You like fiddlin', and that's fine. But, for me (and I think some
others here agree) there's nothing finer than saying "rig here is
homebrew" and getting that "man, that's a good sounding rig" report
in return. The feeling is indescribable.
I can relate to that. First time we cathode-modulated Dennis W5FRS's
homebrew 250TH amp with an Altec 1570B tube amp, we were ecstatic over
the reports. I think the one that got me was "sounds just like a
broadcast station". We did tweek the amp quite a bit to get the
modulation linear, It was originally running AB2, and we had to
increase the bias and drive, and load it a little less, but it sure
did sound sweet. Some pice are here:
http://rawfire.torche.com/~opcom/dbrady1/index.html
Fire is best served in it's natural state? ;-)
Say HI to Dennis for me. I've worked him a few times on 80m CW down at
3.546Mc and once or twice on 7.160Mc AM and just the other day, we
-almost- had a QSO on 7.050Mc CW, from my Viking II at around 50w. It's
the rig that serves as the exciter for the 250THx250TH rig.
I tried to sell a 450TH, appearing as if it was unused, no scratches
on the base - asked in here for $50 for it, in the metal
spring-loaded shipping frame, in a box. No one bit. Put it on ebay
for 7 days, and started it off at 9.95 and it sold for some 3X (plus
shipping) what I was asking for it originally, in here.
A 450TH is a very specialized tube today. Sometimes it takes just the
right buyer. $50's cheep cheep for a new one, I think.
I've got SEVERAL 450TL's that are eating up a lot of space in my shack.
I could use the money, right now. They all have filament glow.
I did go through a few of 5R4's in the modulator bias supply before I
figured out that one of the 304TH's had an intermittent short from
filament to grid, but only with HV applied. Oddly, whatever happened
'blew up' the filament in the 5R4 before any fuses or overloads could
trip, and then the bias supply fuse blew.. This was in a supposedly
good 304TH I bought from ebay. Due to the scarcity and high costs of
304TH's, I have changed to 3-500Z's now, and I need to re-do the bias
supply for 0-30V instead of 0-300V! I think a 10:1 voltage
divider/bleeder drawing about 100mA at 300V should do (The modulator
bias power supply is rated 200mA). That way the rig stays more
original, and the grid supply now has a 300 ohm resistance!
I need to see about locating a 304TH rig I know of. I don't know if
it'll ever be put back on the air, by the present caretaker of that
rig... and it's just the final section.
That thing used be be a pair of 304TH's (or was it TL's, John?)
modulated by (4) 813's in push-pull, parallel. I full kilowatt DC input
to the final, with something like 1100 or 1200v at 750 or 800mA.
Somewhere in that area. As a pulse Radar tube, the 304's will
-certainly- handle the current!
Just for grins and giggles, try putting 2 50 ohm resistors in series
to ground, and then the grid to the center of the two 50 ohm
resistors. It's helped other rigs.
I will try that. I am not sure of the effect it will have with the
grid tank feeding a low resistance load of 100 ohms, but it will point
me in the right direction.
It provides a voltage divider network. Anything stray, goes to ground.
I'm looking at these conditions (they represent much more power than I
will legally use, but should be good for figuring. There were no
figures for 2500V or less.)
Plate Modulated Class C Amplifier [CCS]
Plate Voltage ................................. 3000 V
Grid No. 2 Voltage ............................ 500 V
Grid No. 1 Voltage ............................ -200 V
Peak Grid No. 1 Voltage........................ 340 V
Grid No. 1 Current ............................ 12 mA
Plate Current ................................. 600 mA
Grid No. 2 Current ............................ 145 mA
Driving Power ................................. 12 W
Power Output (approx) ......................... 1390 W
Maximum Frequency ............................. 110 MHz
My question, is from these, how do I figure the impedance of the load
presented by the grid circuit to the grid tank?
The same way you would for the plate tank.
R=E/I
R=340v/12mA
R=28.3k Ohms
The power dissapated is a tad over 4w, so (4) 100kohm, 1-watt resistors
with a parasitic choke wired around 'em would be plenty, I would think.
Run that in series with the grid drive to the pin.
Do the same for G2.
Seperate 120v circuits?
Yes, indeed COL Tucker apparently did not want to install a big ugly
240V outlet, so he apparently used a pair of 20-amp 120V outlets
connected to 240V in the breaker box. Of note in this rig, is that
neutral is bonded to ground. period. That is, there is no ground prong
on the plugs and it is not possible to separate neutral from ground in
this rig without a major rewiring job. So, I will have to take care
that I do use a 4-wire grounding 240V plug, and see that I really do
have a good ground back to the breaker panel. Should present no
problem if done correctly.
Many rigs were built with such circuits as "bias supplies from recified
AC line voltage, with respect to chassis ground". Shoot, you could even
rectify AC line voltage, and make sure it's well filters (clean) and add
it, in series with a DC supply, for an extra 150VDC, or so... "with
respect to chassis ground". I think you're gonna probably have to keep
it on seperate 120v circuits. A single 240v circuit would work, but
break it out into two 120v circuits and make sure the plug is properly
polarized. Yes... AC polarity. makes a difference.
But.. one works hand in hand with the other. In fact, John/WA5BXO
and I have been discussing (at length, and credit to John for
sticking with me and my dumb questions ;-)) audio, it's effects on
the transmitted carrier, how much voltage is neeed to fully modulate
the carrier and what is needed in the way of a reactor to sucessfully
couple the audio to the B+ line to the final amplifier. This
conversation encompassed power supplies, through the modulator and
the final. All components are cruicial.
I picked up a modulation reactor some time ago. There might be room
for it if I get rid of the filament transformers for the 673 rectifers
and put in solid state. I kind of dread building two 15000PIV 1A
bridge rectifiers, but I need to do it anyway.
Find you a pair of 872A's. Mercury Vapor Rectifiers. They'll work,
just fine. I need to do the same, here. I have the rectifiers.
John and I also talk, from time to time, at length about asymetrical
audio. I've achieved it, somewhat.
Sometimes technology is overwhelming.
For less than $100, I bought not only a digital camera that takes
amazing 3.2megapixel pictures, but it also has a 'movie' mode.
No audio, but the movie mode is pretty awesome.
"Hel-l-l-l-l-o test. one, two, three, four."
http://w5omr.shacknet.nu:81/~w5omr/pictures/2005-08-08/000_0049.MOV
The 'noise' you see is a box-fan at full speed. It was over 100°
here, that day.
That is very impressive, I always strive to be rid of power supply noise.
The rig is clean - I mean a box-fan, from about 10ft away, blowing
-hard- on the back of the rig. I mean, it was H-O-T in the shack.
I guess you know, we're generally up and on the air around 4am or so,
when the QSO starts, and probably runs till around 6am when folks get
out - then another group cranks up on 3.890 in North Texas, Oklahoma &
Arkansas and in South Texas, we're on around 7am or so, on 3.880.
We'll have to talk off-list about your choice of Linux distros ;-)
---
73 = Best Regards,
-Geoff/W5OMR
