Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rob_h on September 10, 2013, 11:00:07 pm
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The data sheet for the 6K6GT in the "Maximum Ratings" "Design-center values" gives 315v as the max plate voltage. In the "typical operation and characteristics" 315v is given as plate voltage for one of the three examples they provide. If I understand correctly, in Class-A operation, the inductive collapse from the output transformer will, at max output, double the plate voltage, which would put the plate voltage at 630v. So, in this example is max plate voltage 630v, with max plate supply voltage at 315v?
Also, on down in the data sheet is a reference to 1200v absolute max plate voltage "peak positive pulse plate voltage" in a "vertical deflection application". I assume this is only for high frequency, low duty cycle pulses.
Back to the question: is my assumption close to correct about "supply" vs "peak" plate voltage?
Thanks for answers and insight!
PS, I searched for a couple of hours before asking.
rob_h
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Fender reverb unit with 6K6 has 225v on the plates: http://www.el34world.com/charts/Schematics/files/fender/REVERB_6G15.pdf (http://www.el34world.com/charts/Schematics/files/fender/REVERB_6G15.pdf)
from HotBluePlates (who is very knowledgeable about this stuff):
Q: I thought the 6K6GT was good to 350v on the plates? A: It is. I've also used 6K6's in a tweed Deluxe copy
More info off TAG with a discussion about them:
6k6 has a design center max plate voltage of 315 v
the typical operating values of 285 v at the plate and will make around
10 watts as a push pull pair with a 12k load resistance.
Here is a thread about someone using them in a 5E3 Deluxe:
http://archive.ampage.org/threads/6/dlxgd/338277/5E3_conversion_to_6K6GT_long-1.html (http://archive.ampage.org/threads/6/dlxgd/338277/5E3_conversion_to_6K6GT_long-1.html)
One option is to use Dana Hall's VVR on the power tube plate and screen only to get the tube below the max volts. It's easy to install.
http://www.el34world.com/Forum/index.php?topic=6899.0 (http://www.el34world.com/Forum/index.php?topic=6899.0)
Here is a sound clip of 6K6's in my D'Mars ODS at around 305v (using a PT 275-0-275 & 5Y3GT) :
http://www.soundclick.com/player/single_player.cfm?songid=11527276&q=hi&newref=1 (http://www.soundclick.com/player/single_player.cfm?songid=11527276&q=hi&newref=1)
With respect, Tubenit
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Time is the unspoken factor. If you wave your hand over the stove you will only feel heat. If you leave your hand over the stove for a longer period of time you will bet burned.
Max plate voltage has two separate concepts: "Maximum" + "Plate Voltage". The latter refers to the period of time when the plate is actually functioning in circuit as a plate. During that time the Max plate voltage is 315.
"Plate Voltage" does not refer to any other time, such as the passing moments of inductive collapse from the output transformer. If it did, then the most plate voltage you could use during the time of normal operation of the tube would be 157.5V (= 315 X 1/2). Clearly that is false, per the tube charts.
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Also, on down in the data sheet is a reference to 1200v absolute max plate voltage "peak positive pulse plate voltage" in a "vertical deflection application". I assume this is only for high frequency, low duty cycle pulses.
The vertical deflection application referred was probably a common '60s tv set. The vertical deflection signal would be a 60Hz sawtooth waveform. The sawtooth would slowly rise to a peak value as the beam was deflected from top to bottom on the crt, then it would quickly fall back to it's baseline value to allow for quick retrace of the beam from the bottom of the screen back to the top.
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Back to the question: is my assumption close to correct about "supply" vs "peak" plate voltage?
Once in a blue moon (and mostly on European data sheets) you will find a reference to "Absolute Maximum Plate Voltage"... usually something like 800vdc on an EL34. This is mostly about the maximum supply voltage you can use, and is largely limited by the likelihood of the plate pin arcing to some other pin at the tube's base.
For example, an 807 is same-as a 6L6GB. However, it has a higher plate voltage rating because the plate is brought out to a pin at the top of the tube, where it is less-likely to arc to another pin.
Which makes me wonder the question about the plate voltage rating of the 6K6. Like Tubenit posted, I've used them in a tweed Deluxe copy, and would again without reservation. I'd hesitate before using them in a blackface amp, due to its higher voltage and the fact I'm likely pushing plate dissipation ratings in a 5E3. I never noticed any redplating, though.
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The "315V" is the max you should see on a plain-old voltmeter connected plate to cathode.
This is a wear-out rating. Prolonged operation at a steady higher voltage will shorten tube life.
You can "always" peak near twice that voltage, as long as you also dip near zero voltage.
The tube maker KNOWS (knew?) that power tubes WILL swing higher AND lower. In audio the highs and lows cancel-- the plate-cathode voltage is still on-average 315V. Tube life is hardly affected.
So YES. You feed 300V, swing up/down to 500V and 100V about equally, it's all good.
In non-audio narrow-pulse applications, you could feed 315V through a coil, pull-down to 115V 99% of the time, then kick-up to 20,115V 1% of the time. The average is still 315V. However 20KV will jump all over an Octal base/socket. If not, the back-scatter of 20KV electrons and ions will bombard the cathode and ruin it. Except I bet that 20,000V would just arc-across the internal dimensions of a 6K6. To keep you away from all these problems, they say 1,200V at V-scan duty.
FWIW, the "315V" is a commercial spec and may not be technically precise. 6F6 is for good home radios. Since electrolytic caps only go 450V, and vacuum rectifiers sag, and field-coils were common when 6F6 was new, there was _no_ good reason to claim any higher voltage. This also allowed the makers to use the cheap plate-stuff if they wished. It is not often worth stocking very-cheap plate-stuff, so no doubt many tubes will stand more than rated. Also differences in what "good life" is. A housewife or bar might run the radio 4,000 hours a year. A weekend musician may run the tube only 1,000 hours a year. The domestic user whines about taking the radio to the repair shop every few years. The musician, not so often, and he soon learns to DIY tube replacements.