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

ITPhoenix 5th August 2013 07:51 AM

Quote:

Originally Posted by gingertube (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3585171.html#post3585171)
The OD Gain Pot is VR1? The last 10 degrees of a log pot is at least 1/2 the total resistance. Change the 500K pot for a 220K or even 100K and at the same time you can reduce that 470K grid stop as we are no long overdriving that next stage so badly. Drop that to 47K or even 22K.
Both these things also will help to shunt more grid noise from that next stage because the resistance to 0V looking away from that grid is now much less.

Thed Input Stage is of course the most critical. Yopu want to run it at reasonably high current (say in the 0.7 to 1.0 mA range) whilst keeping grid current low (so that noise from grid current fluctuations is also low). That means keeping the B+ to the first stage high. You want at least +200V on the anode of that first stage, a bit higher is even better. The B+ decoupling resistor for this stage does not have to be right at the end of the chain, you can connect it back to the start of the chain if you like (so it is in parallel with the rest of the chain) in order to keep B+ high.

Yes, VR1 is the OD gain. If you look at the schematic above, OD runs right across the bottom. So V1B is the first input stage, V2 is two overdrive stages and then out to V3B which is followed by the DCCCF.

The normal channel starts with V1B then goes to V1A (around V2), then to the next stage V3B just like the OD. Note that LDR1 is essentially a dead short when in Normal (~200R). This stops the signal from going anywhere.

The B+ is already parallel feed for V1 and V2 and designed for 185v at V1. They both are taken from C which is a little higher. They both use 15k and 10u F&Ts.

Well, I spent a month finding the nastiest oscillation. It was due to a ground loop that made every little imperfection 100x worse than it would have been, but I learned much from the experience. Everything is critical in high gain. Also, I have read it's better to distort the waveform gradually among many stages. In fact, the Carvin Lead channel does just that!

What's another month or two for this problem?

Nigel Goodwin 5th August 2013 07:52 AM

High gain tends to give you hiss, because it's amplifying the noise as well as the signal.

A valve specifically designed for low noise was the EF86, which was a low noise pentode, intended for the front end of tape recorders and mike preamps.

Noisy triodes are usually used in guitar amps because hiss and noise isn't really a problem, if you want high quality use solid state instead of valves.

ITPhoenix 5th August 2013 12:01 PM

Thank you Nigel for your interest and insight.

Quote:

Originally Posted by Nigel Goodwin (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3585223.html#post3585223)
High gain tends to give you hiss, because it's amplifying the noise as well as the signal.

A valve specifically designed for low noise was the EF86, which was a low noise pentode, intended for the front end of tape recorders and mike preamps.

Noisy triodes are usually used in guitar amps because hiss and noise isn't really a problem, if you want high quality use solid state instead of valves.

For now, solid state for the pre is out of the question. I will get it as quiet as possible and have to settle for that. I found the "tone killer" caps' ill-effects can be remedied with tweeters. A large horn or two would be needed in conjuction with 4-12s, I believe. I used a 5" and a 2" bullet and it was ok. Basically, the combo restored what was lost. I had L-pads and adjusted to hear each individually, until one of them burnt up trying to find the hiss at 100% output. The one literally burned the cabinet wood.

I was wondering what that stink was. Also the power amp was cutting out.....

Nigel Goodwin 5th August 2013 03:49 PM

Quote:

Originally Posted by ITPhoenix (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3585462.html#post3585462)
For now, solid state for the pre is out of the question. I will get it as quiet as possible and have to settle for that. I found the "tone killer" caps' ill-effects can be remedied with tweeters.

I don't quite see the reasoning in cutting the treble, then boosting it at the tweeters?.

ITPhoenix 5th August 2013 04:45 PM

Quote:

Originally Posted by Nigel Goodwin (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3585675.html#post3585675)
I don't quite see the reasoning in cutting the treble, then boosting it at the tweeters?.

It is a less-than-optimum way of attenuating the noise without losing the character of the amp. To "fix" the problem, we are taking the upper edge off the tone (all tones in fact), then restoring the harmonics with a more responsive driver in that range. So much for those who believe tweeters cannot be used in guitar amps. I believe there are some cabinets that utilize them. Maybe more for acoustic guitars. Isn't that one big reason why people buy Martins--for the harmonics?

I have found that, in life in general, it's always better to treat the cause rather than the symptoms. This is why I said it would be better to stop whatever it is, whether noise or RFI, at the source.

In the medical industry, it's common practice to treat the symptoms and not the cause. That's one reason why most everyone is sick in the USA.

So I started at the beginning of the chain. I know it's not the instrument, since everything is the same whether plugged in or not. I had reduced R2 (series input resistor to V1B) to 60k. Just a minute ago I paralleled it with a 10k and the hiss was greatly reduced. But the long leads picked up 60 cycle noise and some RFI (everything was open for this test).

There is an RFI cap across grid and anode which would have to be adjusted from 120p to 680p with a 10k. Or maybe eliminate it altogether like the Carvin. However, their scheme parallels a very low value grid leak resistor with the guitar pickups, imposing a considerable load, especially on high gain versions.

Reducing the value of the grid leak there did nothing.

This is where I learned that resistor can be reduced from the traditional 68k value: The Valve Wizard

Thank you very much for your comments.

Osvaldo de Banfield 5th August 2013 05:29 PM

If you have access to an oscilloscope, try if any of the amplifier's stage is self oscillating at an inaudible frequency, it also can give this sound, and also can disappear adding capacitor sin some points at the amp.

Good luck!

ITPhoenix 5th August 2013 07:35 PM

Quote:

Originally Posted by Osvaldo de Banfield (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3585785.html#post3585785)
If you have access to an oscilloscope, try if any of the amplifier's stage is self oscillating at an inaudible frequency, it also can give this sound, and also can disappear adding capacitor sin some points at the amp.

Good luck!

Thanks for that. I really wanted to pinpoint where the areas were, but the only scope I have is Visual Analyser. We can kill the noise with caps, but the tone suffers considerably.

I still don't know if it's Johnson noise, RFI, or ultasonic oscillation. I had the idea of using clamp on RFI filters, but they only attenuate a few hundred ohms. They might be helpful in isolating the area. When I had ground-loop-caused oscillation, I split a ferrite with a Dremel and glued the pieces to tweezers. When closed around one of the ground wires, the oscillation changed its tone.

I just tried reducing R2 to 10k and left the 1M grid leak alone, which did nothing. The Carvin setup made a radio receiver.

If a cap is added to either, grid to cathode, anode to cathode, or if the anode resistor is bypassed with a cap, all result in hiss attenuation.

I am tempted to try those pentodes, at least for V1 and V2 but it sounds like a can of worms for someone like me. Besides, I heard the new productions are no good, and I'm not spending $200 on any valve.

I found a lesson on local feedback for preamp valves. I will study that more. That was mentioned above, but I wasn't sure if he meant on the tube output section which I do not have.

I am exhausted.

Nigel Goodwin 5th August 2013 08:18 PM

Quote:

Originally Posted by ITPhoenix (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3585909.html#post3585909)
Thanks for that. I really wanted to pinpoint where the areas were, but the only scope I have is Visual Analyser. We can kill the noise with caps, but the tone suffers considerably.

I still don't know if it's Johnson noise, RFI, or ultasonic oscillation. I had the idea of using clamp on RFI filters, but they only attenuate a few hundred ohms. They might be helpful in isolating the area. When I had ground-loop-caused oscillation, I split a ferrite with a Dremel and glued the pieces to tweezers. When closed around one of the ground wires, the oscillation changed its tone.

I just tried reducing R2 to 10k and left the 1M grid leak alone, which did nothing. The Carvin setup made a radio receiver.

If a cap is added to either, grid to cathode, anode to cathode, or if the anode resistor is bypassed with a cap, all result in hiss attenuation.

I am tempted to try those pentodes, at least for V1 and V2 but it sounds like a can of worms for someone like me. Besides, I heard the new productions are no good, and I'm not spending $200 on any valve.

I found a lesson on local feedback for preamp valves. I will study that more. That was mentioned above, but I wasn't sure if he meant on the tube output section which I do not have.

If you unplug V1 does the hiss disappear? - it's usual to only use a low noise pentode for the first stage.

Feedback reduces gain, and increases quality - presumably two things you don't want to do?.

However, removing the capacitors on the cathodes will give you local negative feedback, the capacitors also seem rather small? (although I haven't bothered doing the calculations), which would give higher gain at higher frequencies and make the hiss worse.

So try disconnecting the cathode capacitors one at a time and see what effect that has.

tinitus 5th August 2013 08:24 PM

Quote:

Originally Posted by ITPhoenix (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3584911.html#post3584911)
Only problem is that at max gain and max volume.....

is that a normal situation :confused:

Nigel Goodwin 5th August 2013 08:31 PM

Quote:

Originally Posted by tinitus (https://www.diyaudio.com/forums/instruments-and-amps/240300-gain-tube-preamp-hiss-post3585969.html#post3585969)
is that a normal situation :confused:

You don't play heavy metal do you? :rolleyes:


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