Bijou Headphone Amp Build Question
Aug 16, 2010 at 3:03 PM Thread Starter Post #1 of 14

Krayl

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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. 
 
Aug 16, 2010 at 3:23 PM Post #2 of 14

balderon

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The B+ of 138.4 is abnormal. Historically (depending upon the tubes and if you have installed the Regal mod) the 270DAX has trouble driving the B+ to 250v. It's not uncommon to see mid 240's.
 
Do you get the same voltage when you disconnect the power supply from the amp boards and install the load resistor?  If so, I would suspect a bad MOSFET otherwise it may be somewhere in your wiring.
 
Aug 16, 2010 at 11:04 PM Post #3 of 14

funch

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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 
 
Aug 17, 2010 at 12:22 AM Post #4 of 14

Krayl

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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.
 
Aug 17, 2010 at 12:27 AM Post #5 of 14

Krayl

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Quote:
 
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 =)
 
Aug 17, 2010 at 12:47 AM Post #6 of 14

balderon

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Aug 17, 2010 at 2:09 PM Post #8 of 14

funch

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



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.
 
Aug 17, 2010 at 5:05 PM Post #9 of 14

Krayl

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Quote:
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.  
 
Aug 17, 2010 at 6:51 PM Post #11 of 14

balderon

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Quote:
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:
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 remembered somebody had a similar problem a while back. It took a few moments using the search function to find the posts. I hope it solves your problem.
 
It is alarming to read the heatsink is "electrically connected" to the MOSFET.
eek.gif
It is highly recommended isolating the two. I certainly wouldn't want the risk of injury let alone death in case someone accidentally touches the heatsink.
 
Aug 20, 2010 at 7:15 PM Post #12 of 14

Krayl

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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.
 
Aug 20, 2010 at 8:17 PM Post #14 of 14

Krayl

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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". 
 

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