mwl168

@ vvega: check Craig Sawyer's Dec. 16, 2010 post on the KGSSHV thread. His rationale why a thin insulation pad, no matter the voltage rating, would result in arcing in high voltage application makes perfect sense to me.

My bad. I gave Jose bad information. The drop resistor is R19 which is marked 21K on the PCB. I got those two numbers mixed up...

Yes, and, as JimL stated, may also want to increase R12 to maintain (R11 + R12) close to 13.3K to maintain the gain and bandwidth as originally designed . EDIT: attached Megatron schema for reference. megatron.PDF

Jose's mod sets the CCS at 1mA instead of .85mA. This is likely the reason why the 12X7 plate voltage is quite a bit higher than the 245VDC I am seeing on my amp.

It may worth mentioning that, based on the plate characteristic chart of the 12AX7, this higher plate current also puts the tube in the more linear operation zone. Whether the effect is audible is debatable as always.

I spent most of this past weekend working with JimL and Kevin tweaking my Megatron.( I was just the coolie equipped with a soldering iron and a multi meter and Jim and Kevin supplied all the brain power. ) Kevin suggested that I share the information here. My goal was to raise the standing current to the plate of the driver tube (12AX7). It's set at 0.5mA in Kevin's original design. Kevin has mentioned in his June 28, 2012 post that the only thing stable at 0.5mA is the 2SA1968 which has since been obsolete. My build uses KSA1156 in place of the 2SA1968 so this tweak may be more applicable to similar builds. This tweak, at the end, turned out to be the few relatively simple steps below: 1. Reduce R6 and R7 (2.2K) to about 1.3K which sets the plate CCS at 0.85mA. I used 3K to parallel with the 2.2K to get to this value. This higher current also raises the standing plate voltage of the 12AX7 from around 245VDC to about 275VDC (this will vary some with the tubes used). 2. Reduce R11, R23 cathode resistor (3.3K) to about 2K so that it pulls slightly more current and brings the plate voltage down to around 245VDC. I used 4.7K to parallel the 3.3K to get to 1.99K. We need minimum 50V differential between the plate and the B+ (300VDC) to allow for the driver voltage swing without clipping. 3. Because the resulting higher current draw of the front end, you need to adjust R19, the drop resistor, to maintain the 300VDC for front end B+. I like my Megatron’s sound better with this tweak. YMMV. It may be possible to raise the driver current even higher subject to the plate dissipation and CCS transistor limit. I am happy and content with the outcome now but may experiment further in the future. I am also hoping someone may be interested in running a SPICE model and/or observe the outcome with a scope to validate.

I recall reading somewhere that US is imposing a regulation that all-electric cars need to “make” certain level of noise - for safety reasons concerning pedestrians.

Never quite get the "lifestyle" thing. I want to enjoy life. Don't care much about "style".

Some observation/measurements. Using 1M (500K + 500K) input bias resistors as shown on the schematic, with 400VDC rails, gives 18mA output current @ 52C (heatsink temperature) per phase per channel. Being a true push-pull circuit, this is equivalent of 36mA output current of a SE circuit (such as KGSSHV and Carbon)!! Also, the bias is at least as much a function of output transistor temperature as of the bias resistor value. After consulting with Kevin and congo5, I experimented with a few different bias resistor values. With 1.05M bias resistors, I got 14.5mA @ 45C and 17.2mA @ 55C. With 1.1M bias resistors, I got 13mA @ 46C and 14mA @ 49C. I settled on 1.05M bias resistors which gives 14.5mA output current and the heatsink, when standing upright, remains at 45C after fully warmed up. This is a great sounding amp. For the parts cost (when you can get them), this ES CFA probably takes the crown of the ROI of all the ES amps I have built. I would also go on the record that I concur with MLA and congo5; as much as I like my Dynalo and Multiamp, it's the dynamic CFA amp I plug my dynamic headphones in when I crave for some music!

Got my package yesterday too. Thanks for all the great work!

Another electrostat CFA lives. This one born on the 4th of July😄 The PCB is congo5's layout. The amps is powered by my GRHV PSU with ~405VDC rails. Many thanks to Kevin for another wonderful design, to congo5 for the PCB boards and many advices along the way and to sorenb for his generosity of coming to my rescue and providing the missing, unobtanium sands. This build would not have been possible otherwise. I did not match any of the transistors - barely had enough for this one build. The output balance of the two channels sit at 6.6VDC and 4.4VDC after one hour running. The offset is 10.7/4.2VDC and 9.7/5.3VDC. For both channels, the non-inverted output offset is the higher one. The heatsinks are at a toasty 52C after one hour running, about 28C rise from the cold start. Will need to find a chassis with larger heatsinks.

My Grounded Grid sounds very similar to my Blue Hawaii. The Megatron sounds quite different from those two. I agree with Jose that the all-tube front end contributes a lot to the different sound signature. The Megatron is also capacitor-coupled rather than DC-coupled.

Happy Birthday Kerry!

As Kevin said the upper tubes (CCS) need to match the lower tubes (output) on each channel to get the lowest DC offset. I was able to get mine to within 3 -4 VDC using better matched tubes. I paralleled the 21K with another resistor to bring it down to 15K and get the front end B+ to about 290VDC. In my case, the front end tubes draw about 8.5mA total. And where are the pics?

PM me if you are still interested.