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It is currently 30 Nov 2020, 22:27

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PostPosted: 07 Nov 2020, 12:42 
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Joined: 16 Feb 2016, 19:45
Posts: 55
Location: Boston, USA
Got a quad of Tung-Sol KT120s today and installed them. They look and sound very nice. It's hard to do a side-by-side comparison but to my ears the bottom is more solid than the KT77s. Running them at ~65mA for now (the "low") setting until they've burned in a bit, though I did switch to the 90mA setting and everything seemed fine. No circuit mods as my XPWR117 can handle the filament currents OK.

They look nice too. Thanks for suggesting this, and thanks again for a fantastic design!

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PostPosted: 07 Nov 2020, 14:29 
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Joined: 04 Jun 2008, 20:59
Posts: 4285
Location: Arizona, USA
Hi, Looks nice. It reminds me of one of my recent projects. The extra tubes are 12AU7s used for extra gain to compensate for the use of a balance control. The amp as shown is an integrated 25 watt per channel stereo version of the Oddblocks. A heavy beast thus the handles on the sides.

EDIT: BTW the power tubes are the Blue Glass JJ KT88s from my original amps (now 12 years old). They work and test fine. That is one of my design goals to make stuff that has very long tube life.

Good listening
Bruce
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PostPosted: 21 Nov 2020, 18:55 
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Joined: 16 Feb 2016, 19:45
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Location: Boston, USA
Interesting minor issue. I elected to go with AC heaters to simplify things a bit. With KT77 and KT88 tubes this was fine, but interestingly with Tung-Sol KT120s there is audible 120Hz hum directly from the tubes themselves (or possibly the wiring or transformers.) I know it's just the heaters, since I can pull out the relay which switches on the HV and I still hear it, and it sounds to be coming from the tubes.

When I'm playing music it is completely inaudible, but it bothers me a bit. I guess that's an argument in favor of DC heaters, though there is really no audible hum through the speakers.

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PostPosted: 22 Nov 2020, 21:43 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Curious, it would seem that any tubes in the same family (KTs) woul dbehave the same. I wonder if your power transformer is not up to the load and actually the source of the hum. Using DC on mine vs AC the difference is relatively small roughly a difference of about 2 db at a 1 watt. Usually not audible since typical builds have a hum and noise level of around -90 to -95 db at 1 watt. I'm really picky about hum and noise so I always take the quietest route.

Good listening
Bruce

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