Tubelab Simple P-P

[rknize]

Who'd you go with?

I hope you noticed my post in the other thread about the B+. I made an error wiring the board and so my estimates above were off. Bottom line is that 650VAC yields 325V B+, which puts about 310V across the tubes.

Simple P-P "beta builders" thread

[Ian444]

Yes I did see your post. I ordered Antek AN-2T300 and AN-0225, the plan is to use the 2T300 and see what B+ I get, then add the AN-0225 in series if required. Not really an ideal solution but gives flexibility, and meets the 325-0-325 requirement.

Ian.

[rknize]

You could try using one of the 6.3 windings to boost the core as well to get a little more B+.

[Ian444]

Finally got it built. The heater voltages are fine (I rewound both heater windings, 16 turns for 5V, 21 turns for 6.3V). B+ is around 320/325 with the Antek 2T-300 and mains voltage around 248V. Worked straight up, cathode voltages around 11.20 to 11.25V (matched quad JJEL84 from Jim McShane). Using NOS RFT East German 12AT7. Those voltages were measured during the first few minutes, then I had to connect it to some speakers and listen... Running pentode mode, no FB connected yet, been listening a while, sounds very good already. Bass is strong, detail is very good, stereo imaging is very good, no noise/hum, this amp is going to turn out real nice, I'm very happy. 1hr and 15 mins gone by, definitely the best amp in this little house.

Wife is happy with aesthetics and extremely happy (quote: "in love") with the sound quality, and we know the sound quality can only get better from here. The build is a bit "cottage industry" but I'm a newb at woodwork, scavenged the wood from the garbage bin at work, don't know what it is.

http://i198.photobucket.com/albums/a.../SimplePP2.jpg

http://i198.photobucket.com/albums/a...implePP1-1.jpg

Thanks George,
Ian.

[thevoice]

Looks pretty good, not bad for a newbie at woodwork.

Your timber is a pine of some kind, my guess from the photos would be either hoop pine or radiata.

[rknize]

Looks good! Got any pics of the transformer rewinding?

[Ian444]

No pics of rewinding, it took quite a few hours, unwinding and rewinding the clear tape. The low voltage windings on this 2T-300 tranny are the outermost windings, in fact one of the "6.3V" windings was originally a 12.6V CT so I removed the other side of the CT. The 6.3V windings were originally 22 turns so I went 21 turns due to 248V mains here. The 16 turns for the rectifier was checked by soldering the output wires to a socket with rectifier tube fitted and got 5.0V. With the amp powered up and tranny loaded I got 4.9 something, close enough. You need lots of patience and time to rewind it, and strong fingers to keep the wire and tape windings tight. I think winding over the top like George showed would be fine but just spread the windings evenly around the core so you can mount it without putting uneven pressure on the wiring. I'm no expert but I think enamelled copper wire would be the go, and wrap some sort of insulating tape over that.

[Ian444]

Just a quick question regarding cathode to heater voltage limits for the 12AT7, stated as 90V in the datasheets I have read. The voltage on the cathode of both phase splitters measures 106V and 103VDC under quiescent conditions. The heater cct is elevated to +20V DC by the 150K/10K divider R4/R3. Is this a bit close for comfort?

For info, in the above post, the enamelled wire diameter that Antek used for the heater windings in that tranny was measured at 0.049" (1.25mm).

Ian.

[Ian444]

The amp sounds excellent, it is clear to me that the PCB design and layout is very good. I guess that comes from George's lifetime experience building tube amps. I had no idea that a tube amp without sand could sound this good. It sounds good on all types of music. I fitted the NFB resistors, that cleaned it up nicely (running with pentode mode). No NFB caps are fitted yet, I need to buy a selection to test. I guess ceramics are not preferred, what type is to be used? I'm guessing a range from 47pF to 470pF would do the job? Maybe I should scope it first and see what the square wave looks like. I think George said to use a 10kHz square wave, but at what power, 1W?

The Antek AN-2T300 power tranny is getting hot, too hot to hold fingers or hand on it for more than a second and a half after about 4 hours, but it seems to stabilize at this temp. I read recently that a tranny that is too big for the application can run hot. In this case it is around twice the required capacity, but when making buying decisions, for some reason I decided the AN-1T300 would be barely powerful enough. I drilled upper and lower holes to allow air to vent through the middle of the tranny and it seems to have made only a small difference. I ran it 8 hours straight yesterday and there were no hot smells, and the tape wrap around the tranny is not discolored. Maybe its nothing to worry about.

[rknize]

I used silvered mica caps. Mouser sells a large variety of CDE mica caps.