JoaMat

Great! Personally I think the original T2 PSU is a very nice design. A modern SMD version is nothing but awesome.

Regarding the opto servo. Eventually I spotted the fault. Transistor mounted the wrong way. Now servo working and it works very nice. Really like the opto coupler. Many thanks Kevin.

While speaking servos. The small board between tubes on Blue Hawaii (right channel) is a servo a la T2 (picture a few posts up). Did built it after Kerry suggested that one maybe should do it simple when we tried different servos some year ago. That servo works great.

As I posted earlier I rebuilt a Blue Hawaii to GG. And now this new one. I can only say that Kevin’s GG works very well. I’ve the opto servo on the board, didn’t flip (or, did flipped it, depends on ones perspective) the opto coupler when mowing it to the other side of board. After correcting the connections to opto coupler the servo still doesn’t work. Have to find the reason for failure. The opto servo should work, shouldn’t it?

Left channel is a GG. Followed Kevin’s schematic. Intention is to use Emission Labs DHT, didn’t dare to test it with DHT so it’s populated with some modern less expensive EL34. Right channel is an old Blue Hawaii. The combination of BH and GG works terrific.

Have a great day, Wachara !!!

bdent.com seems to have a few in stock. USD14.29 each if you buy 25 pieces or more. You need 28 pieces?

If interested in the story of EL34, I recommend Guide to the EL34 / 6CA7 by Pasquale Russo.

There are all kinds of EL34. Some good and some better. A few years ago I acquired those EL34. Probably manufactured in Eindhoven Feb. 1951(left) and Feb. 1954(right).

Birgir is most certainly right about KT66 and T2. EL34 and equivalent are the one to use. Regarding datasheet. Most of us aren’t competent enough to read them, including me. Anyhow I make a try. KT66 max working voltage is 500V and absolute max is 550V. DC voltage on output tube of T2 is about 450V. You will probably easily overstress KT66 and eventually maybe destroy them. If that happens – please let us know. I’m especially interested in effects on the amplifier. I’m sure that there are members out there that can tell us pros/cons KT66 with T2.

Beautiful!

OK, now it’s done. Swapped the small tubes for solid state adapters. Kept +250V voltage and battery voltage unchanged. The 2SC3840 dissipate some 0.8W each. Might need small heat sinks. If you decrease voltage by 100V you also decrease power dissipation to >1/3W. And yes, they need covers. Working on it.

Didn't found heat shrink wide enough. So printed some covers for the connectors. Not so elegant but it serves its purpose. No, but now that you brought it up - well, why not.....

Managed to open it in DipTrace. Looks very interesting. Anyone capable of making it in .jpg format or similar? Worth showing!

heat shrink, first thing in the morning.

T2 with Emission Lab’s EML 20B-V4. Transformer to left is for filament 4 x 5.0V. Adapters are made by some help of a 3D printer, Teflon tube sockets, slauthered Neutrik XLR connectors, two pole male connectors. All held together with really good Loctite super glue.

If builder decide to use only one series of resistors, then one should probably choose 1/2W. Othervise most of them could be 1/8W.

Pick a lead. Ordered transformers for DHT filament. Not sure the brain was involved.

This is what it looked like with adapter boards replacing the small tubes - ECC88/6922/6DJ8.

Only change to PSU was reducing +250V to +150V. For amplifier I changed batteries from 740V to 640V. Also the 68V grid voltage was reduced to 33V, but that is taken care by the resistor on the small board with two 2sc3840. Maybe one should build a modified T2 with lsk389 for input and EML 20B-V4 for output....

Made those a couple of years ago. All information you need can be found here.

Have listen to the DHT carbon style for three weeks now. It’s bloody good – think I like it more than the T2. Below is a draft of how a smd version of DHT might look.

Thanks! Great have you guys around.

Regarding transformers and humming. Is it the transformer itself that hums? Or is it something else that hums due to the magnetics caused by the transformer?

Will the opto servo be able to control offset without the trimmer/tail resistor?