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Old 30th January 2008, 03:40 PM   #21
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You're not gonna need a Vg of 120V for U/L if triode on;y needs ~80V for that Va/Ia.
If you're gonna go the battery route, and I see no reason why, then proto with an adjustable DC supply.
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Old 30th January 2008, 05:30 PM   #22
Gluca is offline Gluca  Italy
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Quote:
Originally posted by Steven-H
Interesting Stuff ! How do you keep tabs on your battery voltage though ? I would imagine that as the batteries age - that your bias would change ?!

For the filament supplies, I bought the TentLabs 5A supplies. So I suppose that I would connect them to the cathode, then ground one side out right at the tube pin ?

Also - while I have your eyes - I thought that the IT needed to be loaded somehow ?

Lets see if this schematic looks correct....
No, the bias doesn't change that much. I check it every year. Just replace the batteries when they get old. They last as no current is flowing.

I hope my schematic makes the wiring clear

Ciao
Gianluca

Click the image to open in full size.
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Old 30th January 2008, 06:25 PM   #23
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Guys -

That is awesome; Im going to give it a whirl. I will go with 7 batteries so I wind up with a bias of -84v. I'll have to recheck my load-line but I imagine I should still be OK with what I got.

Thanks,
Steve
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Old 30th January 2008, 07:27 PM   #24
Gluca is offline Gluca  Italy
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Hey!

7*12=84V
84+10/2=89V

Yo will bias the tube at 89V as the "virtual" center of the cathode is at +5V (half the filament voltage).

Ciao
Gianluca
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Old 30th January 2008, 07:42 PM   #25
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Yup - sorry about that. That is what happens when you are trying to send out work emails and do tube math at the same time...
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Old 31st January 2008, 08:17 PM   #26
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Guys,
Ok - so; thanks to all of your help it is now becoming clear. Are there any octal tubes that would be a single triode equivalent to the 5687 ?
So I am going to populate the tubelab board, and just mount the sockets to the chassis with short leads to the PCB.
I hope to start the build within a month or so (work is just way to busy); once I nail down the input tube I will be able to make up my top plates for the PSU and the actual amp.

Thanks again,
Steve
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Old 1st February 2008, 12:41 AM   #27
JoshK is offline JoshK  Canada
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5687 came out during the space travel era and therefore doesn't have an obvious octal equivalent. Electro-Harmonix makes an octal version of the 6H30 (which is very similar to 5687) if you really want octal. The 6BX7 is probably the closest original octal to the 5687, in the sense that it is higher gm, but it has much lower mu (~8) and isn't as linear (reportedly).
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Old 1st February 2008, 01:33 PM   #28
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Josh,
Thanks ! I was just looking for an octal so that it would be easier to use up some of my spare octal chassis mount sockets...
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Old 1st February 2008, 04:50 PM   #29
kegger is offline kegger  United States
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The 6BL7 or 6DN7 should do for what your asking.
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Old 2nd February 2008, 06:23 AM   #30
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Paul,
Thanks ! I read through tubelab's site, and found the operating points that he used for the driver section (12mA CCS, 180v), and mapped 'em out on the 6BL7 curves. Using Josh's recommendation of 275v:

Click the image to open in full size.

My only concern here is that the 5842 consumes 300mA of heater current (600mA per pair), while the 6BL7 is 1.5A; so I may have to stick with AC on the heaters for 'em (as I might melt down the heater supply on the board). Everything else I think looks OK.

Onto the power tube. Most schematics I have seen grid bias the tube @ -90v (grey line). As Gianluca helped me out with earlier; I would be right there @ -89v if I used 7 of those 12v batteries (Q=750v@28mA). However, in looking at his schematic, he is at -77v (6 12v batteries, plus the 5v from the heater - blue line). I don't think it would make too much of a difference ?

Click the image to open in full size.

As an amp of this voltage wouldn't be a candidate for a triode-mode switch, I suppose I can easily add/remove a battery when I solder in my resistor, etc

Look OK ?

Thanks guys,
Steve
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