Scott Dorsey wrote:



You're not missing anything. Although I recommend buying only caps
rated for 105'C because they seem also to last longer than the 85'C
spec ones. But that's exactly what I mean.
-scott

The way it was explained to me is that the 105's use a different
electrolyte and tend to have lower ESR, etc. I used to bench repair a
certain power supply that would make short work of ruining an 85...only
a 105 would hold up. And it wasn't a heat issue.
They seem to be worth the few extra pennies in cost.

-Bill

Putting a series resistor would degrade the power supply regulation
performance. This may or may not be important, depending on application.

Yes, but it will degrade it to the same level which using the original
rectifier tube would.
--scott

A vacuum rectifier shows a non linear behaviour, whereby the voltage drop vs. current tends to saturate (until one exceeds the rectifier rating). So, using a resistor to simulate the vacuum rectifier voltage drop, would cause a worse regulation around the rectifier operating point.

But even disregarding the above consideration, putting a resistor instead of a zener diode would mean losing a chance to improve things.

73

Tony I0JX

Here is an example. Let us assume we have 100 volt primary and 100 volt
secondary, and that the normal rectifier has a 10 volt voltage drop. We
replace the tube with 2 series diodes, which have a 1 volt total drop and
then add 9 volts of zeners. That gives us a 10 volt drop, equal to the
tube. Now, assume there is a 10 volt drop in primary line voltage.
Secondary voltage is 90, less 10 volts, which is 80 volts. The output
voltage has dropped by 11%, while the input voltage has dropped 10%. If we
still had the original tube (or a diode and resistor), then the 10 volt
primary drop will result in 90 volts on the secondary - but the voltage drop
will be fairly linear across the tube (and resistor), so will only be
probably 9 volts. Resulting voltage out of the power supply is 81 volts
rather than the 80 volts using the zener diode. In that case, primary
voltage drops 10% and seconddary voltage drops 10%.

Contrary to regulated power supplies, the regulation performance of a non-regulated power supply is defined with regard to load variations, not to mains variations. As a matter of fact a non-regulated supply is inherently unable to tolerate mains variation, whilst it can tolerate load variations if properly designed (think of input-choke power supplies). Regulation (against load variations) greatly benefits from having a series zener in place of a series resistor.

It is however true that, for a mains voltage drop, using a zener diode will cause an extra small output voltage drop.

I use a series zener diode in my car to drop the battery voltage down to about 9V, as required by my VHF handy-talkie. If I would have used a resistor instead, voltage would significantly vary from reception to transmission.

Clearly there is no need at all to use an expensive power zener diode, as a plain power transistor with a small zener connected between collector and base does precisely the same job.

73

Tony, I0JX

Antonio Vernucci wrote:

A vacuum rectifier shows a non linear behaviour, whereby the voltage =
drop vs. current tends to saturate (until one exceeds the rectifier =
rating). So, using a resistor to simulate the vacuum rectifier voltage =
drop, would cause a worse regulation around the rectifier operating =
point.

Ahh! That makes perfect sense! I had not thought about that... I guess
I always just thought about the diode conductance as being a fixed value.

Hmm.... I just got out the old RCA Receiving Tube handbook and the only
rectifier I can find with actual curves is the 5Y3... the curve is pretty
flat, but you're right that it isn't linear. No knee on it, though.

Is this typical? Are the gas-filled types significantly different (other
than the turn-on discontinuity)?

But even disregarding the above consideration, putting a resistor =
instead of a zener diode would mean losing a chance to improve things.

True.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Hmm.... I just got out the old RCA Receiving Tube handbook and the only
rectifier I can find with actual curves is the 5Y3... the curve is pretty
flat, but you're right that it isn't linear. No knee on it, though.

Is this typical? Are the gas-filled types significantly different (other
than the turn-on discontinuity)?

All vacuum rectifiers show a similar behaviour, corresponding to the so-called 2/3 law. The I-V curve is typically S-Shaped. At low currents, voltage drop grows with current. For higher currents, voltage drop does not grow too much with current.. If current exceeds tube ratings, voltage drop again strongly grows with current.

I could not find the curve for a mercury rectifier. An old RCA tube handbook reports:

- for model 83 (mercury rectifier): voltage drop: 15V
- for model 5U4 (vacuum rectifier): voltage drop: 44V at 225mA, 50V at 275mA, 54V at 300mA

73

Tony, I0JX

Bob,
Yes.
I happen to be restoring an HX-50 at this time. I believe the rectifier
tube is a 5R4GYB. The specs can be found on an application at:
http://tdsl.duncanamps.com/ (I use it a lot); There is it specifies a
voltage drop of 63v at 250ma DC. Also PIV=3100; I Cathode is 715ma max;
IRect is 250ma; RMS input is 900-0-900v. Also see ratings chart at:
http://tdsl.duncanamps.com/pdf/5r4.pdf . The final setting of the resistor
will ultimately be determined by obtaining the proper B+.

Some one did the same thing to the unit I am restoring. So I will be doing
the above exercise too.

Were did you get the 500 volt electrolytic?

I hope this helps.

Paul P.

PS - I'd like to hear about your experiences. I am trying to get a clean
schematic. BAMA is missing significant parts of the schematic art work
around the Power supply 5R4.


wrote in message
ups.com...
Is the voltage drop across a vacume tube rectifier significantly higher
than a solid state plug in replacement? Previous owner of my HX-50
changed over to SS. I am changing out capacitors and just curious.

73
Bob
N9NEO