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-   -   High Gain Tube Preamp Hiss (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss.html)

gingertube 6th August 2013 11:40 PM

Just read thru' the last page or so and guess I should correct mistakes.

Nigel, with respect, the EF86 thing is wrong. Pentode input stage will always be more noisy than triode. This is because pentodes suffer from partition noise.

Lowest noise input stage would be a cascode, either 2 triodes or even better (noise wise) a J-Fet Triode cascode.

Cheers,
Ian

ITPhoenix 7th August 2013 12:13 AM

Quote:

Originally Posted by Osvaldo de Banfield (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3586860.html#post3586860)
Did the amplifier anytime worked of properly, or it is a new one?

It is a roll-your-own, hybrid, high gain lead guitar amplifier. Tube preamp, solid state power amp.

It always worked perfectly, I just happened to notice the hiss when a 10" speaker was hooked up to it. I had been working with a cheap 5" midrange car speaker that has a 6k rolloff. High frequencies come out of it but are significantly attenuated; that's why I didn't address the hiss before.

Thank you for your concern.

ITPhoenix 7th August 2013 12:16 AM

Quote:

Originally Posted by gingertube (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3587427.html#post3587427)
...Lowest noise input stage would be a cascode, either 2 triodes or even better (noise wise) a J-Fet Triode cascode.

A J-Fet Triode/Cascode. That sounds interesting. It could be done with a dual opamp with J-Fet inputs?

One Cascode explanation is here: The Valve Wizard

Aiken on the topic:

"The first stage of an amplifier is the most critical; in order to maximize the overall amplifier signal-to-noise ratio, the first stage gain should be maximized. This will raise the signal level farther above the noise floor of following stages. Triodes can be paralleled to increase the signal-to-noise ratio. This is because the noise sources of the two triodes are uncorrelated and add as the square root of the sum of the squares of the individual noise sources, while the signal is correlated and adds directly, giving a 3dB improvement in signal-to-noise ratio. Pentode input stages should be avoided if possible, because they suffer from another type of noise, division noise, caused by the insertion of the screen grid element into the electron path between the cathode and the plate.

"If very high gain is needed, the self-biased cascode stage is the preferred method, as it has the same or higher gain than a pentode, but no extra noise. In addition, the cascode connection does not suffer from the high microphonics of the most common pentode, the EF86. The self-biased cascode approach is better than the fixed biasing of the upper tube grid because it eliminates the direct influence of the DC supply on the upper tube grid, and has a warmer tone."

Osvaldo de Banfield 7th August 2013 12:43 PM

Quote:

Originally Posted by ITPhoenix (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3587449.html#post3587449)
It is a roll-your-own, hybrid, high gain lead guitar amplifier. Tube preamp, solid state power amp.

It always worked perfectly, I just happened to notice the hiss when a 10" speaker was hooked up to it. I had been working with a cheap 5" midrange car speaker that has a 6k rolloff. High frequencies come out of it but are significantly attenuated; that's why I didn't address the hiss before.

Thank you for your concern.

My post pointed to know if there has been a failure in a equipment after a period of time that it was OK, or if it is a new one that never was working OK, then the search in the failure is different.

ITPhoenix 7th August 2013 01:00 PM

Quote:

Originally Posted by Osvaldo de Banfield (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3587989.html#post3587989)
My post pointed to know if there has been a failure in a equipment after a period of time that it was OK, or if it is a new one that never was working OK, then the search in the failure is different.

Your point is a good one, and I wish I had a definitive answer to that. There has been much moving of wires and components. There has been little scientific control during development. Then again, perhaps I am asking for too much, as was implied before.

It appears at the moment, the hiss is not coming from outside radiation. It sounds like white-noise, which also sounds somewhat like a noisy resistor.

So perhaps it's the tubes themselves to some extent and also some RFI getting into the machine from the power line. Right now, capacitors in several locations attenuate the noise to some degree, besides decreasing the value of the grid leak resistors. One valve doesn't even have one.

More experimentation is needed when the parts come in.

Thank you.

Osvaldo de Banfield 7th August 2013 01:06 PM

One idea: let the input to the amplifier with a .1 400V cap ore more. Short both grounds together. The, from the input of the amplifier, put the cap attached to the input of the FA across the signal path in the pre, and locate where it starts to hiss. Then, you can work around this stage only. Thy with and without input signal to the preamplifier.

ITPhoenix 7th August 2013 01:18 PM

Quote:

Originally Posted by Osvaldo de Banfield (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3588009.html#post3588009)
One idea: let the input to the amplifier with a .1 400V cap ore more. Short both grounds together. The, from the input of the amplifier, put the cap attached to the input of the FA across the signal path in the pre, and locate where it starts to hiss. Then, you can work around this stage only. Thy with and without input signal to the preamplifier.

Could you provide a wiring diagram?

Osvaldo de Banfield 7th August 2013 01:22 PM

Yes, but it will be very complex. Travel with the cap attached to the input of an amplifier decoupled with a cap as I mentioned, though the signal path. In someone of them, the hiss can be heard in the amp. Then, this and the following stages may be the defective ones.

Nigel Goodwin 7th August 2013 03:31 PM

Quote:

Originally Posted by gingertube (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3587427.html#post3587427)
Nigel, with respect, the EF86 thing is wrong. Pentode input stage will always be more noisy than triode. This is because pentodes suffer from partition noise.

The EF86 was specially designed as a low noise preamp, specifically for tape and mike front-ends, as triodes were too noisy.

So are you specifically mentioning just other pentodes, or have you tried the EF86?.

Perhaps you've noticed that almost all tape recorders (from the B9A era) use them in the front end?.


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