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Old 18th March 2013, 05:28 AM   #1
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Default D3a-300b amp help!

Friends I need help, I'm already a few days ago with this amplifier can not find the problem..
The problem is with the D3a tube, not give me enough amplification, with 1V peak-to-peak (1 kHz) at input give me 2.5V to the output.
The anode voltage is 190V (10k resistor load) and the cathode is 2.1 V with a resistance of 104ohms (20mA aprox). Filament 6.6V.
In one tube G2 is connected to the anode and g3 to ground and on the other channel g3 to cathode and g2 to anode, in both channel I have the same result.
The tubes test good in my Eico 667 tube tester (95%)

Any idea?

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Old 18th March 2013, 07:40 AM   #2
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Hi!

The output voltage you get from a specific input signal depends on the step down ratio of your output transformer. Since you did not specify that, it is difficult to tell whats going on.

Measure the gain of the D3a stage alone and then go from there.

Best regards

Thomas
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Old 18th March 2013, 07:47 AM   #3
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1.You can connect baypass cap about 500-1000 uFto every cathode resistor. 2.Try to connect in pentode mode /g2 conn. to res. about 50 kohm to B+, g2 conn. to electr. cap about 50 uF to ground/.....and look.

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Old 18th March 2013, 08:09 AM   #4
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Hi Azazello,

when looking at the photos, there seem to by bypass caps at the cathodes.
Before changing the configuration of the tube, I would strongly suggest to first check if it is working correctly. He then still can change it if the gain is not sufficient

Best regards

Thomas
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Old 18th March 2013, 09:18 AM   #5
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Have you measured that the 1V signal is also at the grid of D3a not only at the input connector ?

You have some fault in the circuit. Try to find that.
Do not change to other topology (pentode), because this is not the solution.
Can you post the circuit diagram ?
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Old 18th March 2013, 09:35 AM   #6
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Thank you all for reply!

Sorry for be unclear, the measurement is in open circuit, I put a 100k resistor to gnd for measure the output voltage (anode-cap-resistor to gnd). Please see the picture.

Click the image to open in full size.

The cathode is bypass with 220uf cap, maybe low value?

The voltage signal (1V) is measured in the 1k grid stopper resistor.


Now, time to sleep for me, more to come..
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Old 18th March 2013, 09:46 AM   #7
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Hi.
You should post the schematic.
You should also put a stopper resistor between G2 and the anode.
There is a problem somewhere, you can't get only 2.5 gain - I get 70 with a CCS loaded D3a...

And dont't you feel sorry for your expensive NOS D3a to kill them slowly with the 6.6v heater?
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Old 18th March 2013, 09:48 AM   #8
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Hi!

So you are measuring 1V in and 2.5V out of the driver? Then there is definitely something wrong. Where did you measure the output voltage?

Best regards

Thomas
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Old 18th March 2013, 10:12 AM   #9
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Hi! These two yellow caps in the first picture, right above the 10k resistor. It is my impression they are connected to the 'wrong' side of the plate resistors. They are probably meant for decoupling of DC supply, but it looks as if they have been connected to the anode side of the 10k resistor. This could explain why you are not getting gain. Still, the picture is not very clear.

Other things: are you sure the 100k resistor after cap is indeed 100k (and not 100R)?

These may sound silly suggestions, but I made/make these sort of errors myself as well.

General observations regarding layout. The D3a as high transconductance valve is said to oscillate at the minimum chance. I saw you used grid stoppers, which is good, but I don't know if placing the plate resistors so far and connecting them with wires to the plates is a good practice. And as Vincent said, some care around G2 as well. These are probably not the reasons for getting low gain, but may be reasons for future problems, especially oscillation.

Best regards, Erik
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Old 18th March 2013, 12:28 PM   #10
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Default Problem Solved!!

Thanks to all for reply!

Erik you have good eye!!!

The whole problem is because the wrong conection of these yellow caps

Now is time to improve the layout and put again the g2 grid stopper.

Thank again!

Alberto.
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