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Old 13th May 2005, 11:18 AM   #1
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Default changing longtail

Hi!

At the moment i'm playing music with this amp...
Pre-amp is dc-filament supply.
The psu is tubed (4xpy500a) 40uf-10h-300uf-1k5-82uf
B+ comes from the 300uf (377v)
B++ comes from the 82uf (356v)

As you can see it's been changed...it started with an ecc99 as splitter..now it's a longtail with ccs..it's the only tube-amp i could afford and it's changin almost every motnh (a little bit


The 100 ohm resistors on g2 have been removed (they made the sound sound compressed, now it''s more 'open' )

Well to be honest i'm at the beginning of working with tubes and learning but would like to change the splitter once again...

You'll find it in the next post, sorry for the paint drawing....
Will this work?? will it be better??? it looks like it does, because now there are 2 'mirrors' one for voltage (equal anode resistors) and at the kathode by the second bc546b...

I heard the 2nd bc546b could be replaced by a led! is this true, this would be nice to mount on the chassis, next to the tube base as a sort of 'operating indicator'- are there benfits in doing that???

And....
The 5965 is said in this configuration to need 2v pk-pk for full-output...line=0,707v so... do i need to build a pre-amp??? (still got the ecc99's!)

For replies....please go a little slower than usual, i will understand eventually... but a little explanation why would enable me to learn of this....thnx
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Old 13th May 2005, 11:23 AM   #2
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the 'new' one...
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Old 13th May 2005, 12:59 PM   #3
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If you'll turn that auxillary supply into a negative one, you'd do well to use a cascoded pair of bipoalrs as the CS. A pair of red LEDs forward biased make a fine 3.4V reference for the CS- it's advantageous to use two of them in series, since it allows you to raise the value of the emitter resistor. That increases the output resistance of the CS, though past a certain point, you'll hit diminishing returns.

Why do you use that 100pF cap? It compromises the CS output resistance and reduces CMR and balance at very high frequencies.
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Old 13th May 2005, 01:17 PM   #4
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I expect without that 100pF capacitor, the 317 oscillated at being called upon to deliver such a low current. Whip the 317 out and use it as a 12V regulator (just ground the positive rail of the supply instead of the negative), and make a bipolar CCS.
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Old 13th May 2005, 01:36 PM   #5
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@sy The aux psu is already negative.... the second post is what it has to become.....it has a positive psu...

But are you telling me to use the leds as emittor resistor??, so they are NOT used as replacement of the bc546b?? why a led and not a normal resistor??

@ecc8010: so that's where the 100pf cap is for!! i wondered what it did...

i'm sorry for being slow.... but you are suggesting the 317 as 12v regulator in the new schematic...then i'll have to change the 1k resistor...(to get the 5v i need) and 2 100nf for (in-out) of the 317...


But the question remains, is the second schematic OK????
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Old 13th May 2005, 01:50 PM   #6
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My question about the 100pF cap referred to the second schematic. I don't see what it accomplishes.

The paired LEDs I referred to are used to generate a reference voltage for the CS. So they are in the base circuit, which forces a constant current through the emitter resistor of 3.4/Re. The higher Re, the higher the output resistance at the collector. If I get to a scanner in the next little while, I'll draw you a picture.
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Old 13th May 2005, 02:35 PM   #7
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and the higher the output resistance at the collector the more lineair the tube will behave....

But that's just a guess of what i think you mean, i'm looking forward to see the schematic !

So, the 100p can be removed (i just left it in because it was in the schematic of the 317, thought maybe ss devices needed that.....)
Then i'll get rid of that!
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Old 13th May 2005, 02:47 PM   #8
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Second diagram looks OK at a quick glance (I'm on my way out). Yes, bung the 317 into the second diagram.
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Old 13th May 2005, 03:12 PM   #9
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I've attached a quick sketch. If we assume a -15V rail, 10mA current for the CCS, and 5mA current for the LEDs, then we can quickly calculate resistor values:

Re = (3.4 - 0.7)/10 = 0.27k = 270 ohm

R1 + R2 = (15 -3.4)/ 5 = 2.32k

The last is not terribly critical. We can arbitrarily split the resistances and make R1 equal to 1.2K, R2 to 1K.

The transistors can be just about any small signal high beta type.
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Old 13th May 2005, 04:43 PM   #10
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Thanx... I'll try this one first, because i now still have a negative supply...i see a voltage divider...and the transistors are now in series.... while the leds gives the lower one a bias voltage......

.....this is called a cascode cs??(i'll look that up
I can use it with b++ of 356 (voltage anode 150) and the same 5965 tubes???
I ask just to be sure before i build it and it blows up

very interesting and bount to keep me busy for a while before i figure out how it exactly works....
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