Small? I would guess up to 50W in class AB not a problem with the GE5 and normal anode voltage around 360-370V.
I will use them both as output devices. I do not need a lot o power.
Thomas Mayer has also done experiments with the GE5 in triode SE to replace a 2A3. It looks like one can get in class A1 similar power as a 2A3 in class A2!
But of course I already have 2A3s and will use it in pentode mode + local fb.
With JA5 I would expect at least 16W in class A PP (pentode + modest local fb) with very low THD and triode-like Zout.
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Well, small because I have to keep in mind the constraint of 260V B+ in order to not complicate things by having to use an isolated heater supply for each of the "floating" output tubes. So I'm exchanging lower voltage for higher current. The peak currents thus required exceeds the types listed, while any of the three I listed will handle twice the current. So, if my comment is taken in this context, then yes, small. In your case, of course, if you are going the solidly welded to ground cathode approach, your higher plate voltage will do the trick nicely at the lower allowed cathode current. Hence the qualifier, "for me".
I'm not interested in SE or even triode outputs but, for those of you who are, a replacement for the ridiculously priced 2A3 and such needs to be found. I hope for y'all that the GE5 or some other type will be found satisfactory.
I'm not interested in SE or even triode outputs but, for those of you who are, a replacement for the ridiculously priced 2A3 and such needs to be found. I hope for y'all that the GE5 or some other type will be found satisfactory.
Yes of course mine was a general comment. In this case you are limited if do not elevate heaters.
I bought the last NOS 2A3s about 10 years ago when one could still find them at reasonable prices. The last of that family however were the 6C4Cs for $5 each. I have about 12 of these. NIB in original military boxes with datasheet, all nicely packed. IMHO they are better than NOS 2a3's for both build and performance. Anyway still more convenient that current production crap. If I look at prices now I could sell them at bargain price and still make good money on them! That's the usual speculation. But I won't. Not worth.....
I bought the last NOS 2A3s about 10 years ago when one could still find them at reasonable prices. The last of that family however were the 6C4Cs for $5 each. I have about 12 of these. NIB in original military boxes with datasheet, all nicely packed. IMHO they are better than NOS 2a3's for both build and performance. Anyway still more convenient that current production crap. If I look at prices now I could sell them at bargain price and still make good money on them! That's the usual speculation. But I won't. Not worth.....
One tube has the transformer winding between cathode and ground, the other has not.The DC resistance of those windings can be 20 ...30 ohms or even more.Gives an unbalance in drive voltages.Can be treated with a similar resistance for the other tube.
One tube has that same DC resistance between anode and +B resulting in an asymetrical clipping.Here too a resistor in the anode path of the other tube restores the balance.
But with a current of ½Amax in the tubes you lose 2x 20ohms x ½A=20V at the bottom.
And that with tubes chosen for the ability of low voltage/high current.
About Schade feedback, only possible with capacitors in between.One side commes from a +B source, the other a ground source.
Mona
With the driver load bootstrap for the top tube coming from the top tube cathode, the winding resistance to ground shouldn't have much to do with it. The drive signal is developed relative to the output tube cathode. No?
Looks like the shunt "Schade" N Fdbks both have output signal plus B+ noise in series when they get back to the output tube grid (relative to cathode). So no PSRR indeed. But that's the same as the usual shunt "Schade"
case. No?
The driver stage has to develop about 25 V swing to drive the bottom tube and about 425 V swing to drive the upper tube. Now THAT is asymmetric.
Looks like the shunt "Schade" N Fdbks both have output signal plus B+ noise in series when they get back to the output tube grid (relative to cathode). So no PSRR indeed. But that's the same as the usual shunt "Schade"
case. No?
The driver stage has to develop about 25 V swing to drive the bottom tube and about 425 V swing to drive the upper tube. Now THAT is asymmetric.
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First, on 45's comment about his stash... I think you are right in keeping them since that type appeals to you and what you have is the real deal. Happy for you! For the rest of the folks who would like to have 2A3s but don't, it could be better if a practical sub is found.
On the issue of elevated cathodes. I agree that providing a floating heater supply is not rocket science. Just trying to avoid it, plus these days, a higher current is no big deal and makes transformer choices far easier. For example, a pretty easy to find 130 to 260 isolation transformer will provide around 350VDC. with the scheme I want to use, a well regulated 260VDC with a mains current conduction angle approaching 90 deg per half cycle is quite easy. Good quality but inexpensive transformers are off the shelf. So I'll trade current for voltage and won't need the heater transformers at all.
On the drive question: I do believe Smoking Amp is totally correct, the drive is applied between cathode and G1 so whatever the DC drops may be in the primaries don't matter any more than they do in a conventional P-P approach. This is true for both the one with the winding on the cathode as well as the one with the winding on the plate.
The same is true regarding Schade. One has to visualize the drive being applied from cathode to grid and similarly, one has to visualize the Schade from plate to plate. Perhaps this helps: Call the Schade resistor Rs. Visulalize the voltage across Rs changing in response to the voltage drop across the output and driver tubes combined, with a net effect. Now for the floating stage: the drop across its Rs is doing the same thing. When the output tube is conducting more, the Rs current is flowing from the driver plate to the output plate, out the cathode, winding, B supply. If you simply change the order from winding then B supply to B supply then winding, you have the same thing. A little Kirchoff going on there.
The serious point you brought up, Mona, is that the IR drop is not going to be inconsequential. With a peak current of about 550mA, the DCR has to be kept under control. This is the one and only area that does make me think of a single winding vs two, just to get more copper. On the other hand, the single one has a higher I2R loss because even when decreasing the DCR by 2, all the current is flowing all the time. The effective current with the split approach is about half so for a given DCR one has 25% of the loss per winding, or about 50% of the single. All a trade off as we all know.
As to PSRR, yup. So the B supply will need to be very stiff and very clean. Removing the Schade effect is as simple as removing the resistors. With an active current sink for the drivers, there will be a change in the resting plate voltage and that does need to be watched that all stays within operating limits (voltage swing remains sufficiently symmetrical) but I was not planning on more than 5-10% feedback anyway.
And, if desired, CFB is still available. Just insert the CFB winding between the cathode and the drive reference point. Same caveats about having enough swing out of the driver applies whether P-S or conventional.
Might have time next week to dive into details.
Happy weekend, y'all!
Rene
On the issue of elevated cathodes. I agree that providing a floating heater supply is not rocket science. Just trying to avoid it, plus these days, a higher current is no big deal and makes transformer choices far easier. For example, a pretty easy to find 130 to 260 isolation transformer will provide around 350VDC. with the scheme I want to use, a well regulated 260VDC with a mains current conduction angle approaching 90 deg per half cycle is quite easy. Good quality but inexpensive transformers are off the shelf. So I'll trade current for voltage and won't need the heater transformers at all.
On the drive question: I do believe Smoking Amp is totally correct, the drive is applied between cathode and G1 so whatever the DC drops may be in the primaries don't matter any more than they do in a conventional P-P approach. This is true for both the one with the winding on the cathode as well as the one with the winding on the plate.
The same is true regarding Schade. One has to visualize the drive being applied from cathode to grid and similarly, one has to visualize the Schade from plate to plate. Perhaps this helps: Call the Schade resistor Rs. Visulalize the voltage across Rs changing in response to the voltage drop across the output and driver tubes combined, with a net effect. Now for the floating stage: the drop across its Rs is doing the same thing. When the output tube is conducting more, the Rs current is flowing from the driver plate to the output plate, out the cathode, winding, B supply. If you simply change the order from winding then B supply to B supply then winding, you have the same thing. A little Kirchoff going on there.
The serious point you brought up, Mona, is that the IR drop is not going to be inconsequential. With a peak current of about 550mA, the DCR has to be kept under control. This is the one and only area that does make me think of a single winding vs two, just to get more copper. On the other hand, the single one has a higher I2R loss because even when decreasing the DCR by 2, all the current is flowing all the time. The effective current with the split approach is about half so for a given DCR one has 25% of the loss per winding, or about 50% of the single. All a trade off as we all know.
As to PSRR, yup. So the B supply will need to be very stiff and very clean. Removing the Schade effect is as simple as removing the resistors. With an active current sink for the drivers, there will be a change in the resting plate voltage and that does need to be watched that all stays within operating limits (voltage swing remains sufficiently symmetrical) but I was not planning on more than 5-10% feedback anyway.
And, if desired, CFB is still available. Just insert the CFB winding between the cathode and the drive reference point. Same caveats about having enough swing out of the driver applies whether P-S or conventional.
Might have time next week to dive into details.
Happy weekend, y'all!
Rene
That would be a good idea indeed.
Reading carefully the specs of the 6HJ5 it's like the 6GE5 for cathode-heater peak voltage. It can be up to 200V if no DC otherwise 200V is DC + peak. Max DC is 100V for heater positive respect to cathode.
Less clear in the RCA datasheet but one can realize it knowning the Raytheon's. In fact the limits for DC are the same taking into account the note referred to the heater positive respect to cathode.
Sure thing,, 45, I agree with your interpretation. The "upper" tube has its cathode sitting at ground potential when idle. It will get pushed up and down up to 200V each way with the constraints I have indicated previously. So, the heater/cathode ratings, either peak or DC, will not be violated. I must say that in saying this, the assumption is that when the spec states "DC plus peak" or "DC", that a pure AC with little or no DC bias is the same as what they state. Can't see why not in terms of breakdown, 200V is not nearly enough to be getting into corona effects. But, if someone has more specific information on this subject, please come forward.
Of consideration of course is that they will be pushed right to ratings, no margin other than whatever the tube design inherently has. I suspect the inherent margin is quite large, as over the years I have seen several designs which greatly abused the published ratings with no apparent sudden deaths from this.
I'll say this: If I see that there is too much penalty involved in keeping the B+ low with its concurrent high current, or if I discover any issues in having the cathode swinging so much relative to the heater, I'll put in heater isolation transformers.
I have made a bit of progress with the calcs, a few things are revealing themselves, happy to say no giant dragons so far. But for sure, all kinds of details that will need to be addressed. Fun!
Peace to all
Rene
Of consideration of course is that they will be pushed right to ratings, no margin other than whatever the tube design inherently has. I suspect the inherent margin is quite large, as over the years I have seen several designs which greatly abused the published ratings with no apparent sudden deaths from this.
I'll say this: If I see that there is too much penalty involved in keeping the B+ low with its concurrent high current, or if I discover any issues in having the cathode swinging so much relative to the heater, I'll put in heater isolation transformers.
I have made a bit of progress with the calcs, a few things are revealing themselves, happy to say no giant dragons so far. But for sure, all kinds of details that will need to be addressed. Fun!
Peace to all
Rene
Normally, for the dual primary winding OT case, it would have the same total winding (bifilars in series) as the P-ct-P case. But this design is using lower B+ to stay below the tube heater to cathode ratings. (@ 260 V B+, +/- 200V swing) The "few hundred Ohms" would then be just one of those (bifilar) windings. (so 1/4 Zp-p)
The 6HJ5 can pull the 2X current for +/-200V swing in theory. Remains to be seen how the distortion comes out there. George (Tubelab) liked the distortion profile at 3.3K P-ct-P and higher B+.
For the single winding Totem Pole case, there is just one winding, which would normally be 1/4 Zp-p of a normal P-ct-P OT. For 6HJ5, 3.3K (to 4.2K, Pete Millett's latest 50 Watter) is maybe the normal P-ct-P case with typical +450 V B+ say.
The 6HJ5 can pull the 2X current for +/-200V swing in theory. Remains to be seen how the distortion comes out there. George (Tubelab) liked the distortion profile at 3.3K P-ct-P and higher B+.
For the single winding Totem Pole case, there is just one winding, which would normally be 1/4 Zp-p of a normal P-ct-P OT. For 6HJ5, 3.3K (to 4.2K, Pete Millett's latest 50 Watter) is maybe the normal P-ct-P case with typical +450 V B+ say.
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It was one of their 100 Watter CXPP OTs, around $90 then, plus $40 for the custom (faulty) design. I fixed it myself. (was quite some time ago, maybe they didn't charge extra for the dual wires)
I'm not sure they are doing custom designs anymore. Several designs here that desperately need winding. (center tapped balancing inductors for example, to use with Elliptron designs, which do the Circlotron thing with a single non floating B+ and 28% CFB using the 40% UL taps)
I'm not sure they are doing custom designs anymore. Several designs here that desperately need winding. (center tapped balancing inductors for example, to use with Elliptron designs, which do the Circlotron thing with a single non floating B+ and 28% CFB using the 40% UL taps)
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Folks, there is no difference whatever in the dynamics of the single vs the bifilar. One tube at a time for either one. AC current is flowing in the same manner. The bifilar acts exactly as the single winding for AC. The differences are in the DC area. Where the basic P-S design carries absolutely no DC current (assuming one gets the biasing just exactly right and no drift, one of the reasons I'm not inclined to go that route), there is no current to be carried for G2 or otherwise. If in Class AB, the bias current is made to flow through the two tubes while the 2 winding approach has tube bias current flowing through it (in opposing directions so they do cancel, as in a conventional P-P).
In terms of dynamic loading, I believe them to be identical, as evidenced by no AC differential between the two windings. That is the key, that the windings are operating under the same exact AC conditions.
A quick back of the napkin (or scrap paper in this case) calc puts the plate loading at 363 Ohms for a swing of 200V and a peak current of 550 mA for Class B1. Jury is still out regarding class of operation but Class B will not be hindered by the usual real consideration of the glitch caused by leakage inductance. There can of course be real cross over distortion, which does respond to NFB (the one caused by leakage L does not) and I may not wind up liking Class B in the end, but it is certainly possible with no theoretical fear of the glitch.
thanks again, please keep it coming.
Rene
In terms of dynamic loading, I believe them to be identical, as evidenced by no AC differential between the two windings. That is the key, that the windings are operating under the same exact AC conditions.
A quick back of the napkin (or scrap paper in this case) calc puts the plate loading at 363 Ohms for a swing of 200V and a peak current of 550 mA for Class B1. Jury is still out regarding class of operation but Class B will not be hindered by the usual real consideration of the glitch caused by leakage inductance. There can of course be real cross over distortion, which does respond to NFB (the one caused by leakage L does not) and I may not wind up liking Class B in the end, but it is certainly possible with no theoretical fear of the glitch.
thanks again, please keep it coming.
Rene
You may be right. I tried to get Edcor to quote a custom design and never, ever, got a response back from them. There is another fellow, I'll look up his info, who does. While he spends zero time with you over the phone, he will readily engage in meaningful dialogue over email.
I'll dig up his contact info and post.
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