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Old 6th October 2012, 02:16 PM   #31
Alexsk is offline Alexsk  Canada
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just forget for a moment about signal, cathode capacitor etc.
make sure you have grounded g1, 66v on g2, g3 connected to cathode, 100-110k. anode R, 900-1k cathod R and aroung 400v B+
if you have right pins connection on c3g and tube is ok, there will be 2-2.5 ma anode and 0.4-0.6 ma g2. That brings cathode to 2.5-3v above ground and about 250v drop on the anode R.
BTW, this config (~2.3ma Ia c3m) of the 300b driver has around -2.5dB drop at 20 khz (amplifier output) and more distortions then "correct" 16ma Ia/ 150v g2, but to my ears sounds much better then any higher current c3m implementation and hugely better then 6SN7 driver.

Last edited by Alexsk; 6th October 2012 at 02:27 PM.
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Old 28th December 2012, 10:16 AM   #32
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Hi All
Now I have finished my first clone and damm I have the same problem :-(
325 volt before anode resistor (221K) and 6,4 volt on the anode after ???
The anode voltage should be ~135 volt.
I have 61,2 volt on g2
And 2,8 volt on the cathode

What is going on ???

Please advise
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Old 28th December 2012, 09:21 PM   #33
dinoc is offline dinoc  United States
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It doesn't make sense, only 1.44ma on the 221K resistor, what circuit do you use ??
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Old 29th December 2012, 08:47 AM   #34
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This is my circuit, hope it is readable ?

I think the referance voltage on G2 is to low 61,2V should be 66V
My anode voltage is way to low only 6,4 V should be ~140V
And the cathode voltage is a bit to high 2,78V should be 2,4 volt

What I wrong with my circuit ?
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Old 29th December 2012, 08:54 AM   #35
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A few pictures of the real life circuit
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File Type: jpg DSC02332.JPG (242.3 KB, 443 views)
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Old 29th December 2012, 02:31 PM   #36
kevinkr is offline kevinkr  United States
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Looks very neatly put together.. First step I suppose would be to pull the driver tube and measure the open circuit voltages and make sure they make sense.

Then I would double check that the tube pin out is correct in both your schematic and the build. I can't tell you how many times this has tripped me up.

Next measure the voltage across the cathode resistor with the tube installed and calculate the current.

Either measure the screen current directly or subtract the plate current from the cathode current.

What was the target plate current - something on the order of 0.5mA?

You may need to lower the screen voltage slightly and/or increase the effective grid bias by raising the cathode resistor value.

Might be better to do most of this experimentation external to the amp if you have a spare socket and some parts.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 29th December 2012, 10:21 PM   #37
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I have changed some resistor values, noticed that the SMD resistors in the original were stacked SMD (parallel) so I changed to 100K before the 2x10u to 50K and the 221K on the anode to 100K, this helped.
I now have 83 volts on the anode, and the input stage works.
But now I have a hum issue, annoying, I think I have been really careful with ground.
I have 1,5v 100HZ on B+ (450volt)
2,7 V 100Hz on the power after the MEC50 module, which is bad (I think) but I have tried B+ on the input stage, and no difference ?
I have 0,7mv AC on the 5V DC heater for 300BXLS
2,5mV on the 20V DC heater for C3m

Any advice on what my next step could be ???

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Old 29th December 2012, 11:46 PM   #38
dinoc is offline dinoc  United States
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I am glad to know you solve the problem, you have to do DC operate for the filament, its seems that it is the HV supply problem, do you mind sharing the whole circuit diagram and power supply with us ?

PS: how many current on anode right now after you have changed the resistor value?
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Old 30th December 2012, 10:25 AM   #39
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Hi I tweaked the resistors a little more, with better result.
I still have the hum problem, but the input stage is running with desired levels.

Here are schematics with measurements and a few pictures of the amp.

Ideas for fighting the hum problem are very welcome

Attached Images
File Type: jpg sch1.JPG (242.8 KB, 418 views)
File Type: jpg sch2.JPG (253.7 KB, 404 views)
File Type: jpg In3.JPG (299.0 KB, 245 views)
File Type: jpg In2.JPG (283.6 KB, 184 views)
File Type: jpg In1.JPG (231.2 KB, 142 views)
File Type: jpg Amp.JPG (330.6 KB, 173 views)
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Old 30th December 2012, 04:48 PM   #40
euro21 is offline euro21  Hungary
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1.) I think e-choke does not work perfectly.

461V (4.7V AC) -> e-choke -> 458V (4.3V AC).
4V difference too low, and PSRR only 0.77dB!

In my e-choke simulation the difference is about 28V (at 80mA), and PSRR is 42dB.

2.) Tentlabs wrote:
Mini Electronic Choke
"The MECs .... offer up to 60dB supply ripple reduction."
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