Hey guys, first off I must say what a great site! Here's one for thought, I see a lot of discussion on the 833 and seem's there's some interest for high powered tube amps has anyone given the Eimac 3-500z tube a thought? My background in tube amps is mostly RF transmitters and lately I have been getting more involved with AF amplifiers, and from looking at the 3-500z's specs I dont see why it wouldnt work, if you're brave enough I guess. I have two of them. Any thoughts? P.S.- Tubelab, keep up the great work!
The closest thing that I remember seeing is an article that appeared in Glass Audio in 1997. The author showed an amp that he built using Eimac 4-250 beam tetrodes to produce 175 watts per channel! Although the article was interesting to read, I can't imagine that many folks built one for themselves. The outputs required something like 1250 volts for the plates, and the drive requirements for these tubes must be considerable, since they were biased at -75 volts. In the article, the author covers things like " the cool-down cycle" and notes the power must be supplied from a dedicated 15 Amp circuit! Triode outputs require hefty driver circuits, using audio type tubes, and I'd imagine that using RF output tubes would have even greater demands. While the rewards could be great, I'd think that the costs and circuit sophistication would limit the popularity of such a design.
I pretty much agree with Art about these tubes being impractical. While 1250 volts is doable, these tubes don't cool well by convection and radiation. They were designed to be forced air cooled in a chimney that directs air over the top seal.
And a 3-500Z is a zero bias tube which requires 3000 volts to draw enough plate current to be useful. Otherwise you'd need to bias the grid positive which posses it's own problems including high grid current.
This tube, and other zero bias tubes like it, have a tight grid structure that is very close to the cathode in order to get the needed control. Consequently grid to cathode leakage and shorts occur more readily. Truth be known, most "surplus" 3-500Zs usually have problems as they are easily damaged through misuse or mishandling.
Victor
And a 3-500Z is a zero bias tube which requires 3000 volts to draw enough plate current to be useful. Otherwise you'd need to bias the grid positive which posses it's own problems including high grid current.
This tube, and other zero bias tubes like it, have a tight grid structure that is very close to the cathode in order to get the needed control. Consequently grid to cathode leakage and shorts occur more readily. Truth be known, most "surplus" 3-500Zs usually have problems as they are easily damaged through misuse or mishandling.
Victor
All of these can deliver more than 200watts output power 
I just saw this, since I have been rather busy lately.
Anyone building an amplifier with tubes like these is NOT concerned with practicality anyway. The driver - grid current - positive grid bias issues can be easily handled with a PowerDrive circuit, or even a cathode follower built around a big sweep tube. The 2000 volt or higher power supply should be doable by someone who has built high power RF transmitters using similar tubes. Same with the cooling issues.
The real problem, and the one that stops most monster tube amp builders is the lack of available output transformers. A good transformer winder (real good and experienced) MAY be able to wind a custom transformer for this application. I paid a lot of money to have one made, and the results were not quite HiFi, so YMMV. Hammond makes a 75 watt model, the 1642SE, but I can't seem to find anyone who has actually built a big amp using one. I have asked on this forum with no replies.
When a friend of mine moved up north, he gave me and old HF linear amplifier that contained TWO 3-500Z tubes. I thought about robbing it for parts to build a big audio amp, but another friend saw it, and wanted to put it back on the air, so I gave it to him.
I still have about 20 used 833A tubes, and I know that at least two are good (out of 3 tried). I have some big power supplies, and several killowatt rated HV transformers, so I will get back to my own monster amp experiments someday, but I don't see it happening soon.
Anyone building an amplifier with tubes like these is NOT concerned with practicality anyway. The driver - grid current - positive grid bias issues can be easily handled with a PowerDrive circuit, or even a cathode follower built around a big sweep tube. The 2000 volt or higher power supply should be doable by someone who has built high power RF transmitters using similar tubes. Same with the cooling issues.
The real problem, and the one that stops most monster tube amp builders is the lack of available output transformers. A good transformer winder (real good and experienced) MAY be able to wind a custom transformer for this application. I paid a lot of money to have one made, and the results were not quite HiFi, so YMMV. Hammond makes a 75 watt model, the 1642SE, but I can't seem to find anyone who has actually built a big amp using one. I have asked on this forum with no replies.
When a friend of mine moved up north, he gave me and old HF linear amplifier that contained TWO 3-500Z tubes. I thought about robbing it for parts to build a big audio amp, but another friend saw it, and wanted to put it back on the air, so I gave it to him.
I still have about 20 used 833A tubes, and I know that at least two are good (out of 3 tried). I have some big power supplies, and several killowatt rated HV transformers, so I will get back to my own monster amp experiments someday, but I don't see it happening soon.
"The closest thing that I remember seeing is an article that appeared in Glass Audio in 1997. The author showed an amp that he built using Eimac 4-250 beam tetrodes to produce 175 watts per channel!"
"The real problem, and the one that stops most monster tube amp builders is the lack of available output transformers."
I had looked all over and really couldn't find anything suitable, so
expected to go with a custom wound OPT.
Then I saw these on ebay. They are AES OPTs 8K:8/4 that weigh
about 25 pounds each. They were used in a 175 WPC amp using
Eimac 4-250As ;-) that was written up in Glass Audio in 1997.
I had some email with Triode Mike, the builder of the amp and
decided I would just use these, at least to start with. Mike also
was very helpful with 4-65A op points and use of Eimac tubes.
I don't have much in the way of technical specs beyond 8K Zpri
and 300W power capability. If there are more specs available
or anyone knows what Cary amp might have used these monsters
I could use more info.
I did find this thread so thought I would inquire.
Cheers,
Michael
PS
Q "Why are you guys so fixated on glass envelope tubes?"
A To visually check that the anode is running at the proper color temperature
"The real problem, and the one that stops most monster tube amp builders is the lack of available output transformers."
I had looked all over and really couldn't find anything suitable, so
expected to go with a custom wound OPT.
Then I saw these on ebay. They are AES OPTs 8K:8/4 that weigh
about 25 pounds each. They were used in a 175 WPC amp using
Eimac 4-250As ;-) that was written up in Glass Audio in 1997.
I had some email with Triode Mike, the builder of the amp and
decided I would just use these, at least to start with. Mike also
was very helpful with 4-65A op points and use of Eimac tubes.
I don't have much in the way of technical specs beyond 8K Zpri
and 300W power capability. If there are more specs available
or anyone knows what Cary amp might have used these monsters
I could use more info.
I did find this thread so thought I would inquire.
Cheers,
Michael
PS
Q "Why are you guys so fixated on glass envelope tubes?"
A To visually check that the anode is running at the proper color temperature
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