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6V6 load resistance
6V6 load resistance
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Old 19th January 2018, 11:03 PM   #21
Gnobuddy is offline Gnobuddy  Canada
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Thanks, Dotneck, much appreciate the comment!

-Gnobuddy
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Old 20th January 2018, 12:30 AM   #22
dotneck335 is offline dotneck335  United States
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Now, back to load resistance for tubes in AB1 push-pull operation: Datasheet for 6V6 tubes in this mode state 8000Ω plate-to-plate load resistance; DR transformer is 6600Ω. Same discrepancy found in Pro Reverb: two 6L6s in AB1, datasheet says 5600Ω, transformer used is 4000Ω. Why is this?
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Old 20th January 2018, 02:20 AM   #23
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by dotneck335 View Post
Now, back to load resistance for tubes in AB1 push-pull operation: Datasheet for 6V6 tubes in this mode state 8000Ω plate-to-plate load resistance; DR transformer is 6600Ω.
Here, I can only make what I think is a good guess.

The datasheet curves (attached) are for 285V on the screen grid (G2). With this voltage on G2, notice that anode current never really rises above 110 mA, even with 400 volts on the anode (and zero grid bias, zero volts on the control grid, G1.) Operated within factory specs, with control grid never going positive, you cannot squeeze more than 110 mA out of a 6V6.

We found out in the earlier calculations that you need something like 270 volt peak voltage across each half of the Deluxe OT primary to deliver 22 W to the speaker.

Ohm's law tells us that if we apply 270 volts to 1650 ohms (anode to centre-tap primary impedance), 163.6 mA of current must flow.

And we just found out that according to the manufacturer's datasheet, a 6V6 cannot deliver 163.6 mA, even with 400 volts on the anode, and zero volts on the control grid!

So now we have another connundrum: operated within factory specs, with control grid never going positive, you cannot get a 6V6 to drop 270 volts across a 1650 ohm load. So a pair cannot deliver 22 W into a 6600 ohm OT primary.

So how did Leo manage it? He didn't put 285 volts on the screen grid, he put something closer to 400 volts on them. Increasing G2 voltage has the effect of lifting all the anode current curves higher.

Without measuring one with a curve-tracer, I can't say exactly how much higher the 6V6 curves go with 400 volts on G2. But we must assume that the former 110 mA curve has risen to something well over 163.6 mA. Otherwise, there is no way to get that required 270 V peak voltage, and no way to drive 22 W into the speaker.

So now we know the factory datasheet curves are worthless in Leo's world, a world in which you penny-pinch the cheapest output valve you can find till the anode glows dull red, rather than do the right thing and buy the next size bigger valves.

In Leo's penny-pinching world, the 6V6 curves have all moved far from where the factory told you to expect to find them.

With all the curves moved, the factory load-line doesn't apply, either. With all the curves moving higher (more current at the same anode voltage), you will now need a lower-resistance load.

How much lower? Again, we can't tell without actually measuring 6V6 curves with 400V on G2. But apparently, Leo found that OT's with a 6600 ohm primary did the trick.

Whether Leo did careful testing to find the optimum, or simply bought the cheapest OT he could find that was "a bit less than 8000 ohms", we can only guess. My bet is that it was the latter.

So the short answer to your question is: if you operate the valves far, far, far from factory specs, then the proper load impedance will also be far from the factory specs.

-Gnobuddy
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Old 20th January 2018, 03:59 AM   #24
PRR is offline PRR  United States
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The 1937 suggestions for the 6V6 are very conservative. The idea was to replace 6F6 with essentially the same parts, lower grid drive, lower heater power, and about the same output. These suggestions were never changed; perhaps to avoid eating sales of other less-generic types.

Just scaling B+ from 285V to 400V suggests double the power. 22W makes perfect sense.

Voltage goes about as B+ but current increases more like 3/2 Power Law. Double voltage makes 2.8X current, which suggests 0.7X load impedance. We can't normally double B+, and we should respect Pdiss, but it is reasonable to shift from 8K to 6.6K.
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Old 20th January 2018, 04:45 AM   #25
PRR is offline PRR  United States
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If anybody is still unclear how we have >1000V p-p with only 400V supply....

Draw it out with 1-thumb numbers so the arithmetic is clear.

We have 400V. The tube will pull-down imperfectly, say to 100V. A 300V down-swing. It will also kick-up the same amount, to 700V. So 600V p-p on one side. On the other side it swings up to 700V and down to 100V. Totals as shown.
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Last edited by PRR; 20th January 2018 at 04:48 AM.
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Old 20th January 2018, 06:29 AM   #26
cozido is offline cozido
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Originally Posted by Gnobuddy View Post
Note the factor of 8 in the denominator: this comes from the conversion of peak-to-peak voltage to RMS voltage [divide by 2 times the square root of (2)], and then squaring that to get RMS voltage squared. Squaring [2 x root(2)] gives you exactly 8.
So much for my envelope... I used peak voltage instead of RMS voltage for the calculation, some one should check the checker! Thanks for your patience and taking the time to explain everything.
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Old 20th January 2018, 07:16 AM   #27
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by cozido View Post
So much for my envelope...
I've had a few of those defective envelopes in my hands over the years, too.

And you're quite welcome, glad I could help!

-Gnobuddy
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Old 20th January 2018, 07:32 AM   #28
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by PRR View Post
...current increases more like 3/2 Power Law.
Are there any 6V6 datasheets that shows plate curves at various different G2 voltages? That might let us extrapolate roughly where the 400V G2 lines fall, at least the G1=0 line.

I forgot something when I said earlier that the line would have to move up to at least ~164 mA. There should be something like 20 mA quiescent current on top of that, so the line really needs to move somewhere above 185 mA at a minimum.

Tubelab George's approach of driving G1 into positive grid voltage territory - but with only 250V on G2 - makes more sense to me than Leo's sky-high voltage on everything.

Not that Leo could have taken that approach, though. He didn't have modern power MOSFETs to drive the control grids with.

-Gnobuddy
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Old 20th January 2018, 08:33 AM   #29
jazbo8 is offline jazbo8
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6V6 load resistance
Never come across one with 400V G2, but we can use the conversion chart from RCA or RDH4 to estimate it, which is somewhere ~200mA for Eg1=0V's knee, may be a bit lower.
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Old 20th January 2018, 04:44 PM   #30
Tubelab_com is offline Tubelab_com  United States
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Quote:
Datasheet for 6V6 tubes in this mode state 8000Ω plate-to-plate load resistance; DR transformer is 6600Ω. Same discrepancy found in Pro Reverb: two 6L6s in AB1, datasheet says 5600Ω, transformer used is 4000Ω. Why is this?
Simple explanation from the economics view. The values published in the tube manual come from the company covering the warantee on the tubes. The values used by Fender and others come from the people who refer to tubes as consumables.

Yes I have squeezed 35 watts from a pair of 6V6 tubes with a 3300 ohm OPT and relatively sane B+ and screen voltages. The plate dissipation is only exceeded by a watt or three (depending on which data book you read) on music peaks which don't come too often in a HiFi amp (the application I am working on).

A guitar amp typically drives a speaker which has its resonant impedance peak within the guitar's frequency range. The true impedance of an "8 ohm" speaker in the region near resonance is in the 15 to 40 ohm range. Often a lower impedance OPT can sound louder, since it is seeing a much higher load impedance over a good portion of a guitar's fundamental frequency range.
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