I need some advice on setting bias. Palomar 350z, changed old caps installed new matched tubes. Following the 350z manual from cbtricks. It says set driving tube bias to .16 volts, while measuring across R13. Setting final tube bias to .32 volts while measuring across R2. The four final tubes starting getting very hot and glowing. Backed it back down to .16 volts and stopped. Don’t wont to hurt the new matched tubes . Are the instructions wrong or am I doing it wrong? Any help would be appreciated.
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Palomar 350z bias
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- 350z bias setting palomar
You have discovered the fatal flaw of the 350Z.
Well, one of them. The tubes are never switched off in this model. They pull idle current all the time the power is applied. Add to this the tendency of the bias trimpots to burn out and make the tubes draw excessive current. Makes them overheat.
Gotta work up a suicide-intervention guide for that model some day.
But the 350Z was built to play a never-ending game of russian roullette.
But with five chambers loaded, not just one.
73
Well, one of them. The tubes are never switched off in this model. They pull idle current all the time the power is applied. Add to this the tendency of the bias trimpots to burn out and make the tubes draw excessive current. Makes them overheat.
Gotta work up a suicide-intervention guide for that model some day.
But the 350Z was built to play a never-ending game of russian roullette.
But with five chambers loaded, not just one.
73
It ran cooler with the old tubes and before I jacked up the bias. The pots seam to be testing responding well. I believe it needs to be put on a scope ( which I don’t have) to get it set right. Thanks for the reply .
The 350Z I picked up on EBay already had a plate voltage relay installed so the high voltage is only on the tubes when it's in transmit mode.
But it sounds like you got the bias dialed back enough. I think a good rule of thumb might be a total idling plate current of perhaps 100 mA with no RF drive.
Now I've got a question for you. No matter what I've tried with my 350Z I cannot get more than 250 Watts PEP out on "10 meters" even though it gets an honest 400+ Watts out on 40 meters. I tried nearly new 6LF6s, played with the bias and screen voltages, tried many different tank circuit values, etc. No help on 10 meters. Even bought a second 350Z and had the same behavior; about 250 PEP max out on 10 but easily over 400 out on 40 meters. Now here's the kicker. My Palomar 300A easily hits 400 out on 10 meters. And my neat vintage Palomar mobile 1x3 6LF6 amp hits 375 out. What is up with the 350Z output on 10? I'm ready to rip its drive and tank circuits out and totally copy the 300A design. Madness!
But it sounds like you got the bias dialed back enough. I think a good rule of thumb might be a total idling plate current of perhaps 100 mA with no RF drive.
Now I've got a question for you. No matter what I've tried with my 350Z I cannot get more than 250 Watts PEP out on "10 meters" even though it gets an honest 400+ Watts out on 40 meters. I tried nearly new 6LF6s, played with the bias and screen voltages, tried many different tank circuit values, etc. No help on 10 meters. Even bought a second 350Z and had the same behavior; about 250 PEP max out on 10 but easily over 400 out on 40 meters. Now here's the kicker. My Palomar 300A easily hits 400 out on 10 meters. And my neat vintage Palomar mobile 1x3 6LF6 amp hits 375 out. What is up with the 350Z output on 10? I'm ready to rip its drive and tank circuits out and totally copy the 300A design. Madness!
Love my 300a's but 99% need a couple hundred bucks put into them unless youre handy then its a hundred for some quality parts. If they need toobs its not really worth it unless you really like this amp.
Never messed with the 350z mostly because of what ive read and ive never had the opportunity to buy one.
Never messed with the 350z mostly because of what ive read and ive never had the opportunity to buy one.
Speaking of sweep tubes for these 70's renegade amps; I get about the same output on the 350Z (250 PEP 10 meters and 400+ on 40 meters) whether I run 6LF6's or 6KD6's or the relatively cheap 6KN6's. The 6KN6's really surprised me since they often go for $10 a tube on EBay.
One more comment- I snagged a Palomar mobile 310M recently for cheap. The thing worked to the tune of peaking nearly 400 out and that's with only three 6LF6's. Despite having an SB-220 here I get a bigger kick out of seeing what a few TV sweep tubes can muster. It amazes me to see well over 100 Watts peak power out of a tube with only 33 Watts plate dissipation. It's almost like those vacuum tube designers were radio operators at heart and luckily the tubes also happened to work as sweep drivers in TV sets! I know it was the other way around no doubt but it's cool to imagine they were playing radio man when at home.