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PostPosted: 07 Nov 2020, 12:42 
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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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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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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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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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PostPosted: 21 Dec 2020, 21:21 
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I am planning to build the "octal oddblocks" and I was wondering if the Edcor XPWR264 or XPWR267 would be more appropriate.

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PostPosted: 22 Dec 2020, 08:59 
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Hi, Both are the Edcor part number for ones I use in the commercial kits. The 264 ((EMO757-120) has only 2 amps of heater power and the 267 (EMO813-120-240) has 4 as well as more high voltage current. The 264 is fine for up to KT88s, the 267 is good for up to KT120s. The 267 can be used for a stereo version with KT88s. The heater voltages of the trannies require a relatively low drop bridge rectifier (or individual diodes). If you use a high drop one the heater voltage will be a bit low. If you live in an area with sub 120 VAC mains the EMO757A-120 has 11.2 VAC heaters. For Bigger amps the XPWR222-120 can be used in a similar manner by adding the 6.3 and 5.0 volt windings together. Really you can do that in about 90% of the time with any that both the 6.3 and 5 volt windings as long as the current ratings are sufficient. We (Oddwatt) have allowed Edcor to sell the EMO versions the same as the XPWR ones as a convenience to our mutual customers. We had the transformers designed for our builds but since we encourage DIY it makes sense. No price differences AFIK.

Good listening
Bruce

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PostPosted: 22 Dec 2020, 09:26 
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It's an awesome amp, highly recommended! For what it's worth, I built a stereo version using AC heaters with an XPWR117 powering both channels. Designed for KT88s but currently I'm running KT120s at the 90mA bias setting. No audible (to me) issues with AC heaters. The '120s give an extra low-end boost even without higher bias. Only real drawback to the stereo version is that it weighs a ton :)

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PostPosted: 22 Dec 2020, 10:41 
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Thanks for your input. Maybe thats what I should do, though I am sort of attached to the mono block approach. I'll give it some thought.

Did you ever resolve your direct hum issue? Were you able to confirm that its not from the power transformer as Bruce hypothesized above?


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PostPosted: 22 Dec 2020, 11:14 
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It was an audible hum from the power transformer itself. I unbolted it from the chassis and powered it up, essentially silent.
So I put some grommets in the holes (that was a pain!) and now it is much quieter. My theory is that it's due to the extra load on the filament winding.

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PostPosted: 22 Dec 2020, 13:53 
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Hi, My present set up uses KT120s at 65 ma each. B+ is 450. I use the 8K CXPP25 transformers with them. Power output is just under 20 wrms. The sound it really special. The other configurations are quite excellent, but this one is IMO the best. AC heaters are fine. I also use AC to DC (12V) SMPS in a few and that is excellent as well. You need to be sure that they have sufficient current and can start under a heavy load. Figure about double the heater load for the SMPS. They also need to have the output isolated from the input. If not things go bad quickly.

Good listening
Bruce

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