Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shooter on December 14, 2013, 05:11:26 pm
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Am I reading the datasheet correctly?
Heater Positive with respect to cathode 100v
Heater negative to with respect cathode 200v
I take that to mean if my cathode is at 150v, heater at 6.3v, the heater is 143.7v neg with respect to cathode?
data taken from 12ax7a datasheet
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I take that to mean if my cathode is at 150v, heater at 6.3v, the heater is 143.7v neg with respect to cathode?
That's mostly right. But it depends on the reference on the filaments. If the filaments are referenced to zero volts (CT connected to ground) then you are right. But remember that the 6.3 heater voltage is AC and swings positive and negative around that ground (zero volt) reference. So there's a slight difference in the numbers.
But if your filaments are referenced to say +50VDC (CT elevated), the difference would be about 90v. And if your filaments are floating, no reference to anything, then the voltage difference is meaningless.
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Yes you are. Although we tend to think of them that way, not every 12AX7 exists in a Fender preamp....with "zero" (or 1 or 2 or 3) volts on the cathode.
Plenty of 12AX7 (and its brothers) were used in differential configurations and cascode configs (mostly in lab equipment and power supplies) where the cathode is raised "high". What's high? Well, the cathode on the 12AT7 phase splitter in an AB763 amp is 100 volts in a Twin Reverb. In any given tube, the heater-to-cathode is definitely not meant to deal with the range of voltages that the plate-to-cathode is expected to handle. The filament is a delicate wire, shoved inside the cathode, actually touching it, except for the non-conducting metal oxide coating the filament. So it is prone to breakdown if it is stressed enough. Hence: "do not exceed" warnings on the H-to-K volts. Once we wire up the filaments, we usually forget about them. It's certainly plausible that some designer
might need a 12AX7 running in a ckt with 600-800 volts on the plate...so, he'd look at the tube maximum and try to get the cathode up to 450 volts or so, 200-250-300 down from the plate. Oops, now the filament, running at (essentially) zero volts is 450 volts different from the cathode, and tube can't handle that. So he has to figure out a way to feed a high DC voltage to the heater string (or perhaps just this one tube) so that the H to K insulation doesn't break down.
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Thanks, That is what i'm trying to avoid, blowin up things. My cathode is gonna run between 100 -150vdc off ground. I always read the data sheet as 100v max dif but then I read the 2nd line! I don't want to float the filaments but for no real reason except consistency for my builds I use a ct @ ground or DC @ 6.3. Now that the dc value is acceptable, do I have to consider the AC component developed at the cathode?
Thanks again n if you can link me to the forums charitable wing i'll make my donations promptly!!!
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My cathode is gonna run between 100 -150vdc off ground.
What are you building that has you so concerned?
The Fender Bassman 5F6A has a cathode follower that has 130V on the cathode of a 12AX7. Leo didn't worry about it. I can't say that cathode follower is prone to failure in a 5F6A.
I have a couple amps with high cathode voltage on cathode followers. I just elevated the filament center tap to +15VDC (50VDC would be better) and have had no problems.
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I suspect I will also elevate the filaments, the build, on paper, has a cathodyne? phase splitter that i'm trying to get about 140v pk ac outta to drive p-p kt88's close to 100w pk. Once this forum said yup to max dif of 200v for the dc, I got to thinking, (probably a bad thing!), about the ac signal and now need to find out if the 200v max is ac and dc or I only need to worry about the dc element? thanks much!
dave