Re: AC/DC 6BQ5?

PhattyMo wrote:
Bret Ludwig wrote:
On Jan 15, 8:37 pm, "dre7" <d...@xxxxxxxxxxxxx> wrote:
Thanks! I will.

Andrew

John Byrns <byr...@xxxxxxxxxxxxx> wrote in message

news:byrnsj-3CF5C7.20081415012008@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

In article <00djj.39881$Pv2.21...@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
"dre7" <d...@xxxxxxxxxxxxx> wrote:
Hey RATs:
I'm thinking of building a small potato amp with a couple of 6.3v tubes
and
was considering a transformerless power supply. I understand how to
get my
B+, but filament voltage I'm not so sure about. Can I just string the
filaments in series for 12.6v and use a high voltage cap in series in
front
of that to lower the voltage? I've seen something similar for knocking
filament voltage down in AA5 radios.
Take a look at the 29GK5, it has similar characteristics to the 6BQ5,
but with a 150 mA 29 volt heater which would be a better match to your
application.

A better idea is to use a power transformer with a 6.3 volt winding
and a HV winding. These are readily available and provide better
results than harebrained schemes or using tubes which will probably
never be manufactured again and which are not readily available
through retail channels.



Not to mention the lack of isolation from the line,and the potential for a NASTY shock/surprise. (Smoking interconnect cables are not cool!)

If you DO go ahead with the line-op scheme,make DAMN SURE you've got the Hot/Neutral polarity correct,and earth-ground the chassis -Even with grounding and all of that,It is still possible to get a nasty shock,or worse.

I'd suggest a power trans.For a spud amp you won't need a very big expensive one.
One other altenative is to use two small transformers back-to-back,Something like the pic here.
http://www.dogstar.dantimax.dk/tubestuf/graphics/minib240.gif
The first transformer needs to be large enough to support your filament power,and your B+ (from the second trans.) So,maybe it should be 2x the size(VA) of the second transformer.
I've used this trick before,and it's worked well.

Another 'trick' I've done,is to use a larger power transformer with DUAL primaries. Connect one primary to the line,and use the second primary as your B+ winding. You can usually find such transformers with 6.3 or 12.6V secondaries,so you can have filament power from it also.The one drawback to this approach,is that the transformer has to be a fair bit larger than 'normal' because you're only using one primary to feed it.
this trick has also worked well,aslong as I've used a sufficiently large transformer.
I did this trick once for a small tube amp and it worked well,but the transformer ran a bit hot because I didn't over-rate it,as mentioned above.

Isolation is key..You don't want a speaker,or input jack,etc. floating at 120V!

I'd argue against the dual-primaries trick, on safety grounds.
I'll use the numbers for Australia, convert to (eg) US ones as applicable.
Here the AC supply is 240V, and pri/sec insulation is tested to 3500V RMS. I once heard that figure justified, on the grounds that 3300V is run along the top of the power poles, with 240V on a crosstree just below. Should the 3300V line break, it might touch the 240V one before the breakers trip. So 3300V on a "240V" line. Hence 3500V proof-test.

Anyways, the point is that the two primaries will not be insulated from each other to anything like that much: possibly just 500V. So if you use one as a B+, you have just 500V isolation from mains.
.

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