Tube DIY Asylum

Posted by 91derlust (A) on February 10, 2016 at 03:49:24

Hi again all,

You ask for photos - you get more that you asked for. I hoped to have my chassis design completed so I could post the drawings, but life got in the way. Maybe early next week.

In the meantime I'll post some details about the iron I will be using. I have them, so may as well chat about it now.

I had J&K Audio in Malaysia do the custom winds for me. I would have been comfortable purchasing from Black Art, Magnequest, Intact, Monolith or one of the Japanese legends. In the end, J&K seemed to have the right combination of enthusiasm to help, technical chops, trustworthy ears, locality, and options. Not cheap though and thankfully I ordered before the USD killed off the AUD.

Show me the iron already!

First up, the glamour part: the output transformer. Not sure what people want to know about this. The important stuff to me: 4K2 primary, gapped for 45mA and ideal running 40 - 50mA (max 100mA); rated for 25W. Neat windings on nano-crystalline C-cores, that apparently took some time to get right and consistent. Cased, potted and weighing in at 5.9Kg. Measurements in-circuit loading a 2A3 at 1W: 45H@42mA (20Hz, 5Vrms) and a stated 5Hz - 80KHz +/- 3dB (but the FFT actually shows about -0.2dB at 10Hz and -2dB at 100KHz), with a nice clean roll-off.

Next is the B+ transformer. Hmmm, so much less glamorous. Primaries for both nominal (230VAC) and average (245VAC) wall voltages. Secondaries for rectifier filaments and B+; B+ tapped for voltages suitable for direct-coupled or RC coupled circuit. So, if I mess up direct coupling, I can do it the usual way. Apparently well over-rated, but alas, no low DCR windings (from memory the secondary is about 60R). Super Hi-B C-cores and wound like an output transformer; includes an electrostatic screen; cased and potted.

Sticking with transformers, on to the filaments! Even less glamorous. Primaries for both nominal and average wall voltages. Secondaries are the usual CT for the 6C6 and non-CT for 2A3. The core could have apparently handled a few more taps and I wish that I had added taps for 300B and/or Type 50 filaments. Oh well. Z11 cores and solid - they make the Hammonds I have used in the past seem flimsy.

Finally, the power supply choke. Possibly the least sexy of all iron... 7H/ 10DCR/ 200mA. Same winding quality as the output transformers; same nano-crystalline cores too. Cased and potted as per the output transformers and weighing in at a whopping 6.5Kg. Should be decent. I should be committed for evaluation.

J&K listened to what I wanted, were helpful and responsive, suggested a range of solutions and did not try to up-sell me. Some of their less expensive transformers also seemed to be excellent quality. The transformers were delivered well within the agreed time frames really, really well packed. I would not hesitate to use them again.

Well, that is it for now. The transformer set is probably an indulgence and excessive for the 91-inspired. Still, iron matters and can be re-purposed for other designs.

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

Edits: 02/10/16   02/10/16