New build advice

[martin manning]

Just for grins I ran a couple of "Mullard" reissue EL34's at the proposed idle point (340V Va and 250V Vg2, Vg1= -15) and got pretty much data sheet values (Ia=70mA, Ig2=10mA). So from this you can fill in a few more details:

The cathode resistor needed will be 15V/0.080A = 188R (Std. E24 value 180R).

Knowing screen current and screen resistor value (1k, say) you can estimate the voltage drop at 0.01A*1000R = 10V. The target screen node voltage will then be Vg2-k + Vk + 10V, or 250 + 15 +10 = 275V. The plate node is 355V, plus a few volts for the OT primary.

The resistor from B+1 to B+2 needs to drop 80V at 10mA plus whatever current the preamp tubes draw, so it will be something south of 8k. Add two 12AX7's for another ~6mA, and it'd be 5k.

The load line is plotted below. Notice it will be pretty well centered at 5k Zpri since that load line would hit the vertical axis at ~2x the q-point current.

NOS Mullard EL34 will sound much better, but only if you don't have one ;^)

[didit]

With Martin's able engineering advice the basics are covered except for SE output transformer. Do make rolling plans with KT77 & KT88 along with a few different EL34s.

I've got the MPS OT10-SE, 5K or 7K to 4/8/16. This is installed on what I consider to be an experimental failed build, so I'm ready to pull it. Do you think this might be appropriate for the SE plexi?[...] I'm wanting this to be a bone pile build. I haven't ruled out buying the OT, though. What would you be looking at for a SE EL34 OT?

Suggest this MPS unit for another smaller project -- lower voltage EL84/6V6/etc.

• Output Power: 12VA Maximum
  Maximum Recommended DC Bias Current: 60mADC
  Designed Operating Frequency (+/-1dB): 82 to 15000 Hz
  HiPot Test Ratings 1000Vac minimum
  Input Impedances: 5000 or 7000 Ohms

Static DC 80-90mA, 65 Hz floor & 5K primary seem about right. Perhaps floor suggested is too low. Edcor has something GXSE15, but stock don't come multi-tap on secondaries. Hammond 125FSE might fit, but expensive.

Best .. Ian

[Phil_S]

Martin: I have several of that exact page with red lines on it, trying to understand how to do this. On that note, I my appreciation for you post is at least doubled. 1) It's great to see an example from someone who know how to do it, 2) my drawings look about the same, suggesting I've figure this out.

Ian: Thanks for pointing out the MPS 10W isn't up to the job. I see now.

MPS has a 20W model that is 2300/3000 into 4/8/16, which means I can get 4600 into 8/16, which is pretty good for an OT priced at about $33, in stock and a decent match to the load line.

Edcor has good prices, but drawbacks include "made to order" and single secondary. I suppose they'd be willng to tap 2 more secondaries, but I have a feeling I won't like the price after that. I'm not sure I have the patience for "made to order."

The Hammond 125ESE looks expensive and I'm thinking 80mA/15W rating could get maxed out when I actually build it. The 125FSE looks like it's $80 and that's more than I want to invest.