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#1
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A mystery to me
OK, I admit that my understanding of tube electronics is rudimentary at
best. But one thing really puzzles me. On a Williamson UL PP amp, the screen is at about the same DC potential as the anode (as measured with a DVM). I thought that the screen had to be at a lower potential for normal PP operation. What's up with that? TIA, Jon |
#2
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On a Williamson UL PP amp, the screen is at about the same DC potential as
the anode (as measured with a DVM). I thought that the screen had to be at a lower potential for normal PP operation. What's up with that? A reasonable question. No doubt you'll get a multitude of answers from the folks here, but here's a simple one: If you tied the screen directly to the plate, you'd have - a triode. What the UL tap on the transformer does is put some signal on the screen, but not the full signal that appears on the plate, so the tube runs somewhere between a triode and a pentode. It's actually a lot more complex than that, but that's the basic idea. Note that some tubes cannot be operated in this mode due to screen voltage limitations. The 6146 is an example. If you wanted to run these in UL mode, you'd need a separate winding on the OP trans. |
#3
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"Troglodite" wrote in message
... Note that some tubes cannot be operated in this mode due to screen voltage limitations. The 6146 is an example. If you wanted to run these in UL mode, you'd need a separate winding on the OP trans. Well, it's rated for triode mode, but it'd be much better to have a seperate 25% winding, or parafeed from the same tapping. (I presume you'd want a similarly lower AC screen voltage when reducing DC voltage. Example, +600V plate and +300V screen, with around (theoretically) 500V peak swing on the plates, would give 250V peak swing to the screens, nearly cutting them off, i.e. triode mode, if ran from a typical approx. 50% UL tap.) Tim -- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms |
#4
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"Jon Yaeger" wrote in message
... I thought that the screen had to be at a lower potential for normal PP operation. Nothing says you can't run +150V plate supply and +250V screen supply, however you won't get much power output, distortion will be high and efficiency (especially Ip/Is ratio) will be low. Tim -- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms |
#5
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As some other RATs have already pointed out, this is the so-called
ultra-linear arrangement, which is used by the largest part of HiFi amps using PP tetrodes, particularly these days because it is getting difficult finding OPTs fitted with cathode windings suitable for "distributed load" stages like McIntosh or similar. I suppose You can find the original article written by Hafler back in 1950something describing his findings about this arrangement. The main points about UL are the following: - G2 is operated at the same DC voltage of the anode (in fact 2-3V higher due to the OPT winding resistance), therefore this should be checked when swapping tubes - for the very same reason, tubes allowing a high G2 voltage are best suited to UL (ie. EL34, 6L6GT, or other types where the max V is not very different between G2 and A) - the grid is fed back some signal, and this (negative, local) feedback contributes to reducing output impedance and distortion. In fact, during operation, the applied load creates a very complicated situation which is different from the one measured in static conditions. Look for the old M-O data sheets for the KT88, they show the anode curves in UL mode: they're definitely neither "pentode" nor "triode". - the behavior in terms of distortion products is half way between triode and pentode, depending on the primary tap position - construction is easy, efficiency is very good and so is the sound (IMHO: all my tetrode amps are UL, including some SE types, and I think they're good, but... I'm not an unbiased listener) - a damping resistor should always be connected between G2 and UL tap As per Your understanding of electronics, with that Williamson You managed to get one of the finest pieces of tubed gear ever designed: You don't necessarily need to be a Nobel winner to enjoy it, as far as You have ears. My compliments Fabio "Jon Yaeger" ha scritto nel messaggio ... OK, I admit that my understanding of tube electronics is rudimentary at best. But one thing really puzzles me. On a Williamson UL PP amp, the screen is at about the same DC potential as the anode (as measured with a DVM). I thought that the screen had to be at a lower potential for normal PP operation. What's up with that? TIA, Jon |
#6
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Jon Yaeger wrote: OK, I admit that my understanding of tube electronics is rudimentary at best. But one thing really puzzles me. On a Williamson UL PP amp, the screen is at about the same DC potential as the anode (as measured with a DVM). I thought that the screen had to be at a lower potential for normal PP operation. The screens don't have to be at a lower than anode potential. But with pentode/beam tetrode operation its best that Eg2 *is* no more than the makers recomendations, so an 807 should never have more than Eg2 = 300v regardless of the anode supply voltage. But 807 were amoungst the first to be used for UL amps with 450v applied as the anode supply. With UL, the screen gets signal volatge applied from the anode circuit via the OPT, and the dissipation of heat in the screens is much reduced because of the signal present. With triode op of 807, the G2 diss is at its minimum. With pentode op, when the anode voltage swings low, the G2 current peaks, and when anode swings high, G2 current is reduced; the electrons seek the most easy and attractive target. But with UL or triode, the G2 voltage swings down with anode voltage, so its current input is lower, and when anode swings high, the is still g2 current reduction. Screen current input is about proportional to anode current input though. So in class AB UL amps which are pushed hard with a load which is too low, the screens are probably at some risk. UL operation limits the B+ which can be used because of screen limitations. A pair of 807 can be cajoled into making 80 watts in beam tet mode, with 600 v anode supply but only 300v G2 supply. EL34 can make 100 watts with 900v anode supply, but only 450 G2 supply. Neither of these tubes can be used at the anode voltages for UL, unless a special G2 winding is wound onto the OPT to give the wanted UL signal voltage and supplied with an appropriate G2 supply. This works OK with 6146 type tubes. But if I was trying to make a high voltage output stage I'd either stick to beam T or pentode op, or use cathode feedback windings, which are more effective at reducing the thd and Ro of multigrid tubes, and you then have the option of setti9ng g2 at the lowest efective value which will keep the tubes happy, and the screen current at its minimum, as well as reducing the wanted G1 bias to a minimum. EL34 and EL84 were designed to be able to run with equal anode and G2 voltage supplies, with 450v limit for EL34 and 350v limit for EL84, but I have seen each used with 20% higher voltages for UL amps. Screens are grids which draw current, and they run warm, and will run red hot if their current is eccessive, and then start emitting electrons of deforming in shape and alignment and ruining the tube function. Patrick Turner. What's up with that? TIA, Jon |
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