Noob to tubes- Maco 200

[nomadradio]

The good news is that this specimen was made with a grounded-grid driver tube. The tuned-input circuit that takes the radio's drive power from the input side of the relay feeds it through a yellow wire to pins 2 and 6 of the driver tube. Those are the cathode pins. The grids on the driver tube are grounded.

This setup is easier to make stable.

One end of the output coax IS grounded to the antenna socket ground. So that's not the root of your problem. Bad news is that they used the white foam-plastic insulated coax, so the center-wire insulation can soften and allow the center wire to 'creep' if it gets too hot.

One thing that has helped stabilize twitchy Maco amplifiers is a shield around the driver tube.

Here is the same trick applied to a Maco 300. Much easier for this model. Only takes a flat piece of so-called "hardware cloth". Pretty sure the wires are spaced 5/16 inch on this one.

Wrapping a shield of this stuff around your single driver tube looks a bit clumsier to do. The plate-cap clip and parasitic choke would also have to be placed away from it. A short from either of those to your grounded-metal shield would be a bad idea. It doesn't have to wrap completely around the driver tube, but simply block the direct path between it and the three final tubes that surround it.

Might make a big improvement.

73

 

[dozerman]

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.

Had to look on eBay at my messages and yeah, I bought a kit from you a few weeks ago lol

 

[dozerman]

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.

Yeah.. it was a mistake, I'm aware of that.. as far as a money pit, it may be but I am going to learn about tube amps with this box. It cost me next to nothing, so why not.

 

[nomadradio]

Taking a look at the filter capacitor arrangement shows there are no bleeder resistors. There should be one across each capacitor. 220k 2 Watt works well. A resistance much lower than that will require a larger wattage. Higher resistance is okay, but takes longer to bleed down the caps. Just need to make all three of them the same resistance value.

I see a sketchy angle to the filter caps. To use a string of three caps like that the High/Low switch should be rewired to switch the AC voltage going into the rectifier bridge. This one appears to be using the transformer center tap for the Low side, and switching DC to select High or Low side. That would tend to put too much DC voltage on the filter cap that has the negative side grounded.

But that's why we recommend rewiring the High/Low switch to the AC side of the high voltage before using the 3-capacitor kit we sell for Maco amplifiers.

73

 

[giboni]

Well.

Opinions R like A-holes. Everyone has one. I'm 60+. Been Golden ScrewDriving

sweep tube amps since I was a teenager.

In my opinion. & from first hand experience. "Balancing resistors" Bleeder

resistors R unnecessary in Sweep tube amps with less than 1100v DC at the anode

plate of the tube. That's why I did not include them in kits I sell on flea-bay.

Bear in mind. These C.B. 11 meter Amps were designed & assembled by hams.

Who probably had the highest ticket. They were making a buck.

The most important reason to add balancing/bleeder resistors in series parallel

to the High Voltage series connected stack is this.

Typical 450v to 500v capacitors are not clones of each other. U would be hard pressed

finding two. With the exact capacitance rating. That's why they R + or - %20-%10.

With plate voltages of 1200, 1500, 2000 etc. The series string of Capacitors gets long

& numerous !

Then. It becomes an absolute necessity. To balance the voltage across each capacitor. to avoid...BOOM...BOOM...BOOM.

https://www.illinoiscapacitor.com/pdf/Papers/voltage_balancing_resistors.pdf

Of course. These resistors will discharge the caps !

Now here is the down side.

I have yet. To find a 11 meter Sweep tube Amp that has enough Iron in the HV PSU

The transformers Used. The KVA of these transformers. R just big enough to dead key

x amount of Watts on high. Start talking. U get back swing.

Adding un-needed "Balancing resistors" bleeders". Bleeds your puny HV supply &

makes the transformer work harder. U loose output watts.

Now. To go to my shop. I wound up

cutting the work day short. I did not have the right tools to finish the job.

Lets see if I can find one of those O.D. Green Sniper Amps !

Take pics. Something does not look right from the top view of the above Amp.

Four fingers & a thumb !

 

[nomadradio]

So, uh.

How do you make sure the DC voltage is divided evenly across a series string of electrolytic capacitors?

Not so much a matter of opinion but of physics. Two factors affect this. The capacitance of each unit in the string dictates the 'share' of the total voltage each one will get. The actual measured capacitance of large electrolytics is NOT tightly controlled, and often varies by 20 percent or more. Feel free to consult your local physics textbook for the rule that defines how a DC voltage is divided across unequal capacitance values in a series circuit.

The other factor is the internal leakage resistance. If they are all equal, no problem. If they are not, Ohm's law will apply. Even if you go to the trouble measuring that internal-resistance behavior of each individual capacitor, there's no guarantee that this parameter won't change over the life of the part. Sadly, the capacitor with the highest leakage resistance gets the largest share of the divided voltage. The leakiest capacitor in the string gets the least voltage share.

If the DC voltage is not divided evenly, one of them will end up with more than the rated DC voltage across it.

What happens next is also much more physics than it is opinion.

Oh, and speaking of physics. Three 220k resistors add up to 660,000 ohms. Together they will draw about 1.5 mA from a 1000-Volt DC power supply. Adds up to 1.5 Watts of "stolen" power from your high voltage. Your amplifier has to be pretty small for a Watt and a half to hold it back in any real way.

73

 

[giboni]

Hmm okay.

First thing that is apparent from the photo of the tops of the tubes.

Each vacuum tube must have its own parasitic suppressor.

That is the resistor connected to the anode/plate of the tube.

Not enough photos of the bottom. But. If I remember correctly.

The plate choke of the driver tube hangs down from the top cap "Anode"

just like on the finals. If this is the tube turning RED. Then I was right on that score

in a previous post. AC or better RF. RF is actually AC. But at a much higher frequency.

I guess. The plate choke of the driver tube could be in the chassis ? Well tomorrow

will find one of mine. With a complete Un-Golden Screw Driver-ed specimen.

Perhaps yours can be made to function properly.

Best Four fingers & a Thumb !

 

[giboni]

Hi ! Thank U for ur recent purchase on the Maco 750 kit. It will be in the mail manana.

Finally found 1 of the 3 Maco 200.

Whoo. The last time used one was over 39 years ago LOL ! Did not find my first one.

It's

buried deeper in the garage in a box wrapped in plastic. This one. Picked it up a few

years ago at a garage sale/estate sale. $25.00. At least. two strong 80%8950's ! Am a

sucker for vintage tube gear at garage sales.

A lot of time has passed. Guess not a Newbi Golden Screwdriver anymore.

This box is CRUDE. Poor layout of tubes. Compared to the Palomar Skipper 300.

This is a poorly designed & implemented product electrically.

It is interesting how much it looks like yours !

After finishing the D&A 12 tuber promotion to Warrior. I will tackle these Maco 200's

This one had 1 6KD6 driving 3 8950. Neat "gimmick" with a diode to put approx 6.3

heater voltage unto the 6KD6 fillaments. The remodel of this unit will be.

1) Cutting out a new hole. Repositioning the driver.

2) Proper final tank coil. Possibly driver tank. Removal of the button variable capacitor.

3) 5-40pf small air dialectric variable capacitor. Like the Palomar Skipper.

Have several working Skippers. All I have to do is measure the Inductance of the tanks

on the Skipper. & roll my own. I will experiment by using 1/16 copper tubing that will be

plated with pure silver plumbing solder. Most likey the Iron in the Maco is not as Stout

as the Skipper. Skipper uses a separate heater/fillament tranny.

Parrasitic supressors on all anodes. & hose clamps. The clips on the tubes are just

garden variety hose clamps.

 

[dozerman]

View attachment 22077 Hi ! Thank U for ur recent purchase on the Maco 750 kit. It will be in the mail manana.

Finally found 1 of the 3 Maco 200.

Whoo. The last time used one was over 39 years ago LOL ! Did not find my first one.

It's

buried deeper in the garage in a box wrapped in plastic. This one. Picked it up a few

years ago at a garage sale/estate sale. $25.00. At least. two strong 80%8950's ! Am a

sucker for vintage tube gear at garage sales.

A lot of time has passed. Guess not a Newbi Golden Screwdriver anymore.

This box is CRUDE. Poor layout of tubes. Compared to the Palomar Skipper 300.

This is a poorly designed & implemented product electrically.

It is interesting how much it looks like yours !

After finishing the D&A 12 tuber promotion to Warrior. I will tackle these Maco 200's

This one had 1 6KD6 driving 3 8950. Neat "gimmick" with a diode to put approx 6.3

heater voltage unto the 6KD6 fillaments. The remodel of this unit will be.

1) Cutting out a new hole. Repositioning the driver.

2) Proper final tank coil. Possibly driver tank. Removal of the button variable capacitor.

3) 5-40pf small air dialectric variable capacitor. Like the Palomar Skipper.

Have several working Skippers. All I have to do is measure the Inductance of the tanks

on the Skipper. & roll my own. I will experiment by using 1/16 copper tubing that will be

plated with pure silver plumbing solder. Most likey the Iron in the Maco is not as Stout

as the Skipper. Skipper uses a separate heater/fillament tranny.

Parrasitic supressors on all anodes. & hose clamps. The clips on the tubes are just

garden variety hose clamps.

Yeah I love mine. The tubes are seriously weak in mine but still grunts out 280+ pep. And bad loud. Have it behind a president Dwight D. Yeah the 750, had a guy give it to me. It's been butchered. The hood has been cut on and pieced together. It's schetchy for sure underneath. It's all but dead. It managed to amplify my cobra 1000 by 3 watts. I've not tested the 6lf6's in it yet. Someone took one of the drivers out for some reason, now it's two driving six. Why do that? I had to order new sockets as well. A couple of them are rusted or rotted away. Still haven't a good clue how to re-wire it, but I'm trying to find schematics on it. Hoping I can learn some more with this one. you wouldn't happen to have the top hood for a 750 laying around would you?? Lol @giboni

 

[giboni]

Congratulations;

U R on your way to becoming a Golden Screwdriver !

Here is a short list that will speed up the process of resurrecting the Maco.

1) 2x Driving 5 five. Fogetabout 2x6

2) GG AB1 grounded grid Class AB1 no grid voltage at G3 G3 is grounded to chassis

Note....It is not necessary really to understand conduction angles etc.

Look up on internet G1, G2, G3, Anode, Cathode of tube schematic 6LF6 20LF6

G1 Supressor
G2 Screen
G3 Control Grid
Anode is the top of 6LF6,6KD6 etc
Cathode is the entry point of the driving signal

The easiest conversion will be 6KD6 or 6LR6 or 6LX6

6LF6 is expensive when available.

You will need a Trans conductance GM tube Tester. EMMISION tester is worthless

for Power Beam Compactrons.

The Maco 750 & other Maco's were Grid modulated with neutralization.

Lousy design. Inefficient.

All pins underneath the tube are grounded to the chassis except the Cathode.

And the Heater pins. Filament pins which are Pin 1 & Pin 12 on all the anode cap 12 pin Beam

Compactrons.

It does not get any simpler. GG AB1

Without a working Transconductance Tube checker it will be almost impossible

to work on yur Maco 750.

a VOM is required also. Powerful soldering gun. Surgical tweezers etc.

You will have to ROLL your own driver & final Tank coil. With the iron that is in there.

2x5 will get you 500 watts RMS or 2000 watts AM Pep. 4 watts drive. 12SSBw 600wSSB

Lot of TUBE scammers on Fleec Bay selling worthless tubes.

U R going to need tubes for your project.

good Luck !