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57.66.65.3
In Reply to: RE: Small IDHT tube cathode bypassing posted by 91derlust on July 06, 2015 at 00:44:48
Inaudiable ? He must have tin ears . I don't claim to have golden ears or the ability to discern cable changes over the phone like some here but I do hear cathode bypass caps . Film caps are simply too big , electrolytics leave a nasty footprint on the sound . A combination of electrolytic paralleled with film doesn't sound as good as films . I tend to avoid cathode bypasses altogether favouring LEDs , stacked schottky for drivers and fixed bias for output stages . No matter what people claim about their boutique caps or 'multiple ear-tuned cap bundles' in the cathode is that these are also in series with the grid . I would rather have a smaller , superior quality cap coupling to the grid rather than a coupling cap and a huge cap in the cathode which are both in series with the signal . YYMV and all that
Al
Follow Ups:
Thanks for your thoughts Al.
I suspect my next build - which has a 6C6 pentode input directly coupled to the output tube - will have very high gain. Even when bypassed the plate impedance is so high that the plate R essentially sets the output impedance. So, perhaps I could experiment with running no cathode bypass capacitors... it would lower gain and increase plate impedance, but this may not be an issue. Maybe I should look into that?
Regards,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
High gain ? Use a lower value load and you'll get lower Rout and lower gain from the driver . Why not place the cathode circuit and G2 of the 6C6 within the cathode circuit of your output valve ? You may only need a single bypass cap between output stage cathode and 0V as you'll be running tens of milliamps through the cathode bias stack . I have only tried this with a triode connected D3A but it should in theory work with a pentode . The 'full monkey' used a similar scheme if I remember correctly
Al
Ah, thanks Al... I remember you explaining that when we last "chatted" on the forum. Thanks for the option!
Cheers,
91.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
Knock up a breadboard and give it a try :)
Al
There are a number of techniques available to send the cathode resistor bypass cap. "boogeyman" running. LEDs were mentioned. Another method is the use of a NiMH cell stack between the cathode and ground. Charge the stack up before 1st turn on. The "trickle" of current between cathode and ground keeps the stack charged.
Eli D.
.
"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein
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