I think, the center-tapped coil can be an optimal solution, because the currents flow in opposite direction around core. Two pieces of "C" enough for push-pull, with totally identical coils for CT choke. IMO the original QUAD transformers are museal pieces, they should not be dismantled.
Unfortunately the current flow is essential, because the source of desired "overvoltage" is the inductive energy storage (1/2L*I^2).
I went to Brian Sowter and he made chokes for me. The project was halted but i remember those chokes to be very big and heavy!
Driver tube 300B and output tubes was GM70.
We also made step up transformers 1:3 to get some extra power.
One day.... maybe it will be finished.
Driver tube 300B and output tubes was GM70.
We also made step up transformers 1:3 to get some extra power.
One day.... maybe it will be finished.
I don't disagree, but this is a 200H+ coil which will operate with 2kV on one terminal and 8kV pk-pk on the other terminal.
Not something you fling together on a rainy Sunday afternoon. The QUAD transformers are build for this.
Jan
Unfortunately, the choke must be design for constant idle current which is must be equal or greater than the peak value of current of worst case scenario. Must be implement a necessary wire gauge, which is a lot of bigger, than the transformer's secondary which intended at same peak current (see music crest factor). The thick wire needs bigger bobbin and core. These factors cause higher coil capacitance which must be handle (thicker insulator layers and winding's tricks).
Jan- why do you not want to take advantage of the transformer to its full utility? See below- Tie the center tap of the transformer to ground. Two tubes in push pull tied to the negative supply (no positive supply needed). The transformer DC is balanced out so no saturation issues as long as everything is working. You can operate in class A or class AB by tweaking the bias. You could cross couple (cf MacIntosh) with the transformer secondary tied to the cathodes. Feedback from the plates via caps is a precaution that may not be needed. The drive circuit could be two opamps depending on the tube requirements. Keeping the plates at ground should be a significant safety issue. Add some series air core coils to the transformer to extend the response.
Just measured a step up xfrmr for inductance variation with DC bias (as part of my research into high voltage direct drive with inductive output stage load).
The single ended transformers like the ESL 63 step up could be sensitive to DC bias through the high voltage secondary. Although this measurement is on a different (diy) step up, the effect is very real as shown in the attachment. Even at 5mA DC bias, the inductance collapses. Which is not unexpected given the high number of turns and hence the high field strength even with a small DC bias.
This means I have to go to push-pull with a center-tapped output stage load, where the DC bias currents cancel (when equal). I have an old Final CT output xfrmr as well as a pair of ESL 57 step ups that are also center tapped. More experimentation to do, but the outdoors also beckens ... 😎
Jan
The single ended transformers like the ESL 63 step up could be sensitive to DC bias through the high voltage secondary. Although this measurement is on a different (diy) step up, the effect is very real as shown in the attachment. Even at 5mA DC bias, the inductance collapses. Which is not unexpected given the high number of turns and hence the high field strength even with a small DC bias.
This means I have to go to push-pull with a center-tapped output stage load, where the DC bias currents cancel (when equal). I have an old Final CT output xfrmr as well as a pair of ESL 57 step ups that are also center tapped. More experimentation to do, but the outdoors also beckens ... 😎
Jan
Hi Demian, didn't see your post until now. I am planning something similar to your suggestion, except that the CT will be at +2kV. Total supply 4kV, output 8kV pk-pk, max 6kV pk in the enclosure. Every kV more means more potential trouble.
I'm not planning to use the primary; I drive the tubes at the cathode from a V/I converter. Super linear already without feedback.
Jan
Jan, Instead of using a PP transformer that will make the added harware at least twice as complex, you could also use a SE transformer with airgap.
But for the sort of voltages you have in mind, both types of transformers may be hard to find.
Hans
But for the sort of voltages you have in mind, both types of transformers may be hard to find.
Hans
Hans, the CT type is easy to find: a regular step-up secondary will probably do fine!
I've collected some ESL 57 and Final CT step-ups and will test them.
Though, progress is slow. I have build a high voltage CCS that can be adjusted between 0 and 33mA with a max compliance of 4kV.
That way, I can test a single ended circuit and draw current from the other transformer section to cancel the DC field.
Jan
I've collected some ESL 57 and Final CT step-ups and will test them.
Though, progress is slow. I have build a high voltage CCS that can be adjusted between 0 and 33mA with a max compliance of 4kV.
That way, I can test a single ended circuit and draw current from the other transformer section to cancel the DC field.
Jan
Attachments
Hi Jan,
5 mA isn't much. This does make me wonder if the dc-offset of a power amp, using conventional drive, is even more critical than first assumed? What are your thoughts?
Mogens
5 mA isn't much. This does make me wonder if the dc-offset of a power amp, using conventional drive, is even more critical than first assumed? What are your thoughts?
Mogens
The inductance of a transformer drops with a DC bias, whatever the appication.
I believe most tube builders are aware of the need to balance the output anode currents in a P-P amplifier.
The same goes for a power amp driving the primary of a step-up xfrmr, but a solid state amp worth it's salt has vanishingly small offset so I don't think it is ever an issue. With tube amps driving a step up it isn't an issue either of course.
Jan
I believe most tube builders are aware of the need to balance the output anode currents in a P-P amplifier.
The same goes for a power amp driving the primary of a step-up xfrmr, but a solid state amp worth it's salt has vanishingly small offset so I don't think it is ever an issue. With tube amps driving a step up it isn't an issue either of course.
Jan
Hi Jan,
II was thinking purely on solid state amps driving the primary of the transformer. I haven't checked, but we are perhaps talking 10-20 mV dc-offset is enough to result in 5 mA.
Anyways, just a thought. I look forward to see what you come up with for your next DD-drive.
Mogens
II was thinking purely on solid state amps driving the primary of the transformer. I haven't checked, but we are perhaps talking 10-20 mV dc-offset is enough to result in 5 mA.
Anyways, just a thought. I look forward to see what you come up with for your next DD-drive.
Mogens
It's all about the product of current and number of turns, what we used to call AmpereTurns, that determines the fieldstrength.
5mA through a step-up secondary has such a great effect because the number of turns is huge.
Jan
5mA through a step-up secondary has such a great effect because the number of turns is huge.
Jan
When I started (12 years ago) to get acquainted with electrostatics, then on one forum, where participants were from all over the former Soviet Union, the guys did a tube direct drive without the use of winding products (transformers even with a low transformation ratio), but they didn’t find nothing except the Soviet GU-48 lamp to implement this project, the maximum voltage it can produce is 3 kV, but the power supply was such that two people needed to carry it.
Somewhere there were even photographs of this monster; its owner was delighted with the sound of the electrostats.
I have never seen a direct drive with a similar output voltage on the Internet, maybe I was looking wrong?
Mine also does without iron and delivers about 4kV pk-pk.
https://audioxpress.com/article/you-can-diy-edd-a-direct-drive-amplifier-for-electrostatic-speakers
Easy to carry even with one hand ;-)
Jan
https://audioxpress.com/article/you-can-diy-edd-a-direct-drive-amplifier-for-electrostatic-speakers
Easy to carry even with one hand ;-)
Jan
Have a look at this application sheet regarding direct drive amplifier!
chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://apexanalog.com/resources/appnotes/an56.pdf?_hsenc=p2ANqtz-_O3lML0k_PHO_E4RCoHuFds201DECoch2iCbvOo5bspFgD7qyP1WMahWmzV4kqGO2NU-su82OYIM0n58750IT_v6OZIkOBJlCTUQyg9y7fQ0McIn0&_hsmi=327048527
chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://apexanalog.com/resources/appnotes/an56.pdf?_hsenc=p2ANqtz-_O3lML0k_PHO_E4RCoHuFds201DECoch2iCbvOo5bspFgD7qyP1WMahWmzV4kqGO2NU-su82OYIM0n58750IT_v6OZIkOBJlCTUQyg9y7fQ0McIn0&_hsmi=327048527
It's a unity gain buffer apparently, so you still need to come up with the high voltage signal to drive it.
It doesn't strike me as the problem solver I have been working on.
Jan
It doesn't strike me as the problem solver I have been working on.
Jan