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 It is currently 27 Oct 2020, 02:18

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 Post subject: 12ax7 cathode bypass cap valuePosted: 11 Jul 2020, 16:56

Joined: 23 Jun 2020, 18:22
Posts: 2
Trying to re-build or at least make some new boards to use in my Radford SC3 based prep-amp and 88-50 Wireless world 1957 based power amp circuits that have been very good for many years.
They are using Tantalum 50uf cathode bypass capacitors – only problem is not able to find sensible priced tants any more.

I can find 47uf or 56uf – been way too long since calculated (12ax7) triode circuits but I’m assuming 47uf will increase the low cut and the 56uf would lift the low end – An sure I’m missing something here so any help would be useful.

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 Post subject: Re: 12ax7 cathode bypass cap valuePosted: 21 Jul 2020, 08:53

Joined: 03 Jun 2020, 11:06
Posts: 83
Location: France
Hi @halibut,

The cut-off frequency (-3dB) is given by : Fc=1/2Pi.RC
Where R ohms = cathode R and C farads = bypass cap.
If it was me, I would test a 100µ or over, but between 47 and 56µ, there won't be much difference IMHO.

-tbln.

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 Post subject: Re: 12ax7 cathode bypass cap valuePosted: 02 Aug 2020, 12:15

Joined: 23 Jun 2020, 18:22
Posts: 2
Many thanks!
Got some 68uf Tants - hopefully ok.

The 50/88 power amp was very sensitive to speaker type (maybe crossovers) but uncontrolled oscillation @ supersonic frequencies was observed when I built it - like output of 20v at 30khz - change of speaker and modification of feedback cured it - so was nervous of changing the bypass uf

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