JoaMat

A few points regarding the boards Michael have had made on my gerbers. They are basically the same as my kitchen made boards I made five months ago. The small tube footprint has no hole for the center pin. So far I’ve cut away the center pin on the small tube socket after checking the pin holders to be not to tight nor to loose. One time I got tube sockets where the tubes fell out when turning upside down. So either you have to cut pin or drill a hole. Be aware that upper tube has a trace between the grids that might be to close to center pin. Right board 270K resistor has a 0805 pad (intention was 1206). It’s possible to solder a 1206 resistor on the 0805 pad. If +220V you get about 130 V cross the resistor so Vishay CRCW0805 is OK here. But with +250 V you get close to CRCW0805’s limit of 150 V. Some of component values on silk screen are poorly place and almost unreadable. The blame is entirely on me, I’m very sorry. There are four octagons on bottom side. One at each big tube socket center hole and one each side of small tubes filaments connections. Here I have standoffs. A few comments on updated layouts. Small tube footprint with center hole. 270K resistor pad corrected - now 1206. Silk screen remade. Pleas notify me if there is any left that needs additional attention. Now “value” of connections on both sides (except heaters). No terminal blocks on my boards. Top right. Updated 2019-02-27. Bottom right. Updated 2019-02-27. Top left . Updated 2019-02-27. Bottom left. Updated 2019-02-27. I call above version .2. My kitchen made and Michaels are version .1. Regarding this amplifier - there is nothing I've invented. I just have picked up different things and tried to put them together. As: Opto servo (the only offset servo you need) by Kevin. 01N100D/TL1021 current source by Kerry. Schematic of Stax T2 provided by Kevin, Et. al. Schematic of Stax T8000 provided by Kevin and Birgir. Etc. etc. Thanks for all knowledge, information and inspiration from you DIYers and Head-Case members. Please look for errors or other issues.

OK – Connected an unmodified T2 PSU. Raised voltages to +250 and +/-500V. Top 6922 anode voltage sits at 227V and grid at 78V. So far it works. Will run this for a while and check for darkening spots on the boards.

I want to make some minor changes to the layouts. After that I think more boards will be available here. Should not take long.

Board size 160mm x 100mm. Sure it's possible to shrink it, but that will be the micro T2, then we have nano, pico ….. Thinking about the possibility to use an unmodified T2 PSU. If applying +250V instead of +220V you get +79V at upper 6922’s grid and +230V at anode (with no changes on the board). T2 has +69V resp. +200V. Power at HN4A51J goes up from 2 x 76mW to 2 x 87mW, so still OK I think. Question is if the higher tube voltages make any difference. But I don’t think it’s wise to put 48V cross HN4A51J when bringing down anode voltage to +200V (with +250V supply). As for +/-500V; no problem with 01N100D it’s a 1000V device. I do think it might be possible to use original DIY T2 PSU. Has to be proven.

I don’t really want to make any comments on how things sound and compare them. Mainly because it makes me feel uncomfortable and I also lack the ability to analyze the sound. Anyhow I’m very happy with “mini T2” and I want to think that it is quite similar to THE DIY T2. Now I have put the original DIY T2 in the closet and I don’t miss it. I still have my modified T2 in our summer cottage and I like that one as much as the “mini T2”. But I have three more original T2 boards and all sands for them – no duck feeding the next couple of years.

Oops. Schematic posted above uses 2sa1486 in the current tunnel – my first boards had the 2sa1486. Schematic in the link uses twin ksa1156 in the tunnel and the boards have the twin ksa1156. The boards Micheal and Jose have had made are created from the linked schematic.

Ajaj! That -400V is something from earlier schematics I worked with and I obviously forgot to remove it. The -400 V potential is created by voltage divider R53 – 56. Otherwise the schematics are identical except for that components have different coordinates (I hope, crossing fingers). Thanks for noting it, Steve.

You can use +/-450V for +400V and -460V. But I suggest you find a good way to achieve +220V.

Oh! I forgot Kerry’s micro GR - that would really be something. Link

Well, I changed the voltages as said above on a T2 PSU that is a bit different from the original. I used those lower voltages for my T2 for some time before I built mini T2 and it worked great. So if you dare to lower the voltages on your T2 PSU and remove the 7812 and 7912 and put in the https://www.amb.org/audio/sigma78_sigma79/ set to 15 volts. Then you have an universal PSU. I haven't tried AMB sigma78/79 in T2 PSU but someone has to...???

I’m using a modified T2 power supply. +250V reduced to +220V. +/-500V and -560V are reduced to +/-400V and -460V. +/-12V increased to +/-15V. Filaments as original. But the -400V is not used at all with this amplifier.

A very early layout attempt. Have to figure out how to spread the parts. Board size 2.65in x 0.87in.

Increasing trimmers resistance always increase corresponding side’s current and vice versa. If upper trimmer (ksa1156) increases corresponding side’s output voltage increases and the other decreases. If lower trimmer (ksc2752) increases corresponding side’s output voltage decreases and other side increases. So far I’ve preadjusted all eight trimmers to the same resistance. Resulting in roughly the same current of all for channels and in an output voltage of some positive volts - all good and not needing any further action. To me it has been a bit tricky to turn all trimmers to achieve better balance and offset than when the preadjusted.

OK, now I’ve reached further. Replaced all eight 2K resistors with 2K trimmers. Makes it easier to change the current. Set at 1.6K current start at 2.5 mA and climbs to 4.5 mA. IMO it’s still an interesting amplifier and I believe it’s possible to build an all SMD version.

Reduced the 2k resistors at base of input transistors by 100 ohms steps. 1.7k give approximately 7.5 mA and I have decided to keep it there until further…

Thanks folks. No matching. I use our kitchen table as workbench and to avoid some nasty conflicts with wife….

Nice! and thanks for files.

Modushop heat sink 300 mm X 80 mm and Modushop Galaxy Maggiorato front and back panels. I buy them as spare parts. Brackets and aluminium plates for top and bottom I get from a local shop. I've drilled and tapped M4 holes at both ends of heat sinks to fasten the panels. Below is a board attached to 200 mm X 80 mm heat sink - temperature stabilazed at about 50 degrees Celsius. With 300 mm heat sink temperature stabilized at 42 degees. Current stabilized at 20 mA with resistors as silk screen.

Amplifier built on Gary’s (congo5) boards. I really like this one . Cheap and easy to build - and it sounds really good.

More Sort Guld. A Søren built amplifier is easily recognized. Very nice!!

Nice work indeed. I like the two box approach. The transformers – are they DIY made?

Maybe one could change resistors on Alexey’s board to achieve -20 V.