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Old 22nd June 2003, 06:55 PM   #1
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Default Plate curve calculations - am I doing this right?

Hi,

I'm looking at the 2A3 plate curves. My operating point is 290V plate-cathode voltage, 62mA plate current, about 55V bias voltage (cathode biased). I plotted this point on the plate curves (and drew the 2500 ohm load line), and am trying to calculate u, gm and Rp. Rp is the slope of the plate curve at the operating point, so I drew a line that's tangential to the nearest curve at the operating point. That gives me a value of around 660 ohms, which is close enough to the 800 ohm nominal value found in the data sheet (which is probably valid at a different operating point). I've read advice that it's more accurate to calculate u and then get gm from u and Rp, than trying to calculate gm straight from the plate curves, because the lines are further away in the vertical direction. So, to get the value of u, I draw a horizontal line through the operating point, intersecting the nearest plate curves on either side. This gives me a plate voltage change of about 40V, for a grid voltage change of 10V, or u = 4, and so gm = u * Rp = 2640.

Do those numbers look right? Does a 2A3 at this operating point have a voltage gain of just 4?

The hum balance pots on my amp have taken care of almost all the 60Hz hum, I have some 120Hz hum still left though, and I'm thinking of trying the Tubecad idea of adding a capacitor from B+ to cathode to act as a voltage divider with the cathode bypass cap to inject the right amount of B+ hum into the cathode so that it cancels out the hum on the plate (article link: http://www.tubecad.com/april99/page2.html). To do this, I need to know the value of u.

On a related note, is this circuit pretty similar to the ultrapath connection circuits I've seen? I've seen some that remove the cathode bypass cap and only leave this cap from cathode to B+. Those talk about making the output circuit loop smaller and not making the signal go through the last PS cap. They don't talk about cancelling hum, and the appropriate cap value is usually describes as "from 7uF to 100uF, about 40uF works best", at least in the few posts I've read. I'm wondering if both circuits do similar things?

Thanks in advance,
Saurav
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Old 23rd June 2003, 02:00 PM   #2
Joel is offline Joel  United States
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Default Re: Plate curve calculations - am I doing this right?

Quote:
Originally posted by Saurav
...Do those numbers look right? Does a 2A3 at this operating point have a voltage gain of just 4?
Sure, it could be that low. It's a power tube afterall, not a voltage amp.
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Old 23rd June 2003, 03:29 PM   #3
arnoldc is offline arnoldc  Philippines
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The mu of the 2A3 is 4.16, 300B 3.85, 45 is 3.52 for reference.
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Old 23rd June 2003, 03:58 PM   #4
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Thanks, that makes me feel better. I know that preamp/driver tubes will have a higher mu, but I didn't know that power tubes were this low.
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Old 23rd June 2003, 04:09 PM   #5
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Hi Saurav,

Your reading of the plate curve data is correct - sounds like you have that nailed! If you had called up 2A3 data you would see mu quoted as 4.2 at normal operating points and as you can see by the curves your slightly hotter than traditional point is close to that.

On the other question, Ultrapath and the Tubecad hum reduction technique are not the same and don't do the same job. As you state Ultrapath bypasses the powersupply and cathode bypass cap to create a short and well defined ac signal return path. As such it doesn't do much to reduce hum. The tubecad technique does directly affect hum by introducing a measured amount (via the cap divider) of anti-phase hum into the valves amplifing path and so providing active cancellation - to a degree. It is possible to get better than 20dB of cancellation by this 'trick'. Funny how a capacitor in the same place can do two jobs according to it's value... (It does, of course do both jobs but it does one better than the other according to how you choose it's value)

Hope this helps

ciao

James
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Old 23rd June 2003, 04:19 PM   #6
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Quote:
It does, of course do both jobs but it does one better than the other according to how you choose it's value
Ah, I see. Thanks, that makes things clearer.
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Old 24th June 2003, 09:33 PM   #7
PRR is offline PRR  United States
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> I know that preamp/driver tubes will have a higher mu, but I didn't know that power tubes were this low.

For a given heater power, a tube's Gm near zero-bias is pretty much "fixed". Some of the 1960 tuner tubes do a little better than the ancient 1930s tubes, but not a lot.

The plate resistance is Mu/Gm. We should mind the difference between large-signal Rp and the incremental Rp, but for most triodes the difference is not large and the trends are valid.

For best power from a fixed supply voltage, we want the lowest possible Rp. But for a given heater power, Gm is roughly fixed. (Assuming your grid design is not stupid, Gm is proportional to cathode area, and heater power is proportional to cathode area.) So the only way to get a small Rp is to move the grid and plate to get a low Mu.

You use Mu=20 to 100 for voltage gain where current and power output is small. But for POWER, you must use low Mu tubes and accept the low gain (high drive voltage).

But Mu=1 or Mu=2 is absurd. The required drive voltage is larger than the plate supply on the output stage. Since we usually design the power supply for the output stage, since it takes the most power, it is annoying and costly to add another higher-voltage power supply just for the driver.

If you assume some rules-of-thumbs: peak RC-coupled driver output is 0.20*B+, output stage bias is about 0.63*(B+/Mu), then the lowest practicable Mu for a tube with R-C coupled driver on same supply as the output is about Mu=3.

You hate to whack a driver that hard, and you would like a cheap R-C ripple filter on the driver (so the main B+ does not have to be as clean as the driver B+). Then the optimum is about Mu=5.

All the classic audio power triodes have Mu=4 or 5, in that range: 2A3, 300B, type 50, etc.

(Pass Regulators work a little different and can use a lower Mu, such as 6080/6AS7 Mu=2. The difference is that the pass-tube only sees a small part of the total power supply voltage, but the reference amplifier can use the whole supply voltage.)

In a somewhat related field, TV Vertical Sweep amplifiers, the power triode on a wide-deflection design runs a Mu of 6 or 7. See 6EM7. They need big current and high efficiency at low suply voltage, but need more-than-minimum gain to keep the sweep feedback working at extremes of voltage and current. (There is an older class of V-Sweep tubes with Mu=20, but these were for narrow-angle CRTs that did not need huge yoke current, or for TV sets that had large supply voltage available.)

And interestingly: the huge TV Horizontal Sweep pentodes also have Mu=5 or so when triode strapped. They needed maximum efficiency, so they needed low large-signal plate impedance, which in a pentode is approximately the triode-strap plate resistance.

OTOH, the venerable 6L6 and its many kin run triode-Mu around 10 or 20. Maximum current is a little less, but the pentode mode allows this current to be available at fairly low plate voltage swing, and the higher Mu means less driver voltage and distortion.
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Old 24th June 2003, 10:05 PM   #8
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Hi,

Interesting post but...

Quote:
But Mu=1 or Mu=2 is absurd. The required drive voltage is larger than the plate supply on the output stage. Since we usually design the power supply for the output stage, since it takes the most power, it is annoying and costly to add another higher-voltage power supply just for the driver.
Frankly, I don't much care about that since I feel it to be a better approach to use independent supplies, some even regulated, for input and outputstages.

From your reasoning all CF outputstages are out since they don't have any gain at all, in fact their gain is even a negative figure.
Or are you just reffering to the stated mu in the datasheets which is a topology dependent figure anyway?

Anyway, I feel you are always better off keeping PSUs for different stages as separate as possible and I always use totally independent supplies on a pro stage basis...obviously I don't care about the cost, just the performance.

Cheers,
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Old 25th June 2003, 05:49 AM   #9
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Quote:
Originally posted by fdegrove
From your reasoning all CF outputstages are out since they don't have any gain at all, in fact their gain is even a negative figure.
Hi Frank,
Indeed, CF's are the most famous inverting stage known!

Your friend, Tim, the pedant

(Ok I'll add something factual. You mean less than unity (i.e. <1), but of course not less than zero.)
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Old 25th June 2003, 06:00 AM   #10
EC8010 is offline EC8010  United Kingdom
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Default Pedants unite!

Tim, mind you, in dBs, CF does have negative gain (even more pedantic).

PRR, your points about output valve mu were very interesting, and I'd not thought about it in that way. It's always nice to know why things are the way they are. Thanks for that.
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