Bottlehead Kits > Kaiju Stereo 300B amp

DC filament / humpot / DC flip-flop reverse

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alejon:
Merry Christmas and a Happy New Year to everyone!
Finally got time to assemble my Kaiju kit and in a mean while got few questions.

1. When I install the DC-filament kit, do I still need humpots?
2. If yes, why?
3. Have seen some discussions on the 'net about randomly reversing / flip-flopping polarity of the DC filament of the DHT tubes, to improve the longevity of the cathode and to "compensate for the voltage gradient effect DC heating introduces"
4. What is your opinion on that?

Many thanks!!!
Best regards,
alejon

Paul Joppa:
1. When I install the DC-filament kit, do I still need humpots? --- maybe


2. If yes, why? --- the humpots cancel any differential-mode noise, distortion or hum from the DC supply. This is important for simpler, unregulated supplies, such as used in the Stereomour DC upgrade kit. With the regulator, the residual noise is probably inaudible, but in case you doubt that (many do) the pot is retained. It is also easier to install that way.


3. Have seen some discussions on the 'net about randomly reversing / flip-flopping polarity of the DC filament of the DHT tubes, to improve the longevity of the cathode and to "compensate for the voltage gradient effect DC heating introduces"
4. What is your opinion on that? --- In practice it's never seemed significant - it reduces one form of aging degradation; there are many others. The practical approach it to wire the sockets in opposite polarity and swap tubes annually. This adds to socket and tube pin wear and tear, of course.

2wo:
I just got the DC mod, I am planning to wire the sockets in opposite polarity, just to keep my options open...John

alejon:
Dear Paul,
Great thanks for your reply!
Also found your old post from 2007 on audioasylum on this issue. I am posting it below to keep the info in one place. It may be interesting for other members, too.

"The DC gradient will affect the emission life along the filament. If there were no mechanical turn-on/turn-off stresses, no cathode stripping, no filament or grid warping, no mechanical stresses from moving, tube swapping, or shipping - i.e. no other causes of tube demise except cathode exhaustion, this would be a concern. But even then, the difference can be estimated. Cathode current is proportional to (Eb-mu*Ec) to the 3/2 power. Assuming 70 volts bias and 350v plate to cathode (fairly typical), a DC filament would be 67.5v one end and 72.5v at the other end. The currents would be about 43% different by my quick calculation. In other words, one end might fail 21.5% early, while the other end woudl last 21.5% longer compared with AC heating.

Other effects, for example variation in the uniformity of the initial cathode coating, are probably larger than that - certainly tube lifetime is highly variable between tubes, with a standard deviation usually larger than 21.5% even in controlled tests."

Best regards!

Paul Birkeland:

--- Quote from: alejon on December 27, 2023, 01:59:41 AM ---1. When I install the DC-filament kit, do I still need humpots?

--- End quote ---
In the Kaiju, there just isn't enough residual gunk leftover for the hum pots to be useful, so I'd say no.

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