I did some curve tracing of the "small" RCA 12HL7 versus a Sylvania full size 12HL7 and surprisingly they are fairly similar except the "small" 12HL7 behaves like it has about +1 Volt added to its grid1. The full size 12HL7 had slightly better triode curves and less screen grid current in its B pentode curves.
Then I traced an 11HM7 tube (which -looks- like the small RCA 12HL7 ) and its curves are much more like a 12GN7 except the 11HM7 behaves like it has about +1 Volt added to its grid1.
Then I traced an 11HM7 tube (which -looks- like the small RCA 12HL7 ) and its curves are much more like a 12GN7 except the 11HM7 behaves like it has about +1 Volt added to its grid1.
The +1V tubes are more turned on for the same external bias condition. So they might need another -1V added to the bias to behave the same.
I notice the 11HM7 has an rp of 40K while the 12HL7 and the 12GN7 have an rp of 50K. The smaller plate structure of the 11HM7 and of the "small" RCA 12HL7 are placing the plates closer to the cathode, so plate voltage alone is pulling more current.
I notice the 11HM7 has an rp of 40K while the 12HL7 and the 12GN7 have an rp of 50K. The smaller plate structure of the 11HM7 and of the "small" RCA 12HL7 are placing the plates closer to the cathode, so plate voltage alone is pulling more current.
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Yeah, that could be a problem with designing an Amplifier with "fake" tubes, and then the "real" ones don't work right.
Warning Label: this product -must- operate with Fake tubes only.
I wonder what the "fake" 12HL7 tubes did to the video response of old TV sets. At least most TV sets would have a Brightness control to fix things.
Warning Label: this product -must- operate with Fake tubes only.
I wonder what the "fake" 12HL7 tubes did to the video response of old TV sets. At least most TV sets would have a Brightness control to fix things.
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There is the 15/6HB6 power pentode, which is rated for 10 Watts, 20,000 gm, mu 33, roughly 40+ mA DC and has a plate twice as big as the "real" 12HL7. The triode curves aren't as good, but at least it will keep working. Draws more Htr current though.
I think I would keep 12HL7 to below 6.5 or 7 Watts, like a 12BY7, and 30 mA DC. They just don't seem to hold up at 50 mA DC.
I think I would keep 12HL7 to below 6.5 or 7 Watts, like a 12BY7, and 30 mA DC. They just don't seem to hold up at 50 mA DC.
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We mustn't forget that tube makers were/are manufacturers in the truest sense of the word. Welding together a tube's internal structure was/is strict manual labour, hence prone to large tolerances in the outcome.
...and a contrast control, too, which I think is more important to compensate for gain tolerances in different exemplars of nominally the same tube.
Best regards!
passing the 6es8 for a 6dj7 makes the latter a fake tube, although na former is not a fake tube...
there are other examples i am sure..
i remember, with TV tubes of old, even the owner himself can fix his tube by simply replacing the tube.
i remember my father usually did that with our Admiral 21" bnw tv set in the 60's....
None of the above Video tubes gives any explicit current ratings for max DC or peak current, just a suggested operating point and graphs with current for the Vertical scale. So I had been using the approx. rule ( for Sweep outputs ) of peak current about 3 to 3.5 times the DC max. Taking the graph maximums for peak. But HF video is a different thing from 60 Hz deflection. And HF noise would be an issue too. The manufacturers of video tubes may have used a different oxide/coating scheme for video tubes. Maybe smoother surface or different oxide combo to keep HF noise down, but sacrificing max DC current. Less porosity maybe? At any rate, looks like the suggested operating current level may be more important for these Video types. The "small" RCA 12HL7 variants do appear to have a slightly smaller cathode too. The GE 12HL7 data sheet seems to indicate 76 mA peak operating current and 25 ma Operating point. (while the graphs go up over 150 mA)
6HZ8 is another Video tube with an 8 Watt actual pentode. 29 mA suggested operating current, with what looks like a 90 mA peak current in the operating data.
By the way, the 6HB6 tube is listed as a Beam Pentode, Vertical Defl. type on the GE datasheet, but I have some RCA 6HB6 tubes (but not all the RCA ) that are actual Pentodes. Look like the same cathode size for either. Looks like 40 mA suggested Op. and 150 mA operating peak for 6HB6.
6HZ8 is another Video tube with an 8 Watt actual pentode. 29 mA suggested operating current, with what looks like a 90 mA peak current in the operating data.
By the way, the 6HB6 tube is listed as a Beam Pentode, Vertical Defl. type on the GE datasheet, but I have some RCA 6HB6 tubes (but not all the RCA ) that are actual Pentodes. Look like the same cathode size for either. Looks like 40 mA suggested Op. and 150 mA operating peak for 6HB6.
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Ah, that could explain what I found in my circuit. I was running the 12HL7s way above their recommended operating point (at over Ip = 40mA, I think).
12GN7A looks like it's designed for slightly higher plate plus screen current...
But according to its GE datasheet, 12HL7 isn't far behind...
However, look at the values for Rk. Very similar operating points (Vp = 250V, Vg2 = 150V, Ip+Ig2 = 31 to 34mA) but Rk for 12GN7A is 56 ohms, while Rk for 12HL7 is 122 ohms.
In my little (triode) headphone amp, Rk = 62 ohms, Vp+g2 = 155V.
I think if I want to make this circuit work with 12HL7-triode, I'd need to increase the value of Rk, perhaps to 82 ohms or maybe 100 ohms.
Or leave it with 12GN7A-triode in there. It seems to be working well that way.
12HL7-triode did 'sound better' to me...
The 12HL7 may have it's max DC current level up around 36 mA. Of course, one should de-rate from that some for reliable operation. The "small" RCA 12HL7 however may have it's max DC level just above the 25 mA suggested level.
Originally, I tried to estimate a 12HL7 tube max DC current level from the 6BQ5/6GK6 tube datasheet ratings, which DO give a max DC current rating of 65 mA, using a 0.76 Amp Htr versus 0.6 Amp Htr equiv. for the 12HL7. That ratio gave 50 mA estimated for 12HL7. However, it is apparent now that that is still too high an estimate.
Looking at the 6HB6 tube, which is essentially just a 6BQ5 with a fine mesh grid1, for getting gm up to 20,000, the 6HB6 suggested operating current drops to 40 mA from the 48 mA for the 6BQ5. The finer grid mesh limits current some. The 12HL7 is at a similar 21000 gm with a frame grid. Combining Htr and grid1 mesh effects gets expected max DC current for the 12HL7 down to 40 mA. There is still some additional in-efficiency of a smaller Htr due to the same heat loss from wire supports etc, so it will be a little below the 40 mA max for the 12HL7. Maybe 38 mA then.
An alternative approach is to look at the 6BQ5 max DC current versus suggested operating point. At 65ma/48mA = 1.354 so that gives 1.354 x 25 mA for the 12HL7 or 33.85 mA max DC current for the 12HL7. Another data point.
And another approach is to look at the plate curves for suggested Vg2. 6BQ5 has the top knee at 140 mA and 12HL7 has the top knee at 97 mA. 97/140 = .6928 so that times the 65ma max for the 6BQ5 gives 45 mA max, but we still need to take into account the finer grid mesh. So 40/48 = 0.83 so dropping that 45mA to 37.5 mA max DC. Another data point.
Averaging the 3 data points gives around 36 mA max DC current for the 12HL7. Good enough I think. They were cheap when I bought them.
Still a good idea to de-rate some for reliability. Usually the data book recommends 70% derating. Hmmm, 0.7 x 36 mA gives 25 mA, the suggested 12HL7 operating point!! So they knew what they were doing.... ( OK, so I figured out what they were doing. )
So 36 mA if you bought them at $1 each and have a box of them. ( the Flash-Bang threshold? )
And 25 mA if you have to buy them still.
Originally, I tried to estimate a 12HL7 tube max DC current level from the 6BQ5/6GK6 tube datasheet ratings, which DO give a max DC current rating of 65 mA, using a 0.76 Amp Htr versus 0.6 Amp Htr equiv. for the 12HL7. That ratio gave 50 mA estimated for 12HL7. However, it is apparent now that that is still too high an estimate.
Looking at the 6HB6 tube, which is essentially just a 6BQ5 with a fine mesh grid1, for getting gm up to 20,000, the 6HB6 suggested operating current drops to 40 mA from the 48 mA for the 6BQ5. The finer grid mesh limits current some. The 12HL7 is at a similar 21000 gm with a frame grid. Combining Htr and grid1 mesh effects gets expected max DC current for the 12HL7 down to 40 mA. There is still some additional in-efficiency of a smaller Htr due to the same heat loss from wire supports etc, so it will be a little below the 40 mA max for the 12HL7. Maybe 38 mA then.
An alternative approach is to look at the 6BQ5 max DC current versus suggested operating point. At 65ma/48mA = 1.354 so that gives 1.354 x 25 mA for the 12HL7 or 33.85 mA max DC current for the 12HL7. Another data point.
And another approach is to look at the plate curves for suggested Vg2. 6BQ5 has the top knee at 140 mA and 12HL7 has the top knee at 97 mA. 97/140 = .6928 so that times the 65ma max for the 6BQ5 gives 45 mA max, but we still need to take into account the finer grid mesh. So 40/48 = 0.83 so dropping that 45mA to 37.5 mA max DC. Another data point.
Averaging the 3 data points gives around 36 mA max DC current for the 12HL7. Good enough I think. They were cheap when I bought them.
Still a good idea to de-rate some for reliability. Usually the data book recommends 70% derating. Hmmm, 0.7 x 36 mA gives 25 mA, the suggested 12HL7 operating point!! So they knew what they were doing.... ( OK, so I figured out what they were doing. )
So 36 mA if you bought them at $1 each and have a box of them. ( the Flash-Bang threshold? )
And 25 mA if you have to buy them still.
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