This will sound dumb but how does one read tube specs? I see things like "Ia(o), Ia(b). Is that the current draw at idle and full load? What's a
good link for a rosetta stone on tube specs?
Some of the figures I see do this number on me- 
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Reading Tube Specs
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 jbmojo |
Reading Tube Specs |
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Posts: 471 (11/07/09 21:39) |
Ninty-nine percent of failures come from people who have the habit of making excuses.
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StoSoStu |
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Posts: 683 (11/08/09 05:30) South of the Border..way South! |
Probably the best instruction manual on electronics from the valve (tube) era is the US Navy NEETS Course, here's the module on valves,
http://www.phy.davidson.edu/instrumentation/Files/NEETS/Mod06%20-%20Electronic%20Emission%20Tubes%20and%20Power%20Supplies.pdf
You can find the whole course from there, well worth reading the lot.
Bob
The road goes on forever and the party never ends |
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jbmojo |
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Posts: 472 (11/08/09 08:35) |
Hell Bob, that is like the Rosetta Stone. Now I'll have plenty of new reading in my "study" in the mornings. Definitely fair dinkum.
Muchas gracias for this one, mon ami.
Me I gotta go, cuz we got another one o' dem damn hurricane head at us so I gwine go out to batten dem damn hatch down again. Ninty-nine percent of failures come from people who have the habit of making excuses.
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StoSoStu |
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Posts: 685 (11/08/09 18:04) South of the Border..way South! |
No worries, jb. Keep safe from them big winds, senor.
Bob
The road goes on forever and the party never ends |
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SG123 |
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Posts: 8474 (11/09/09 17:59) |
Happy to hear Ida's been downgraded to a tropical storm.
So many
frequencies, so little time...
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Old Tele man |
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Posts: 1478 (11/09/09 19:18) Techus Maximus |
...if anybody is interested, I wrote an article for Pentode Press about how to "read" between the lines of a Data Sheet (using RCA 6L6GC as example).
Interested?
"...Renovatio, non Exauguratio..."
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StoSoStu |
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Posts: 687 (11/09/09 22:14) South of the Border..way South! |
OTM, I for one am very interested.
Bob
The road goes on forever and the party never ends |
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jbmojo |
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Posts: 477 (11/10/09 00:13) |
Hell yes I'm interested! Talk to me.
Ninty-nine percent of failures come from people who have the habit of making excuses.
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Andy ZZ |
Interested?? | ||
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Posts: 2617 (11/10/09 00:57) |
Ya Mon! Gimme da stuff!
It aint the years, it's the mileage...
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SG123 |
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Posts: 8475 (11/10/09 19:18) |
I'm also standing in line with my reading glasses on.
So many
frequencies, so little time...
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jbmojo |
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Posts: 481 (11/11/09 07:43) |
Cm'on Tele, yer movin' like old people fu...umm... move.
Ninty-nine percent of failures come from people who have the habit of making excuses.
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Old Tele man |
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Posts: 1480 (11/11/09 18:49) Techus Maximus |
...OK, my guinea pig "example" is the RCA or GE 6L6GC data sheet Class-AB1 55W example listing:
PUSH-PULL CLASS AB1 AMPLIFIER, VALUES FOR TWO TUBES Plate Voltage................................ 450 Volts Screen Voltage............................... 400 Volts Grid-Number 1 Voltage........................ -37 Volts Peak AF Grid-to-Grid Voltage................. 70 Volts Zero-Signal Plate Current.................... 116 Milliamperes Maximum-Signal Plate Current................. 210 Milliamperes Zero-Signal Screen Current................... 5.6 Milliamperes Maximum-Signal Screen Current................ 22 Milliamperes Effective Load Resistance, Plate-to-Plate.... 5600 Ohms Total Harmonic Distortion.................... 1.8 Percent Maximum-Signal Power Output.................. 55 Watts ...everybody familiar with this example? Two points to make first: (1) the data describes a "composite" tube (ie: two tubes), and (2) although not stated some values are DC and others are RMS. ...ready to continue?
"...Renovatio, non Exauguratio..."
Last Edited By: Old Tele man
11/12/09 18:38.
Edited 6 times.
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StoSoStu |
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Posts: 691 (11/11/09 19:53) South of the Border..way South! |
I am a little confused by the Zero-Signal Plate Current of 116 mA, even allowing for the fact that it is the sum of the two plates, it is still 58 mA each,
which seems awful high to me. By my figuring that is 26 watts per valve at idle. What am I missing/getting wrong? Also, should the second "Maximum Signal
Plate Current 22 mA" be the max sig screen current?
Bob
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jbmojo |
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Posts: 487 (11/11/09 20:07) |
I've seen that before and wondered about it myself
Ninty-nine percent of failures come from people who have the habit of making excuses.
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Old Tele man |
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Posts: 1481 (11/11/09 21:03) Techus Maximus |
...very astute of both of you! Yes, it *IS* a very HOT idle, 87% of 30W to be exact:
Ipq = 0.116A(dc)/2 = 0.058A Vpq = 450V(dc) Ppq = 0.058A*450V = 26.1W %W = 26.1W/30W = 0.87 or 87% (very HOT)...but (later) we'll discover that it's STILL much, much closer to Class-B regime than Class-A! ...as you've noted, the Zero-Signal plate (or screen) current value is for two tubes, hence when considering a single tube, the 0.116A value must be halved. ...such is NOT the case however with the Maximum-Signal plate (or screen) current value which is the RMS-current value delivered by each tube during *its* portion of the output cycle, this value is NOT halved. Q: What about the small periods of overlap when BOTH tubes are conducting? A: as one is increasing the other is decreasing so their net ac-values cancel each other in the push-pull windings. ...from the plate-to-plate value Rpp = 5600 ohms, the 'reflected' value that is presented to each tube is Rp = Rpp/4 or 5.6K/4 = 1.4K ohms; however, that's NOT the "effective" reflected Rp' value because it hasn't (yet) been adjusted (tweeked) to account for the "mutual loading" affect that the tube rp has upon Rp and vice versa. The "quick & dirty" way to determine what the true "effective" reflected Rp' value is, is to work backward from the power (55W) and the maximum-signal plate current (dIp.rms = 0.210A): Rp' = 55W / 0.210A^2 = 1,247 ohms, which is certainly NOT that close to Rp = 1,400 ohms. ...why the difference? Mutual loading or "load factor" %R = Rp' / Rp, which is basically: "...what you GET versus what you SHOULD'VE GOT." Which, in this case is: %R = 1247/1400 = 0.8907 or roughly 89%. The 'direct' equation for determining %R is: %R = (rp/(Rp+rp))^2 ...OK, so far?
"...Renovatio, non Exauguratio..."
Last Edited By: Old Tele man
11/11/09 21:07.
Edited 1 times.
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StoSoStu |
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Posts: 693 (11/11/09 22:25) South of the Border..way South! |
Was I correct that, the "Maximum Signal Plate Current 22 mA" should be the max sig screen
current?
Bob
The road goes on forever and the party never ends |
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jbmojo |
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Posts: 488 (11/12/09 09:16) |
Is "rp" in "rp/(Rp+rp)" the same as Rp' ?
and is tube rp = %R Just trying to keep my terms in order, other than that I'm still with you. Ninty-nine percent of failures come from people who have the habit of making excuses.
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Old Tele man |
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Posts: 1482 (11/12/09 18:40) Techus Maximus |
StoSoStu wrote:Yes, that was a 'typo' error on my part (correction made).
"...Renovatio, non Exauguratio..."
Last Edited By: Old Tele man
11/12/09 18:48.
Edited 1 times.
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Old Tele man |
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Posts: 1483 (11/12/09 18:46) Techus Maximus |
jbmojo wrote:Legend: rp = tube dynamic plate resistance, for this 6L6GC it's 23,500 ohms, which is slightly higher than spec value of 22,500 ohms. Rp = reflected resistance presented to the tube by the PP-OT (Rp = Rpp/4), here it's 1,400 ohms: Rp = 5.6K/4 = 1.4K Rp' = effective reflected load due to mutual loading (Rp and rp), the "load factor" %R = 0.8907 %R = Rp' / Rp ...does this help? ...and, here's how we backsolve to determine the actual tube rp value is 23.5K for the example 6L6GC's: rp = Rp*SQR[ %R ] / (1 - SQR[ %R ] ) = 1.4K*SQR[0.8907] / (1 - SQR[0.8907]) = 23,497 ~ 23.5K ohms (about 4.4% above spec)
"...Renovatio, non Exauguratio..."
Last Edited By: Old Tele man
11/12/09 18:59.
Edited 2 times.
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jbmojo |
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Posts: 492 (11/14/09 11:15) |
OK, I follow you now...(this can get very confusing very fast if I don't pay very close attention)
Ninty-nine percent of failures come from people who have the habit of making excuses.
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Old Tele man |
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Posts: 1486 (11/14/09 14:49) Techus Maximus |
...here's the "where & what" sequence that I recommend for understanding push-pull Zoo, Zo, Rpp, Rp, Rp' and rp:
Zoo = Zo*(Np/Ns)^2; where Zo = speaker impedance at the OT-secondary and Zoo is the reflected impedance at the OT-primary. Rp = Rpp/X; where Rp is the portion of the reflected impedance that the tube "sees"; where X=2 for Class-A, X=4 for Class-AB,B; note: Zoo = Rpp, samething, different words. rp = output tube operating characteristic, dynamic plate resistance Rp' = %R*Rp; where %R is the mutual loading factor and Rp is what the tube "sees" if there were no loading factor %R = (Rp'/Rp) = (rp/(Rp+rp))^2; the 'mutual' loading factor of tube rp upon OT-Rp and vice versa.
"...Renovatio, non Exauguratio..."
Last Edited By: Old Tele man
11/14/09 17:07.
Edited 1 times.
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- The Break Room
- Fender Amps
- Tech Bench
- Tubes
- Speakers
- The Instrument Room
- Mesa Amps
- Marshall Amps
- Small Amps
- Boutique Amps
- For Sale
- Other Major Amp Brands
- Introduce Yourself
- Solid State Amps
- Effects
- Other British Amps
- Sound Reinforcement
- Recording
- Important Announcements
- Hammond Organs & Leslie Cabinets
- Shameless Promotion
- The Boiler Room
