Hmmm... Johan, may be I am wrong, but the essence on the whole discussion is that feedback on screen grids must be delivered from separated windings and voltage on it must be less than on anode according to the tube geometry.
Another option is to use the same UL taps, but do not connect grids directly to them, instead use zeners or some another way to shift voltages, such as power MOSFETs.
If power MOSFETs are used we do not need taps anymore and can play with variations using potentiometers.
Another option is to use the same UL taps, but do not connect grids directly to them, instead use zeners or some another way to shift voltages, such as power MOSFETs.
If power MOSFETs are used we do not need taps anymore and can play with variations using potentiometers.
"It doesn't. That's why I suggested it."
I have the RCA datasheet for the 6973, is this the one you have?
It does not give any graphs, but just a single operating point for 50% and 43% UL. What they state is class AB1 operation, and a very high primary resistance xfmr compared to the pure pentode data case. Operation at 95 or 84 mA for both tubes at peak signal.
I came up with this data from other parts:
Gm1 = 4800 at 46 mA (2*46 mA => 92 mA so applicable for here)
Gm2 = 30mA/50V = 600
Mu_triode = gm1/gm2 = 8
Rp_triode ~ = 1/Gm2 = 1666 OHM
Rp_pentode = 73000 OHM
this would give Rp_effective = 1666 / .43 => 3874 OHM and Rp_effective = 1666 / .5 => 3332 OHM
If we also include the small pentode plate factor in the above calcs these numbers drop to 3788 and 3258 OHM respectively
For class AB1 the tube sees about 1/4 of the P-P xfmr impedance.
So 13000 / 4 => 3250 OHM (43% case)
and 12500 / 4 => 3125 OHM (50% case)
So these seem to be roughly in line with load Z matched to the effective plate Rp again. With some overlap in conduction (class AB) the load Z might be averaged up a little too for the higher Z seen in class A.
They don't give any drive signal amplitude, so I cannot check the effective Mu #. Without any graphs, one cannot tell whether these points are minima to distortion or maxima to power or what. Limited conclusions here.
Hi Johan,
None of the papers on UL seem to treat the case for any other triode plate Rp other than the 6L6. Nor do they treat different xfmr primary Z.
They simply did not finish the analysis. That's why we are all still here scratching our heads today.
The manufacturers data on tubes in UL seem to all be for tubes that are extremely close to the 6L6 in triode_plate resistance (KT88 6550 ...). In fact, most of the common audio tubes around are closely related to the 6L6. The only exception I have seen with UL data so far, being the EL34.
The Maths on the other hand DO give suggestions that the %UL would be different for a different triode configured Rp or primary Z. They are simply not constrained enough. Now there COULD be some additional constraint to be uncovered yet, but no one has anounced it yet to my nowledge.
Being DIYer's, we are NOT obliged to use the common audio tubes recommended by the manufacturers historically, and like to experiment outside the box. Having no satisfactory historical theory on this, one feels a certain need to check this out. Do I sense some concern that the Emporer (UL 43%) might be found lacking in the clothes department?
On the screen current distortion issue, another concern has not been mentioned yet. P-P will only cancel even harmonic distortion if the two tubes are matched. Now how many tubes get matched for identical screen current? (probably none, and certainly not dynamically over the signal range) I'm sure the H-K tests used the best matched tubes they could find, plus class A operation, to present the best results possible.
The screen current drawn by pentode tubes depends critically on the alignment of the g2 grid wires behind the g1 wires. Little windows often being present in the plate for visual grid alignment purposes. Having checked this same window on a number of tubes, I can often see visually obvious misalignments. Just dropping an amplifier or tube on the table may cause grid wire misalignment too. So lab results are not sufficient to characterise practical UL amplifiers.
Now, don't get me wrong on this, I am not dumping on UL, I just think more needs to be characterised, so that good designs can be attained with confidence. And whether any new flexibility in good design exists. And whether any problems identified need to be addressed, which I'm confident fixes can be found for IF necessary. (the screen current issue already has a fix previously noted)
Well, I need to spend some time on getting the test arrangements set up here.
Don
I have the RCA datasheet for the 6973, is this the one you have?
It does not give any graphs, but just a single operating point for 50% and 43% UL. What they state is class AB1 operation, and a very high primary resistance xfmr compared to the pure pentode data case. Operation at 95 or 84 mA for both tubes at peak signal.
I came up with this data from other parts:
Gm1 = 4800 at 46 mA (2*46 mA => 92 mA so applicable for here)
Gm2 = 30mA/50V = 600
Mu_triode = gm1/gm2 = 8
Rp_triode ~ = 1/Gm2 = 1666 OHM
Rp_pentode = 73000 OHM
this would give Rp_effective = 1666 / .43 => 3874 OHM and Rp_effective = 1666 / .5 => 3332 OHM
If we also include the small pentode plate factor in the above calcs these numbers drop to 3788 and 3258 OHM respectively
For class AB1 the tube sees about 1/4 of the P-P xfmr impedance.
So 13000 / 4 => 3250 OHM (43% case)
and 12500 / 4 => 3125 OHM (50% case)
So these seem to be roughly in line with load Z matched to the effective plate Rp again. With some overlap in conduction (class AB) the load Z might be averaged up a little too for the higher Z seen in class A.
They don't give any drive signal amplitude, so I cannot check the effective Mu #. Without any graphs, one cannot tell whether these points are minima to distortion or maxima to power or what. Limited conclusions here.
Hi Johan,
None of the papers on UL seem to treat the case for any other triode plate Rp other than the 6L6. Nor do they treat different xfmr primary Z.
They simply did not finish the analysis. That's why we are all still here scratching our heads today.
The manufacturers data on tubes in UL seem to all be for tubes that are extremely close to the 6L6 in triode_plate resistance (KT88 6550 ...). In fact, most of the common audio tubes around are closely related to the 6L6. The only exception I have seen with UL data so far, being the EL34.
The Maths on the other hand DO give suggestions that the %UL would be different for a different triode configured Rp or primary Z. They are simply not constrained enough. Now there COULD be some additional constraint to be uncovered yet, but no one has anounced it yet to my nowledge.
Being DIYer's, we are NOT obliged to use the common audio tubes recommended by the manufacturers historically, and like to experiment outside the box. Having no satisfactory historical theory on this, one feels a certain need to check this out. Do I sense some concern that the Emporer (UL 43%) might be found lacking in the clothes department?
On the screen current distortion issue, another concern has not been mentioned yet. P-P will only cancel even harmonic distortion if the two tubes are matched. Now how many tubes get matched for identical screen current? (probably none, and certainly not dynamically over the signal range) I'm sure the H-K tests used the best matched tubes they could find, plus class A operation, to present the best results possible.
The screen current drawn by pentode tubes depends critically on the alignment of the g2 grid wires behind the g1 wires. Little windows often being present in the plate for visual grid alignment purposes. Having checked this same window on a number of tubes, I can often see visually obvious misalignments. Just dropping an amplifier or tube on the table may cause grid wire misalignment too. So lab results are not sufficient to characterise practical UL amplifiers.
Now, don't get me wrong on this, I am not dumping on UL, I just think more needs to be characterised, so that good designs can be attained with confidence. And whether any new flexibility in good design exists. And whether any problems identified need to be addressed, which I'm confident fixes can be found for IF necessary. (the screen current issue already has a fix previously noted)
Well, I need to spend some time on getting the test arrangements set up here.
Don
Wavebourn said:
Anatoliy,
I understand that part of it; we are on slightly different paths somewhere. What you say above is part of the discussion, and I was trying to state that I can not see the necessity for "voltage on it" must be less! It is not clear how I failed to register my point, but apology where needed. Some did mention tube geometry, but it is the same tube for pentode/triode - why not the necessity for separate voltages there?
Unless I got mixed up with the sweep tube affair! Yes, surely there. But unless I am obtuse (and please do not hesitate to point that out!
), much of the discussion was also about normal audio tubes. After all KT88, EL34, 6L6 et all frequently came up. I cannot make it clearer than before: I simply see no reason for this sudden decree that Vg2 must be lower than Va in UL, but it is fine for, let us say, 0% and 100% g2-taps. The question is stil begging: Are all the manufacturer's specs suddenly wrong - or where am I missing what?To (try to) be clear, I have no objection to a lower Vg2 than Va at high voltages. In my 100W Quad type circuit I use an effective Vg2 of 460V and Va of 560V. But that "it must be" (i.e. under all conditions) - that is my question.
Kind regards
Your point is interesting- most audio pentodes/beam power tubes cluster around the same general characteristics. And there's an evolutionary reason for that.
The 6973 data I got was from the sheet on Frank's.
http://frank.pocnet.net/sheets/049/6/6973.pdf
No UL curves, but plenty of beam power data.
The 6973 data I got was from the sheet on Frank's.
http://frank.pocnet.net/sheets/049/6/6973.pdf
No UL curves, but plenty of beam power data.
Constraints on %UL
Seeing as how this %UL factor is still unresolved, let me suggest some possible constraints on it.
(The equivalent triode model does not care about the %UL. Setting %UL to some # simply causes it to crank out a primary xfmr Z to use with the given tube so that Zload = Zplate_effective _triode for maximum power transfer to the load.)
Usually I see low Mu triodes specified for regulators with minimum conduction loss (ie. best efficiency). This I believe is due to the plate being useable for high current down to lower voltage drops. This efficiency criteria would constrain %UL's on the low side (high MU's) as too inefficient. Maximum power output cannot afford to waist power in the tube.
The Ik^(-1/3) factor in the triode effective_Rp formula (it's also in the normal triode formula too) may constrain %UL's on the high side. I don't know why yet. Workin on it. Maybe it causes the cathode current limit to be reached?
I think it is important to understand the nature of any constraints on the %UL, so that if a designer does wish to vary this for some objective, he/she will understand the tradeoffs. Many would be comfortable with the loss of some power or efficiency if lower or more benign distortion, for example, could be gained for small change.
Don
Seeing as how this %UL factor is still unresolved, let me suggest some possible constraints on it.
(The equivalent triode model does not care about the %UL. Setting %UL to some # simply causes it to crank out a primary xfmr Z to use with the given tube so that Zload = Zplate_effective _triode for maximum power transfer to the load.)
Usually I see low Mu triodes specified for regulators with minimum conduction loss (ie. best efficiency). This I believe is due to the plate being useable for high current down to lower voltage drops. This efficiency criteria would constrain %UL's on the low side (high MU's) as too inefficient. Maximum power output cannot afford to waist power in the tube.
The Ik^(-1/3) factor in the triode effective_Rp formula (it's also in the normal triode formula too) may constrain %UL's on the high side. I don't know why yet. Workin on it. Maybe it causes the cathode current limit to be reached?
I think it is important to understand the nature of any constraints on the %UL, so that if a designer does wish to vary this for some objective, he/she will understand the tradeoffs. Many would be comfortable with the loss of some power or efficiency if lower or more benign distortion, for example, could be gained for small change.
Don
Gee, the constraint on the higher %UL has been staring me in the face. It's obvious.
The effective triode model shows that the effective MU drops with increasing %UL, or alternatively, more triode % means more internal feedback, so less gain. The input signal to drive it simply gets so big that it runs into grid1 current.
(10 lashes for not seeing that one a few days ago!)
The previous Hi-MU/lowefficiency constraint on lower %UL is not sufficiently convincing yet.
Don
The effective triode model shows that the effective MU drops with increasing %UL, or alternatively, more triode % means more internal feedback, so less gain. The input signal to drive it simply gets so big that it runs into grid1 current.
(10 lashes for not seeing that one a few days ago!)
The previous Hi-MU/lowefficiency constraint on lower %UL is not sufficiently convincing yet.
Don
I should have been off already - this thread is addictive (and that from a body that was born in a wine-producing area!)
This runs fast; I was typing when Don's last post came through and cancelled to read first.
Don, I do agree with your, well then, two last posts (if you are not already typing further!) You make stimulating remarks; rest assured that I am also thinking further despite some considerable previous experience (the scourge of researchers - they always have questions). I also owe you some explanations that I have not forgotten about. Kindly see my previous post.
But then already a further question now! Yes, the possibility of overdriving (too large an input signal). But how much of a probability is that? I do presume that that constitutes such obvious sign of overload - mostly there will be NFB that will then "discontinue" etc, that it signifies the amplifier output limit (i.e. max. output) without any room for doubt. When testing one looks with a scope at all signal points anyway (at least I do). Do you mean that some people will do this without recognising it? (Oops, let us not dwell on what some will do. Anything is possible these days.)
Kind regards (I am now off for a while.)
This runs fast; I was typing when Don's last post came through and cancelled to read first.
Don, I do agree with your, well then, two last posts (if you are not already typing further!) You make stimulating remarks; rest assured that I am also thinking further despite some considerable previous experience (the scourge of researchers - they always have questions). I also owe you some explanations that I have not forgotten about. Kindly see my previous post.
But then already a further question now! Yes, the possibility of overdriving (too large an input signal). But how much of a probability is that? I do presume that that constitutes such obvious sign of overload - mostly there will be NFB that will then "discontinue" etc, that it signifies the amplifier output limit (i.e. max. output) without any room for doubt. When testing one looks with a scope at all signal points anyway (at least I do). Do you mean that some people will do this without recognising it? (Oops, let us not dwell on what some will do. Anything is possible these days.)
Kind regards (I am now off for a while.)
Well, its clear from H-K's paper that THEIR constraint on lower %UL was low level distortion rising. But this is because their xfmr load Z becomes too low compared to the effective triode Rp, causing 3/2 power distortion. A higher primary impedance should allow %UL to continue to drop without a distortion penulty. And so no limit appears to be really stopping lower %UL.
My conclusions from this would be that a broad range exists for %UL. On the high %UL end, power must be sacrificed as effective Mu lowering runs into g1 current. But distortion does not shoot up as long as one stays below the g1 current level.
On the low %UL end, practical transformer primary winding problems for high Z limit the %UL tap. Also, screen current distortion will increase as %UL lowers since the screen current will be using less of the winding. (the closer the screen current enters the winding to the plate current, the less the alteration in power output due to it siphoning off plate current)
Sounds good to me. Anyone see any problems with this?
Don
My conclusions from this would be that a broad range exists for %UL. On the high %UL end, power must be sacrificed as effective Mu lowering runs into g1 current. But distortion does not shoot up as long as one stays below the g1 current level.
On the low %UL end, practical transformer primary winding problems for high Z limit the %UL tap. Also, screen current distortion will increase as %UL lowers since the screen current will be using less of the winding. (the closer the screen current enters the winding to the plate current, the less the alteration in power output due to it siphoning off plate current)
Sounds good to me. Anyone see any problems with this?
Don
"When testing one looks with a scope at all signal points anyway (at least I do). Do you mean that some people will do this without recognising it?"
Well, H-K's paper (figure 2) show's the high level distortion skyrocketing as the triode end of the chart is reached. Probably what is happening is the distortion begins to rise rapidly as g1 signal peaks get close to 0 volts, with out-right grid current occuring somewhere where it goes off the chart. Obviously, triode wired pentodes for normal amplifiers do not cause distortion like they show it, so they are clearly overdriving it.
That was my complaint earlier about their data, they should have lowered the drive on the triode end and so lowered the power out curve more toward the triode end too. Its the old trade-off of more power and more distortion - or - less power and less distortion.
An observation would be that as one increases the %UL toward triode, it pays to increase the screen DC voltage (well, up till B+ is reached). Since the tap is moving closer to the plate, where screen current will do less dist. harm, and higher screen voltage will hold off g1 overdrive (can lower g1 DC level, ie larger bias).
Conversely, toward lower %UL, it would pay to lower the screen DC voltage, due to the tap moving further from the plate, and the increasing MU at lower %UL allows g1 DC voltage to come up to compensate the lower g2 DC (less g1 drive signal required, so more headroom).
AS far as testing %UL effects, its clear that 100% UL works fine, just less power there. The low %UL with higher primary Z to compensate is really the issue lacking data support.
Don
Well, H-K's paper (figure 2) show's the high level distortion skyrocketing as the triode end of the chart is reached. Probably what is happening is the distortion begins to rise rapidly as g1 signal peaks get close to 0 volts, with out-right grid current occuring somewhere where it goes off the chart. Obviously, triode wired pentodes for normal amplifiers do not cause distortion like they show it, so they are clearly overdriving it.
That was my complaint earlier about their data, they should have lowered the drive on the triode end and so lowered the power out curve more toward the triode end too. Its the old trade-off of more power and more distortion - or - less power and less distortion.
An observation would be that as one increases the %UL toward triode, it pays to increase the screen DC voltage (well, up till B+ is reached). Since the tap is moving closer to the plate, where screen current will do less dist. harm, and higher screen voltage will hold off g1 overdrive (can lower g1 DC level, ie larger bias).
Conversely, toward lower %UL, it would pay to lower the screen DC voltage, due to the tap moving further from the plate, and the increasing MU at lower %UL allows g1 DC voltage to come up to compensate the lower g2 DC (less g1 drive signal required, so more headroom).
AS far as testing %UL effects, its clear that 100% UL works fine, just less power there. The low %UL with higher primary Z to compensate is really the issue lacking data support.
Don
%UL constraints revisited
Well, I woke up this morning and realized that my previous conclusion that grid1 overdrive limits output on the triode end of %UL was a little off the mark. Right answer, wrong reasons actually.
Both effective MU and effective Rp formulas have a %UL term acting identically in each. So if the xfmr primary Zload is matched to Rp effecitive in all cases, the voltage output required drops at the same rate as Mu drops, and no g1 overdrive occurs. They track.
The 1st outlaw factor turns out to be the Ik^(-1/3) term, which appears in only the effective Rp equation. This causes a mistracking between the two (Rp and Mu). Ironically, it favors the triode end. Lower Rp and Zload at higher %UL increases current causing the IK^(-1/3) term to lower Rp a little further (and so too our matched Zload ). This requires LESS g1 drive for the same Mu and power out, and in addition, the gm1 formula has Ik^(+1/3) in it to produce more current out for the same g1 drive. This term is actually INCREASING g1 drive headroom toward the triode end. But being only a 1/3 power law, it is a weak one compared to a linear effect.
There clearly has to be some other factor, and I believe it is the fixed DC voltage on the screen grid. The reason pentode mode can put more power out versus triode mode is due to the triode running out of current drive steam at low plate voltage peaks. Whereas the pentode still has the screen voltage effect to deliver current on those low plate voltage peaks.
If we look at the AC screen voltage versus %UL, it decreases from 100% Vout for triode to zero for pentode. But the DC screen voltage is staying constant. Clearly, this is a factor that is not tracking %UL, giving the pentode end the power advantage. This relative increase in DC voltage on the screen versus AC voltage leads to increased g1 headroom on the pentode end, since G1 bias can be increased. This is effect is linear with %UL [well actually with (1-%UL)], so is stronger than the Ik^(-1/3) effect on Rp or g1 headroom.
This leaves us in practice with power increasing toward pentode mode, and only Hi Z transformer difficulties preventing one from going all the way to pentode.
The fact that their are two competing factors however is quite interesting. The linear screen DC factor is like the rabbit, and the Ik^(-1/3) factor is like the tortoise. For a short race the rabbit wins, which is the case in practical tubes. But power law factors, even weak ones, when taken to extremes, always eventually win out over linear factors (linear factors are K^0 power, with K varying). Now IF we had a triode (same wattage dissipation as the pentode) with extreme low Rp, and consequently low Mu, this current factor could tip the balance to the triode. And such a tube would no longer need an output xfmr either, driving the load directly. Just a thought.
Don
Well, I woke up this morning and realized that my previous conclusion that grid1 overdrive limits output on the triode end of %UL was a little off the mark. Right answer, wrong reasons actually.
Both effective MU and effective Rp formulas have a %UL term acting identically in each. So if the xfmr primary Zload is matched to Rp effecitive in all cases, the voltage output required drops at the same rate as Mu drops, and no g1 overdrive occurs. They track.
The 1st outlaw factor turns out to be the Ik^(-1/3) term, which appears in only the effective Rp equation. This causes a mistracking between the two (Rp and Mu). Ironically, it favors the triode end. Lower Rp and Zload at higher %UL increases current causing the IK^(-1/3) term to lower Rp a little further (and so too our matched Zload ). This requires LESS g1 drive for the same Mu and power out, and in addition, the gm1 formula has Ik^(+1/3) in it to produce more current out for the same g1 drive. This term is actually INCREASING g1 drive headroom toward the triode end. But being only a 1/3 power law, it is a weak one compared to a linear effect.
There clearly has to be some other factor, and I believe it is the fixed DC voltage on the screen grid. The reason pentode mode can put more power out versus triode mode is due to the triode running out of current drive steam at low plate voltage peaks. Whereas the pentode still has the screen voltage effect to deliver current on those low plate voltage peaks.
If we look at the AC screen voltage versus %UL, it decreases from 100% Vout for triode to zero for pentode. But the DC screen voltage is staying constant. Clearly, this is a factor that is not tracking %UL, giving the pentode end the power advantage. This relative increase in DC voltage on the screen versus AC voltage leads to increased g1 headroom on the pentode end, since G1 bias can be increased. This is effect is linear with %UL [well actually with (1-%UL)], so is stronger than the Ik^(-1/3) effect on Rp or g1 headroom.
This leaves us in practice with power increasing toward pentode mode, and only Hi Z transformer difficulties preventing one from going all the way to pentode.
The fact that their are two competing factors however is quite interesting. The linear screen DC factor is like the rabbit, and the Ik^(-1/3) factor is like the tortoise. For a short race the rabbit wins, which is the case in practical tubes. But power law factors, even weak ones, when taken to extremes, always eventually win out over linear factors (linear factors are K^0 power, with K varying). Now IF we had a triode (same wattage dissipation as the pentode) with extreme low Rp, and consequently low Mu, this current factor could tip the balance to the triode. And such a tube would no longer need an output xfmr either, driving the load directly. Just a thought.
Don
smoking-amp said:
Interesting thought. In other words, you are trying to preserve the ability of the plate to swing to a lower voltage for a given current, in a pentode-like fashion? I am guessing that what you mean by this is WRT your previous statement that for lower %UL it would pay to decrease Vg2 to lower Ig2 distortion.
It seems to me that it logically follows that one can try to optimize the swing of Vg2 so that it's 'top' coincides with the maximum swing of the triode connection with B+ = maximum Vg2DC. Let me explain - suppose the plates swing +-200V off a 250V B+. At 100%UL = triode, Vg2 would also swing +-200V, i.e. from 50 to 450V. At 80% UL, G2 swings +-160V, so you use a 290V Vg2, to make the top of the G2 swing again 450V, and the bottom 130V, in order to preserve the beneficial 'pull' of G2 and lowering of the saturation voltage (i.e. how low the plates can swing).
It occurs to me that this would mean a big balancing act regarding finding an appropriate tube WRT max Vg2, not to mention G2 current and dissipation. On the top end of the swing the tube would behave very close to triode (Vp = Vg2), on the bottom end, closer to a pentode with Vg2>Vp. It could be an interesting way to get more power out of a PP stage at B+ voltages lower than Vg2max.
This brings me to another thing - unusual 'UL-like' taps. I recall vaguely someone mentioning a connection where G2 taps were reversed in PP tubes WRT plates, the name 'isolinear' keeps popping up in my head. Negative %UL? Positive feedback? Another one would be over 100% UL 'taps'. Not sure about the usefulness, but since you are exploring, why not make it the exploration of 'the general case'?
hey-Hey!!!,
McIntosh was the first to connect the g2 to the opposite plate in their PP amps. This preserved the pentode operation in what would have been a 50% U-L given their equal CFB and anode windings.
There was the name E-Linear, but that involves feeding a LTP/diff amp plate loads from the g2 taps of the output TX in order to do away with the need for a loop of global FB.
cheers,
Douglas
McIntosh was the first to connect the g2 to the opposite plate in their PP amps. This preserved the pentode operation in what would have been a 50% U-L given their equal CFB and anode windings.
There was the name E-Linear, but that involves feeding a LTP/diff amp plate loads from the g2 taps of the output TX in order to do away with the need for a loop of global FB.
cheers,
Douglas
There have been 8 pages of theory and conjecture in this thread. It is time to make some smoke! I ordered 100 of the "magic mosfets" along with some other types. I will test them as a possible replacement for the discontinued 2SK2700 that I have been using in my PowerDrive circuit. After that I will try some of the ideas outlined here. I also want to see if there is any clean power to be found by allowing the tube to enter A2. I will devise a test setup with everything variable.
I also ordered some non inductive resistors in .1, .2, .5 and 1 ohm values. I will make a dummy load that goes from .1 ohms to 12 ohms in .1 ohm steps. This makes finding the optimum load impedance real easy. I even got my HP8903A audio analyzer fixed. This makes distortion VS power for different load measurements easy.
I still have some $3 Chinese 6L6GC's that take some serious abuse without failure. They don't sound bad either. I can offer these up to science without worrying about blowing up expensive tubes. The only one that I have actually blown up was the one that I used for the expeirment outlined in the EDN article:
http://www.edn.com/index.asp?layout=article&articleid=CA302240#ref
I wasn't paying attention to the tube itself, but the screen grid must have actualy melted. The distortion kept rising and then there was fireworks. You can't run the screen grid at 100 volts more positive than the plate for long. The results may have been different if the tube had perfectly aligned grids, but it was a $3 tube.
The parts won't be here until Monday, so this will likely happen the following weekend since I don't have time during the week for experiments.
I also ordered some non inductive resistors in .1, .2, .5 and 1 ohm values. I will make a dummy load that goes from .1 ohms to 12 ohms in .1 ohm steps. This makes finding the optimum load impedance real easy. I even got my HP8903A audio analyzer fixed. This makes distortion VS power for different load measurements easy.
I still have some $3 Chinese 6L6GC's that take some serious abuse without failure. They don't sound bad either. I can offer these up to science without worrying about blowing up expensive tubes. The only one that I have actually blown up was the one that I used for the expeirment outlined in the EDN article:
http://www.edn.com/index.asp?layout=article&articleid=CA302240#ref
I wasn't paying attention to the tube itself, but the screen grid must have actualy melted. The distortion kept rising and then there was fireworks. You can't run the screen grid at 100 volts more positive than the plate for long. The results may have been different if the tube had perfectly aligned grids, but it was a $3 tube.
The parts won't be here until Monday, so this will likely happen the following weekend since I don't have time during the week for experiments.
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