Contex 500 Tube Amp

[Shadetree Mechanic]

I also found a contact pulled off the selector switch. This apparently measures the current at the load knob. I am sure there is more to it than that, but that is what it is hooked to. I am going to leave the cover off for a while so I can keep coming back throughout the day with fresh eyes. I am sure I will find more.

 

[Shadetree Mechanic]

Ok so what does the Drive knob do? Well the way it is hooked up it looks like a whole bunch of nothing. Only one wire is connected to it. The other tabs have no solder on them and appear to have never been connected. I am sure it does something, the Tune control is the same way.

Oh I see now, maybe the movable plates are grounded through the shaft.

Ok I just confirmed with my DMM that is the case.



The wire from the Drive control goes to a capacitor and a coil. The capacitor goes to the top cap plate connection of the driver tube. The coil goes to the pin 3 Cathode of the final drive tubes that are all tied together.

 

[Shadetree Mechanic]

I just realized that the other picture of the bottom that I posted didn't show the whole thing. Here is a better picture.

 

[Shadetree Mechanic]

Here is a page from the Palomar 350z manual that describes how to tune the 3 knobs. Basically adjust them all for Max power. I have read that this will give a full power dead key. Some people turn the load clockwise a little to lower the dead key to about 1/4 power so it can modulate to full power. Would this be the correct way or would this hurt the tubes?

 

[Shadetree Mechanic]

I fixed the switch and it was time to try it out. I took it out to the garage and hooked it to my Starduster and Stryker 440 (echo and Roger beep turned off) and an MFJ dummy load. With the 440 turned all the way down, the amp would not even key. With 5 watts on the Stryker dead key I tuned the amp up and was able to get about 400w on my cheap meter. So this gave me a 400w dead key coming out of the amp. I tried advancing the Load and it brought the power down but it still didn't have hardly any swing. So I put the Load back where it was and turned the Drive down a little. This was able to bring the dead key down to about 200w and swing up to about 375w. I put the rig on the local channel 20 and asked for a radio check. Someone came back about 20 miles away saying it pegged his meter and it sounded real good like I was running Ham equipment.

I thought I could get more swing out of it but how much swing do I really need? Am I doing this right?

 

[Sonar]

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.

I agree for the most part. I'm not exactly sure why the sound good/excellent but I've gotten nothing but stelar reports on ssb and Am with every single sweep tube amp I ever used. Palomar, d&a, and more recently my thunderbolt 305 which I personally don't like. As far as splater not a single complaint in almost 4 years. If it's happening no one has ever let me know. No rfi neighbor complaints either. I've often heard good in good out. Is that true? I don't know. But it seems to have been working for me. I've challenged some (without spectrum scopes) to tell me if I was stock or running my phantom while a couple of channels up or down and 1 s-unit to no splatter was detected (buy four stations testing) from 15 miles away with 650 watts (driven with my madison.) Some locals running class C kill my locals from a channel away while I'm not heard while having a qso with the class C guy from the same one channel away.

Off subject a bit.
But that class C guy splatters me 1 s-unit (if that) wheather on my Sonar or Madison. Nothiing that would take out but the weakest station. While the others are getting hammered so hard they can't even hear guys 10 miles away doing s-10. I should mention that all whove complained about getting hammered we're and are on black box modded exports. Many don't understand that to my understanding once those rci's, connex, ect ect are modded for 11 it seriously affects the receive to reject adjacent channel second harmonics. Those receivers aren't all that good on the band in which they were designed for ("10") but once modded it's worse.
If I'm wrong please correct me. At first I didn't understand why anyone was complaing until I heard a gate. And wow! I see why they conscidered moving to a new home channel. One they've been operating on since their dads were 40 years ago. If you purchase and modify an export get ready for the splatter. If you need an exsport because you need 100 or 200 watts output buy a Uniden 980 and get an amp. Or better yet buy an oldie like a 2000 or Washington, and an amp. Size does matter! especially when it comes to receiving filters. This clip was certainly eye opener.

 

[Sonar]

Here is a page from the Palomar 350z manual that describes how to tune the 3 knobs. Basically adjust them all for Max power. I have read that this will give a full power dead key. Some people turn the load clockwise a little to lower the dead key to about 1/4 power so it can modulate to full power. Would this be the correct way or would this hurt the tubes?

View attachment 22847

I don't understand why the 350z's tunning manual doesn't suggest over coupling the load.
The palomar 300a's manual not only suggets it but makes it clear why over coupling the load is necessary.
If the 350z load knob isn't slightly turned clockwise in order to drop the amps final tune you wind up with a dead key of about 500 watts. No forward Modulation is allowed unless one over coples. Not only is there now forward modulation but the transmitter sounds horrible!
It sounds as if the amps being way over driven.
The 350z manual although mentions nothing about over coupling it has to be wrong. If I tuned my 350z per manual tunning specs I'd wind up with a 500 watt dead key and no forward Modulation. I think Palomar designers we're running out the back door as the fcc we're breaking down the front door. This must've happened before correcting the mistakes in the manual where over coupling should've been addressed.
If I'm tunning my 350z wrong I hpoe someone corrects me. The way I tune it now shows a final dead key of 125 and 500 pep. With excellent reports. I hope nomadd chimes in. 73's

 

[Shadetree Mechanic]

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

Thanks for sharing the schematic, I have been following your Kris Power Pump thread. I am starting to put the pieces together in my head. I keep going back through your thread and what Nomad wrote here and now the SB200 thread that Nomad is doing. Now I need to take the cover off my amp again to look at it some more. There is a thread about a Palomar amp that Sonar has that Nomad explained quite a bit about. I need to find it again. I am trying to get my feet wet on this amp because I have a big transmitter in storage that belonged to my grandfather and the last time I looked at it years ago, I had no idea what I was looking at. Now I might have a half an idea. I will be sure to start a thread this spring when I bring it home. Thanks!

 

[unit_399]

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

Matt -

You may well be right. My Contex 500 had a single 3-400Z triode. The drive control matched the input to the radio. The sweep tube 500s may be different.

- 399

 

[Shadetree Mechanic]

I just got an Autek WM-1 meter and I had to try it out. With the peak reading, I can easily get 400w out of this amp. Unfortunately I think the AVG power setting on the meter doesn't work right on the 2000w scale. I can set the dead key at 100w by advancing the Load on the 200w scale and then switch to the 2000w scale and put it on peak. On modulation it will go up to 400w so I guess it helps to have a true peak reading meter. I got a radio check on channel 28 from a guy about 20 miles north of me. He said I was hammering his meter, but I was breaking up. I immediately reduced the power on my radio and he said it cleaned up real nice. I plan on getting a scope in the future to see what I am doing.

 

[Shadetree Mechanic]

The dummy load I was using is an MFJ-267. There is a switch on the back that switches between the antenna and the load. When I was trying to get the modulation swing set up switched to the dummy load, I said "Check 1,2,3." Into the mic and a local came back "4,5,6". This was while switched to the dummy load! I almost didn't know what to do I was not expecting this. I quickly switched to the antenna and started talking to him. The switch in this thing must not be much in the way of RF. Maybe there was so much RF inside the box that some still leaked out to the antenna?

 

[Sonar]

The dummy load I was using is an MFJ-267. There is a switch on the back that switches between the antenna and the load. When I was trying to get the modulation swing set up switched to the dummy load, I said "Check 1,2,3." Into the mic and a local came back "4,5,6". This was while switched to the dummy load! I almost didn't know what to do I was not expecting this. I quickly switched to the antenna and started talking to him. The switch in this thing must not be much in the way of RF. Maybe there was so much RF inside the box that some still leaked out to the antenna?

View attachment 22876

Sorry to hear there's an issue with the Autek. I would suggest you contact the seller. If the average scale is something you're going to use request a full refund. If he refuses open a case with eBay. If it's not a tremendous issue that the average reading of the meter doesn't work then have the seller refund a partial amount of what you paid for it. If he refuses simply request a full refund and or open a case with eBay. Another wm1 will come along. You need not settle. When we pay good money for items that are described as 100% in working order, and they're not we shouldn't have to settle. And you don't. I hope everything works out. Once again I'm sorry that the meter isn't working 100%. I should check the average on both my wm1's. I never used the average setting on either one of them and don't know if they actually work properly or work at all for that matter. I hope everything works out. 73 my friend.

 

[Sonar]

I just got an Autek WM-1 meter and I had to try it out. With the peak reading, I can easily get 400w out of this amp. Unfortunately I think the AVG power setting on the meter doesn't work right on the 2000w scale. I can set the dead key at 100w by advancing the Load on the 200w scale and then switch to the 2000w scale and put it on peak. On modulation it will go up to 400w so I guess it helps to have a true peak reading meter. I got a radio check on channel 28 from a guy about 20 miles north of me. He said I was hammering his meter, but I was breaking up. I immediately reduced the power on my radio and he said it cleaned up real nice. I plan on getting a scope in the future to see what I am doing.

View attachment 22875

I don't know how everyone else does it, but I myself tune all my amplifiers on the 2000 watt keeep scale. I would assume that 1 driving 4 (6lq5's) it should be easy to tune up in the 2000 watt pep scale.

 

[Shadetree Mechanic]

I don't know how everyone else does it, but I myself tune all my amplifiers on the 2000 watt keeep scale. I would assume that 1 driving 4 (6lq5's) it should be easy to tune up in the 2000 watt pep scale.

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?

 

[loosecannon]

When you tune your tube amplifier you want to tune it for a modulated peak. If you tune it for maximum dead key your modulation will be low. I like to use a tone for this since it is constant and not varying like the human voice. You can just use your phone and look up a YouTube video to get the tone there are a bunch of them on YouTube. Just do a search on YouTube for 1khz test tone.
LC