Contex 500 Tube Amp

[543_Dallas]

The reason I was switching scales was because I used the AVG mode to set the dead key. Only because I don't know what I am doing. Do most people set it with the peak mode?

Tune it with peak power. If you don't the amp ends up being undercoupled. Grid current is high, tubes fail and things can arc if you tune up at lower drive than you intend to use. The 1k tone works good for cb radios and is usually the best method. Yelling audio or whistling into the mic is tricky because it's hard to keep the audio level steady while tuning.

You can also tune the amp up with a carrier equal to or slightly greater than the pep power you are going to drive it with. Amateur radio operators normally tune an amplifier like this. HF rigs often have a tune function that dumps the set power into the amp for tuneup. You can also use AM or FM mode with the carrier cranked up. It shouldn't take more than a couple of seconds.

 

[Shadetree Mechanic]

I got my antenna up and I am having a problem with the SWR between the radio and the amp. The SWR was over 5:1 so I opened up the amp to see if I could tune the input. This cap is on the driver tube. It is 1 driving 4. The cap felt like it was already turned full clockwise and I was able to give it another half turn before I felt like I was going to break it. This was able to bring the SWR down to about 3:1 but I would like to get it lower if at all possible. The radios I tested it with are my Sonar 2340 at 1.5w dead key swinging to 12w and a President Madison with 2w dead key swinging to about 14w. Both give the same SWR going into the amp. The SWR of the antenna is below 1.4:1. Does turning the screw clockwise increase the capacitance or reduce it? How can I figure out what it needs?

Can anyone tell me what I can do to this amp input to get a lower input SWR?
Thanks
Chris

 

[nomadradio]

The white trimmer cap at the very top of the pic is directly below the driver tube.

This is the only adjustment they gave you to adjust the input-side impedance of the amplifier. You need to tune up the amplifier for best modulated swing before checking the input-side SWR. That trimmer cap may or may not reduce the input-side SWR reading below 2 to 1, but that's what you have to work with.

A fancier input-matching circuit with a coil and capacitor both tends to perform better, but if that adjustment gets it below 2 to 1, I would stop there.

73

 

[Shadetree Mechanic]

The white trimmer cap at the very top of the pic is directly below the driver tube.

This is the only adjustment they gave you to adjust the input-side impedance of the amplifier. You need to tune up the amplifier for best modulated swing before checking the input-side SWR. That trimmer cap may or may not reduce the input-side SWR reading below 2 to 1, but that's what you have to work with.

A fancier input-matching circuit with a coil and capacitor both tends to perform better, but if that adjustment gets it below 2 to 1, I would stop there.

73

Thanks Nomad, the best I can get is 3:1 with the trim cap turned full clockwise. I assume this is the maximum value of capacitance? If so then would it be possible to add a capacitor in parallel to it to bring it back into its normal adjustment range? I am not sure at what point I would need a coil. I assume the coil would be in series with the coax signal? I just ordered some 468 trimmer caps from RF Parts for the input of my Texas Star DX350. Maybe I could use one of these in place of the existing 462? I am not sure exactly what the number means as far as the adjustment range.
Thanks again,
Chris

 

[Shadetree Mechanic]

Ok here is what I have so far:
The amp has a 462 and is 10 - 80pF
The trimmers I ordered for another project are 468 and 175 - 680pF
It was suggested to me that because the trimmer was already full clockwise that the mica plates might be damaged. And then I gave it another half a turn. So I am thinking about trying the 468 since that's what I will have on hand and if I need to I will get the 462. Does anyone think this is a bad idea?
More to follow.
Chris

 

[Dmans]

STM,
I think that you could parallel the trimmer with another cap to give more tuning range. My suggestion would be to FIRST make sure that the installed trimmer is not damaged and to use a Silver Mica capacitor of at least 500V rating. 1Kv would be better. You can attach this with a clip leads (BE EXTRA CAREFUL NOT TO SHORT TO ANYTHING!) to insure the value selected lowers the input match where you want it, then solder it in once the correct value is found.

Ceramic trimmers are available in different values and the added capacitance of your selected parallel Silver Mica can be added/calculated to find the correct trimmer with no parallel capacitor added.

Your Sonar will not care about a 2:1 input tune. Your Madison will not appreciate it!



73's
David

 

[Shadetree Mechanic]

STM,
I think that you could parallel the trimmer with another cap to give more tuning range. My suggestion would be to FIRST make sure that the installed trimmer is not damaged and to use a Silver Mica capacitor of at least 500V rating. 1Kv would be better. You can attach this with a clip leads (BE EXTRA CAREFUL NOT TO SHORT TO ANYTHING!) to insure the value selected lowers the input match where you want it, then solder it in once the correct value is found.

Ceramic trimmers are available in different values and the added capacitance of your selected parallel Silver Mica can be added/calculated to find the correct trimmer with no parallel capacitor added.

Your Sonar will not care about a 2:1 input tune. Your Madison will not appreciate it!

View attachment 23780

73's
David

Thanks Dmans,
I was hoping it would be as simple as adding more capacitance. If the trimmer is not damaged then I will try more capacitance. I will put some Silver Mica caps on my list for my next order. I will probably get a small assortment as well as a few different values of trim caps. When I ordered the 468 trim caps from RF Parts I had to order $20 minimum. So I got 3 of the caps and some other stuff I didn't really need. Then I ran into this problem like 3 days later. LOL
Thanks again,
Chris

 

[Dmans]

STM,
Not to knock RF Parts(I have ordered from them before with great results), but I have ordered a handful of these trimmers and other items from Ken's Electronics with no minimum order $, quick shipping and he is a nice guy. The website is a bit outdated but a quick call to him will confirm stock.

73's
David

 

[Shadetree Mechanic]

Ken's has a lot of good information.

https://kenselectronics.com/charles/linear.htm

Free Technical Repair Tip: Tube replacement notes: The 6KG6/EL509 tube still made in Europe & Russia can be used as a heavy duty replacement for a 6LQ6 & many other "sweep" tubes. A 6KG6 is about 1/2" taller than a 6LQ6 so you first need to see if you have room for a conversion. The socket pin connection change is easy: Pin 3 of the 6LQ6 was the cathode while pin 9 of a 6KG6 is the cathode; move the lead formerly on pin 3 to pin 9 (not used on the 6LQ6); add a jumper from pin 3 to pin 2 on the socket. You may need to squeeze or swap the plate cap to fit the 6KG6. The pins of a 6KG6 are a little larger than the pins of a 6LQ6; however--with a used socket--the pins just fit a little tighter in the socket! In some units it may be necessary to shorten the "pi-net" coil by 1 turn or add a 100 to 200pf Silver Mica 1000V capacitor in parallel across the antenna-load capacitor to bring the "tune" & "load" capacitors into the proper tune-up positions.

 

[nomadradio]

The trimmer-cap only solution almost never works terribly well. But it's simple, and if the trimmer you have goes full tight, this means it has reached its maximum setting without showing you a minimum reflected power reading. A trimmer with more capacitance should at least reach that 'minimum' setting and travel beyond that, where the reflected power starts to rise as you turn it farther clockwise.

That lowest SWR you see at that 'minimum' setting may be too high to tolerate, but it's the simplest thing to try first. The designer of this amplifier wasn't interested in high performance, just keeping the cost down. Tube-type base radios were pretty forgiving about driving an amplifier with a high input-side SWR. Solid-state radios not so much.

As a general rule, you need a coil and a trimmer cap both to achieve a low SWR feeding into a single driver tube. But that means finding a way to mount it.

Don't have any pics on file showing where to place a coil with the input coax feeding into one end of it, and the other end of the coil attached to pin 3 of the driver-tube socket.

For this hookup, the trimmer cap would have the hot end attached where the input coax center wire meets the coil, and its other end grounded. Good chance the 462 would do the job in that setup.

And if just swapping the trimmer cap for one that has more 'travel' does the job, no reason to get any fancier than that.

Always try the simplest thing first.

73

 

[Shadetree Mechanic]

OK quick update. I received the 468 trim caps in the mail and installed one in place of the 462. I was able to turn the screw back and forth to zero in on the sweet spot. This leaves me at 2:1 SWR.

How much of a coil would I need to lower the SWR even further? Maybe 16awg wire wrapped around a pencil 5 times? I have no idea how much inductance would be needed. The way the trim cap is now, it is in series with the coax center conductor. Would the coil go from the center conductor to ground?
Thanks again everyone for your help.
Chris 73

 

[nomadradio]

Rule of thumb I'm used to is 5 to 7 turns wound on half-inch diameter. A single tube tends to take 7 turns. Driving two tubes calls for fewer turns.

The coil would go where the trimmer cap is now, from the input coax center conductor to pin 3 of the driver tube. The trimmer would go from the input coax center conductor on one side of the cap with the other side of the cap to ground. The original cap is probably the better choice to use with a coil. The 468 would probably adjust at the very-loosest end of travel, and might not turn down far enough.

There is a back-and forth adjustment for lowest input SWR, stretching or squeezing the spacing of the turns on the coil, along with setting the trimmer cap. Changing the coil will also change the trimmer's setting for minimum reflected power. If the coil gets the turns squeezed together as close as they'll go without getting a low reading, the coil may need an additional turn.

And if stretching it WAY out reduces the SWR, removing one turn will allow reducing the space between turns.

It's totally a 'cut-and-try' process.

Once it's below 2-to-1, you'll find that the driver tube runs hot with the radio's carrier turned down to a reasonable level. Adding grid bias helps this, but it will change the input-matching circuit from what works best with zero bias.

Somebody (else) needs to post a YT video how-to for adding grid bias to these old amplifiers.

73

 

[Shadetree Mechanic]

Just a quick update, the amp has been running well. I have been easy on it or so I thought at about 350w pep. Then the other day I got a little long winded and heard a tapping noise kind of hard to explain. Maybe a "tink, tink, tink". Then I noticed the power was only about 200w pep so I switched it into standby and then shut it down. The next day I powered it up again and retuned it. The power came back up but the drive knob is turned up a little more than before. Would I be correct in suspecting the driver tube is hurt? The driver is a 6ME6 and it drives four 6LQ6 tubes.
Any help is appreciated, Thanks!

 

[kopcicle]

plate choke is lookin a lil dark

 

[Shadetree Mechanic]

plate choke is lookin a lil dark

This is a picture from when I first got it. It might look worse now, I will get another picture when I take the cover off.