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Pal 750B questions.

walterjn

Well-Known Member
Mar 15, 2019
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Yeah, yeah I know, it's a tube amp. Tube amps are old crap, boat anchors, the hams let them die for a reason..... Read a bunch of similar comments online; I've also read a bunch of comments saying that tube amps are still worth while. I have been offered a working Pal 750B for $100. Is it worth that much? I know, eventually it will need either new rare 8950 tubes, or be wired to work with tubes that are not as scarce. I'm looking at this more as a learning experience and project. Thank you all for your time.
 
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Not so different from being offered a 50 year-old english sports car for 500 bucks. That's the cheapest part of the thousands you'll spend before it's a daily driver.

As a learning experience and a project? See if you can knock him down on the price even more.

Just kidding. If it really is "working" the market value of the tubes is probably several times the seller's asking price.

This is the dilemma I point out when someone asks me to put one of these back on the air. So long as the customer does the dirty work of obtaining sufficient tubes, we can talk. And that includes the tubes that flash brightly blue and go "SNAP!" the third or tenth or thirtieth time you key it.

Remember that the more tubes you tie together in one parallel "stage", the better they have to balance. The strongest tube of four, or six will pull more than it's intended share of the load current, cherry up and fail.

Two tubes in parallel can only get out of balance by a factor of two-to-one. If one of them is a bit stronger, you can just dial it down a bit to keep the stronger tube at a safe temperature.

Six tubes in parallel must means that the balance between tubes is three times as important. Dialing down enough to keep one strong tube safe out of six? Probably would hold you to about half power, more or less, depending on how much stronger the 'new' tube is compared to the other five.

Sure don't want to count on that discipline being maintained. But hey, seeing this kind of dynamic play out first-hand would be a learning experience.

Go for it!

73
 
Not so different from being offered a 50 year-old english sports car for 500 bucks. That's the cheapest part of the thousands you'll spend before it's a daily driver.

As a learning experience and a project? See if you can knock him down on the price even more.

Just kidding. If it really is "working" the market value of the tubes is probably several times the seller's asking price.

This is the dilemma I point out when someone asks me to put one of these back on the air. So long as the customer does the dirty work of obtaining sufficient tubes, we can talk. And that includes the tubes that flash brightly blue and go "SNAP!" the third or tenth or thirtieth time you key it.

Remember that the more tubes you tie together in one parallel "stage", the better they have to balance. The strongest tube of four, or six will pull more than it's intended share of the load current, cherry up and fail.

Two tubes in parallel can only get out of balance by a factor of two-to-one. If one of them is a bit stronger, you can just dial it down a bit to keep the stronger tube at a safe temperature.

Six tubes in parallel must means that the balance between tubes is three times as important. Dialing down enough to keep one strong tube safe out of six? Probably would hold you to about half power, more or less, depending on how much stronger the 'new' tube is compared to the other five.

Sure don't want to count on that discipline being maintained. But hey, seeing this kind of dynamic play out first-hand would be a learning experience.

Go for it!

73
I do understand the example of the 50 year old British sports car.
I guarantee that the 8950 tubes cost a whole lot more than the asking price. I have been seeing that the socket can be rewired to work with the 6LB6 tubes. Lower output, however they cost less than 1/2 of what I'm finding 8950s for. I was looking to see if someone made new 8950s for today's market (if there is one).... Probably not a snowball's chance that they can be rebuilt...
 
The 8950 was never made outside the USA, and not since the very-early 90s.

The 6LF6, 6KD6, 6LB6 tubes are not all that different on RF power. Just need an even number of them, to split 12-Volts across the heaters of series-connected pairs.

73
 
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The 8950 was never made outside the USA, and not since the very-early 90s.

The 6LF6, 6KD6, 6LB6 tubes are not all that different on RF power. Just need an even number of them, to split 12-Volts across the heaters of series-connected pairs.

73
Just wondering,o wit be possible to run a resistor inline for one to reduce the voltage?? Or is that not acceptable?
 
Using a resistor to drop 12 Volts down to six for a single tube works with the right resistor value.

But if that tube is in parallel with others that are using the "split 50/50" hookup, it will probably not balance properly. Don't know just why, but this is what we found with D&A amplifiers. They took the older designs with 4 driver tubes and moved one of them over as a "crystal oscillator" stage controlled by a morse-code key jack. Found you needed that fourth tube to balance the filament voltage. Never could get the "odd" driver tube of three to balance with the other two any other way.

Doesn't mean it can't be done, but the only results I ever had were futile.

73
 
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Using a resistor to drop 12 Volts down to six for a single tube works with the right resistor value.

But if that tube is in parallel with others that are using the "split 50/50" hookup, it will probably not balance properly. Don't know just why, but this is what we found with D&A amplifiers. They took the older designs with 4 driver tubes and moved one of them over as a "crystal oscillator" stage controlled by a morse-code key jack. Found you needed that fourth tube to balance the filament voltage. Never could get the "odd" driver tube of three to balance with the other two any other way.

Doesn't mean it can't be done, but the only results I ever had were futile.

73
Well that is good to know. I asked as the one I'm looking at has 9 tubes. I believe 2 driving 6, & 1 for a pre amp. If I'm doing this, I'm looking into replacing all of them if need be. So I need to have a plan to deal with the lone pre amp tube before I realize that all the tubes are half bad... Now, when I said it is a working amp they guy who currently has it has had it on the air the last 2 days, & he said it's putting out 500 on SSB. About 1/3 what the info I have found online said it should do.
 
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I would recommend losing the single "pre-driver" tube, and driving the two-tube stage directly.

Three stages of sweep tubes is just too much gain.

It will live a lot longer that way.

73
Would that be done by just removing that tube? Or rewiring? Can this amp then be run with a 1-2 watt am dead key?
 
Well, I got to stop by & take a look at that amp today. Got to look inside as well. I'm thinking that the tubes are starting to go bad. Then again, they all had Pal electronics silkscreened or painted on them. Either they have never been changed, or the previous owner was able to buy replacement tubes directly from Pal electronics. I checked all mine of them & got a box of the easiest to see.
 

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Seen both already, thank you, downloaded the manual & schematics. Reading the thread, again, trying to learn as much as possible. Is there anything else I can read to learn more about tubes, something that 'breaks it down Barney style'?? I seem to learn best when something is made as simple & plain as possible.
 
That tube has an "open" 4-digit date code. Tubes sold in bulk were marked this way, two digits for the year followed by two digits for the week of that year, 01-52.

The tube in the pic says 1973, 30th week.

Tubes sold in individual retail boxes one at a time would normally have an encoded date, usually two letters. This way, a retail customer would not gripe about a 'new' tube having an 'old' date when he bought it. The tubes that PAL bought to make this box were delivered in 'egg-crate' style trays in a box of 144, typically. Those tubes usually had an open four-digit date marked on them.

The age alone tells you only so much. The mileage counts for a lot more.

Bypassing the single predriver tube starts with removing the choke coil that supplies high voltage to the tube. The output circuit for the predriver tube gets rearranged to now serve as a 50-ohm input-matching circuit for the two driver tubes. Nearly always some cut-and-try to do that. Don't have a ready-made setup to copy for this model.

73
 
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From the looks of the one driver 8950, that tube shows zero signs of having suffered any abuse. The silkscreen is not dark, the getter flash marks are completely intact with no migration off the glass and the entire envelope is clear, even around the focal point of the beam plates. If it's dead, the cathode "fuse" on one of the two pins (8950's have two chances) probably opened or it had a nice easy life with enough hours to finally deplete the oxide coatings of the cathode. I'll bet it's still good. If the rest look the same, get them tested before replacing.

One thing to note about the 6LB6 is that it has about the same plate dissipation as most larger sweep tubes. However, the difference is in the peak cathode current ratings and they are lower on the 6LB6 because it has a much narrower cathode than the 6KD6. It's all about surface area here and you can see the difference in the middle of the tube and on the PEP watt meter in many amplifiers. It is not enough to notice on the signal meter and these tubes are still good substitutes when found economically.

You might want to take and post more pictures because one of us may be able to spot whatever the issue is, in the event it's not the tubes.

PS: More info on another replacement tube for these amps can now be found here: https://www.worldwidedx.com/threads/sweep-tube-option.258338/
 
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