Palomar 90A vintage classic CB sweep tube linear amplifier
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- Опубліковано 4 чер 2016
- Quick vid of a Palomar 90A vintage sweep tube linear amplifier from the 70's. It uses (2) 6lf6 sweep tubes that are expensive. However with a good set of tubes (the 2 here test over 100) recapped, gone thru, tested and aligned, makes a good little baby driver amp. Does what its supposed to. Note that in this vid, I am overdriving it with 40 watts peak/30 RMS. Clean and quiet. I may use this one for a while instead of the Blackcat.
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Had a 300A once. External Transformer. Damn thing needed the transformer rewound every so often. Finally sold it and bought a Golden Eagle 700 and ran that with a Modified Johnson White Face viking messenger one. 5 Channel, that I had a box built that worked all 40 channels, and hooked to a solid state receiver via a tolroid. Damn that radio was loud. Talked all over the world with it..
After a lot of research and then refurbing a couple of mine: The reason you said and in some other comments too (Rich B below), shows that the 300A (and other multi sweep tube amps) had stability issues. Palomar fixed that in the 300a by using a relay to shut off the HV until the amp is transmitting.
Nice little amps. I call these the 350Z's little brother. Look very similar. My favorite Palomar is the Skipper 300, the black/grey face and chrome cover just looks slick. Speaking of which I have have one of those to restore for a customer.
Wish someone would start manufacturing one or two of the popular sweep tubes. Blasted things are starting to go for crazy money. Hate to see customers brick a perfectly good amp because it needs a set of tubes and they can't justify the cost. Gets rather pricy in the 10 and 12 tube amps.
Just got one of these
Flat getting it
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Thanks..... I think.
About like the 200x. it has no fan either.
My 90A has fan
Got 2 new tubes for mine. Less than $10.00 each. Does its job again
You got a really good deal on the tubes!
That is the ONLY Palomar amp that I refused to work on because they had a serious design flaw that could burn your house down.
And the repairs to fix the problem the best way would cost more than the amp was worth.
I did not want to be responsible for anyone's fire damage!
It would not fail when you were using it; but when you put it on standby and went to make a fresh pot of coffee.
If you use it, turn it OFF every time you walk away from it and you will probably be ok.....but a repair guy can't count on a customer remembering to do that; or to keep curtains and other flammables FAR away from it...
#12
Hmmm what exactly is the design problem?
I looked the schematic over myself. I think I see what you are talking about. I see that that amp uses the tubes as Tetrodes, not grounded grid, and hence it has -Bias on Grid 1 and also the Plate and Screen are applied to the tubes at all times. I see what you mean as with all the voltages on the tubes all the time, if something goes, it going to really go with nothing but the 5 amp fuse to stop it.
With that said, the project(s) I am working on right now are Palomar 300A amps. I have 8. It started with me just trying to find the difference between the HV transformer one and the Low Voltage voltage doubler one. I have found many things and many differences, but all of mine use the low voltage voltage doubler configuration. I found 1 transformer that is about 10% larger than the rest, 2 transformers have the low voltage 12V coil fused, a few have the HV relay mounted off the board, 1 few more have the HV relay on the HV PCB. I have found that other than some are on the PCB and some the relay is off, there is NO other difference in them. 3rd, now get this one, some do not have the HV relay at all! Those are kinda like the 90A you speak of. HV is connected to the tubes at all times! I wonder if they had problems with them going up in smoke and thats why they added the HV relay. Only reason I can thin of as the 300A is grounded grid and its Grid 1 has -Bias going to it in standby and the grid is grounded only when the amp is keyed. Since the Grid is not grounded when the amp is not keyed, the tubes are not conducting, so why else would you need to put a relay on the HV that also only actuates when the amp is keyed? I have also found 2 versions of the -Bias boards. 1 has totally separate -Bias for the drivers and the finals and the other the -bias for the drivers ties into the -bias for the finals and just uses a couple resistors to tap it. Video coming soon.
@@tramdr I think Palomar went to the relay switched HV on the plates of the tubes for one simple reason. Back then there were many brands of sweep tubes and not all of them were made to the same specs. I would bet that they had problems with some tube brands oscillating in the amps due to insufficient cut off bias. So, in order to fix it quick and dirty without redesigning the entire amp, they just added a relay to switch off the HV. This makes it virtually impossible for any brand of tube to oscillate with no B+ on the plate. The negative cut off bias on the grid is a leftover from the original design and they just kept building them the same way to avoid confusion to the folks building them. I would bet the folks who built these 50 years ago had no idea how the amp worked, they just soldered and installed parts in a box. The Yaesu FL2100B has this issue with Chinese tubes. They were made with Cetron 572B tubes and the stock -18vdc cut off bias is good enough for these tubes. To use Chinese tubes in the 2100B you need to modify the bias circuit to get more bias to keep the tubes from taking off on you. On the ham amps, the B+ is always on the tubes so you have to bias the grid or filament / cathode to cut off the tube. In the case of an FL2100B or the SB200, they use the grids to bias the tubes to cut off. In the AL 82 or SB220, the filament/cathode of the 500Z's is how they bias the amp to cutoff. The grids in these are grounded to chassis.
I agree with you 100%. Only will add that some others say the cut off bias is not high enough. I measured -16V on mine and a few other blogs on the net say it should be at least -24V. Would not be an easy way that I know of to get -24 (or more) bias.
On the Drake L4B originally the amp has +cutoff 120v Bias on the cathode. This 120 is derived by 2 50w dropping resistors from the plate HV. Lots of wasted heat, plus not the safest way to do it. From the Harbach mods, I get rid of that and just use a high value dropping resistor going to ground on the cathode for bias. Works well plus no wasted plate watts and heat, and much much safer. Amp design has came a long way in 50 years.
@@tramdr
THE PROBLEM WAS CAUSED BY EXCESSIVE GRID LEAK FROM HOT, CONSTANTLY OVERDRIVEN TUBES; NOT TAKEN INTO ACCOUNT IN THE ORIGINAL DESIGN...
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As the tubes get beat up the grids have far less control over the plate current.
Grid leak causes lowered grid bias, allowing more plate current, causing more heat and more grid leak, lowering bias even more, etc, until something goes up in flames.
The fuse does not blow because you are within it's normal rating when the self destruct occurs.
NOTHING BREAKS THE PLATE CURRENT PATH WHEN THE UNIT IS ON STANDBY.
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So you beat the crap out of it talking skip for an hour or two, switch it to standby, and leave it unattended for a coffee or bathroom break, and that's when the SHTF!
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And worse yet; only one bad tube can cause the destruction of the other one, even if it was new!
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Only solution is to open the cathode to ground path of both tubes when on standby and hope the tubes are not gassy and arc over from plate or screen grid to the control grid when you do it..
i have a new one
whats the big deal running without a lid?
Smarter than thou Hams would say "Death Trap"
hmmm