30 Aug
2016
30 Aug
'16
1:55 p.m.
Still having efficiency issues with my K2RIW amplifier. I can get 500W out in Class C, but
I seem to be stock at about 45% efficiency, which would be OK for AB1 but is low for C.
I've tried cleaning and polishing the output stripline, the fingerstock connectors,
double checked the RF chokes and tuning and loading flaps, but no significant improvement.
One thing I do notice is that the output coupling flap is just about at the end of
it's range (minimum capacitance - maximum distance from the stripline) using the
original dimensions. I tried making it slightly smaller, but that didn't improve
things either.
So I'm thinking of maybe going for fresh tubes to replace the current ones which were
used and pulled from service sometime back in the 1970s. I'm wondering what a fair
price these days would be for a pair of 4XC250R tubes, "new old stock", made 20+
years ago. They are hard to find, so I'd like to get a ballpark figure of what they
typically sell for in case I find some and need to make a quick decision. I resume they
sell at a premium over the 4CX250B, which seem to go for anything from $50-$200 each on
ebay depending whether they were made in the USA, Russia or China!
Maybe new tubes won't improve the efficiency, but it might be worth a shot. 500W out
is OK, but at only 45% efficiency I'm pushing my Variac and power supply quite hard
and I'm getting close to tripping the 15A braker on my 120v line...
30 Aug
30 Aug
2:26 p.m.
Bob,
Like a lot of things these days, price depends on want or need more than value most of the
time...I got lucky and found a NOS Eimac matched pair at a hamfest in a big box of NOS
tubes I paid 30 bucks for. They still show up in sealed cans from Eimac and Amperex that I
have seen although there are likely other old-school mfgrs productions around too.
Amperex 7580Ws(iirc) are highly recommended....73...Jordan VE6ZT
----- Original Message -----
From: Bob Atkins
To: moon-net(a)mailman.pe1itr.com
Sent: Tuesday, August 30, 2016 11:55 AM
Subject: [Moon-Net] Fair price for NOS 4CX250R?
Still having efficiency issues with my K2RIW amplifier. I can get 500W out in Class C,
but I seem to be stock at about 45% efficiency, which would be OK for AB1 but is low for
C. I've tried cleaning and polishing the output stripline, the fingerstock connectors,
double checked the RF chokes and tuning and loading flaps, but no significant improvement.
One thing I do notice is that the output coupling flap is just about at the end of
it's range (minimum capacitance - maximum distance from the stripline) using the
original dimensions. I tried making it slightly smaller, but that didn't improve
things either.
So I'm thinking of maybe going for fresh tubes to replace the current ones which
were used and pulled from service sometime back in the 1970s. I'm wondering what a
fair price these days would be for a pair of 4XC250R tubes, "new old stock",
made 20+ years ago. They are hard to find, so I'd like to get a ballpark figure of
what they typically sell for in case I find some and need to make a quick decision. I
resume they sell at a premium over the 4CX250B, which seem to go for anything from
$50-$200 each on ebay depending whether they were made in the USA, Russia or China!
Maybe new tubes won't improve the efficiency, but it might be worth a shot. 500W out
is OK, but at only 45% efficiency I'm pushing my Variac and power supply quite hard
and I'm getting close to tripping the 15A braker on my 120v line...
------------------------------------------------------------------------------
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Moon-Net posting and subscription instructions are at
http://www.nlsa.com/nets/moon-net-help.html
5:06 p.m.
Bob- My suggestion is to acquire some NOS Y-400 Eimac (several pairs on eBay right now
for around 150.00, not a bargain but a fair price) or NOS Eimac or Amperex 7580/4CX250R.
The 4CX250R's show up NOS pretty regularly, at bargain prices sometimes, but I do
recommend the Y-400. Byron W5FH
On Tuesday, August 30, 2016 1:30 PM, Jordan Arndt <Outposter30(a)shaw.ca> wrote:
#yiv7583484886 P {MARGIN-TOP:0px;MARGIN-BOTTOM:0px;}Bob, Like a lot of things these days,
price depends on want or need more than value most of the time...I got lucky and found a
NOS Eimac matched pair at a hamfest in a big box of NOS tubes I paid 30 bucks for. They
still show up in sealed cans from Eimac and Amperex that I have seen although there are
likely other old-school mfgrs productions around too. Amperex 7580Ws(iirc) are highly
recommended....73...Jordan VE6ZT
----- Original Message ----- From: Bob Atkins To: moon-net(a)mailman.pe1itr.com Sent:
Tuesday, August 30, 2016 11:55 AM Subject: [Moon-Net] Fair price for NOS 4CX250R?
Still having efficiency issues with my K2RIW amplifier. I can get 500W out in Class C,
but I seem to be stock at about 45% efficiency, which would be OK for AB1 but is low for
C. I've tried cleaning and polishing the output stripline, the fingerstock connectors,
double checked the RF chokes and tuning and loading flaps, but no significant improvement.
One thing I do notice is that the output coupling flap is just about at the end of
it's range (minimum capacitance - maximum distance from the stripline) using the
original dimensions. I tried making it slightly smaller, but that didn't improve
things either.
So I'm thinking of maybe going for fresh tubes to replace the current ones which were
used and pulled from service sometime back in the 1970s. I'm wondering what a fair
price these days would be for a pair of 4XC250R tubes, "new old stock", made 20+
years ago. They are hard to find, so I'd like to get a ballpark figure of what they
typically sell for in case I find some and need to make a quick decision. I resume they
sell at a premium over the 4CX250B, which seem to go for anything from $50-$200 each on
ebay depending whether they were made in the USA, Russia or China!
Maybe new tubes won't improve the efficiency, but it might be worth a shot. 500W out
is OK, but at only 45% efficiency I'm pushing my Variac and power supply quite hard
and I'm getting close to tripping the 15A braker on my 120v line...
_______________________________________________
Moon-Net posting and subscription instructions are at
http://www.nlsa.com/nets/moon-net-help.html
_______________________________________________
Moon-Net posting and subscription instructions are at
http://www.nlsa.com/nets/moon-net-help.html
6:38 p.m.
Hi Bob,
Unlike transistor amplifiers, tetrode amplifiers are fairly easy to
understand. Capacitances do not change when voltage is changed.
When running in class C you have some grid current. For full
power something like 25 mA in each tube with -90V on the
grids. This is easy to measure.
If g1 is OK, then monitor screen grid current and plate current
as you tune the tank circuit. The screen grid is a voltage probe
for the plate. If your load is not coupling well enough, the AC
voltage on the plate will go very high and the voltage will
periodically be much lower than the screen grid voltage. In such
a case the screen grid current becomes very high at the point
of maximum output power. In such a case you need stronger coupling.
If, on the other hand coupling is too strong, the plate will always
be at a higher potential than the screen grid in which case the
g2 current would be low or negative. In this case you need weaker
coupling.
The nice thing with tube amplifiers is that you can remove all
voltages and run the amplifier cold. Put a suitable resistor
between plate and screen grid. It should have the value of the
output impedance of the tube at the desired operating conditions.
Then send some (low) power into the plate resonator through the
antenna connector. Tune for a good VSWR. Then replace the resistor
with one of half the resistance and verify you can match that
one also and finally verify that you can match a resistance
twice above the desired.
For details, look here: http://www.sm5bsz.com/power.htm
73
Leif
Still having efficiency issues with my K2RIW
amplifier. I can get 500W out in Class C, but I seem to be stock at about 45% efficiency,
which would be OK for AB1 but is low for C. I've tried cleaning and polishing the
output stripline, the fingerstock connectors, double checked the RF chokes and tuning and
loading flaps, but no significant improvement. One thing I do notice is that the output
coupling flap is just about at the end of it's range (minimum capacitance - maximum
distance from the stripline) using the original dimensions. I tried making it slightly
smaller, but that didn't improve things either.
So I'm thinking of maybe going for fresh tubes to replace the current ones which were
used and pulled from service sometime back in the 1970s. I'm wondering what a fair
price these days would be for a pair of 4XC250R tubes, "new old stock", made 20+
years ago. They are hard to find, so I'd like to get a ballpark figure of what they
typically sell for in case I find some and need to make a quick decision. I resume they
sell at a premium over the 4CX250B, which seem to go for anything from $50-$200 each on
ebay depending whether they were made in the USA, Russia or China!
Maybe new tubes won't improve the efficiency, but it might be worth a shot. 500W out
is OK, but at only 45% efficiency I'm pushing my Variac and power supply quite hard
and I'm getting close to tripping the 15A braker on my 120v line...
31 Aug
31 Aug
4:18 a.m.
Maybe a 4CX400 can be used for 70 cm ??
Anode is bigger than 4cx250 but pin-out is the same
http://www.g8wrb.org/data/Svetlana/pdf/4CX400A.pdf
http://www.ur4ll.net/gs-36b.html
From: moon-net-bounces(a)mailman.pe1itr.com [mailto:moon-net-bounces@mailman.pe1itr.com] On
Behalf Of Bob Atkins
Sent: Tuesday, August 30, 2016 7:56 PM
To: moon-net(a)mailman.pe1itr.com
Subject: [Moon-Net] Fair price for NOS 4CX250R?
Still having efficiency issues with my K2RIW amplifier. I can get 500W out in Class C, but
I seem to be stock at about 45% efficiency, which would be OK for AB1 but is low for C.
I've tried cleaning and polishing the output stripline, the fingerstock connectors,
double checked the RF chokes and tuning and loading flaps, but no significant improvement.
One thing I do notice is that the output coupling flap is just about at the end of
it's range (minimum capacitance - maximum distance from the stripline) using the
original dimensions. I tried making it slightly smaller, but that didn't improve
things either.
So I'm thinking of maybe going for fresh tubes to replace the current ones which were
used and pulled from service sometime back in the 1970s. I'm wondering what a fair
price these days would be for a pair of 4XC250R tubes, "new old stock", made 20+
years ago. They are hard to find, so I'd like to get a ballpark figure of what they
typically sell for in case I find some and need to make a quick decision. I resume they
sell at a premium over the 4CX250B, which seem to go for anything from $50-$200 each on
ebay depending whether they were made in the USA, Russia or China!
Maybe new tubes won't improve the efficiency, but it might be worth a shot. 500W out
is OK, but at only 45% efficiency I'm pushing my Variac and power supply quite hard
and I'm getting close to tripping the 15A braker on my 120v line...
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11:44 a.m.
On 30 August 2016 at 18:55, Bob Atkins <ka1gt(a)hotmail.com> wrote:
Still having efficiency issues with my K2RIW
amplifier. I can get 500W out
in Class C, but I seem to be stock at about 45% efficiency, which would be
OK for AB1 but is low for C.
Is 45% unusual at 432 MHz? I used to get 60% from a pair of 4 CX250Bs on
144 MHz in class ABI, and about 70% in class C. But 432 MHz is almost
certainly well outside the area for full ratings of that tube. You might
have to expect some degradation in efficiency.
Also, what are you using as a power meter? A Bird 43 is usually accurate to
10% of FSD, but assume 15% of FSD if you want to be 95% sure. Forgot all
the crap about 5% of FSD on the Bird specification, as that is simply not
true in my experience, having tested several hundred elements in a
standards laboratory run my the MoD in the UK.
I bought a Bird 43 element for 432 MHz. Took it into work, and found it was
out of spec. I then took it to the UK distributor, who adjusted it while I
waited at 432 MHz, as I said that was what I wanted. I took it back to the
lab, and found it was ok.
IMHO, if you want to measure power accurately, you need a VNA, a high power
attenuator, and an HP or similar power meter.
Many hams seem to worship the Bird 43, yet in my experience, it is like
Chinese test equipment - you can take the specifications with a pinch of
salt.
Dave, G8WRB.
2467
days inactive
2468
days old
5 comments
6 participants
participants (6)
-
Bob Atkins -
Byron Tatum -
Dr. David Kirkby (Kirkby Microwave Ltd) -
Jordan Arndt -
Leif Asbrink -
Rick Muller