Bijou Headphone Amp Build Question

So I ordered the kit for the Bijou from glassjar audio and the build went smoothly. The amp is working, but after probing around with the multimeter I'm curious about something...

With the Amplifier boards disconnected from the power supply, the b+ voltage reads exactly 250.0V, however, when I connect the boards and have the amp running (no input connected, just in standby) I read 138.4V from the b+ wire. 

Is this standard? or is there a problem with the amp boards drawing too much amperage for the voltage regulator to keep up?

I'm using a 270DAX and an EZ81 for the rectifier, 2x JJ E88CC and 2x 6N6P for the amp boards. 

Well, I tried a 100k ohm resistor across the B+ and ground and I'm reading a stable 250V. But the second I connect one of the amp boards the voltage starts to drop. It takes a few seconds to go from 250v to 138v, I assume due to the caps slowly discharging. Interestingly, with one board, it only drops to 148v. I'll re-examine all the wiring and see if I can track down the problem.

Quote:

Originally Posted by funch 

 

I'm using the 270DAX in my Bijou, and have never had a problem getting a stable 250V, running single ended.  My problem with that tranny is that

the heater coil is only rated at 3.5A. When it was my only tranny, it ran very hot with 6H30's and ECC99's. Now I only use the 270's heater coil for

the EZ81, and a separate 4A/6.3V tranny for the amp heaters. FWIW 

Yeah my 270DAX is getting SERIOUSLY warm. Like 50-60°C. I think I'll follow your lead =)

Wow, that's probably it. Excellent job finding that thread balderon, thanks very much!

I'll replace the MOSFETs and let you know if that works. =) 

Quote:

Originally Posted by funch 

At present, I'm using a Hammond 166N6, but have ordered a 167Q6 (6A) to replace it, mostly

for better aesthetics (and a little more current headroom). Each of my tubes uses almost an amp

each for the heaters, so the N6 runs just a tad warm, but nothing like the 270 did, trying to keep

up with five tubes.

Awesome! Thanks for the info funch, that saved me a lot of time scouring catalogs XD I just ordered a 167Q6 from angelainstruments.

I must say though, the Bijou sounded pretty damn good even with a screwed up B+ voltage and an over worked transformer (although I don't dare turn it on again until I've fixed the PS board). I can't wait to hear it when it's running properly.

Thinking back, I do recall accidently shorting the metal screw in the voltage adjustment pot to the heat sink of the MOSFET with my screwdriver (and the gate is electrically connected to the heatsink because I didn't insulate them) while the ps was operating. ...I'm thinking that might be what did it.  

I got my new MOSFETS today just got them soldered in to the PS board (insulated from the heatsink this time). That fixed it! 

With the amp boards connected and running, I have no trouble adjusting the B+ voltage to 250.0V

Thank you both for your help! I couldn't have fixed it without you.

Yes I did. Mostly in the treble. It's much sharper and has much more energy. The difference in sound resembles using Adobe Audition's mastering effect called "Energizer - Mild".