I had one of these back in the 60s, but it had an EIMAC 3-400Z triode in it. As I recall it required 50W drive to reach full power. Great amp.
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Contex 500 Tube Amp
- Thread starter Shadetree Mechanic
- Start date
Well...I have two different cb type tube amp's...A RF Linear Kicker 500 (same thing as the Firebird 500) which is a 1x3 (6lq6's) and a Palomar 300A (2x4) and although some keep saying on these forums they are nothing but trashy box's...I have always got great feedback on just how clean my station sounds! On AM and SSB... Think it depends more on the radio, than the box on just how clean your going to sound using one of these old tuber's.
They do get hot, specially the Kicker 500 since it has no fan, but after years of talking on them, Only problem I have ran into was a leaky cap on the Kicker 500... replaced the 3 caps a couple years ago, along with the tubes that came in it when I bought it, and haven't had a problem since...
The Palomar still has the tubes in it that came with it when I bought it, and seem to still be strong...
Good luck with that Contex, I'm sure it still has plenty of life left in it.
They do get hot, specially the Kicker 500 since it has no fan, but after years of talking on them, Only problem I have ran into was a leaky cap on the Kicker 500... replaced the 3 caps a couple years ago, along with the tubes that came in it when I bought it, and haven't had a problem since...
The Palomar still has the tubes in it that came with it when I bought it, and seem to still be strong...
Good luck with that Contex, I'm sure it still has plenty of life left in it.
Well, that's the dilemma.
If you set the carrier high enough for the tubes to be comfortable, you'll see "backwards swing" on an average-reading wattmeter. Turning the carrier down to get little or no forward swing is where the danger zone begins. Turning it down enough to get any forward movement from an average-reading wattmeter will tend to work the tubes too hard. Any lower than that, and the tubes' gray anodes become dimly incandescent, glowing first red, then orange.
Until something goes "POOF!" or "POP!".
Just for reference, this kind of tube is designed to remain below that "cherry" temperature. The plates should remain dark and gray when the tube is running at full rated power.
The current through a tube is controlled by the voltage on the control grid. The electrostatic repulsion of the negative-charged electrons flowing from cathode to the positive-polarity anode gets opposed by a negative-polarity voltage. The grid is literally a fence, with gaps that the electrons flow through. Like-voltage polarities repel each other. A high-enough negative-polarity voltage on the wires of the grid will shut off the flow of current from anode to cathode completely. The textbook for this when it happens is oddly enough called "cutoff".
Your signal changes the voltage difference between the grid and cathode, causing the same signal to appear in the anode's current flow. But at a higher voltage and current. This is what gives the tube power gain. A small change in grid voltage becomes a large change in anode current. This is why the Brits call these things "Electronic Valves".
We routinely take pins 2 and 6 of the tube sockets loose from ground. A .01uf disc capacitor goes from each of the two pins on each of the four sockets to ground.
---(Interruption)---
It's late, and I have stuff to pack and ship, so I can't get carried away with a complete procedure for this right now.
Short version is that we rectify/filter the 6.3 VAC heater power to get a negative-polarity 8 Volts DC. That gets fed to the grids. The disc capacitors make the grids appear to grounded for RF currents, even though those pins of the tube will now have that negative DC potential on them. This reduces the tubes' zero-signal current to a safe level.
They left this out for the obvious reasons. First cost, and second there were no radios with a carrier control when this thing was designed.
And like I said, a SSB switch would be a marginal fraud to include, at best.
I'll check back later and see if I have this mod already written up in the can somewhere. I'm sure I've described it in detail with pictures for D&A Maverick and Phantom amplifiers some time since the last hard drive crash.
73
If you set the carrier high enough for the tubes to be comfortable, you'll see "backwards swing" on an average-reading wattmeter. Turning the carrier down to get little or no forward swing is where the danger zone begins. Turning it down enough to get any forward movement from an average-reading wattmeter will tend to work the tubes too hard. Any lower than that, and the tubes' gray anodes become dimly incandescent, glowing first red, then orange.
Until something goes "POOF!" or "POP!".
Just for reference, this kind of tube is designed to remain below that "cherry" temperature. The plates should remain dark and gray when the tube is running at full rated power.
The current through a tube is controlled by the voltage on the control grid. The electrostatic repulsion of the negative-charged electrons flowing from cathode to the positive-polarity anode gets opposed by a negative-polarity voltage. The grid is literally a fence, with gaps that the electrons flow through. Like-voltage polarities repel each other. A high-enough negative-polarity voltage on the wires of the grid will shut off the flow of current from anode to cathode completely. The textbook for this when it happens is oddly enough called "cutoff".
Your signal changes the voltage difference between the grid and cathode, causing the same signal to appear in the anode's current flow. But at a higher voltage and current. This is what gives the tube power gain. A small change in grid voltage becomes a large change in anode current. This is why the Brits call these things "Electronic Valves".
We routinely take pins 2 and 6 of the tube sockets loose from ground. A .01uf disc capacitor goes from each of the two pins on each of the four sockets to ground.
---(Interruption)---
It's late, and I have stuff to pack and ship, so I can't get carried away with a complete procedure for this right now.
Short version is that we rectify/filter the 6.3 VAC heater power to get a negative-polarity 8 Volts DC. That gets fed to the grids. The disc capacitors make the grids appear to grounded for RF currents, even though those pins of the tube will now have that negative DC potential on them. This reduces the tubes' zero-signal current to a safe level.
They left this out for the obvious reasons. First cost, and second there were no radios with a carrier control when this thing was designed.
And like I said, a SSB switch would be a marginal fraud to include, at best.
I'll check back later and see if I have this mod already written up in the can somewhere. I'm sure I've described it in detail with pictures for D&A Maverick and Phantom amplifiers some time since the last hard drive crash.
73
Thanks for taking the time to explain this to me. Your help is much appreciated. Thanks!
Here is the GE data sheet for the Beam Pentode 6LQ6.
So many grid pins.
EDIT: Five grid pins to be exact. Pentode.
So many grid pins.
EDIT: Five grid pins to be exact. Pentode.
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I agree that you have to start with a clean signal, its the same way with stereo equipment too. I plan on buying an Oscope to monitor the modulation with. I am an Industrial Electronics Technician by trade and I don't even own a scope so its time to get one anyway. Thanks for the encouragement, I'm sure there is plenty of life left in the amp if I don't kill it first. LOL
I have read the amp tune up post that is pinned in the amplifier section but it is a little un clear to me. My amp has a Drive, Tune, and Load knob and the write up told how to adjust the Load and Tune but I am unsure what to do with the Drive. Does the Drive adjust the input power from the radio? Also there is a safe current of 250ma on the plate? I have been unable to find a manual for this .Sorry for the noob questions any help is appreciated.
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Should have been more specific. Only grid 1, also called the control grid gets the bias voltage. Grid 2 and Grid 3 remain connected to ground.
73
73
Chris. I hate to use this fourm as it's off subject, but I haven't received a pm from you concerning that wm1 I've sent you several PM's about. Please let me know. This guy went out of his way a bit to try and hold it for uyo. Let me know if you want it. If not illI take it myself if just to krekmy word to him. My friend will repair my illuminated one for little money and although I don't really need it I will buy it anyway. I guess one could never have enough backups. Thanks
The drive control is there to match the input impedance of the amp to the output impedance of the radio. The TUNE and LOAD controls match the output impedance of the amp to the impedance of the antenna.
- 399
Thanks for clarifying that. Being able to easily tune the radio to the amp input will be nice. Can you tell me what the Drive, Tune, and Load selector under the meter does? Thanks for your help.
Far be it from me to disagree with Jim (399) because he knows a heck of a lot more about this stuff than i do, but i think the DRIVE control tunes the match between the driver tube and the final tubes.
a schematic would answer this quickly, but looking at the bottom of the amp, i think one of those trimmer caps is the actual input tuning control for the match between the radio and the amp.
i am not sure about that though!
one thing about tuning for minimum SWR going into an amp is that you have to also watch the power coming out of the amp to make sure that the reason for a lower input SWR is not due to you accidentally decreasing the power going into the amp.
the bias that Nomad is talking about is the same method he helped me with when i restored and modded my kris power pump from a one driving two to a straight three tube amp that now has bias.
here is the schematic that i drew up for my amp, and it may help you understand what he is talking about regarding your amp.
LC
a schematic would answer this quickly, but looking at the bottom of the amp, i think one of those trimmer caps is the actual input tuning control for the match between the radio and the amp.
i am not sure about that though!
one thing about tuning for minimum SWR going into an amp is that you have to also watch the power coming out of the amp to make sure that the reason for a lower input SWR is not due to you accidentally decreasing the power going into the amp.
the bias that Nomad is talking about is the same method he helped me with when i restored and modded my kris power pump from a one driving two to a straight three tube amp that now has bias.
here is the schematic that i drew up for my amp, and it may help you understand what he is talking about regarding your amp.
LC
I pulled the cover off again to take another look. Now i have half an idea what i am looking at. At some point I will draw up a schematic for it, but for now I just want to get it going. I discovered that the driver tube is not a 6LQ6 as I previously stated. It is a 6ME6 and I can't edit my first post so hopefully nobody will be confused by it.
It's a good thing I pulled the cover back off. I found two tabs from the keying relay touching. This would put RF and the coil of the power relay together. Hopefully it made a bad connection or it wasn't really touching, just very close. I pushed them apart because it's too close for comfort.
