Hefty, but doable.
Triode cascode with paralled E288 driving a single BX7 system could easely drive, not only g1 to kathode of the gm70, but additionally quite a portion of that transformers primary.
Reduced anode voltage swing may well be worth it.
Also bootstrapping the driver may be worth considering, especially if it turns out that the transformer peaks undesirable and/or one wants to decrease flash over risks at higher anode voltage . At 800V propable ok but this toob would not complain against more n that...
Triode cascode with paralled E288 driving a single BX7 system could easely drive, not only g1 to kathode of the gm70, but additionally quite a portion of that transformers primary.
Reduced anode voltage swing may well be worth it.
Also bootstrapping the driver may be worth considering, especially if it turns out that the transformer peaks undesirable and/or one wants to decrease flash over risks at higher anode voltage . At 800V propable ok but this toob would not complain against more n that...
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Ah, did not consider, directly heated, better to forget about split load and go with schade if troublefree feedback is needed
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Thats my fault, should be 1125ohm..
Internal resistance Ri=Rp=u/S
u - gain coefficient
S - mutual conductance (transconductance)
regarding to https://frank.pocnet.net/sheets/084/g/GM70.pdf
S = 6 mA/V
u = 6.7
so Rp=1116 ohm
regarding to https://frank.pocnet.net/sheets/018/g/GM70.pdf
S = 4.6 to 7.2 mA/V
u = 5.6 to 8.2
so Rp=1217 to 1138ohm respectively
Marek,
Based on my experience with GM70, treat it as a tube with at least 1500 ohm internal resistance, and do transformer calculations accordingly.
With most used operating points you will not be close to S 4.6, and the aging tube will loose transconductance anyway.
When ~ 1200 ohm would be the Ri to base calculations upon, primary Z could be ~ 5k instead of the 7k which is "normal" for GM70.
Based on my experience with GM70, treat it as a tube with at least 1500 ohm internal resistance, and do transformer calculations accordingly.
With most used operating points you will not be close to S 4.6, and the aging tube will loose transconductance anyway.
When ~ 1200 ohm would be the Ri to base calculations upon, primary Z could be ~ 5k instead of the 7k which is "normal" for GM70.
Wow cool software!
FWIW, I used an ECC8010 to drive a GM70 and that work well. I had a lot of trouble with heater noise but at that time switching power supplies were expensive and hard to find for that device. Today that option is more available but may or may not work better That would have to be tried to determine. .
Also, FWIW, In an unrelated project I used a well known C core interstage for SE to PP and found phase problems at high frequencies 10Khzis which I attributed to the physical space between windings using that core type. IMO, EI cores might be better for SE operation. C core might work well for PP operation. YMMV.
And again FWIW, I worked with a small old time transformer manufacturer on a custom 4K SE output and used the 1P, 1S, 1P, 1S, 1P interleaving Ps in series, Ss in parallel using old time craft paper and varnish construction and got well over 20K bandwidth with good square wave.
Good luck.
Scott
FWIW, I used an ECC8010 to drive a GM70 and that work well. I had a lot of trouble with heater noise but at that time switching power supplies were expensive and hard to find for that device. Today that option is more available but may or may not work better That would have to be tried to determine. .
Also, FWIW, In an unrelated project I used a well known C core interstage for SE to PP and found phase problems at high frequencies 10Khzis which I attributed to the physical space between windings using that core type. IMO, EI cores might be better for SE operation. C core might work well for PP operation. YMMV.
And again FWIW, I worked with a small old time transformer manufacturer on a custom 4K SE output and used the 1P, 1S, 1P, 1S, 1P interleaving Ps in series, Ss in parallel using old time craft paper and varnish construction and got well over 20K bandwidth with good square wave.
Good luck.
Scott
This is your first OPT, and going straight for the hardest configuration - SE, high voltage, high primary impedance? Well, my only transformer winding is adding a 6.3V winding to a toroidal, so doesn't count, but wouldn't it be better starting with something simpler, like something for a 2A3? Involves another core, of course.
Dialing in the 845 on this calculator, came up with figures that match those given on the RCA data sheet, which presumably means that the calculator is reasonably accurate.
Then dialing in the GM-70 with stated design Ip max of 100mA, even at a very low Vp of 250V the rp is at 1.9K. Calculated rp of ≈1200Ω appears to be pretty optimistic. Unless the goal is to squeeze as much power out as possible (without much concern re: distortion) I'd guess it more reasonable to aim the design higher. Or at very least, test the tubes themselves before designing the transformer.
I'm no expert and haven't used the GM70 for more than a quick test so please forgive if I'm not right about this. Thanks.
BTW. diyAudio thread related to the calculator is here .
Then dialing in the GM-70 with stated design Ip max of 100mA, even at a very low Vp of 250V the rp is at 1.9K. Calculated rp of ≈1200Ω appears to be pretty optimistic. Unless the goal is to squeeze as much power out as possible (without much concern re: distortion) I'd guess it more reasonable to aim the design higher. Or at very least, test the tubes themselves before designing the transformer.
I'm no expert and haven't used the GM70 for more than a quick test so please forgive if I'm not right about this. Thanks.
BTW. diyAudio thread related to the calculator is here .
I've build my GM70 amp years ago by myself around SAC Thailand OPT and VT-25 driver. Now I want ultimate OPT. Monolith Magnetics makes some decent OPTs but 1450€/pair for amorphous version is to much, nano-x core would be probably more than 2000€. For this price I can buy numeric winding machine (already done that), nano cores and OCC copper and still save some money...
I'm an electronic engineer and can handle such things but need to get some additional knowledge.
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Hi Marek,
I understand your motivation, and hopefully other people can help you out with the lacking bits and pieces. Good luck!
Erik
I understand your motivation, and hopefully other people can help you out with the lacking bits and pieces. Good luck!
Erik
You'll need some amount of patience gathering all this knowledge, as it is all scattered around. Buying the winding machine is just a small beginnining, but in order to have a smooth winding operation, you'll need to study other details, such as acquiring the winding tensioners suiting your purpose, build your own tooling with precision, tweak your wire guides. Winding transformers is a learning process that takes forever. All insulation materials and wires will also differ in winding behavior, for example different wire brands will require different tensions, as the copper heat treatment in them differs. Similar thing goes to the insulation. Some insulations are easier to work with due to better wire laying, others are not.
You'd also want to pre-calculate your capacitances and leakage inductance before starting and determining carefully your design criteria. All secondaries in parallel as a start is a good recipe for success.
Marek need just 2 transformers, not serial manufacturing, so this over-complication is not necessary. He can use just hands and gloves to tension wires.
Who knows, when one owns an automated winding machine, might become enthusiastic for more work? If I had to wind only a pair of transformers, I'd go with a hand winding machine.
I bought this machine not just for making only this one pair. It gives me ability to make OPT for any tube I will potentially switch to in the future, nano ITs, anode chokes and transformers for my audio friends.
This is also great part of this hobby!
Tensioners can by added in the future but for now built in screw type "tensioner" 😉 should be sufficient.
This is also great part of this hobby!
Tensioners can by added in the future but for now built in screw type "tensioner" 😉 should be sufficient.
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I have at least 6 different tensioners and my Meteor tensioners i stll don’t use. Even brand X will do the job
I measured permeability of large (~6KG) nanocrystalline C core I have. Its in range 7500 - 10200 without air gap. Its going to be used in PP output transformer, but anyway, will need an air gap.
What specific nanocrystalline material your core is made of? Permability is quite low so I guess it is some of the flat loop type?
The C core cutting clearance is also a permeability limiting factor for the nanocrystalline material.
Material is from China, didn't remember its exact mark. Material permeability from data sheets measured on toroid, for C core after immersion in the glue and cut, permeability drops to 3000 - 10000 in average as was quoited by manufacturer. Quality of the cuts are none but excellent, may be with wire EDM.
For PP transformer permeability over 2000 is not needed, as it may lead to core saturation because of output tube idle current imbalance.
For PP transformer permeability over 2000 is not needed, as it may lead to core saturation because of output tube idle current imbalance.
Not if you know how to deal with it.
When the transformer works within it's limitations, the core type and material, and the winding scheme, and the grade of wire, and the alloy of wire, and the impregnation (or not), and the potting, and, and, and.....all influence the final sound character.