Tube challenge!

Harry Bissell harrybissell at prodigy.net
Mon Nov 22 09:07:10 CET 1999


Hi Bill (et al)

I'm familiar with the "decoupled nodes" idea... every guitar amp I've
ever seen does it. This has two advantages
1) the highest voltage is usually at the output stage, where you also
get the lowest power supply impedance (good)
2) Each preceeding stage gets lower voltages (and higher impedances from
a power supply standpoint) So headroom is always increasing (good) and
progressivly smaller caps (and lower voltage caps) become more
effective...

OTOH: there is still a lot of interaction on the supply. A good current
pull in the output stage will usually sag the supply a lot, as well as
"reflect" the current waveform up the line... each "decoupled" node will
get this surrent wavform.... only phase shifted so to speak. This might
or might not give audible artifacts depending on the time constants
involved...

OTOOH: These artifacts are what give the tube circuit its "character"
and transform a "sterile" sounding guitar amp into a  "toneful" unit...

Just like solid state... good decoupling is hard to find and often NOT
found at all... but it is easier to prevent solid state devices from
interacting than tubes. As you noted, the 78xx450 is NOT a readily
available component (at least not in a TO-220 package)
:^) Harry (who contrary to popular belief is not 'anti-tube' otoh he is
not planning a 'tube-synth' in the near future) Bissell



Bill Layer wrote:

>  Hello All,
>
>
>> With the high impedance of tubes... if you want to get "zero"
>> interaction through the supply you should go separates.
>
>
> Don't confuse the impedance of the tube's grid-cathode circuit with
> the source impedance of the power supply.. Tube sections are actually
> LESS likely to interact, as the currents they draw are less than
> similar solid-state devices. Assuming supplies of similar impedance
> (probably not safe :) interactions should be less.
>
> The interaction fear stemms from the fact that it is a little harder
> to regulate a 400-600V circuit than a 12V circuit in a world of
> 7812-like  devices.
>
> In an audio circuit, it is not uncommon to have several progressive
> stages all powered off of the same supply, or at least decoupled nodes
> of the same supply.
>
>  +----------------------------------------------------------+|
> "The" Bill Layer - Frogtown, Minnesota. U.S.A.      || Vacuum tubes,
> Analog, Motorcycles and Other Alternatives
> |+----------------------------------------------------------++---------------------+
> +---------------------------------+| <blayer at uswest.net> |  |
> <b.layer at vikingelectronics.com> |+---------------------+
> +---------------------------------+




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