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In Reply to: RE: What is the value of the current limiting resistors in the output section? posted by Ralph on May 13, 2020 at 11:54:58
The best place to place the resistors is in the cathode circuit of each section. We use 5 ohm 5 watt devices and what might sound weird here is that they will actually lower the output impedance of the amplifier because they will cause the power tubes to work more harmoniously together. If a power tube is hogging current it will limit the output power and it will be more likely to fail.
So this is a common sense improvement to the original circuit.
The other thing I would do is pre-condition the power tubes. 6080s and 6082s need about 4 days and 4 nights of pre-conditioning prior to use. You can't precondition a used tube- this only applies to new tubes or NOS. Pre-conditioning is essentially running the filament circuit and no B+ whatsoever during the process. It can double the life of the tube and vastly reduce arcing due to flaking of the cathode coating.
Follow Ups:
I just re-tubed one monoblock with all new 7241s. Do you recommend for whole days for pre-conditioning the filaments of that tube type. I did it for 6 hours or so and then was impatient to hear music. So, listening now, but could go back and do more conditioning. Thanks.
Isn't it 'conditioning' (whatever that is) even as you listen?
It's not like the filaments shut off when you turn on the music.
Too much is never enough
Well actually, the goal is to condition the filaments before you apply B+ from plate to cathode. So while you are applying B+, the filaments are having to do their job of producing electrons, and therefore at least in theory they are not ".conditioning".That's the way I think about it. It's kind of like letting your car idle until the oil and water warm up sufficiently before asking it to perform at a high-level. I could be off base. But in researching this topic online, and with respect to a generic triode, the recommendation of another expert was 5 to 6 hours. But Ralph has vast experience with the particular power triodes used in his amplifiers, and I have no doubt that his recommendation to do it for 4-5 days is based on his long-term results. My question is whether that applies to the type 7241 power triode, which can pass many times more current than a 6AS7.
I have no idea what that means, but if it makes you happy, carry on.
The metals from which the various conductive parts of a tube are made MAY benefit in some fashion, perhaps increase lifetime, from the stress relief of heating and cooling, but I have not idea.
Also? once the getter has done its work and the vacuum in the tube is as good as it will get, than THAT should be it on that front, too.
Too much is never enough
I have no dog in the fight for filament conditioning. You asked a question and I tried to respond based on my own poor understanding. This is the sort of topic where even EEs might disagree.
The vacuum tubes I currently own, and I DO have a few, are in a 1950s Zenith Trans Oceanic Radio.
And they are difficult to source tubes, I'm told. But also last 'forever'.
I'd love for an EE to weigh in. Lot's of points in modern hi-end audio which are less than accessible to many people. And I think works to the detrement of the hobby as a whole when somebody goes to a store and is told they need XX for cables and the tubes in the amp 'are OK' and 'you should upgreade them ASAP'......
I'd love to compile a list....
Too much is never enough
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