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Can someone give some info on this tube circuit please?
Can someone give some info on this tube circuit please?
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Old 18th September 2018, 01:58 AM   #21
TonyTecson is offline TonyTecson  Philippines
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Can someone give some info on this tube circuit please?
i use tubes with mu in the 20's range, so that you do not need global feedback to trim gain...

the marantz 7 and the sp3 line stages used high mu tubes and then used global negative feedback to trim gains to line levels, still has many following till today..
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Old 18th September 2018, 07:27 AM   #22
georgehifi is offline georgehifi  Australia
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Quote:
Originally Posted by rayma View Post
His circuit uses higher B+ (200V vs 140V). Try 20k first, that should work. The plate current will increase by the reciprocal factor, as the grid voltage is constant. Is the power supply just passive dropping resistors? One or more of those may have to be decreased.

The input section has low plate current, another limitation of this circuit.
Here's the whole thing.
Thanks.
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Old 18th September 2018, 07:29 AM   #23
TonyTecson is offline TonyTecson  Philippines
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Can someone give some info on this tube circuit please?
why not build it as per schematics and learn from it later?
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Old 18th September 2018, 07:32 AM   #24
georgehifi is offline georgehifi  Australia
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I plan on doing that but I thought if there is something that could be easily bettered then now is the time before the board is stuffed and mounted. (especially output drive to <68kohm loads).

Just posted the full schematic with ps above.

Cheers George
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Old 18th September 2018, 08:07 AM   #25
TonyTecson is offline TonyTecson  Philippines
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Can someone give some info on this tube circuit please?
how will you know better when you have never seen the start...?

you have to start somewhere and decide for yourself if what you heard is good or not good to YOUR EARS......not our ears as we are not listening to your finished amp.....

if you assume from the start that such a circuit can be better, then you do not learn anything...you have no reference, just the say so of others...

a cathode follower in that amp has low enough output resistance, 68k loads will be peanuts...

a 12au7 cathode current at around 2mA, gives transconductance of about 322 ohms if my calculations are correct...others can correct me.....http://www.mif.pg.gda.pl/homepages/f...3/1/12AU7A.pdf
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Old 18th September 2018, 10:06 AM   #26
georgehifi is offline georgehifi  Australia
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"how will you know better when you have never seen the start...?"

I hoping tube gurus may see obvious problems that are measurable rather than just relying on listening.
I'm one that like the specs/measurements right first in solid state before drawing subjective conclusions, after all that's how things are designed in the first place by the ones in the know, get the specs/maths and measurements right first then listen. Not the opposite.

" 68k loads will be peanuts..."
Your saying now 68kohms is peanuts, other say it's not great, who am I to believe now? I 'll wait for more info from others to see who backs who as you have just asked "others can correct me."

Cheers George
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Old 18th September 2018, 10:49 AM   #27
DF96 is offline DF96  England
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Quote:
Originally Posted by georgehifi
So even though it's a couple hundred ohms output impedance, the 33kohm cathode resistor is the controlling factor of what it can and can't drive without any impediments.
Yes and no. The cathode resistor in the CF sets the CF current, which affects the load driving ability. Unfortunately it also affects the distortion. You need to decide what balance you want to strike: low distortion into a high impedance load (high cathode resistor) or not too high distortion into a lower impedance load (low cathode resistor).

As I said, this circuit is hampered by low supply rail voltage and poor valve choice. You can fiddle with component values to pick a different compromise from that chosen by the designer, but if you want better performance you need to address the real problems.

Another possible problem is noise. If this circuit is used in a high gain setup, so the volume control stays at the low end, then thermal noise from the 100k input resistor (about 6uV) might become an issue. If used with high volume control settings then it should be OK.
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Old 18th September 2018, 11:24 AM   #28
martyh is offline martyh  United States
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Quote:
Originally Posted by 6A3sUMMER View Post
martyh,

I am surprised by you suggestion that for the two circuits, post #1 and post #4
decreasing the load resistance will null the distortion....
I was surprised myself when I stumbled upon it. Iím really not the guy to explain why as I had ask what was going on here. Here is a link to the original thread.

unexpected harmonic cancellation
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Old 18th September 2018, 11:55 AM   #29
TonyTecson is offline TonyTecson  Philippines
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Can someone give some info on this tube circuit please?
even with op amps, driving 68k load is peanuts when your opamp output impedance is 600 phms..

the 33k at the cathode of the cathode follower is there just to bias the tube or set its operating point, as DF96 pointed out, the output impedance of a cathode follower is its cathode internal resistance, in this case 1/transconductace, i estimated it at 322 ohms which i arrived at by dirty calculations, this is what i mean when i said i could be wrong....but cathode followers have such low output impedance.....
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Old 18th September 2018, 12:00 PM   #30
TonyTecson is offline TonyTecson  Philippines
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Can someone give some info on this tube circuit please?
Broskie's CCDA works on the premise that the common cathode plate load is the same as the cathode follower resistor and that the plate voltage of the common cathode gain stage is at 1/2 the B+....the reason being the net ac current drawn from the psu cancelled...

so the 100k plate load resistor for the first stage is indeed old fashioned...
make them the same 33k or even 22k and adjust the cathode resistor of the first stage so that you get half the B+ on the plate...

tubes are fun to work with.....
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