JoaMat
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Posts posted by JoaMat
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Happy Birthday!
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8 hours ago, Shawn said:
What specific differences do you notice after adding the 100 µF caps?
Higher gain and in my opinion better frequency response. I haven’t tried other than the 100uF electrolytics. With the capacitors the amplifier is an absolute favorite.
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Happy Birthday!
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I put 100uF/16V electrolytics in those positions in my Megatron with solid state CCS.
I think it made a big difference.
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Happy belated Birthday!
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Thank you everyone. Had a nice one with my wife in our summer(winter)cottage. Now I’m preparing for Boxing Day +6. Birthday of both our children and one grandchild. Happy New Year everyone!
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Happy Birthday!
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A frendly Head-Caser has kindly lent me four EL156,
So, I made myself some EL156 to EL34 adaptors.
Here a Megatron SS CCD something with the four EL156.
Same friendly fellow has built himself a couple of filament supply for DHT similar to the one in above post.
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Perhaps you are right, but that’s nothing I can verify.
It’s correct that pos input is dancing. Now I think that the neg input is a perfect dancing partner which results in a working servo. With the servo the amplifier behaves similar to the ones without servo except that the bias doesn’t drift.
And cross talk? Not a problem - I’m married and used to cross talk.
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8 hours ago, jokerman777 said:
thermal: Is it essential to mount all the transistors in darlington triples to heatsink?
I’ve a smd version of CFA3. Only output transistors on heat sink. Bias is servo controlled. Works all right. Output stage with servo looks like this…
On 7/27/2024 at 11:36 AM, JoaMat said:Have had a new bias servo working with my CFA3smd something for a few days. Schematically it looks like this,
and the new thing is in red square. Now I get a good reference point to the servos. Don’t understand why I didn’t figure this out earlier – slow brain perhaps. All four servos are fed from the same point. So, one trimmer and full control over the BIAS of all channels.
Diagram for new servo - 6 minutes from cold.
and diagram for old servo (in mA for the first 40 minutes from power on)
Please have in mind that the graph for no servo is for my CFAsmd something. I guess the original CFA3 have a different and better behavior – where all tripple Darlington and vbe transistors are mounted on heat sink.
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Only in fantasy world...
Filament supply for DHT.
9VAC in and 2.5/5.0VDC out. Size 2.07in x 0.7in.
All you need is to connect a transformer and a tube to heat.
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A small 120 V supply for above grid voltage control. 6.3VAC Hammond 229A transformer and LT8365 DC/DC converter. Out 120V . Size 2.3in x 2.3in.
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Seems simmconn reads shematics. Regarding Q12, schematics and gerbers don't match, I think.
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Completed servo controlled grid voltage.
Works with EL34, 2A3, 300B and EML 20B-V4.
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Did this today. A two-trimmer control board. Here along with three one-trimmers.
One trimmer affects both 2A3 and 300B filament voltages and the second just the 300B voltage.
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The one-trimmer control board above adjusted voltage for both 2A3 and 300B at the same time. No way to adjust voltage for one without effecting the other.
I assumed that it should be possible to set voltages for one tube and have the other unaffected. But as my brain isn’t as fast as the two comparators in LM2903 I made a test board with seven trimmers.
After some trimming back and forth I found one trimmer that adjusted voltage for both tubes and an another that just affected 300B,So, next is a two-trimmer control board, same size as the one-trimmer. If you don’t understand how a circuit works, the trial-and-error method might help you.
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Three kinds of switching mode filament supplies made in my kitchen.
Right: Traco brick, only 5V with some fine adjustment, designed by Kevin Gilmore and slightly modified by me.
Left: DC-DC Step Down Converter XL4015, adjustable voltage and current limiter, have used it for 2.5V to 6.3V.
Middle: TPS62913 Low Ripple Buck Converter (green board) My own design but done with help of TI datasheet (all info needed is there). Copper color board is an Elekit regulator with 2A3 and 300B auto detection. I’ve added a trimmer for fine adjustment of voltage.
All three seem to work all right with my electrostatic amplifiers.
I also have Pete Millett, Tentlabs and Coleman filament regulators. They are all bulky with heat sinks and produces a lot of heat.
On 10/14/2025 at 7:57 PM, simmconn said:It’s a pretty ingenious design.
Yes, I do agree.
On 10/14/2025 at 7:57 PM, simmconn said:It should be okay for a DIYer to build a clone for his/her own use, but integrating the same in a “DIY” unit that sells for an atrocious price at “the marketplace” would probably be a different matter.
Yes, at some point it (probably) will be a different matter.
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Made a control board for filament smps today.
Some pictures from my kitchen.
Idle voltage – no tube
2A3
300B
and EML 20B-V4-
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I believe R41, FB2, C16 and C17 are connected as red line shows.
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Thanks a lot.
I've been thinking of a 300K resistor cross the h11d to limit the upper grid voltage.
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Schematics auto switching. Not sure it's accurate, but that's what I've done.
New Grid Voltage Control boards.
One trimmer for offset and the other for balance so, both sides are controlled simultaneously. Two jumpers. For 300B/2A3, EL34 and EML 20B-V4. Works al right, quite easy to use.The -510V is achieved with help from GRHV 110V supply. Here is one with Hammond 2VA trafo on board - 2.75in x 2in.
New idea - servo controlled grid voltage.
Okey, I've tried servo controlled Megatron (with solid state CCS) before and that failed...-
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A new daughter board to my Megatron something. It sets the grid potential of the output tubes. A few resistors, one trimmer set offset and the other the balance plus a small 120V GRHV supply completes the whole thing.
So, I removed cathode resistor along with its decoupling capacitor. EL34 has Vgk 40 V and 2A3 has Vgk 92 V. The amplifier has solid state CCS (no top tubes).
Works? so far so good.
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From a post somewhere else.
I also tested HA-3FXX pins, but I like above pins better. Disadvantage is I had to destroy good Neutrik connectors to get the pins out.
Made in Sketchup.
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If you need 2SC4686a – check this link. German company claiming they have 4000 pieces available. I bought a couple of hundreds 10 years ago and they were genuine.
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Megatron Electrostatic Headphone Amplifier
in Do It Yourself
Posted
I tie cathode direct to -HV, -400 V in my case. Works with EL34, 300B, 2A3, EML 20B-V4 and EL156.
The datasheet for my Electro-Harmonix 2A3EH Gold says 450 V maximum plate voltage.