- There is a book
that was produced by Eimac back in 1967 called "Care
and Feeding of Power Grid Tubes". The
complete book is available, in sections, in pdf format from
the following address (click on the "Care & Feeding"
link on the left): http://www.cpii.com/eimac/
This should be required reading for anyone
contemplating building an amp. Section 4 is of particular
interest and Section 4.1.1, page 19, applies to the triode
tube.
- This is a complete
scan of the original Care and Feeding document in PDF. It
is 174 pages and over 40 megabytes - CLICK
HERE
http://www.medphys.ucl.ac.uk/~drkirkby/data/Eimac/care_and_feeding_of_power_grid_tubes.pdf
- IMD - InterModulation Distortion - What is it and how do I prevent it? IMD is the result of mixing within the amp. Read more HERE.
- How do I tap my coil in the plate tank? What size caps should I use for C1 and C2? This is a question that many builders never understand. One of the biggest mistakes made is thinking that the amp should operate with the plate tuning capacitor, C1, set to mid range on each band. There are several good resources for determining the right place for your coil taps and the value of the plate tune, C1, and load, C2, capacitors. Before you begin, remember that these things are determined through a process that insures that your tank circuit will operate with proper Q. The importance of maintaining the proper Q can't be stressed enough. If your Q is too high, you will have very high circulating currents in the tank. These high currents can heat the components to the point that they melt the solder right out of the connections. If your tank Q is too low, tuning will be extremely broad, efficiency will be low and supression of harmonics and other spurious emissions will be poor. Design your tank for a Q between 12 and 15 for best operation.
The links below will give you tools to calculate values for your tank circuit:
- How do I test my power transformer? Most folks can't test much beyond output volts without some kind of test equipment. I have built a High Voltage Dummy Load which comes in very handy. Here are some pictures of a test performed on a hamfest transformer. This transformer was tested to 1.5 Amps output and load tested for ten minutes at 1 Amp output. CLICK HERE
CLICK HERE for information on the dummy load.
- What happens
if B+ shorts to ground? Where does that
place B - (which isn't directly grounded)? This will cause
B- to rise above ground to the full High Voltage power supply
potential! CAUTION! This will blow up meters and metering
circuits and bias supplies; possibly cause cathode to grid
(ground) arcs, destroying tubes; plus, it can kill you!
Remember, the B- is NOT normally at ground potential but
is below ground potential by some amount determined by your
bias circuit. There is a good bit written about providing
protections for this event. Most of them detail using a
diode with a large surge current capability to ground from
the B- rail. This may actually be a string of diodes but
the most important thing is that it carries a large surge
current, shunting the rising voltage to ground and blowing
a fuse. There is a high likelyhood that the diode will be
shorted as it sacrifices itself to save you and the rest
of your amp. Expect to have to replace the diode, your glitch
resistor and fuses in the HV primary and / or secondary.
- Are we going
to use a HV interlock? Where? Is it
going to short B+ or just turn it off? I think that an interlock
that does any more than turn it off is a waste and dangerous.
Shorting the HV with any kind of interlock is asking for
damage to the supply. This doesn't mean that an interlock
shouldn't be used. On the contrary, interlocks can help
save your life but you can't depend on the interlock to
replace your common sense. Most importantly, keep your
hands out of the hot box!
- Are you going
to put a meter on the power supply indicating B+? I
am... and a bright RED LED too.
- Are you going
to put any other indicator other than a meter on the power
supply indicating that there is HIGH VOLTAGE? What
would that be? Perhaps a bright red LED powered through
a portion of the bleeder?
- Filament
Burn In - What is burn in and Gettering? Why do we
have to do it? Burning
in or conditioning a tube is the process of applying filament
voltage and cooling air only to the tube in stages to slowly
remove molecules of air that accumulate in the tube over
long periods of non use. Read the links below for details.
- Hi-Pot
Tester - Tube conditioning - Arcing - is it the same
as conditioning and burn in or Gettering? Do we need to
do that too? We have two different issues.
The burn in or gettering is covered in the topic above.
The other issue is covered here. There is plenty of evidence
that New Old Stock (NOS) tubes need a period of conditioning
with a very high voltage source connected between the anode
and nearest element (grid). The voltage being raised slowly
and leakage monitored and closely watched for signs of internal
discharge (sparking). There is evidence that this kind of
conditioning HELPS bring these old tubes back to new production
performance. A hi-pot (high potential) tester or Breakdown
Voltage Tester (BVT) is used to test with. The hi-pot tester
is also used to test vacuum variable capacitors and vacuum
relays. These tests can help prevent catastrophic failures
when high voltages are applied to these components.
- BANG!
What's the significance of these pictures? These
pictures show a tube that was damaged by an internal arc.
This arc could be from not conditioning the tube properly
or from another situation like parasitic oscillation. In
either case, the tube is fried. A glitch resistor may help
prevent this damage but not the cause.
- What is skin
effect? The vast majority of the RF
current flows on the surface of the conductor. That is why
a large conductor is needed in the plate tank to prevent
heating losses. A #12 wire may handle 20 amps at 60HZ but
not nearly that much at 14 MHz. In high power RF environments,
bigger is better. Silver plating helps lower the IR problem
at VHF and UHF but has little effect at HF other than to
make the tank circuit look nicer.
- What is a
"glitch resistor"? It may
be the one thing that can save your tube(s) when (not if)
there is a flashover inside. The glitch resistor limits
the amount of energy available at the instant of the flashover
and hopefully will keep things under control, even at the
expense of a destroyed glitch resistor.
- What is high
voltage leakage and how do I test it? see
the discussion above about tube conditioning
- How much
iron is enough? If you have enough iron,
you can get enough power:
- How much
HV filter capacitance is enough? A minimum
of 20uF and preferably at least 45uF should be used (from
his parts list) for these voltages. Higher voltages may
use less capacitance. Be careful, larger capacitors store
more energy and stay charged for long periods of time. They
can be charged and dangerous for days after removing the
power from the mains.
- What if you
want to use one power supply for 2 (or more) RF decks?
Then you have to consider some things that
will help protect you, the amps and facilitate powering
both of them. A few of these things might be:
-
Making sure that the blower for the proper amp is running
(you do plan separate blowers don't you?)
-
Use a B2B vacuum relay to switch the high voltage to
the proper amp; One for each and mounted in the power
supply.
-
Lockout in the power supply which senses which amp is
"on" so the other one can't be powered up
(at least for B +) unless your supply is large enough
to handle both at the same time.
-
Will you need a triode control board in each RF deck?
You will have to think this through giving consideration
to BIAS, METERING, LED STATUS INDICATORS, and control.
-
W4ZT - Some say share the board, some say don't. My
gut feel is dedicate a board to each RF deck because
of the differning bias requirements of each tube.
- Physical
layout of the RF deck will be limited by the components
you have available. Once you have a
layout that will work then the front panel layout has to
be considered, including meters. If you look at all the
pictures, you will see layouts in many different styles
like meters across the top, meters stacked on one side,
meters side by side in the middle. Try to visualize the
placement of the two caps and the coil behind the panel
and how they will limit where the meters can be placed.
The front panel will also have switches and LEDs. The front
panel layout is what you will see and live with every day
so it deserves a lot of time making it what you want.
- How do you
test the High Voltage power supply under load without risking
damage to your tube(s)?
Can this be done? Can it be done safely?
For how long? Can it be done without spending a fortune?
Does it need to be done at all? Why?
W4ZT - I think it needs to be done to insure that your power
supply can and will provide the HV that you expect without
a sudden breakdown with your tubes in circuit. It can be
done safely and inexpensively with a little planning and
work. Can you
say high voltage dummy load? Click here for pictures.
- Do we need
a parasitic suppressor on the anode of the GS-35B? This
topic is hotly debated and opinions can be found that differ
greatly with one another. It is worth noting that many grounded
grid GS-35B amps have been built without parasitic suppression
and appear to have worked fine. The key to prevention of
the parasitic oscillation is elimination of stray inductance
in the grid circuit and making sure that it is solidly grounded,
ESPECIALLY for RF. The anode circuit should not have any
high Q VHF resonances. Read the information at the links
below:
- Do we need
to use a tuned input? Some people say
that the GS-35B will present your exciter with an acceptable
load but is that all there is to it? NO... there's more.
The most important issue is overlooked when one doesn't
use a tuned input. The issue of Inter Modulation Distortion
(IMD) products stands proud. The tuned input circuit reduces
the IMD a very significant amount. It is a wise thing to
use a tuned input on your amp. Eimac's Care and Feeding section 6.1 says: "In a grounded-grid circuit the cathode, or input circuit, is in series
with the plate circuit. Because of this, any change made in the plate
circuit will have an effect on the input circuit. Therefore, the driver
amplifier does not see its designed load until the driven stage is up
to full plate current." Two important things come from that. First, the RF current path between cathode and ground will be through the tuned input. That means that if you depend on the output tuned circuit of your tranceiver, the amplifier RF current must flow through the coax to the transceiver. Second, the cathode input impedance is very dynamic and the tuned input with its flywheel effect helps smooth that out for the driving transmitter while keeping the RF current path short and contained within the amplifier.
- One
of the best discussions of the need for a good tuned
input is by Rich Measures, AG6K, in his article about
the Heathkit SB-220. You can read the entire document
at http://www.somis.org/SB220ci.html
and scroll down to the "Improving Input SWR"
topic about 2/3 down in the page. In part Rich says
the following:
"The
job of the tuned-input circuit is more complicated
than just matching 50-ohms to the input-resistance
of the amplifier-tubes.
Here's why: The instantaneous input-resistance
of a grounded-grid amplifier fluctuates wildly
during the positive and negative voltage swings
of the sinewave input signal.
When the input cathode-voltage swings positive,
the grounded-grid looks negative with respect
to the cathode, and the current is completely
cut-off, making the input-resistance nearly infinite.
During the negative swing in input voltage, the
grid looks more positive, and a large current
flows in the tube, making the input-resistance
very low.
For example: a pair of 3-500Zs. When the driving
voltage is peaking at negative 117v, the anode-current
is at its maximum peak, and the instantaneous
anode-voltage is swinging to its lowest point
of c.+250v, the total, peak cathode-current is
3.4a.16 Thus, the driving resistance at this point,
Rin c. 117v/3.4a c. 34.5-ohms, and, incredibly,
Ppeak c. 117v x 3.4a c. 397w.
Thus, the resistance swing is from near-infinite
with positive driving voltage, all the way down
to 34.5-ohms. [17] The instantaneous drive power
requirement varies from 0w to 397w at the positive
and negative peaks of the sinewave input voltage.
This is not the type of load that makes for contented
transistor-output transceivers.
During the positive swing in input voltage, there
is virtually no load on the driver, so the tuned-input
circuit must store the energy until it is needed
the most, during the negative crest in the input
voltage.
Thus, the tuned-input circuit's job is to act
as a flywheel/energy storage system, and a matching
transformer.
Q is like the mass of a flywheel. More Q makes
for a better flywheel, which does a better job
of averaging the wild swings in input-resistance,
giving a lower input-SWR. The tradeoff is that
more Q means less bandwidth. This means that,
with a high Q, the input SWR may be near-perfect
at the center of the band, but too high at the
band edges. Thus, a compromise must be made..."
(AG6K) |
This is great
reading and everyone should take the time to read the
text to get an understanding of the need for a tuned
input circuit.
- What is the
input impedance of the GS-35B cathode in Grounded Grid?
This is a great question! There is no
published data to tell us what it is. There are lots of
opinions and you will have to consider each on their own
merits. Seems that most folks think the actual effective input
impedance is somewhere between 25 and 90 ohms. You should remember one important point. When a tube manufacturer specifies an input impedance for a particular tube that is the effective impedance that you should use to DESIGN a network to interface to. It is NOT to be considered RESISTANCE to drive directly with your exciter. The network that should be designed to match that impedance should be a pi-network. The pi-network provides some energy storage (the flywheel effect) which smooths the impedance match reflected back to your exciter. Your amp should never be run without a tuned input for this reason. See the tuned input values above. See the
following thread for more:
- Should the
chimney be removable from the anode side of the chassis?
This could aid in tube removal and installation
but wont really be an issue with a short chimney. You will
have to remove the anode cooler to deal with the grid ring
clamps no matter what design you have chosen.
- Filament
/ Cathode choke - rod or toroid? This
discussion has taken many of us 'round and 'round. bottom
line is that you need a core, whether rod or toroid, which
will provide the inductance while not saturating when subjected
to all the currents through the windings. The common practice
of using a rod is certainly the most popular method of making
the filament choke. A toroid with a slit cut in it will
do exactly the same thing only wrapped into a smaller length.
For the 3 Amps of filament current required by the GS-35B, a bifilar choke made with #18 enameled wire on a 3-1/2" long 3/8" diameter ferrite rod works fine.
- What kind
of door knob capacitors are suitable for RF power versus
voltage? This is an interesting topic.
- What kind
of chimney should I use? The vast majority
of users seem to be using PVC for the chimney. I personally recommend PTFE (Teflon). CAUTION: I have documented proof that there is a problem using PVC as a chimney. CLICK HERE for pictures and more information.
There are
some other materials in use and here are some links with
info on them:
- What kind
of Blower should I use? Cooling the
GS-35B is very important. The minimum air flow is 90 cfm.
The datasheets on most blowers will show the zero back pressure
air flow and then a chart with air flow at different amounts
of back pressure. A blower that will do 100 cfm at zero
bp wont be enough. It will take one that will do about 140
cfm at zero bp. A blower of that size will have an air outlet
that is near that of a 2" diameter round hole. There are
several different types of flanged fittings that can be
used for a hose if you mount it separately. The only down
side to mounting the blower on the RF deck is noise. Of
course that is how a large number are done and it will depend
on the blower RPM too. I like a blower that turns around
1500 to 1700 RPM as opposed to a 3000 RPM blower. Grainger
lists a large number of Dayton blowers and there are others
to choose from as well. Take a look here: http://www.grainger.com/Grainger/searchresults.jsp?mfgindx2=Blowers&mfgindx1=Dayton&search_type=mfgindx
Take an example like item number 2C647,
click on that number to see the data sheet. For that blower
at 1500 RPM you will see that the air output is 134 CFM
at zero bp but still 96 CFM at 0.4 inches of water. This
would be a good candidate for your blower.
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