Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Quickie => Topic started by: earwaxxer on March 26, 2012, 01:09:05 PM

Title: Cathode bypass cap options?
Post by: earwaxxer on March 26, 2012, 01:09:05 PM: Hey Paul - I was trying to learn a bit more about the electrolytic cathode bypass. It seems like the approved approach these days is to use a large value like 1000uF in the Quickie. What I'm wondering is how much lower I could go and not loose bass. I'm thinking about how to replace the electrolytic with a film cap. Not real practical at 1000uf, but certainly 500uf is doable. Bypassing the electrolytic seems a bit questionable in terms of sonic benefit. Not sure as well about how much ESR would benefit by paralleling the load over 3 caps. Just thinking. Any info would be appreciated!

thanks - Eric
Title: Re: Cathode bypass cap options?
Post by: Paul Joppa on March 26, 2012, 02:43:21 PM: I think around 40uF should be satisfactory. The 1000uF is just something big enough to not lose on the workbench :^) seriously, at that low a voltage the cap is very cheap.
Title: Re: Cathode bypass cap options?
Post by: earwaxxer on March 26, 2012, 05:34:05 PM: wow! how cool its that! Thanks Paul.
Title: Re: Cathode bypass cap options?
Post by: Grainger49 on March 27, 2012, 01:37:23 AM: In the Quickie the voltages are so low capacitors will be inexpensive. Even an electrolytic Black Gate would be affordable.
Title: Re: Cathode bypass cap options?
Post by: corndog71 on March 27, 2012, 06:08:36 AM: Ooh! Good to know!
Title: Re: Cathode bypass cap options?
Post by: Dr. Toobz on April 12, 2012, 11:59:47 AM: If one is using the PJCCS to keep the tube current constant as the battery voltage drops, does that now open up the possibility of using LED bias vs. the resistor/bypass cap combo? Seems like many common LED's would result in the usual 2VDC at the cathode.
Title: Re: Cathode bypass cap options?
Post by: Paul Joppa on April 12, 2012, 01:27:28 PM: The PJCCS does not keep the current constant, it reduces the current in proportion to the battery voltage. It also adjusts the bias voltage in the same proportion, since that current flows through the bias resistor. This way the plate voltage is maintained at about the same proportion of the battery voltage so the circuit continues to operate throughout the battery life.

If you wanted an LED bias, you could add one or two 9-v batteries and use a regular C4S current source. The extra batteries are needed to keep the C4S input voltage well above the plate voltage as the batteries fade out.

A reasonable alternative to extra batteries is to use NiCd or NiMH batteries, which have a relatively flat discharge. Unfortunately, many of them run low, like 7.2v, and you'd want the full 36v - so you are back at five "9-v" batteries - or a custom battery pack and associated charger. Starting to get complicated now...
Title: Re: Cathode bypass cap options?
Post by: Dr. Toobz on April 12, 2012, 02:28:09 PM: Very interesting, and very clever. Thanks for the explanation of how the PJCCS works; it's different than I had assumed.