Genalex Gold Lion KT88 tubes that were in there.
2. The Genalex Gold Lion's have just a little more top end "air" than the Tung-Sol's..
3. The Tung-Sol's seem to have a more "full" sound with more mid-bass content than the Genalex tubes.
4. Both tubes seemed pretty much equal in soundstaging in both width and depth."

Note "1" - agree fully, "2" - agree, my KT88's have slightly better air and a brighter top end, "3" - I agree, more bass weight, fuller sound right through the spectrum,"4" - disagree here, the KT120s give a slightly deep sound satge.

Remember my amp is a SE UL amp his was a four tube push pull amp so over-all performance has to be slightly different but interesting how closely our notes agree. Also he was comparing Gold Lions and I, JJ Blue Glass KT88. All I can say here is both tubes are amongst the best of their kind. Common point is they are both KT88s.

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PostPosted: 18 Jul 2010, 14:39 
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Hi Everyone, I still abusing the quad of KT120s. No problems, bias is stable in the OddBlocks. I have them set with 180ma a pair at 450B+. Solid, powerful sound. Excellent sound stage and detail. Lots of bottom end. For anyone who may be using these in an oddwatt design, be sure to use the HV version of the LM317. The typical bias point on the KT120s is about 45 volts and is above the rating of the standard 317. BTW if you go to 220 ma a pair the bias drops (as expected) to about 40 volts.

I wonder if a KT200 is possible?

Good listening
Bruce

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PostPosted: 18 Jul 2010, 16:25 
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Bruce, what is your plate to cathode voltage?

You are certainly pushing some current through the tubes. What is the current rating of your power tranni HT?? Also you have upped the HT I see. I thought I was pushing current at 112mA!!

What is the current rating of your OPTs? I agree with your comments about the sound, they really are impressive and you are using a large driver tube too.

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PostPosted: 18 Jul 2010, 17:11 
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Hi, The plate to cathode is 410-415 depending on the phase of the moon. The power trannie is good for 475 B+ (with SS rectifiers and capacitor input) at 240-250 ma. So 180ma is quite comfortable, but 220ma is pushing it more than I like. To run the KT120s at the higher levels a bigger trannie is in order. I have a pair that will deliver about 450 ma at the correct B+. That is the next project. I can't decide on if it will be dual monos on one chassis (probably weigh 30kg) or separate mono blocks. BTW, for those new to the KT120s you can run a pair at over 120 watts dissipation and still be in spec. I personally think 100 pd is max for long lasting amps and tubes. That would be 500 volts cathode to anode at 200ma a pair. Or where I am running them at just over 400 volts you could go to 250ma a pair. Awesome tubes.

Good listening
Bruce

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PostPosted: 18 Jul 2010, 23:05 
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Bruce are you saying a pair in push-pull will do 120W. So a total of two tubes for 120W!!

That would make a splendid guitar amp if the case.

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PostPosted: 24 Sep 2011, 19:55 
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hello to all
i am in the process of replacing my kt90 valves with 120 valves can anyone tell me where to get a pair in australia please(postage from usa is a bit excessive)
also what is musical terms brighter,darker etc i dont understand how a valve can make a sound darker although i did get rid of a rotell system and get valves because the power amp system sounded tinny (my term) tinny as in not smooth but brighter and darker ,are they the same sort of analogy.


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PostPosted: 24 Sep 2011, 23:24 
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brianleon wrote:
hello to all
i am in the process of replacing my kt90 valves with 120 valves can anyone tell me where to get a pair in australia please(postage from usa is a bit excessive)
also what is musical terms brighter,darker etc i dont understand how a valve can make a sound darker although i did get rid of a rotell system and get valves because the power amp system sounded tinny (my term) tinny as in not smooth but brighter and darker ,are they the same sort of analogy.

For me dark is where the mid-range to lower treble is not bright and more bassy then neutral. The sound more leans toward the bass end of the spectrum. Tinny for me is bright or metalic sounding. A poor SS amps or Class D might sound tinny or metalic.

Bright is (for me) an excess of treble in the mids into the treble. Tipped-up may explain bright.

Private message me and tell me where in Australia you hail from.

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PostPosted: 28 Sep 2011, 22:49 
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Hi Everyone, I am really abusing the KT120s now. I looked back over the earlier posts and thought an update was in order. The KT120 Oddblocks now exist and sing loudly. A pair is being circulated for critical evaluation. General specs are max output before things really go tilt is 50 WRMS. I would rate them at 45 though. Distortion at half power is right around 1%. B+ is 440 with an idle current of 310ma per pair. (You need two LM317s to handle this) Yes it gets rather warm. With the cathodes at approximately 38 volts the input is 124 watts (both anode and screen currents make up the 310ma). Total power consumption is around 175 watts. Remember this is class A push-pull ultra linear configuration. Folks in cold climates can turn down their thermostats when these are running :hot:

Good listening
Bruce

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PostPosted: 29 Sep 2011, 02:04 
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Bruce: Good work as usuual.

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