Noob to tubes- Maco 200

[dozerman]

Hello all. I hope I'm posting this in the right forum. I am brand new to tube amplifiers. I've always run transistor amps and can get my way thru one of them decently. I moved to west Tennessee a while back and everyone here runs tubes. So when in Rome... ok so I bought a Maco 200 to learn on. Box still looks new! I had to recap it, as it had a bad hum over the air. After, audio was clean. But I had bad reflection. Swr was 1.2 with amp keyed on high and would swing to 1.7 or 2. Wasn't long one of the tubes was glowing. The plate I mean. And soon there after, shot sparks. Coincidentally, my swr dropped when that happened. Bought another 8950 and put it in. Let it cook, and keyed the amp to tune. Swr was back up. And after a while, it's plate would start glowing. There is a faint blue-ish light coming from inside of it too when I modulate. My question... I've got two tuning controls, I assume, one on the bottom back and one beside two of the final tubes. The one by the tubes will raise and lower the key and the one in the back doesn't really do much. What are their functions and how can I keep from popping these expensive tubes! Thanks in advance

 

[DainBramage]

Maco 200,........................Is this by chance, an amp that originally used 6LB6 tubes?

 

[Tallman]

Does it look like this?
http://www.cbtricks.com/Amp/maco/maco_200b/index.htm

 

[nomadradio]

This model is a hot rod with no "OFF" switch for the nitrous bottle.

The driver stage is what's called a 'grid driven' circuit that requires much less drive power than the more-common "grounded grid" circuit found in nearly every other CB base amplifier. This also makes it more sensitive to unwanted feedback problems, since it takes a lot less "leakage" from the final tubes' output power back into the driver to make it misbehave.

The high SWR you saw was from the amplifier oscillating, and putting out multiple extra frequencies besides the one the radio feeds into it.

Those 'extra' frequencies are not the ones your antenna is tuned for. This causes the SWR reading to rise, even though the reading with just the radio alone is fairly low.

The single biggest thing that would serve to settle down this model and make it behave had to do with the coax cable that runs from the Load control to the relay in the rear.

The shield on this coax is not connected in the factory setup. This turns that length of coax cable into a transmitting antenna.

INSIDE the amplifier. Causes a portion of your output power to "leak" back into the input of the driver tube. This causes RF feedback, the source of your unwanted 'extra' frequencies.

Simply adding a ground wire to the braid on that coax at each end has been shown to tame this amplifier at least some of the time. Ground it to the frame or the ground lug of the Load control at the front end, and to the "Antenna" socket at the rear end. Stripping 3/8 of an inch of the outer jacket to expose the braid at each end, then wrapping/soldering the stripped end of a hookup wire will let you ground the other end of the hookup wire. This will work as well as any method of grounding both ends of the coax braid.

Cheapest thing to try first, at least.

And if the clear plastic that insulates the center wire of that coax looks like it has been hot enough to soften and "sag", this will be trouble once it sags far enough to dead-short the cable's center wire to the outside shield braid layer.

You'll just need to replace that piece of coax altogether if this is the case. We got in the habit of using teflon-insulated coax for this purpose. Won't melt from the heat.

Just don't use foam-center insulated coax in this spot. It's even more liable to melt.

73

 

[giboni]

8950's cost is steep. 6KD6 or 6LB6 best.The Maco 200 does not really have enough

iron

for 8950,6lx6 or 6LF6. But......Replace the "Parasitic Suppressors" connected to the

Anode of tube. US made carbon resistors radically change value with age in that spot.

Check the RF choke on the Input. ( L4 ) If its toasted. duh replace. Check on each final

tube "R7" connected to G1 control grid. Again carbon resistors. The finals from the

above schematic are in "AB2".

Grid current flows. The finals require a - bias voltage to prevent "runaway" of the tubes.

Carbon resistors ohm's increase drastically with age. This would result in the - negative

voltage being even less than it is supposed to be. Translation. The plate current of the

tube is not limited. Hence runaway. Blue I'm blue is good when U modulate. Not Red.

Lastly how many Watts are U pushing into this aged Maco ?

Could be AC getting into the plate of the tubes ? Take a few pics. top & bottom ?

Without a tuned input filter & a Pi-L tank circuit + an over modulated CB. U bet. I.M.D. all over the Band !!!!

Maybe a hundred watts on the frequency the CB is on. The rest from 2 meters to 80. A chunk of

splatter on 10 meters Ha Ha !

Best PuppetMaster

 

[nomadradio]

One last thought. This thing was designed to burn rubber with the comparatively "tiny" transmitters of the early 70s when it was designed. 16 or 18 Watts peak, max.

Radios are now two, three or four times the wattage of an average base station from those days. A solid-state radio with two finals will drive it straight to the junkyard.

Just too much drive power.

73

 

[dozerman]

Maco 200,........................Is this by chance, an amp that originally used 6LB6 tubes?

That I have no idea. According to the schematic, it was 6lb6. I e been told to not change the sockets to take those tubes. Would turn a decent amp into a door stop.

 

[dozerman]

Does it look like this?
http://www.cbtricks.com/Amp/maco/maco_200b/index.htm

Yeah, I've got the green weenie. It's still a really good looking box with the only wear showing on the transformer. Plates are kinda dirty.

 

[dozerman]

@giboni @nomadradio i will get some pictures up and let y'all see what y'all think. Like I said, I've no idea yet what's going on in there.

 

[dozerman]

One last thought. This thing was designed to burn rubber with the comparatively "tiny" transmitters of the early 70s when it was designed. 16 or 18 Watts peak, max.

Radios are now two, three or four times the wattage of an average base station from those days. A solid-state radio with two finals will drive it straight to the junkyard.

Just too much drive power.

73

I had it hooked up behind an old cobra 25 then I hooked it to a single final 3300. It only does 17 pep or so, I just wanted the audio from the connex over the cobra

 

[dozerman]

 

[giboni]

Ah yeah. I have two of them.

OD Green. Ready for COMBAT ! They will fit M-2057 ! But....really not enough iron.

I hate that puny air variable on the bottom ! Years ago was going to drill a hole & mount

Palomar Skipper 300 style air variable on the back somewhere. I think it might be a

304.on the bottom. Don't remember. Maybe will look at one in a few days. Its on a shelf

in the back reachable. Test the 8950's in it . Re-Cap. Put that air variable in the back.

Could put it next the Phantom in my car ! Ugly. But menacing.

Ah memories LOL !

Best PuppetMaster

 

[dozerman]

Ah yeah. I have two of them.

OD Green. Ready for COMBAT ! They will fit M-2057 ! But....really not enough iron.

I hate that puny air variable on the bottom ! Years ago was going to drill a hole & mount

Palomar Skipper 300 style air variable on the back somewhere. I think it might be a

304.on the bottom. Don't remember. Maybe will look at one in a few days. Its on a shelf

in the back reachable. Test the 8950's in it . Re-Cap. Put that air variable in the back.

Could put it next the Phantom in my car ! Ugly. But menacing.

Ah memories LOL !

Best PuppetMaster

What are the two variables on the box? One on the back and one by the tubes? The one in the back doesn't do much of anything that I can see on a meter, but the one by the tubes will run the dead key up and down? And where would I get the big resistors and chokes for this rascal. The one on the driver looks pretty bad. Was looking at it earlier and got my first taste of electrocution. My neck is still twitching...

 

[giboni]

Oh' boy. Still happens to me. It is dangerous. If U have to. Make a sign clearly visible on

your work space. DISCHARGE HIGH VOLTAGE CAPS BEFORE Touching anything.

Hmm Bet that's one of the kits I sell on Ebay ! Insulated handle screw driver. Touch the

caps & ground the other end to the chassis. A small loud POW only. The button air

variable in yours. Appears to be mounted in the back of the case. Ah forgot. It has two button variable

air capacitors. The one in the back. Adjusts the output impedance of the CB. 50 Ohms. To the

input. Cathode of the first stage. Driver tube. Impedance. Which is probably lower than 20 ohms.

Could be wrong on that. Could be higher than 50 ohms. For full transfer. Of the driving RF from the

CB. There has to be a Matcher. Have to check the Skipper. See how its done in that Amp.

The bottom button variable.

Adjusts the impedance between the driver tube anode "output" to the input of the next

stage which is the "Cathode". Hence 1 x driving 3 x. The Palomar Skipper has it in the back.

Very convenient. Lets say. Going from 27.185 to 27.515. U have to reload & tune.

This tube box is GG. grounded grid. Might be AB2 ?

Give me a few days to pull mine out. I have been considering upgrading all the kits

with the resistors used in the parasitic suppressor's. Especially after the beat down on

my time with a 24LQ6 powered Phantom 500 I rebuilt. This Phantom 12 tuber in my car

certainly needs new parasitic resistors. Its cool. But a bit disconcerting. When I hear a pop

while talking & I see arcing. Jumping across one of the parasitic connections in the 12 tuber.

Waiting for the Bulk order to arrive. Other kits.

Maybe upgrade with the various RF chokes. If the RF chokes are still RED not black.

They R still Good.

Dc blocking caps might be issues after 55 + years. It was in that 24LQ6 Phantom 500.

Give me a few days to pull mine out. Check the value of parts in it. Re-cap. & fire it up !

Check the tubes. Yup 8950's. Hmm maybe I stuck M-2057's in it ?

In this forum. Go to...

How to tune a tube amp.

 

[543_Dallas]

I was going to suggest you sell that money pit and buy an amplifier with tubes designed to amplify RF. Maybe not a good idea because that kind of amplifier wouldn't have just shocked you.

Most of the time there is never a good reason to be in a hot amplifier. Discharge the thing and put a jumper from b+ to ground while you're working on it as a safety. It could save your life.

 

[Tallman]

Give this a read and see if it helps on tune up.
1http://www.worldwidedx.com/threads/how-to-tune-a-tube-amp.26225/