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From: wtm@neoucom.UUCP
Newsgroups: sci.electronics
Subject: Re: Power Supplies for Tube Circuits.
Message-ID: <622@neoucom.UUCP>
Date: Thu, 9-Jul-87 10:37:12 EDT
Article-I.D.: neoucom.622
Posted: Thu Jul  9 10:37:12 1987
Date-Received: Sun, 12-Jul-87 07:41:27 EDT
References: <12887@rose.UUCP> <8240@utzoo.UUCP> <485@jetsun.weitek.UUCP>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 51
Summary: DC power for tube heaters


Hi,

While we're on the topic of tube preamps...  I learned a lot about
the nearly lost art of tube construction methods by looking at old
Tektronix 500 series oscilloscopes and their letter series
plug-ins.  The Tek scopes were about as far as the state of the art
of tube technology went.

In their hight gain amplifers, they often used DC on the tube
heaters to help reduce 60 Hz modulation of the signal being
amplified.

The high gain preamps were also usually isolated from the chassis
by rubber or springy metal strips.  The idea was probably to combat
microphonics from picking up vibrations of the fan.  (Quite a big
fan was necessary to cool down a 551 with its complement of 111
tubes! -- and it had a separate power supply with about a dozen
more tubes for regulating all voltages).  Shock mounting the amp
would probably be useful in combating feedback from the speakers.

One thing that has puzzled me is that some Tektronix plug-ins have
flat rubber coated resistors.  I've wondered what the purpose of
doing so was.  I've never seen any references as to why.

One thing to watch out for in tubes is the potential difference
between the heater and the cathode.  12AX7s permit about 100 vdc
between the heater and cathode.  The main impact is on the life of
the heater, and possible dielectric breakdown of the barrier
between the heater and cathode.  You should be aware of this if you
are designing a circuit that runs one or more of the stages with
the cathode significantly above 0v.  If so, you can float the
filament xformer.  The got-cha is if you feed DC to the heaters,
and the DC supply is ground referenced.

I like the idea of using a switcher to supply the B+.  By using a
switcher, you eliminate bulk and the higher frequency means that
smaller capacitors can be used to provide equal ripple removal.  Be
sure to use good construction, to prevent RF from escaping.  Also,
you need to use good quality capacitors with a high Q rating.
Regular aluminum electrolytics aren't very good.  A small mylar
capacitor in parallel can be used if you can't locate high grade
electrolyic capacitors.  (Save your $$$, and don't bother buying
esoteric polyproylene caps for your power supply).  Who knows, the
polypropylene capacitors may make a difference in interstage signal
coupling-- you be the judge of that.  To me, mylar caps sound
equivalent for audio coupling.

Hope this was of some use.
Bill
(wtm@neoucom.UUCP)