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Old 13th April 2012, 03:26 AM   #1
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Question K-12M extra caps??

Woo! First post on DiyAudio! I have built my first amp, the K-12M! I completed the typical upgrades (snubbers, changing coupling caps, going to put in UF4007 diodes)

I know there are bajillion threads on the K-12M amp, but I don't think anyone has addressed this idea...

In another thread someone posted as a mod to the K-12m:

"The unused pads are most likely meant for bypass caps for the common cathode resistors (R8 and R17) for the push-pull pairs.

Adding a pair of 50uf 25V caps will improve the bass response, although some people might cringe with the mere thought of adding an electrolytic cap into the signal path."

(Page 7) S-5 Electronics K-12M (11MS8) 8 wpc PP Tube Amp Kit ($139)

Can anybody back this guy up? Or disprove him? I can't see anything wrong with doing this. I want to try it and see for my self if anything changes, I just don't want to blow anything up. I don't know much about circuits though, I'm new to this. Does anybody think this is safe? worthwhile? silly?

Thanks for any info,

Jeff
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Old 13th April 2012, 10:18 AM   #2
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Hi Jeff, just watch the polarity of the electrolyte caps and everything will be fine, you won't blow anything up. The caps you mention are necessary to restore the full gain due to the cathode bias resistor. Both will form a low pass filter, so, yeah, you are right it will improve
Bass response.

Hope this will help you.

Stef.
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Old 13th April 2012, 10:24 AM   #3
DF96 is online now DF96  England
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A cap here won't make much difference to frequency response, as the signals from the two output valves are antiphase. It might affect distortion (either way) as it will reduce the second-order products at the cathode (which are in-phase).
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Old 13th April 2012, 03:25 PM   #4
akimmet is offline akimmet  United States
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That is a common place to put a capacitor on a cathode bias amplifier. In my experience, it dose indeed help.
Anything 47uF - 1000uF 25V-50V will work here for this purpose. So you can almost use what ever you may have on hand to try it for your self. Just make sure you make sure the + side of the capacitor is connected to the tube's cathode pin and the - side is connected to the ground.
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Old 13th April 2012, 05:03 PM   #5
HpW is offline HpW  Switzerland
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Jeff,

Quote:
Originally Posted by Bambooda View Post

Can anybody back this guy up? Or disprove him? I can't see anything wrong with doing this. I want to try it and see for my self if anything changes, I just don't want to blow anything up. I don't know much about circuits though, I'm new to this. Does anybody think this is safe? worthwhile? silly?
The R + C is the usual auto bias circuit. The R defines the idle current and the cap will define a fixed voltage (stabilized) over a given frequency band given R & C. The drawback of the cap or nature is the absorption voltage. Worst on electrolytic caps and better on film types (just google to get more details on this).

I modified some weeks ago my Cary SLM-100 (old design), who had an auto bias with an 150E & 220u electrolytic cap (4 * KT88 on a single mono block ~ 280mA idle current), to a fixed bias using some trim-pots.

The sound change immediate to a clear & defined sound stage. OK you may argue the cap was about 20 years old. Could be, but having electrolytic cap's in a signal path this should be avoided at all!

This was my learning session on tube amps.

Cheers
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