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From: neal@weitek.UUCP
Newsgroups: sci.electronics
Subject: Re: Power Supplies for Tube Circuits.
Message-ID: <485@jetsun.weitek.UUCP>
Date: Tue, 7-Jul-87 07:53:06 EDT
Article-I.D.: jetsun.485
Posted: Tue Jul  7 07:53:06 1987
Date-Received: Thu, 9-Jul-87 06:11:39 EDT
References: <12887@rose.UUCP> <8240@utzoo.UUCP>
Reply-To: neal@jetsun.UUCP (Neal Bedard)
Organization: Weitek Corporation, Sunnyvale
Lines: 42

In article <8240@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:
>> ... I could obtain the less than 330 volts needed by
>> directly rectifying the line current and putting it through a voltage
>> doubler, eliminating the power transformer. [...]
>
>The trouble is that the negative end of the power supply is not isolated
>from the AC line and hence can pretend to be "ground" only if the whole
>gadget is self-contained and well-insulated, and you avoid connecting
>things like oscilloscopes to it. I believe some TV sets do their power 
>supplies this way, much to the irritation of TV service people who need to
>haul around an isolation transformer if they want to connect test gear.

They don't call 'em "hot chassis" for nuthin'! Yeow!

> [...] For a pre-amp, you are presumably going to be connecting it to other
>equipment.  Isolating those connections is going to be hard, probably too hard
>to be worthwhile.  Accordingly, I would give this a big black NOT ACCEPTABLE.
>Use a transformer.

One way to reduce the bulkiness (but not neccessarily the cost) of the setup
is to rectify the AC directly, and use that as the power supply of a DC-DC
converter. Not only do you get the isolation (thx to the ET transformer) but
you also get better regulation on the B+ (300V) supply. However, you still need
filament voltage (a small/cheap transformer can do this.)

Another approach is to use a low (filament) voltage transformer with enough
excess capacity to drive a "flyback" DC-DC converter for the B+ supply. You get
isolation, simple B+ regulation, and filament voltage too.

However, if you're lazy (or dislike working with switchers), you can probably
scrounge an all-in one B+/filament transformer pretty cheap from a surplus
house. You'll have to filter the B+ supply with some big HV caps, tho'. Ick.
One advantage: transformers tend to "swallow" line spikes.

Henry's right, tho' - whatever you do, do not hook the circuit innards directly
to the AC line. I wouldn't recommend the use of an "auto-transformer" either,
since these connect you to at least one leg of AC (50-50 chance it'll be the
"wrong" one.) Be careful.



-Neal