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Old 27th July 2008, 11:17 AM   #31
Fuling is offline Fuling  Sweden
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The paint is dry, though it wasnīt when I put all 20 tube sockets in place so now itīs a little wrinkled around the bolt heads...
FYI: Wiring the heaters for 20 tubes is not the funniest part of a DIY project...
Havenīt heard from Lars for a couple of days.
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Old 28th July 2008, 09:22 PM   #32
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Sorry guys, have been in Arvika, Hotland. Back in town five minutes ago.

Will come back about spajs tomorrow.
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Old 29th July 2008, 02:03 PM   #33
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Was it something like this you had in mind? High sensitivity and local feedback to get Zout lower

The Tubecad idea seems like an extremely elaborate and also totally worthless solution when spiced. Drawing error?
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Old 29th July 2008, 02:29 PM   #34
Fuling is offline Fuling  Sweden
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Hi reVintage!

That is pretty much what I have in mind, but there is one thing that doesnīt match Tubecads schematic:
R13/14 makes a voltage divider together with R1/3 which seems to cut the feedback signal down to ~25% of what it should be.
Remove R13/14 and increase R1/3 to 370R and see what happens...?

Thank you for "spajsing" this circuit for me!
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Old 29th July 2008, 04:06 PM   #35
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This is how I would do it!

It was NOT ment to match Broskies schematic as it seems to be f-cked up! Must be a drawing error as I said in my previous post. Or also I could be the jerk, not to understand his strike of genius! Anyway I will give it another try tonight as I might have missed something. It could be as simple as that the gnd reference resistors are missing.

My circuit will not work changing R1/R3 as these sets the current through the 6N1Ps to 8mA. And you must have R13/14 as these are for GND reference.

I recommend using a low-mu tube like 6H30 to lower sensitivety.
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Old 29th July 2008, 04:42 PM   #36
Fuling is offline Fuling  Sweden
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Sorry, forget the part about increasing R1/3 to 370R. Tankevurpa
I still donīt see the point in R13/14 though, the output stage would still have ground reference through R1 and R3.
Of course Iīm talking about shunting R13/14, not removing them!

Regarding the high sensitivity Iīm going to use 6N6P (ĩ~20) in the input and driver stages.

The only weird thing I can see in Broskies circuit is the bias arrangement, I donīt understand the need for a -300V rail when the output tubes only needs something like -25V and cathode bias would be perfectly sufficient for the driver tubes.
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Old 29th July 2008, 05:08 PM   #37
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Sorry Fuling,

Thought it was 6N1P, will do another sim tonight with the right tube. As 6H30 and 6N6 are "cousins" they might work well.

About gnd reference I ment that it is missing in Broskies circuit.
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Old 29th July 2008, 05:59 PM   #38
Fuling is offline Fuling  Sweden
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I thought about using 6N1P at first but I prefer amps with low input sensitivity (and low feedback if possible).

Now I understand what you mean is wrong with the broskie circuit! He explains the need for ground reference in an earlier article about circlotrons so I guess he just left them out in this one to since the focus is on the feedback circuitry.
What I mean is that those reference resistors can do double duty as cathode (bias) resistors for the driver tubes.
The floating power supplies will then be raised a couple of volts above ground but as they are floating it shouldnīt matter.
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Old 29th July 2008, 06:28 PM   #39
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Have spent the last hour trying to sim Broskies circuit with no success.

Anyway, my circuit(below) seems to work. The sim indicates a Zout under 3ohm and a sensitivity of ca 0,4V rms. Thought output before clipping should be higher as this is indicated in TCJ PP calculator. I get 3-4W into 8ohm. With less feedback, higher R13/14, Zout gets higher. 8mA through all 6N6s, 110mA through each 6C19.
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Old 29th July 2008, 06:48 PM   #40
Fuling is offline Fuling  Sweden
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Thanks again reVintage! May I ask: What happens if you decrease R13/14 ( towards zero)? And what happens if the quescent current through each output tube is cut down to 50-60mA?
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