kevin gilmore

that cannot work with the + inputs of the opamps grounded.

quite a bit more power and heat and bigger power supply etc

the second schematic is not going to work, there is nothing to keep the output at +300, the left side of R4 needs to go to some negative value and AGAIN, the dn2540 will BLOW UP when the voltage across it hits 400v use a bsp125 in the to220 package if you want a 600v power supply but much better is a c2m1000 and a 900v power supply

the massdrop is a liquid carbon. not a liquid gold. lower voltage power supplies, less heat, less power etc a correctly built liquid gold is at least 50 watts of heat. but hey its cheap

the 2 channel pot to do balanced is the same crap eddie current uses. it can be made to work, but causes all sorts of problems. All 4 wires are with respect to ground so not what you think. Also you need a reverse log pot for it to work right, and no one makes those. electrostat, dn2540 goes boom at anything over 400v. and its a depletion mode part, so not a great idea. what you are doing is a hev70 copy, look at the original for better ideas. also the opamps driving the mosfets have the feedback wrong.

its always better to match parts, less work for the servo

those JJ tubes have only 1 hole in the plate per side, the originals have 3 holes and a different configuration of the plate, more square than rectangular. my T2 is still using the same tubes as when I demo'd it at CJ010. I recently did a 100 to 120V conversion on a real T2 for someone on HF. It has the original tubes, 22 years old. (pictures can be supplied if anyone has a real T2 and wants to convert voltages, better than $250 in shipping both ways) The gold aero's are labeled 6dj8

so on the input tubes the input tube has a plate voltage of roughly +75 volts and a cathode voltage of -3 volts the cascade tube has a plate voltage that would be a max of 250v, but its a current source, and is typically +200V and a cathode voltage of +75 volts and the maximum voltage swing is about 15v so both tubes well within specifications closer look at the original pictures and the original tubes are el34G they look absolutely identical to the Richardson tubes currently made in china which are labeled as 6ca7

The real original has el34 with the Richardson electronics National Logo made in USSR about 1994 the 6dj8's were gold aero

lots of floating filaments

although a lot more parts, and not all tubes, the T2 battery appropriately adjusted for voltage to replace the loss across the 0a2. of course you could just use a 1uf cap across the gas tubes

ssdynahi has a lower input impedance and would be fully balanced.

r19 in parallel with r20 is the gain resistor

that definitely works, but the price is a bit goofy. probably what happens when you spend 10 years trying to make better parts only to make parts identical to the originals

lh0033 is a diamond buffer, and is VERY obsolete. I have a couple of originals somewhere.

That was a jpg picture, the schematic above is the latest

that is the correct schematic with q27 and q28 replaced with surface mount on the back of the board

lg.zip note: phase splitter not included you need 4 for balanced

so after thinking about it a while, something is wrong with the hv delay circuit which turned on with the tubes cold. this has to be fixed first. when the tubes are cold, the output voltage is going to go to +500v, and thru the 100k feedback resistor is going to put 500v on the cathode of the input tube. With the filament of the input tube grounded, there is 500v cathode to filament. Guaranteed to cause a light show and permanently trash the input tubes.

I always bring up the low voltage power supplies first. and turn them off last

it would be nice to come up with some universal layout that would take all the available options that340, 4 x single jfet,2 x dual jfet,2 x dual bipolar... yes pars posted the picture I was thinking of, I even found the board

I did post a picture quite a while ago of the 4 jfets soldered to a milmax socket. definitely the easy way finding the picture might be harder

that definitely does suck, I had one noisy one out of a batch of 10. birgir is making a little mini board, and Justin bought a pile of dual transistors, might be a better solution the layout of the amp is such that you can replace the that340 with jfets if you can get them

you either add a unbalanced/balanced to balanced input stage which adds lots of parts and space, or supersymmetry which forces a low input impedance. so let me explain further, lets say you have an amp with a gain of 5 now you put 1v on both the + and - xlr inputs. Output voltages are +5 for both. Difference is zero, but if you have unbalanced headphones, then 5v on the headphones which is bad now you do the same thing to a true differential amplifier. Output voltage is now 0 for both channels which is why a true balanced amp removes any common mode voltages and noise this does not help you with a single ended input. or a single ended input converted to balanced. In the first case there is 5v on the SE output. In the second case the + output has +5v and the - output has -5v

no that is not correct. Its 4 single ended amplifier channels. No input dc rejection. Its a balanced amplifier, but not differential. its wired different than the liquid gold in that a single ended input results in a single ended output.