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Old 3rd August 2013, 02:25 AM   #1
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Default Hot Cathode Follower Resistor

I have two DC coupled cathode followers; one with 100k and the other with 47k off the cathode.

Both resistors are hot to the touch even when idling. Measurements show they are drawing less than half a watt. I changed one of them to 2W but it is too hot as well.

There is also fairly nasty hiss at high gain settings. The heaters are DC and regulated. Because of the poor power supply regulation, the voltage going in had to be raised to avoid dropout when the amp is pushing full power.

As a consequence, the rectifier and the regulator are running so hot, eggs could be fried on the heatsinks. In fact, the diode pack didn't even have a heatsink when I got it. I wonder if thermal noise is being generated there.

Otherwise, the amp works and sounds perfect. I know the regulator situation has to be remedied, and it will. But why are those resistors so hot?

Any ideas much appreciated.
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Old 3rd August 2013, 05:21 AM   #2
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Schematic please....
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Old 3rd August 2013, 05:26 AM   #3
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Thanks.Pre 2 .JPG

They both have these, too.

DCCF3.jpg

The V4 has the 47k. C = 350V
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Old 3rd August 2013, 06:28 AM   #4
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How did you measure the dissipation? Not clear to me which resistors you talk about; when you say 'off the cathode' do you mean the cathode resistor or some load resistor?
I see in one schematic a 100k cathode resistor with 212V across it - that would be less than 0.5W.
A 47k with 350V across it would dissipate some 2.6W and that one would definitely be hot.

What is the total current the amp draws from the supply? Did you measure that?

In the upper schematic, V3a output seems heavily attenuated and that could account for the high noise. But the circuit doesn't seem to be right - why the high attenuation?

jan
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Old 3rd August 2013, 07:19 AM   #5
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Thanks for answering. This is very bewildering.

C = 340

PS DC Amps = 8mA Two other 12AX7 dual triode gain stages are also being powered.

V across R25 100k on V3A is 167V = 0.28W That one is not noticably warm with 2W metal film resistor.

V4A Divider.JPG

V across the 15k on V4B is 43v = 0.12W Also not warm with 3W resistor.

V across the 47k on V4B is 133V = 0.37W Too hot with the same 2W metal film resistor as R25.

The attenuation on V3 output is to bring the output to pro equipment level which I understand is about 2V. Tapping further down, or reducing the 2.2k to ~ 200R will bring it closer to instrument level (~100mV) at high gains and medium inputs.

That resistor is currently a 0.5W carbon pot.
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Old 3rd August 2013, 08:05 AM   #6
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Quote:
Originally Posted by ITPhoenix View Post
Thanks for answering. This is very bewildering.

C = 340

PS DC Amps = 8mA Two other 12AX7 dual triode gain stages are also being powered.

V across R25 100k on V3A is 167V = 0.28W That one is not noticably warm with 2W metal film resistor.

Attachment 363933

V across the 15k on V4B is 43v = 0.12W Also not warm with 3W resistor.

V across the 47k on V4B is 133V = 0.37W Too hot with the same 2W metal film resistor as R25.

The attenuation on V3 output is to bring the output to pro equipment level which I understand is about 2V. Tapping further down, or reducing the 2.2k to ~ 200R will bring it closer to instrument level (~100mV) at high gains and medium inputs.

That resistor is currently a 0.5W carbon pot.
Maybe your idea of 'too hot' is too restrictive. If you can barely keep your finger on it, that will be about 45 degrees Celcius which is not hot at all for a resistor.
Are you sure the resistor value is correct? It's easy to make a mistake with one of the color rings and be a factor of 10 off.

Still puzzled about that 1st stage. The attenuation on V3a appears some 45 times, that's a waste and may be the cause of the noise.
You should chuck the gain stage and use just the cathode follower, much better!

Edit: taking away C11 on V3b will give better performance and lower gain, that will help already a lot!

jan
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Old 3rd August 2013, 09:09 AM   #7
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Quote:
Originally Posted by jan.didden View Post
Are you sure the resistor value is correct? It's easy to make a mistake with one of the color rings and be a factor of 10 off.
It has no bands, but it's the size of a 1/4W. I think Mouser made a mistake. One time I got a bag with no resistors at all!

Quote:
Originally Posted by jan.didden View Post
Still puzzled about that 1st stage. The attenuation on V3a appears some 45 times, that's a waste and may be the cause of the noise.
The level has to come down somehow. I don't know how else to do it. Perhaps the total resistance could come down, then readjust the divider. If 100k is faithful, and 47k clips half the waveform, then I guess ~85k would do no violence

Quote:
Originally Posted by jan.didden View Post
You should chuck the gain stage and use just the cathode follower, much better!
Can't do that, that stage colors the sound and would upset the chain. Better to leave well enough alone.

Quote:
Originally Posted by jan.didden View Post
Edit: taking away C11 on V3b will give better performance and lower gain, that will help already a lot!
I will try that.

Thank you so much. Back to rubbernecking....
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Old 3rd August 2013, 09:54 AM   #8
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Quote:
Originally Posted by ITPhoenix View Post
It has no bands, but it's the size of a 1/4W. I think Mouser made a mistake.
Well how do you know it is the right value then?? Has it text?

jan
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Old 3rd August 2013, 10:20 AM   #9
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It says 47k and measures the same. I looked up the code in Vishay Dale and it says it's 1W.

Ok so it's not 2W, but it's still more than twice the rating necessary, yet too hot.
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Old 3rd August 2013, 10:20 AM   #10
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Quote:
Originally Posted by ITPhoenix View Post


The level has to come down somehow. I don't know how else to do it.
Either don't amplify (as Jan suggested, remove the gain element) or put a volume control at the input. Your scheme increases noise and output impedance.

Other thing to note is that cathode followers can oscillate easily- they're operating under 100% feedback. This is a common source of noise; you can't hear the RF, but you hear the effects. You want local bypassing of the plate, and grid stoppers placed physically as close to the grid pins as possible.
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