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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: 16 Feb 2018, 15:44 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, You can use KT120s as KT88s. I don't recall what that particular trannie can do for heaters... The 120s do require more than the KT88s.

On the question about an all wood chassis...It ought to be OK as the gain is rather modest in the amplifier. I prefer metal, at least on the tops for strength and shielding but it might not make a great deal of difference.

Good listening
Bruce

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PostPosted: 16 Feb 2018, 17:39 
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Joined: 18 Jun 2016, 06:52
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Hi Eric,
In my Oddblocks I'm running KT120s and KT150s at 92mA bias setting, because of the ever lurking red plating at higher settings. In my (strictly personal) view the sound is more relaxed (which I prefer) at a lowish bias level, whereas at higher levels the sound is more muscular and taut. So, strictly down to taste.

Jan


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PostPosted: 19 Feb 2018, 06:58 
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Joined: 01 Apr 2017, 19:56
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Jan, just my point or view about your experiences. Please feel free to reply back.

JanG wrote:
In my Oddblocks I'm running KT120s and KT150s at 92mA bias setting


Be carreful not to fry the LM317HV. On this bias setting the voltage across it will be rather high, approaching the 57V max voltage spec. This with a grounded input. Any signal will swing an AC wave over 57V magnitude. Not to forget the power dissipation will be high!

JanG wrote:
because of the ever lurking red plating at higher settings


A KT120 should be able to dissipate 60W of power. That means about 145 mA bias current with 410V Plate-to-Cathode voltage. A KT150 is good for 70W, so 170 mA bias current. Maybe there is something wrong with the tubes or other component in the circuit?


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PostPosted: 19 Feb 2018, 09:03 
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Hi, You will find there is a thread on that issue on the forum already. Yes there has been a serious issue with new tubes over about the last two years or so. They red plate at well below ratings. My suspicion is that a number of batches were not up to standard. Those they sold off to companies that supply diyers. Good tubes went to the companies that supplied big manufacturers of tube gear. It seems that now the supply of junk is mostly gone. A licensee of my designs found that both KT120s and KT150s were only able to handle about 60% or so of their ratings for dissipation. I have a quad here that I just got back about 6 months ago as a replacement for a quad that didn't meet spec....the new ones are not much better. This is from the supposed to be trusted supplier. The newest ones are only able to handle about 75%-80% of the ratings. If you are buying the KT120s now...try to get ones with either very old production dates (before say 2011) or new ones with current dates and going for ones with lower gm ( below 7500 not all suppliers provide that info though) seems to be useful as well. A curious issue as my originals and the ones we (Oddwatt) got in quantity a while back that work fine have gm up to 8500. Go figure.

You are correct about the spec ability to dissipate power. I have my original KT120 amps running at 145ma and 430 VDC. No problems. They have been doing this for over 5 years. I recommend for diy use no more than 135 ma to guard against unusually high mains voltages which will in turn up the B+. I intentionally have set mine high to test the longevity of the tubes in this use (push-pull- Ultra linear class A) as it does work the tubes fairly close to max spec (85-88%). I have 1000's of hours on them and they still test fine.

Typically the voltage across the LM317 is about 40 volts with 130ma. If you reduce the current the voltage will need to be greater. The LM317HV is rated at 57 volts so be aware not to exceed it. Initially I was concerned that the AC swing on it caused by high signal levels would fry them. This has not ever been the case.

Good listening
Bruce

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PostPosted: 19 Feb 2018, 15:14 
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Joined: 18 Jun 2016, 06:52
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Hi Victor,

Rest assured. I'm aware of the risk and when lowering the bias I measured the voltage. I don't remember the exact value but it was not more than 45V.

Jan


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PostPosted: 21 Feb 2018, 07:21 
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Joined: 01 Apr 2017, 19:56
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It is finally ready and working good! It sounds amazing and really loud with my 92 db tower speakers.
It's a Stereo version of the Oddwatt plus a Forewatt with 3 inputs. Homemade (and overrated) output transformers.
The whole thing weights arround 18kg (40 pounds)!
Bruce, thanks for the great project and support! All in this forum, thanks for the support as well.


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PostPosted: 21 Feb 2018, 20:48 
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Hi, nice looking build. It is good to have satisfied diyers.

Good listening
Bruce

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PostPosted: 06 Mar 2018, 21:39 
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Joined: 03 Feb 2018, 00:47
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Location: Brisbane, Australia
Hi all,
I have ordered my Edcor OPT's and PT. Can't wait. It (She?) will be a stereo single chassis behemoth, possibly using blue glass EL34's because I don't need large amounts of power.
On the power supply side, the plain rectified DC after the PT seems to be close to 500 volts (509 theoretically with no load I gather). The caps readily available seem to be 500V rated. Isn't that running a bit close to the maximum rating? I would normally run a cap at more like 70% of its rated voltage. I guess you would know if there were any issues by now. My PT is XPWR117 180-0-180.
Also, the first smoothing cap is 100uF, I assume I double this one for double the load and duplicate the rest of the circuit for each channel?
Thanks,
Glenn.


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PostPosted: 07 Mar 2018, 07:17 
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Joined: 16 Feb 2016, 19:45
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Location: Boston, USA
Hi Glenn-

I built something similar and love it! See some details on my page:
http://ohm.bu.edu/~hazen/TubeAmp/. I used an XPWR117
to power both channels. If I were to do it again I think I'd try to find
a slightly larger power tranny as mine runs quite hot.

I started with KT77s in mine which sounded wonderful, now I'm
running KT88s (Shuguang) which have a little more punch.

Happy building!

Image

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PostPosted: 08 Mar 2018, 04:49 
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Joined: 03 Feb 2018, 00:47
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Location: Brisbane, Australia
Thanks Eric. Pretty much exactly what I am after. Even the same Hammond chassis by the looks. I gather the Edcors do run hot, but that doesn't bother them. I am tossing up whether to use the XPWR117 12V heater winding, or use DC SMPS to keep as much load off the transformer as I can. Bruce suggests DC to minimise the hum, probably a good suggestion, although it grates to use such a modern device inside my highly expensive retro amplifier. I intend to use EL34's to keep the load down and because I don't need the extra power, but based on your experience, maybe KT88's will do just fine.
I see you specified 600V filter caps. May I ask where you found those? (If you did).

Thanks,
Glenn.


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