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Bijou All Tube Futterman Headphone Amplifier - Page 165

post #2461 of 3976
4/8/09 at 2:51pm
Thanks guys, looks like I'll be going from no tubes at all, ever, to lots of them in very short order
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post #2462 of 3976
4/9/09 at 4:00am
My Bijou casework has taken much longer than expected bit I am nearly completed. I have been saying to myself for weeks... "just one more hole to drill" haha

I decided to construct the case using all 2.5mm aluminum alloy (5052-H32). Already the case is quite heavy by itself. I am now wondering if I should of used lighter material on the sides and top? One thing is for certain - it's not going anywhere once I place it on my desk.

I can finally clean off my desk and heat up my soldering iron!
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post #2463 of 3976
4/9/09 at 11:43am
[QUOTE=balderon;5593569]I have been saying to myself for weeks... "just one more hole to drill" haha
[QUOTE]

perpetual optimism builds character in the face of failures - at least thats the b.s. i tell myself when i end up throwing tools across the room, cursing mad as hell.

i'm really jonesin' to re-build my Bijou.
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post #2464 of 3976
4/10/09 at 1:01am
Pardon my lack of circuit knowledge, but I need to know if the signal
through C1 is going - to + on the schematic, or + to -. I'm going to be
replacing C1 with Auricaps, and they are 'directional' (signal going from the
black lead to the red lead). Thanks.
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post #2465 of 3976
4/10/09 at 1:07am
Interesting question. Signal goes from the output to the junction of R5 and R6 which means it goes from + to -.
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post #2466 of 3976
4/10/09 at 1:10am
So it's involved in feedback?
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post #2467 of 3976
4/10/09 at 1:39am
Yes. It is not the normal type of NFB, however.

This feedback compensates for the fact that the cathode of the upper triode is moving with the O/P signal while the cathode of the lower triode is at a constant voltage (ground).

C1 feeds back the O/P signal to the phase splitter where it appears immediately at the cathode of the splitter and where it adds itself to the normal signal introduced at the grid of the splitter. This causes the grid drive on the upper triode to be much larger than the grid drive on the lower triode. The larger drive on the upper triode exactly compensates for the fact that the cathode is moving with the signal.

The net result is that the grid-to-cathode signal voltages on both triodes are identical ensuring that they are exactly in push-pull balance.

This was Futterman's invention although I'm not sure that his orignal amps actually employed the technique completely correctly.
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post #2468 of 3976
4/10/09 at 10:40am
"My Bijou casework has taken much longer than expected..........."

This is the story of my life at present as well. Mine's more practical than flash (Hammond steel rectangular with perforated cover) but still requires a fair amount of design & much drilling. I need more hours in the day.

Now my Bijou is in bits awaiting the re-build, I got distracted by Adamus's comparo of the Bijou & SOHA II. I've never tried or even properly listened to a hybrid but the concept is very intriguing & the SOHA looks like a good starting point. So I'm going to order a kit from Jeff Rossell. Its about the same price as the basic Bijou kit but includes a transformer & is quite a bit simpler as everything except transformer is on the board. No plans to go overboard with "nice bits" (as with the Bijou) but it will be an enjoyable project & satisfy my curiosity about hybrids.

runeight, thanks for the explanation about the Futterman feedback cap & finer points of the circuit's operation. I'd searched for such an explanation of how it worked, including going back to some of Futterman's notes, but still couldn't work it out. IMO, its a brilliant design that's still not properly understood or appreciated by many audiophiles.
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post #2469 of 3976
4/20/09 at 10:49am
I've read through 140+ pages before coming to a conclusion to DIY this amp.

I have some questions though.
1) Can I use CAT5 cable for all my internal wiring except for the signal cable?
2) Some of you said something about bleed the high voltage cap. Does it mean run a resistor across the Caps' leg?

Sorry if i have used the wrong terms, haven't got the time to fully indulge in DIY but i'm getting to it.
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post #2470 of 3976
4/20/09 at 11:42am
SBoy - If the CAT5 is rated for 300v or better then it wouldn't hurt to try it.

Without modifications the high-voltage caps of the Bijou's power supply should already bleed, perhaps through R9/R10? Not sure exactly.
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post #2471 of 3976
4/20/09 at 12:02pm
I've been caught up with home repairs and probably won't have any time to do any modifications to my Bijou as I had suggested before. I did take some time last night though and I put in a three position switch so I can select bias for 2.4kohm (6bq7), 2kohm (6922), and 1.5k ohm (6N1P). Nice.

I'll post the V1 voltage readings later, but the 6bq7 measured about the same as the 6N1P with appropriate bias settings.

Listening impressions using AKG K240 Monitor headphones, and no major mods to circuit aside from bias. My own opinions may suck so bear with me:

Valvo PCC88 = yuck. But it's better than nothing. I would expect it would do better with the appropriate 7v heater voltage (7.6v???).
6N1P-VI = Thick bass, but a bit loose. Kinda reminds me of listening to the neighbor's old console radio in the basement while playing "Santana" and playing pool.
RCA 6bq7 = Best so far. Detailed and tight. Smooth as butter. Typical RCA blackplate awsomeness.

I'm curious about 6922 now. The 6922 is an improved 6dj8 which is an improved 6bq7. I can't remember where I read it but hearsay says that the improvments aren't noticeable until you're pushing the current limits of the tubes and it's probably more RF related. So anyway that's my propoganda for the day.

Someone should publish this thread as a book.
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post #2472 of 3976
4/20/09 at 12:08pm
Well.. then i suppose i can't use CAT5 for the internal wiring since 300v would instantly melt it. Can anybody clarify my 2nd question? =)
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post #2473 of 3976
4/20/09 at 1:18pm
The 7308 is an 'even better' 6922 - may want to give them a look, though I don't use them here. I also have 6n23p-ev to try as well, but use them in the Raven with 6h30-DR.

I kinda like the 6n1p, but also need that switch to change the bias for the different tubes... one day.
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post #2474 of 3976
4/20/09 at 7:12pm
SBoy, if you are on a shoestring budget, Radio Shack or automotive hookup wire will work. 22ga signal, 22ga HV, 20ga power, 20ga heaters. Welcome to the Bijou Project!

Quote:
Originally Posted by SBoy View Post
Well.. then i suppose i can't use CAT5 for the internal wiring since 300v would instantly melt it. Can anybody clarify my 2nd question? =)
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post #2475 of 3976
4/21/09 at 1:03am
Quote:
Originally Posted by SBoy View Post
I've read through 140+ pages before coming to a conclusion to DIY this amp.

I have some questions though.
1) Can I use CAT5 cable for all my internal wiring except for the signal cable?
2) Some of you said something about bleed the high voltage cap. Does it mean run a resistor across the Caps' leg?

Sorry if i have used the wrong terms, haven't got the time to fully indulge in DIY but i'm getting to it.
SBoy, nice to see another Bijou Builder.

1. As mentioned, ethernet cable is not good for HV wiring. Use wire with at least 600V insulation stand off.

2. If you need to debug the PS then you should discharge across the output of the PS using a 100k 1W resistor before you start poking around. If you don't you might get a nasty shock. And if you short the PS when it is fully charged you will wipe out the mosfets.

Good luck.
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