I'm curious about what is invloved in converting boatanchor
(specifically my NC-125) power supplies from tubes to solid state bridge
rectifiers. I understand that most guys will mount the bridge and
dropping resistor on a tube socket so that the conversion is easily
reversable. How do you determine the difference between (a) the
voltage drop of the tube and the solid state bridge, and (b) the working
current so that you can calculate the appropriate values of the dropping
resistor and its power rating ??
Is there any point to converting the 6.3 volt filament string from
from the ac bus, running all over the chassis spreading 60 hz hum
wherever it goes, to 6.3 volt filtered DC, a rather easy thing to do
these days. Has anyone done this ?? Is it worth it ?? what issues were
involved ?

William Mutch wrote:
I'm curious about what is invloved in converting boatanchor
(specifically my NC-125) power supplies from tubes to solid state bridge
rectifiers. I understand that most guys will mount the bridge and
dropping resistor on a tube socket so that the conversion is easily
reversable. How do you determine the difference between (a) the
voltage drop of the tube and the solid state bridge, and (b) the working
current so that you can calculate the appropriate values of the dropping
resistor and its power rating ??

One approach...

Often the increase in B+ is not enough to fret over. But if you want to
do it "right" its a simple matter of measuring the B+ with the tube and
then remeasuring with the ss rectifier to see how much you want to drop.
Start by using say a 100 ohm, coupla watt resistor and you can
calculate the current draw and then calculate a final value accordingly.

This also gives the advantage of KNOWING the ACTUAL B+ current draw
which may point you to other problems the set may have like leaky caps,
too much audio tube current draw, etc.-

-Bill

Regarding the conversion of vacuum tube rectifiers to solid state, I believe
there are serious issues that need to be addressed.

I have repaired a number of Collins 75A4 receivers that had leaky mica
coupling caps after a solid state rectifier is used. The reason is that the
solid state rectifier puts out the full voltage prior to the vacuum tubes
warming up. Thus, there is very little load on the power supply and the
voltage soars to something around 500 volts, which causes the mica coupling
caps in the if stage to become leaky, causing the grids to go positive and
reducing sensitivity.

A second problem is that high voltage is applied to the tubes and current is
drawn before the tube is warmed up. We do not do that to indirectly heated
transmitter tubes, and for long life we should not do it to receivers,
either.

The higher voltage is a secondary issue, although that can be easily solved.

I built a solid state supply for my KWM-2A. It has one minute delay prior
to applying high voltage and all the voltages are within original specs. I
have not changed tubes since about 1980, when I made the power supply, even
though it was on daily for over 10 years.

DC on the filaments can be another long topic. There is some evidence that
dc on the filaments will shorten the filament life of tubes, because the
electron emission is off one end of the filament. Regular changing of
polarity would reduce that problem. As to what that practical effect is, I
do not know. A pilot light manufacturer rated some pilot lights for so many
hours of use for military lighting. After most failed to come close to the
life specifications, the manufacturer studied the problem and realized all
the tests had been on ac rather than dc. When run on dc, the life was
diminished.

I do not know of any studies that apply the same effect to vacuum tubes, but
it probably does. However, very few tubes fail because their filaments
fail, so maybe it is not a big issue.

However, a number of things can be done to reduce the hum, when using ac -
such as applying a dc bias to the filaments, which are otherwise isolated
from ground, or grounding only through a ct transformer and have both sides
of the filament above ground.

73, Colin K7FM


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COLIN LAMB wrote:

Regarding the conversion of vacuum tube rectifiers to solid state, I believe
there are serious issues that need to be addressed.

I have repaired a number of Collins 75A4 receivers that had leaky mica
coupling caps after a solid state rectifier is used. The reason is that the
solid state rectifier puts out the full voltage prior to the vacuum tubes
warming up. Thus, there is very little load on the power supply and the
voltage soars to something around 500 volts, which causes the mica coupling
caps in the if stage to become leaky, causing the grids to go positive and
reducing sensitivity.

I won't dispute your findings but if the caps are bad (leaky) they
should be replaced as a matter of course in refurbing. The higher
voltage wouldn't appear to *make* them "become leaky". There's better
ways to check them rather than "smoke test".
The adages from the old days that "mica caps don't go bad" are falling
to the wayside these days. They may not be as circumspect as a paper
cap but the failure rate nowadays is becoming more than negligible. I
don't trust them anymore in a 50-60 yr old radio but I haven't reached
the point of shot-gunning them like paper caps. I suspect that the time
will soon come where that is an efficient way of dealing with them. In
the meantime, Q-challenged circuits will simply have to deliver poor
performance due to leakage until the hapless 'restorer' sees some real
visible sparks flying to believe there is an issue with these old boys.

A second problem is that high voltage is applied to the tubes and current is
drawn before the tube is warmed up. We do not do that to indirectly heated
transmitter tubes, and for long life we should not do it to receivers,
either.

A CL-90 inrush protector can go a long way where this is a concern.

-Bill M

Other comments well taken, I have in the past simply dropped in the
octal, solid-state replacements into various equipments without any
undue effect(s)... or so it would appear. Having said that, I no longer
convert to solid state, nor build up power supplies for older/surplus
equipment without the ability to turn-on filaments first. The original
tubes are still cheap and readily available. What I have found is that
if I run my vintage equipment at the rated input voltage (typ 110v) via
variacs in my distribution bus, I do not have the heat problems I did
when simply plugging them into my house 125v service, directly. The
heat difference was quite amazing!

COLIN LAMB wrote:


I have repaired a number of Collins 75A4 receivers that had leaky mica
coupling caps after a solid state rectifier is used. The reason is that the
solid state rectifier puts out the full voltage prior to the vacuum tubes
warming up. Thus, there is very little load on the power supply and the
voltage soars to something around 500 volts, which causes the mica coupling
caps in the if stage to become leaky, causing the grids to go positive and
reducing sensitivity.

Most rectifier tubes have directly heated cathodes and warm up in a
matter of seconds...way before the other tubes with indirectly heated
cathodes warm up enough to draw current. Other than a reduction in
inrush current during the rectifier warmup, you will have the same
problem with tube rectifiers. A possible solution is the use of a
NTC thermistor in the AC line.

73, Roger

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Remove tilde (~) to reply

Remember the USS Liberty (AGTR-5)
http://ussliberty.org/

Roger has a good point about the warm up time of most rectifier tubes,
although I have found the delay is a matter of seconds and helps a lot.

For better protection, it is easy to substitute indirectly heated rectifier
tubes for the 5Y3. The 5V4 and the 5Z4 are two tubes that can be
substituted - although it is important to verify that the output is taken
from pin 8.

The 5AT4 may be substituted for the 5U4 in many cases, but be sure to look
at the specifications to confirm it will work. I recall that Eico used the
5AR4 in some of the ham rigs, but the tube manual I grabbed does not have
that one.

We used to consider mica caps as "forever", but the smaller ones are now
fair game.

73, Colin K7FM


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"William Mutch" wrote in message
ell.edu...
I'm curious about what is invloved in converting boatanchor
(specifically my NC-125) power supplies from tubes to solid state bridge
rectifiers. I understand that most guys will mount the bridge and
dropping resistor on a tube socket so that the conversion is easily
reversable. How do you determine the difference between (a) the
voltage drop of the tube and the solid state bridge, and (b) the working
current so that you can calculate the appropriate values of the dropping
resistor and its power rating ??
Is there any point to converting the 6.3 volt filament string from
from the ac bus, running all over the chassis spreading 60 hz hum
wherever it goes, to 6.3 volt filtered DC, a rather easy thing to do
these days. Has anyone done this ?? Is it worth it ?? what issues were
involved ?

Why would you want to use a solid state power supply in the first place.
The original power supply is not as efficient but unless you are operating
from batteries the extra 10 watts or so will never be noticed.

Hi all,

I've heard and read a lot of discussion on the "problem" of the
B+ being applied before the tubes can warm up. However, plenty
of tube-type HF rigs came from the factory with solid-state power
supplies: Drake, Swan, Heath, National, Galaxy, et al. Did they
do something to make their gear more tolerant? Just curious.

73,
Dean K5DH


In article . net,
says...

Regarding the conversion of vacuum tube rectifiers to solid state, I bel
ieve
there are serious issues that need to be addressed.

I have repaired a number of Collins 75A4 receivers that had leaky mica
coupling caps after a solid state rectifier is used. The reason is that
the
solid state rectifier puts out the full voltage prior to the vacuum tube
s
warming up. Thus, there is very little load on the power supply and the
voltage soars to something around 500 volts, which causes the mica coupl
ing
caps in the if stage to become leaky, causing the grids to go positive a
nd
reducing sensitivity.

A second problem is that high voltage is applied to the tubes and curren
t is
drawn before the tube is warmed up. We do not do that to indirectly hea
ted
transmitter tubes, and for long life we should not do it to receivers,
either.

The higher voltage is a secondary issue, although that can be easily sol
ved.

I built a solid state supply for my KWM-2A. It has one minute delay pri
or
to applying high voltage and all the voltages are within original specs.
I
have not changed tubes since about 1980, when I made the power supply, e
ven
though it was on daily for over 10 years.

DC on the filaments can be another long topic. There is some evidence t
hat
dc on the filaments will shorten the filament life of tubes, because the
electron emission is off one end of the filament. Regular changing of
polarity would reduce that problem. As to what that practical effect is
, I
do not know. A pilot light manufacturer rated some pilot lights for so
many
hours of use for military lighting. After most failed to come close to
the
life specifications, the manufacturer studied the problem and realized a
ll
the tests had been on ac rather than dc. When run on dc, the life was
diminished.

I do not know of any studies that apply the same effect to vacuum tubes,
but
it probably does. However, very few tubes fail because their filaments
fail, so maybe it is not a big issue.

However, a number of things can be done to reduce the hum, when using ac
-
such as applying a dc bias to the filaments, which are otherwise isolate
d
from ground, or grounding only through a ct transformer and have both si
des
of the filament above ground.

73, Colin K7FM


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.783 / Virus Database: 529 - Release Date: 10/25/04

If you anyway wish to replace the vacuum rectifier with a solid state =
one, using a voltage dropping resistor is a silly way to go, as it would =
impair the power supply regulation.

My suggestion is: after measuring the voltage increase you have with the =
solid state rectifier, just put an appropriate zener diode in series =
with the DC line. The zener diode causes a fixed drop and does not =
impair regulation.

If, for your application, a high-power (and hence expensive) zener diode =
would be needed, then use the well-known zener diode emulation circuit =
(a power transistor and a small zener).

73

Tony, I0JX

"William Mutch" ha scritto nel messaggio =
ell.edu...
I'm curious about what is invloved in converting boatanchor=20
(specifically my NC-125) power supplies from tubes to solid state =
bridge=20
rectifiers. I understand that most guys will mount the bridge and=20
dropping resistor on a tube socket so that the conversion is easily=20
reversable. How do you determine the difference between (a) the=20
voltage drop of the tube and the solid state bridge, and (b) the =
working=20
current so that you can calculate the appropriate values of the =
dropping=20
resistor and its power rating ??=20
Is there any point to converting the 6.3 volt filament string from =20
from the ac bus, running all over the chassis spreading 60 hz hum=20
wherever it goes, to 6.3 volt filtered DC, a rather easy thing to do =

these days. Has anyone done this ?? Is it worth it ?? what issues =
were=20
involved ?