I'm considering adding a stand-by switch to my Audio Innovations S500.
Currently it can only be turned on or off, and since it's class A with 390V
plates it's sucking a lot of juice and adding unnecessary wear to the tubes
just to warm them up, which I like to do for a good 45 minutes before
playing. I've got the schematics, and I'm reasonably handy with a soldering
iron, so I assume I can pull it off myself.

Here's the power supply schematic:
http://www.drtube.com/schematics/ai/ai500-89-4.gif

As I understand it, stand-by mode delivers power to the heater filaments
only. The way I see it, all I'd need is a 5A switch to interrupt the PT tap
at the top of the schematic (everything but the filament lines). Am I on
track here?

I'd also like to know if it's a bad idea to replace the main power switch
with a 3-position switch to handle all power functions, which would be nice
and tidy, but would mean there'd be a 390V line from the PT to the power
switch and back. I'm also considering a relay that would be powered from a
filament line in stand-by mode, would that be worse for sound quality than
the extra wiring? Or should I play it safe and mount a dedicated switch
near the PT?

Based on what I've posted here, is this something I can figure out myself,
or is it complex enough that I should have it done professionally?

I appreciate any help!

Sounds like you have a plan to me - you may want to consider adding a cap
across the switch to mitigate arcing - but I believe you have it! You could
use an all in one switch, but I'd keep it simple in case you want to switch
it back. I'm not clear why you need a relay.




--
JAMES RUGGIERI
"Sugarite" wrote in message
...
I'm considering adding a stand-by switch to my Audio Innovations S500.
Currently it can only be turned on or off, and since it's class A with
390V
plates it's sucking a lot of juice and adding unnecessary wear to the
tubes
just to warm them up, which I like to do for a good 45 minutes before
playing. I've got the schematics, and I'm reasonably handy with a
soldering
iron, so I assume I can pull it off myself.

Here's the power supply schematic:
http://www.drtube.com/schematics/ai/ai500-89-4.gif

As I understand it, stand-by mode delivers power to the heater filaments
only. The way I see it, all I'd need is a 5A switch to interrupt the PT
tap
at the top of the schematic (everything but the filament lines). Am I on
track here?

I'd also like to know if it's a bad idea to replace the main power switch
with a 3-position switch to handle all power functions, which would be
nice
and tidy, but would mean there'd be a 390V line from the PT to the power
switch and back. I'm also considering a relay that would be powered from
a
filament line in stand-by mode, would that be worse for sound quality than
the extra wiring? Or should I play it safe and mount a dedicated switch
near the PT?

Based on what I've posted here, is this something I can figure out myself,
or is it complex enough that I should have it done professionally?

I appreciate any help!

Sugarite wrote:
I'm considering adding a stand-by switch to my Audio Innovations S500.
Currently it can only be turned on or off, and since it's class A with 390V
plates it's sucking a lot of juice and adding unnecessary wear to the tubes
just to warm them up, which I like to do for a good 45 minutes before
playing. I've got the schematics, and I'm reasonably handy with a soldering
iron, so I assume I can pull it off myself.

Here's the power supply schematic:
http://www.drtube.com/schematics/ai/ai500-89-4.gif

As I understand it, stand-by mode delivers power to the heater filaments
only. The way I see it, all I'd need is a 5A switch to interrupt the PT tap
at the top of the schematic (everything but the filament lines). Am I on
track here?

That would work, but I'd switch the negative lead of the bridge
rectifier (the upper one, of course). That way, at least one end of the
switch is always grounded, and the other end is grounded during operation.

I'd also like to know if it's a bad idea to replace the main power switch
with a 3-position switch to handle all power functions, which would be nice
and tidy, but would mean there'd be a 390V line from the PT to the power
switch and back.

Yes, a DPDT (double pole, double throw) center-off switch would work,
but would be awkward because "off" is in the middle...

I'm also considering a relay that would be powered from a
filament line in stand-by mode, would that be worse for sound quality than
the extra wiring?

A relay is just a switch, actuated by a magnetic field instead of a
fingertip. There is no sonic advantage. The only advantage is that the
switch would only see low voltage, to actuate the coil. The actual
high-voltage switching is done by the relay.

Relays are generally only advisable if you want to implement delays, or
simple logic circuitry (e.g. a interlock switch, etc.)

Or should I play it safe and mount a dedicated switch
near the PT?

That would be my vote.

Based on what I've posted here, is this something I can figure out myself,
or is it complex enough that I should have it done professionally?

If you are comfortable with drilling the hole, mounting the switch,
cutting the line to the negative lead of the bridge rectifier, and have
the soldering skill to close the break with two wires going to the
switch, you should be alright. ;-)

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

Good question. I have employed numerous methods in addressing this issue.
Your initial idea is simple, requires no big modification or drilling and
will work just fine provided you use a good switch. I recommend putting a
..01uf/ 2000v disc ceramic cap right across the switch contacts to discourage
arcing and soften the potential "pop" you might get in the speaker. Also
keep in mind depending on where you decide to interrupt the HV circuit, if
you lift the load away from a filter cap, the voltage across it will
increase and definitely over-voltage that cap. According to your schematic
you have a first stage Ebb of 390v across a first filter of 400v rating. If
it was my rig, I think I would increase that first filter rating to 500v as
it is too close for comfort. I don't know for sure but - you may want to
change out your bridge rectifier while you are in there for a higher peak
current rating. You are going to subject it to a higher peak current now
that you are "hot switching" a load to it rather than waiting for the B+
current to ramp up with the filaments.

Another method could be to insert a large value resistor in series with the
B+ supply and have your standby short this resistor out for a semi "soft
start" condition. This is also very simple to employ and could be a bit more
gentle on your system when switching from stby to on.

For another aproach altogether - I have employed relays to do this with a
simple 555 timer and a transistor switch driving the relay. You can set this
up with a small handful of parts and a little PC board although the relay
itself should be nice & hefty IMO. The "time out" could be adjusted with a
particular combo of 1 resistor and 1 cap at the timer chip to provide the
apropriate B+ delay. This circuit is dead simple and should be available in
any basic timer cookbook or a number of "elec circuits" cookbooks.
Todd