Re: Recommend tutorial on testing ss parts

On 7 Mar 2007 07:57:45 -0800, "Andy Evans"
<performanceandmedia@xxxxxxxxx> wrote:

On Mar 7, 3:34?pm, nos...@xxxxxxxxxx (Don Pearce) wrote:
On 7 Mar 2007 07:22:48 -0800, "Andy Evans"

<performanceandme...@xxxxxxxxx> wrote:
Is there a schematic somewhere? It would be interesting.

d

--
Pearce Consultinghttp://www.pearce.uk.com-Hide quoted text -

- Show quoted text -

I emailed you a generic schematic. This shows two LEDs at the bottom,
where Morgan Jones version has one LED and a sensor resistor between
the bottom BC459 and the -15v supply.

There's a big problem there, Andy. You need a resistor in Q2 emitter
to determine the current in this source. At the moment Q2 is
conducting hard and not setting anything.

I do have a resistor between Q2 and -15v supply. It's not shown in the
diagram I sent, which is from somewhere else.

The circuit appears to need 10mA shared between the two valves.

that's a typical current, yes

Assuming that Q2 emitter is 3.4V above the -15V line (two LEDs, minus
a base-emitter junction), the resistor should be about 350 ohms.

In Morgan's design which I use there's one LED from the -15v supply,
with -13.3v on the other side of it. Sensor resistor is around 200
depending on the valves.


Two is better - more thermal stability.


Finish off with an electrolytic capacitor between Q1 base and -15V to
prevent any hum modulation of the valve current, and it should all
function a bit better.

Hmmm - that's not in Morgan's design - this is a good tweak? Thanks
for all this. Doesn't explain the BC546 issue though. According to the
datasheets I've seen, BC546-550 are all NPN. My BC546s seem to have
the right pinout.


I've seen it both ways on two web sites - do check.

d

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Pearce Consulting
http://www.pearce.uk.com
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