Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on October 18, 2011, 12:57:22 pm
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I am intrigued by this 6MB8 tube but I will confess that I don't understand some of the tube data. I am hoping this is a smaller version 6BM8 with less power?
The 6MB8 is a sharp pentode. The 6BM8 is a power pentode.
http://www.nj7p.org/Tube4.php?tube=6MB8 (http://www.nj7p.org/Tube4.php?tube=6MB8)
http://www.nj7p.org/Tube4.php?tube=6BM8 (http://www.nj7p.org/Tube4.php?tube=6BM8)
Q1) What is the difference between a sharp pentode and power pentode?
Q2) Can a 6MB8 be used push/pull like a 6BM8 can?
Q3) What current ma draw would there be with 270v on the plate?
Q4) Does a 47R/5w resistor sound reasonable for a shared cathode resistor?
The p/p 6BM8 is about 7-9 watts. IF the 6MB8 is a similar type pentode/triode where I can get a great musical sound at 4 watts ........... that would be fantastic!
With a PT that is 65ma and 190-190, this could be a "perfect" tube from that vantage point, I think?
Anyone here have any thoughts or experience with 6MB8 p/p amps?
With respect, Tubenit
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6MB8
Pentode Class A Amplifier
Plate Voltage ................................. 125 V
Grid No. 2 Voltage ............................ 125 V
Grid No. 1 Voltage Derived from
Cathode Bias Resistor ....................... 33 Ω
Plate Resistance (approx) ..................... 125000 Ω
Transconductance .............................. 12000 µMho
Plate Current ................................. 10 mA
Grid No. 2 Current ............................ 2.8 mA
So B+/Pi = 125/.01 = 12500 ohm transformer impedance. That seems cool but
Pi * Rk = .01 *33 = .33V Vbias. That means a signal of 1/3 of a volt drives the tube to full power. :cussing:
Somebody please verify this but I don't think this Pentode is suitable for an output.
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The 6MB8 is a sharp pentode. The 6BM8 is a power pentode.
I guess you could say that another way, as "6MB8 is a "not-power" pentode. :icon_biggrin:
"Sharp cutoff" means it is a small-signal pentode, generally for preamp or voltage amplifier use, which has characteristics you're used to. A "remote-cutoff" pentode is also sometimes called a "variable-mu" tube, and is difficult to completely cut off. They are most often used with a variable bias provided by a radio's automatic gain control (AGC) circuit, to allow weak and strong stations to sound equally loud. Some clever folks have used them in tube compressors.
A power pentode is meant for, well, power output applications.
Q2) Can a 6MB8 be used push/pull like a 6BM8 can?
You can use any tube in push-pull. The real question is what are you trying to accomplish? That, and do you happen to have a bunch of 6MB8's in hand waiting to be used? I do happen to have a bunch of 6AU6's in hand, without a home, so my interest lies more in your original thread.
Q3) What current ma draw would there be with 270v on the plate?
Something between 0mA and infinity. I'm really not trying to be flip; for any pentode, plate voltage hardly matters, but screen voltage is essential in determining plate current.
But since we have only short-form data (I hate short form data), let's note that max plate dissipation is 2w. So the most current we could have at 270v is 2w/270v = 7.4mA. We know from the specs given that we have either 10mA (-0.422v bias) in pentode mode with 125v on G2, and Gm is 12,000 micromhos (1.2mA/v). Gm will drop with less current, but as a first guess, we need 2.6mA less so 2.6mA/1.2 = 2.17v more bias is needed. We could then guess that 270v plate, 125v screen, and -2.6v bias will work.
Q4) Does a 47R/5w resistor sound reasonable for a shared cathode resistor?
It would be smart to plan for at least 3v of bias. According to the data sheet, the ratio of plate to screen current is about 3.6:1. For 7.4mA of plate current, screen current should be about 2mA. Cathode current is then 9.4mA, so the cathode bias resistor will be about 3v/9.4mA = 320 ohms; 330 should be close enough. For sharing between two tubes, tend to the high side with a 180 ohm resistor. 18.8mA * 180 ohms = 3.38v * .0188A -> 0.06w, so a 1w resistor should be plenty.
You'll probably have a separate power supply node for the screen supply. Take these numbers as an example to work based on that node's voltage. We have 270v, and need 125v on the screen (if we had more complete data, or breadboard testing, we could use other screen voltages). We know the screen will pass about 2mA; 145v/2mA = 72k, 75k is close enough. You'll need a bypass cap to ground from the screen pin, as you see in preamp pentodes. A .1uF cap is -3dB down at F = 1/(2*pi*.0000001F*75k) = ~21Hz. A 1uF cap would be a solid bypass and help keep hum out of the screen.
Will continue separately with possible good OT primary impedance.
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Gentleman, thanks for the replies about the 6MB8.
HBP, always appreciate your input my friend (& amp building mentor)!
I think I'm headed back towards the 6AU6 direction. :think1:
Doesn't sound like a 6MB8 is what I am looking for and I don't have any either. :l2:
With respect, Tubenit
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> this 6MB8 tube ... a smaller version 6BM8 with less power?
The letters are mostly meaningless. Both are 6V heat with 8 active elements. Could be a common-cathode duo-triode + duo-diode. Yes, it does seem that both are triode-pentodes.
> Can a 6MB8 be used push/pull like a 6BM8 can?
ANY tube can be paired-up push-pull.
> What is the difference between a sharp pentode and power pentode?
Power pentode is bigger. Specifically: enough current to work well with OTs, which don't like to be wound-up higher than 5K or 10K.
Power pentodes are, generally, "sharp", because that's what you want in high-power uses. The alternative, remote-cutoff, makes sense in some small-to-very-small signal applications, mostly for adjusting radio-set gain for near or far stations.
> a smaller version 6BM8 with less power?
So why not run the easily-available 6MB8 with lower supply voltage? Under-cooking a tube won't make it sick.
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> Gm is 12,000 micromhos (1.2mA/v).
12mA/V. (Dang slippery decimals.)
> a signal of 1/3 of a volt drives the tube to full power.
No. That condition is Max Gain, _not_ high output. Work the 1/3 Volt against 12,000uHmho transconductance: another 0.3V brings Ip from 10mA to 13mA. 3mA up-swing is pathetic. Since 13mA seems the best it will do (at 125Vg2), for "hi power" we would bias more like 7mA, so as to swing down to 1mA and up to 13mA, 6mA either way. (This also slants the best-load to more like 20K.)
It is a Chroma Amp. A specific sub-set of Video Amps. The intended load is three resistors to three grids, something like that; not even a CRT cathode.
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> Gm is 12,000 micromhos (1.2mA/v).
12mA/V. (Dang slippery decimals.)
It should've occurred to me that's more like a 12AX7 than a hot pentode. Thanks!
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So why not run the easily-available 6BM8 with lower supply voltage? Under-cooking a tube won't make it sick.
Good question. Answer is the PT only can handle 65ma. The 6MB8 p/p would come under that but the 6BM8 exceeds it. '
Withr respect, Tubenit