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Grid resistance for SET 2A3
I've found one instance in an old audio book that talks about the size
of the grid resistance. RCA indicates 50K max for fixed bias and 500K max for cathode. Like most of the old audio handbooks, it generalizes too much, leaving out technical details. Essentially it states that positive charged ions from gassy tubes are repelled by the plate and are attracted to the negative grid. As they collect on the grid, one needs small grid resistance to bleed off the charge, else it leads to a positive grid and increased bias current. The handbook states that this is not a problem with cathode bias as the increased plate current self-compensates with the biasing action of the cathode resistor. So you can get away with the larger grid resistance. Ok, I'm cool with that. Question 1: Is this a really big problem? With modern production tubes? I'm trying to run Sovtek 2A3's at about 20W static. Question 2: Right now I'm strictly running fixed bias and want to try to stay with it for adjustibility. Can one add a small cathode resistance to compenstate for the grid buildup, if any, and keep the fixed bias setup? I guess a small cathode resistor would have to be at least big enough to develop a sizable drop across it for compensation, but how big in general? As indicated by the book, the lower grid resistance values load my 6SL7 gain stage badly and I want to kick up the grid resistance back up to around 500k. Any suggestion? TIA Gilbert |
#2
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Gilbert Bates said:
I've found one instance in an old audio book that talks about the size of the grid resistance. RCA indicates 50K max for fixed bias and 500K max for cathode. Like most of the old audio handbooks, it generalizes too much, leaving out technical details. Essentially it states that positive charged ions from gassy tubes are repelled by the plate and are attracted to the negative grid. As they collect on the grid, one needs small grid resistance to bleed off the charge, else it leads to a positive grid and increased bias current. The handbook states that this is not a problem with cathode bias as the increased plate current self-compensates with the biasing action of the cathode resistor. So you can get away with the larger grid resistance. Ok, I'm cool with that. I was under the impression that the effect you describe is caused by loose cathode particles that get attached to the grid, and are heated by the adjacent cathode enough to start emitting electrons by themselves, making the grid more positive. Question 1: Is this a really big problem? With modern production tubes? I'm trying to run Sovtek 2A3's at about 20W static. I've observed the above with some pairs of Chinese KT66s and KT88s. They were used with Rg of 330 kohm, which was enough to let them drift away from their (cathode-biased!) working point. Lowering Rg to 100 k solved the problem. Question 2: Right now I'm strictly running fixed bias and want to try to stay with it for adjustibility. Can one add a small cathode resistance to compenstate for the grid buildup, if any, and keep the fixed bias setup? I guess a small cathode resistor would have to be at least big enough to develop a sizable drop across it for compensation, but how big in general? Dunno if a cathode resistor would entirely eliminate this effect, due to what I said above. As indicated by the book, the lower grid resistance values load my 6SL7 gain stage badly and I want to kick up the grid resistance back up to around 500k. Isn't 100...220 k enough? How about a somewhat beefier driver configuration? 2A3 isn't that hard to drive I'd guess? -- "Audio as a serious hobby is going down the tubes." - Howard Ferstler, 25/4/2005 |
#3
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On Mon, 20 Jun 2005 21:43:57 +0200, Sander deWaal
wrote: Gilbert Bates said: I've found one instance in an old audio book that talks about the size of the grid resistance. RCA indicates 50K max for fixed bias and 500K max for cathode. Like most of the old audio handbooks, it generalizes too much, leaving out technical details. Essentially it states that positive charged ions from gassy tubes are repelled by the plate and are attracted to the negative grid. As they collect on the grid, one needs small grid resistance to bleed off the charge, else it leads to a positive grid and increased bias current. The handbook states that this is not a problem with cathode bias as the increased plate current self-compensates with the biasing action of the cathode resistor. So you can get away with the larger grid resistance. Ok, I'm cool with that. I was under the impression that the effect you describe is caused by loose cathode particles that get attached to the grid, and are heated by the adjacent cathode enough to start emitting electrons by themselves, making the grid more positive. Question 1: Is this a really big problem? With modern production tubes? I'm trying to run Sovtek 2A3's at about 20W static. I've observed the above with some pairs of Chinese KT66s and KT88s. They were used with Rg of 330 kohm, which was enough to let them drift away from their (cathode-biased!) working point. Lowering Rg to 100 k solved the problem. Question 2: Right now I'm strictly running fixed bias and want to try to stay with it for adjustibility. Can one add a small cathode resistance to compenstate for the grid buildup, if any, and keep the fixed bias setup? I guess a small cathode resistor would have to be at least big enough to develop a sizable drop across it for compensation, but how big in general? Dunno if a cathode resistor would entirely eliminate this effect, due to what I said above. As indicated by the book, the lower grid resistance values load my 6SL7 gain stage badly and I want to kick up the grid resistance back up to around 500k. Isn't 100...220 k enough? How about a somewhat beefier driver configuration? 2A3 isn't that hard to drive I'd guess? Please exscuse my ignorance on the matter. I've already configured my chassis for one octal preamp tube while using 1/2 of a 6SL7 as a no gain buffer for a sub-woofer output, which leaves me with only one triode section for the 2A3's drive requirements. After I think about it now, I could wire the sub-out directly without the triode stage directly off the volume? The 2A3 needs a sizable current drive for the large input capacitance. The 6sn7 would be a better drive candidate but doesn't have enough voltage gain with just one stage and I'm running the 2A3 at higher B+ voltages than the traditional 'sweet-spot' point at around 250V. So with the higher B+ I have more negative bias voltage requirements which should leave room for higher drive voltage swings. Initially I've been playing with high gain configurations with the 6sl7 but results in low idle (drive) current and high plate resistance in the drive stage. That was my reasoning with the need to get the 2A3 grid resistance up and not load the plate resistance of the 6sl7 so much. I guess I'll see this through using just one triode section of the 6sl7 to drive the 2A3. I'll just have to accept reduced voltage swing for increased drive current. At this point, I believe I'll move the 6sl7 plate resistor value down to 100k and live with the resultant gain so that 100k - 220k grid resistor on the 2A3 won't load it so badly. So I still have to figure out an acceptable combination of cathode/fixed biasing so that I can have increased grid resistance and enough cathode resistance to keep the drift under control (if any) |
#4
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Gilbert Bates said:
After I think about it now, I could wire the sub-out directly without the triode stage directly off the volume? You could try that, and parrallel both halves of the 6SL7. That allows you to halve the Ra and thus leave you with enough voltage swing to drive the grid of the 2A3 and its grid resistor easily. I'm building a 2A3 PP amp right now, and am using a 6SN7 phase splitter and 2 x 6SN7 drivers, both halves in parallel feeding each a grid of the 2A3. With 300V B+ I reach over 80 Vp-p swing at the anodes, more than enough for the 2A3s. -- "Audio as a serious hobby is going down the tubes." - Howard Ferstler, 25/4/2005 |
#5
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Gilbert Bates wrote:
Any suggestion? Yes. Don't operate gassy tubes. Which you will recognize by their degraded performance, and check out, long before they start going postive-grid. |
#6
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Gilbert Bates wrote: I've found one instance in an old audio book that talks about the size of the grid resistance. RCA indicates 50K max for fixed bias and 500K max for cathode. Like most of the old audio handbooks, it generalizes too much, leaving out technical details. Essentially it states that positive charged ions from gassy tubes are repelled by the plate and are attracted to the negative grid. As they collect on the grid, one needs small grid resistance to bleed off the charge, else it leads to a positive grid and increased bias current. The handbook states that this is not a problem with cathode bias as the increased plate current self-compensates with the biasing action of the cathode resistor. So you can get away with the larger grid resistance. Ok, I'm cool with that. Question 1: Is this a really big problem? With modern production tubes? I'm trying to run Sovtek 2A3's at about 20W static. By modern production do you mean tubes made in Russia or china after 1993? I have not had much problem with grids going positive in respect to 0V with cathode bias or with respect to the -ve fixed bias supply. NOS GE6550A were quite prone to +ve grids, and the more Ia there is, the larger the rise in +ve voltage. Its a weird phenomenon, becayse in a radio with a 6AV6 detector triode, the cathode is usually grounded, and the grid is biased perhaps with 10Mohms, to get a grid bias of -1v at the grid. A very small grid current = 0.1 µA flows; electrons are being collected by the grid and flow towards the bias supply, ie, 0V. But with a power tube the grid starts off being -ve when new, then slowly starts to go +ve. The dozen GE5550A I bought in 1995 all did this over time. The rise in grid voltage is limitable by Rg = 100k. In one amp I used FB and Rg = 390k to begin with and there was up to 4 volts across the 390k, and since the grid was slightly +ve, it meant that electrons were being emitted by the grid, like they are in grid current conditions, only not in such great quantities. But perhaps the very +ve screen support rods in the mica and right beside the grid1 support rods allow a tiny leakage of electrons to the screen. I am still not 100% sure why the g1 goes +ve though. Some say +ve ions, some say leakage across the mica inside the tube, whatever, but it happens, and its not a condition to be confused with leaking coupling caps, which exhibit a very similar sympton. Question 2: Right now I'm strictly running fixed bias and want to try to stay with it for adjustibility. Can one add a small cathode resistance to compenstate for the grid buildup, if any, and keep the fixed bias setup? I guess a small cathode resistor would have to be at least big enough to develop a sizable drop across it for compensation, but how big in general? Using an Rk large enough to regulate the cathode current as the normal Rk does in cathode bias amps would mean that Rk would have to be about at least 1/2 the usual value of Rk for CB. I do this myself in my 300 watt amps using 12 x 6550, but I use Rk about 2/3 the size of the normal full value for cathode bias. I apply some fixed bias which is 1/2 the value of the actual cathode bias obtained. This means the Rk dissipation in the amp is low, since Pd = E x I and if E is 0.7 times normal, Pd will be substantially lower, and the effective Ea will be usefully higher. it is essential to bypass any substantial Rk used to regulate the Ia, since the Ra of the tube will effectively increase with Rk unbypassed, so that Ra' ( effective Ra ) = Ra + ( µ x Rk ). The power tubes unbypassed Rk would make the Ro of the amp a lot higher than you'd want. As indicated by the book, the lower grid resistance values load my 6SL7 gain stage badly and I want to kick up the grid resistance back up to around 500k. I wouldn't use a 6SL7 to drive any power tube grid. I would use a 6SN7, in which can flow a much higher Ia, so a large V swing across a lowish Rg is possible. The load seen by the driver anode is its DC supply RL in parallel with the cap coupled Rg of the following tube. When you draw the loadline for the combined parallel load at the anode of the driver, and passing through the selected Q point for the anode, the combined loadline will intersect the hrizontal axis below the Ea value for driver triode. The Vswing can be thus limited by this, and that's why the RDH4 and other books recommend that the cap coupled RL is should be greater than 3 times the DC RL, especially for a driver tube driving an output triode. So with a 6SN7, the DC RL might be 39k, and the 2A3 bias R could be as low as say 120k, and you still should get a decent low distortion drive signal. The 6SL7 is incapable of doing the same thing with thse loads. The 6SL7 is a very nice sounding input tube where a fair amount of gain is to be used and you want to use NFB around a 3 tube line up like SL7, SN7, 2A3. I use 6SL7/ECC35, ECC32/6SN7, 13E1 ( SEUL, 25 watts ) line up with 16 dB of global NFB and the owner is one very happy dude. Each of the small input tubes has both halves paralleled. This effectively lowers Ra for the stages, increases bandwidth and voltage drive ability and reduces thd. The outcome is blameless sound devoid of high levels of artifacts, and thus allows the tubes to really sing for their supper well. Another very nice driver tube for any SET output tube is a 6V6 in triode with about 12 mA of Ia....... Patrick Turner. Any suggestion? TIA Gilbert |
#7
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Patrick Turner wrote:
NOS GE6550A were quite prone to +ve grids, and the more Ia there is, the larger the rise in +ve voltage. Yet the NOS GE 6550A has quite a robust G1 when intentionally driven + in AB2 hi-pwr (not so hi-fi) applications, & there's hardly another 6550 made that holds up as well in that service. Some others melt, horror show. But I admit that's venturing out of audiophile territory. It's hard to beat the 8417 for unexpected G1-pos catastophies in otherwise good gear. Whether the mechanism stated above as its cause is accurate or fully understood, is up for grabs, but 8417's testing gasfree on reliable equipment have done it. |
#8
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Hi Gilbert !
I never tried modern Sovtek's 2A3 so I can't say. But I got the same problem time ago with an amp I was working on : http://www.arrakis.es/~igapop/6b4g_push_pull_amp.htm using old Svetlana's 6B4G I just soldered 120k grid resistors and had no problem at all. But 500k - hmm, don't know... Best regards, -- Igor http://www.arrakis.es/~igapop "Gilbert Bates" escribió en el mensaje ... I've found one instance in an old audio book that talks about the size of the grid resistance. RCA indicates 50K max for fixed bias and 500K max for cathode. Like most of the old audio handbooks, it generalizes too much, leaving out technical details. Essentially it states that positive charged ions from gassy tubes are repelled by the plate and are attracted to the negative grid. As they collect on the grid, one needs small grid resistance to bleed off the charge, else it leads to a positive grid and increased bias current. The handbook states that this is not a problem with cathode bias as the increased plate current self-compensates with the biasing action of the cathode resistor. So you can get away with the larger grid resistance. Ok, I'm cool with that. Question 1: Is this a really big problem? With modern production tubes? I'm trying to run Sovtek 2A3's at about 20W static. Question 2: Right now I'm strictly running fixed bias and want to try to stay with it for adjustibility. Can one add a small cathode resistance to compenstate for the grid buildup, if any, and keep the fixed bias setup? I guess a small cathode resistor would have to be at least big enough to develop a sizable drop across it for compensation, but how big in general? As indicated by the book, the lower grid resistance values load my 6SL7 gain stage badly and I want to kick up the grid resistance back up to around 500k. Any suggestion? TIA Gilbert |
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