K-502 Filament supply (or K-12M) - diyAudio
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Old 26th June 2009, 04:21 AM   #1
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Default K-502 Filament supply (or K-12M)

Hi,
Was wondering if anybody ever rectified the filament current for the K-502 kit. I just purchased this kit and I was reviewing the schematics for it and it appears to have a straight AC current to the filaments. I wan thinking of rectifying the filament supply and running it through a voltage regulator to set the voltage right at 12V. I was wondering if anybody has tried this and if to any benefit. I have read all kinds of reviews about this kit and it appears to be a solid amp, esp for the money. There appears to be a ton of mods, the ones that most impressed me were at Voltsecond (sp?). Anyways I wanted to start up a thread again for mods to these kits as it appears the current threads are a bit dated (as far as I can find). I welcome any input to the modification of this kit (K-502 or K-12M as I hear they are practically the same).

Thanks alot!
Jeff M.
Lawton, USA
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Old 26th June 2009, 05:22 AM   #2
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Why would you want to rectify the filament voltage?
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Old 26th June 2009, 03:44 PM   #3
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That sounds more like an objection than a question. I have seen it done and have heard it can make a difference, I actually own a pre-amp where this is the way it is configured. From what I have read this may be more important for pre-amps than amplifiers but seems to how this amp is "intergated" I thought it may apply here too. I am certainly open to feedback here. If it sounds like a bad or useless idea, let me know why.
Sincerely,
Jeff
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Old 26th June 2009, 04:26 PM   #4
DHT112A is offline DHT112A  United States
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My assumption would be if you can't find where anyone performed the modification it probably isn't necessary. DC filaments are usually implemented due to excessive hum from AC on the filaments. Apparently this has not been a problem so why bother?
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Old 26th June 2009, 05:54 PM   #5
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I'd measure the loaded output of the power transformer you are using...

So the 10GV8 used in the K-12M was rated for 11.6 volts on the heater. If yours is measuring over 12, then DC may bring you down much closer to the rating of the tube. Some tubes can sound better with slightly less voltage on the heaters. (DHT especially) YMMV.
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Old 17th November 2009, 11:02 PM   #6
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Just for the record, my version of the K502 came with Japanese 11BM8's.Lovely tubes.I believe the 11BM8'S are the equivalent to 6BM8'S (ECL82) but needing around 11volts on the heater filaments instead of the 6.3 volts that the 6BM8's use.

Bill
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Old 18th November 2009, 12:59 AM   #7
Jeb-D. is offline Jeb-D.  United States
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I don't think it's worth the effort to switch to DC filaments.

-Put it in a chassis
-Add a bleeder resistor across one of the reservoir caps.
-Replace the coupling caps. I used Wima MKP4's 400V .22uF, they are available in a pin spacing that fit's perfectly in the the existing hole spacing. They are fairly inexpensive and I think they sound very nice. I also replaced the cap in the feedback path with a Wima, though I forget which value off hand.
-If the existing amount of hum bothers you, add a choke to the power supply and/or switch to soft recovery rectifiers.

The 11BM8 is indeed a 6BM8 with an 11V filament. They are very pleasant sounding tubes IMO.
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