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211: 75W or 100W?

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crazyfrog

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Joined 2009
#1
I wrote to Jack Elliano (Electra-Print) recently. In his 211 A2 amp, he runs the triodes near 100mA plate dissipation. Since the datasheets I saw indicate 75W, I asked...

Q About your A2 amp, I'm surprised to see 100W of plate dissipation. On GE datasheet, 75W is indicated for class A (but I know some versions are 100W like 211C or something).

R RCA transmitting tube manual says, 100 watts. GE was mostly TV tubes and got in the habit of rating lower to increase reliability, guess it carried on to these also. No matter, just watch for red plate. If you use a blower on them, they will go to 150 watts. We have been an Amateur Radio operator since the 1950’s so this is common practice for us.

All power triodes work best at near or full plate dissipation, this is the only way to achieve A2, if you check, A2 was called “extreme class A” back in the 1940’s.
 
#9
FullRangeMan said:
Why settle for the 211/845 if the 805 is much better and has 125W plate 🙂
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Because the 805 has too high plate resistance. It was made specifically for class B. In SE is not really a good choice because you need high primary impedance and high current at the same time if you want to use all its power. With the 211 I can also go for 20K primary load now (Lundahl LL1691B) and one only needs 45-50 mA at 1250V for more than 20 watts output. 75W plate dissipation is more than enough!
 
#12
Vinylsavor said:
75W!

RCA manual clearly states 75W max plate dissipation for Class A:

http://frank.pocnet.net/sheets/049/2/211.pdf

If you want the tubes to last, stay with 75W!

Thomas
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That's what it claims all right. Be best to keep the Pd to 75W. 100W won't sound all that much louder anyway, and the 211's will last longer if not pushed so hard. That 100W looks like an ICAS rating, or an intermittent rating for Class AB/B/C for operation that's not "brick on the key" service.
 
#13
I have an A2 amp that will run 211's or 845's.

When I use old RCA or GE tubes I set the bias current at 70 mA with 1050 volts on the plates. This is about 73 watts. I do this because the old tubes are rare and expensive, and I want them to last.

Be best to keep the Pd to 75W. 100W won't sound all that much louder anyway
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It may not sound "louder" at all. Increasing the current in a preperly set up (sufficient bias current and optimum load impedance) class A1 or class A2 amp will not increase the power output. It will increase the damping factor due to the lower plate resistance, and usually will lower the distortion if the OPT can handle it. Simulations with Tubecad SE Amp Cad, and real world measurements on several amps support this. Compare two amps at exactly the same power level into the same speakers. Most people would say the amp with the higher distortion was "louder."

I got several sets of Chinese Shuguangs back when I could get them for $26 each. I have seen no issues running these tubes as hot as 125 mA which drags the plate voltage down to 1000. I have seen no red plate even in a dark room....well how dark can the room get with those filaments glowing.

All power triodes work best at near or full plate dissipation.......
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To some extent this is true. Damping factor and distortion is improved. Power output does not change.

....... this is the only way to achieve A2, if you check, A2 was called “extreme class A” back in the 1940’s.
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This is totally untrue. The textbook differences between A1 and A2 ONLY involve grid current. A class A1 amp does not permit grid current to flow at all during any part of the audio cycle. A class A2 amp permits grid current to flow during some or all of the audio cycle. Tube dissipation or current is NOT a factor.

I usually run my amp at 70 mA. It makes 42 watts per channel at this level, and enters A2 at about 28 watts. The amp does not draw any grid current until the power level reaches 28 watts, so if you never turned it up past this point, it is an A1 amp. The mosfet driver is capable of pushing the grid to +100 volts (needed with an 845) but it never gets above 0 volts until 28 watts of output is reached.

When the current is turned up to 125 mA the grid is closer to zero at idle. Now the amp enters A2 at about 20 watts. Maximum power is about the same. If I remember correctly is goes down about a watt. The distortion is slightly lower, but the bass quality is improved dramatically on my 15 inch OB speakers. Yes, you can play Pink Floyd or Depeche Mode very loudly on an SE amp.
 
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