JoaMat

Happy Birthday!

Someone told me D3a is a good tube, Did this in LTspice… Modified Megatron – one less capacitor in signal path and no feedback. Works in LTspice. That's always something.

No particular reason behind the jfets, aside I like the to-71 package and that it’s fairly easy to build the layout around them SMD transistors because they are small. I had to decrease current at some locations to reduce heat. I see this amplifier merely as an experiment. I wanted to test if it was possible to make a small board. The bias servo is probably needed while moving from heatsinked MJExxx to small smd transistors. I believe the bias servo previously posted by Kevin, with dual op amps, is preferred. So, another experimental board…

Attached ZIP-file with gerbers, shematics and some 3D views. CFA3smd.zip I’ve used both gerbers when I milled the boards (kitchen made) and that is about three years ago. Board size 160mm x 74mm to fit on Modushop 200mm x 80mm heat sinks. Take a look at the files and see if/how the boards could be a part of your project. I think one should make a few changes to attached gerbers before considering sending them to a fab house.

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.

Happy Birthday!

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.

Happy Birthday!

I put 100uF/16V electrolytics in those positions in my Megatron with solid state CCS. I think it made a big difference.

Happy belated Birthday!

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!

Happy Birthday!

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.

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.

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…

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.

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.

Seems simmconn reads shematics. Regarding Q12, schematics and gerbers don't match, I think.

Completed servo controlled grid voltage. Works with EL34, 2A3, 300B and EML 20B-V4.

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.

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.

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. Yes, I do agree. Yes, at some point it (probably) will be a different matter.

Made a control board for filament smps today. Some pictures from my kitchen. Idle voltage – no tube 2A3 300B and EML 20B-V4

I believe R41, FB2, C16 and C17 are connected as red line shows.

Thanks a lot. I've been thinking of a 300K resistor cross the h11d to limit the upper grid voltage.