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ITPhoenix 4th August 2013 10:45 PM

High Gain Tube Preamp Hiss
 
2 Attachment(s)
Hello everybody. I'm a "tube noob" and having trouble with this first amp. It sounds great, and all the controls respond as they should.

Only problem is that at max gain and max volume, there is nasty hiss from the speaker, more so on the OD channel.

Experimenting with caps, it has been found that placing the following across the specified terminals (grid to cathode), the hiss is reduced considerably, but the upper edge of the tone is compromised, which is undesirable.

Affecting both channels:
V3 7-8 470p
V4 7-8 270p

Norm:
V1 2-3 270p

OD:
V2 2-3 270p

The tubes are all 6N2P-EV
Attachment 364204
Attachment 364205

Any ideas or pointers much appreciated.

JonSnell Electronic 4th August 2013 10:58 PM

If you use a low value between the grid (not gate) and cathode, usually that means the stage before is producing the hiss. If there is no stage before, use a quieter valve or negative feedback and more amplification later.

JonSnell Electronic 4th August 2013 11:02 PM

I do also notice that there is a lot of HF gain ... the cathode bias capacitors are very low value. Normally use 22 to 47uF.

ITPhoenix 4th August 2013 11:23 PM

Quote:

Originally Posted by JonSnell Electronic (Post 3584924)
If you use a low value between the grid (not gate) and cathode, usually that means the stage before is producing the hiss. If there is no stage before, use a quieter valve or negative feedback and more amplification later.

Thanks. What do you mean by "negative feedback"?

Any recommendations on quieter valves?

ITPhoenix 4th August 2013 11:28 PM

Quote:

Originally Posted by JonSnell Electronic (Post 3584927)
I do also notice that there is a lot of HF gain ... the cathode bias capacitors are very low value. Normally use 22 to 47uF.

Marshall uses 0.68u. It's worth an experiment, though.

I also have regulated DC heater supply, which is currently not using fast recovery diodes, but they are bypassed with 0.1u. Moreover, because voltage regulation at full power is so poor, I had to increase the dropout margin considerably. This is causing the rectifier bridge and the regulator to run rather hot. I was wondering if this thermal noise can make it into the signal path.

Thank you for this idea, too.

gingertube 5th August 2013 03:30 AM

Hiss generally comes from 2 sources, resistor thermal noise particularly resistors on the grid (grid stops and grid leaks). This is proportional to the size of the resistor.

Random variations in grid current, this is best handled by shunting to 0V by use of low value resistance from grid to 0V (looking away from the grid back toward the preceeding stage). Again, low value grid stop and grid leak is your friend.

This is not a good design with respect to noise. The interstage attenuators are not agressive enough and the resistor values are too high. Also some of the grid stop resistors are too high, probably in an attempt to tame high frequency response ( they form a Low Pass Filter with the Miller Capacitance).

A better way is to use lower grid stop and grid leak to shunt grid noise to 0V and to handle the high frequency roll off by adding a cap from anode to 0V. Adding these HF roll off caps, particularly in the early stages (where there is a lot of gain down stream) should help.

If looking for a highish gain design that was done reasonably well (with respect to low noise design), have a look at the Carvin Legacy Lead Channel (just one example)

Cheers,
Ian

ITPhoenix 5th August 2013 04:39 AM

Thank you very much for you input, Ian.
Quote:

Originally Posted by gingertube (Post 3585071)
Hiss generally comes from 2 sources, resistor thermal noise particularly resistors on the grid (grid stops and grid leaks). This is proportional to the size of the resistor.

All the resistors are over-size, overkill.

Quote:

Originally Posted by gingertube (Post 3585071)
Random variations in grid current, this is best handled by shunting to 0V by use of low value resistance from grid to 0V (looking away from the grid back toward the preceeding stage). Again, low value grid stop and grid leak is your friend.

This is not a good design with respect to noise. The interstage attenuators are not agressive enough and the resistor values are too high. Also some of the grid stop resistors are too high, probably in an attempt to tame high frequency response ( they form a Low Pass Filter with the Miller Capacitance).

A better way is to use lower grid stop and grid leak to shunt grid noise to 0V and to handle the high frequency roll off by adding a cap from anode to 0V. Adding these HF roll off caps, particularly in the early stages (where there is a lot of gain down stream) should help.

How low?

Quote:

Originally Posted by gingertube (Post 3585071)
If looking for a highish gain design that was done reasonably well (with respect to low noise design), have a look at the Carvin Legacy Lead Channel (just one example)

Cannot find schematic.

Ok, assuming it's not resistor noise, then it must be either tube noise, RFI, or noise from other electronics.

Testing for RFI suceptibility, turned everything up and switched a flourescent light on/off that was plugged into the same socket. A click was heard. A microwave oven made really loud clicks when it started and stopped.

I read here Spice and the art of preamplifier design, Part 1 a particular common mode power supply filter worked good. I have one but it was out of an ungrounded TV set. It is also where I learnt the RFI test.

This one has two "secondary" caps that are grounded in between. It also claims differential mode attenuation. http://www.mouser.com/ds/2/337/85103004-24804.pdf

My cabinet is completely enclosed and the preamp and sockets bottoms are fully shielded with copper lined box. I mentioned all this since it would be better to prevent RFI from coming in (if that's what it is) than to attenuate it within the circuit. Caps definitely kill highs and harmonics.

Here's one schematic http://www.carvinservice.com/crg/sch...00%20REV-E.pdf

What do you think?

gingertube 5th August 2013 04:53 AM

1 Attachment(s)
When I say resistor size I actually meant resistor value, higher resistance mean higher thermal noise.

Resistance from the grid back to 0V should be kept low - that means that a interstage divider of 470K + 1M would be better as a 220K + 470K or even 100K + 220K, since from the point of view of the following tube, the grid to 0V resistance is lower and so more grid noise will be shunted to 0V. Actually the 470K +1M pot in your circuit means that at max gain you have 2/3 of the signal passing through. A 470K + 500K pot or even a 470K + 220K pot would most likely be better (more minimum attenuation) - ? Do you ever use the last few degrees of that pot ?

I have attached what I have of the Carvin Legacy schemo. Note that they put the high frequency roll off caps across the anode load resistor. This is identical to putting the cap from anode to 0V but the anode to 0V way of doing it has better inmunituy to any noise on the power supply itself. Also note that while not particularly low, the resistances back to 0V when looking away from the tube grids is never more than 470K. There aare probably better example circuits to highlight what I said but this was one which came to mind and I knew I could find.

Cheers,
Ian

ITPhoenix 5th August 2013 05:56 AM

With the 500K OD gain pot, the last 10 degrees are practically useless.

I like the idea of bypassing to ground with the anode caps, if that will reject any PS noise.

Thanks. I got you on the resistor values. Less resistance = less noise.

I also noted with the Carvin the non-traditional input scheme. No gridstopper at all, and the bypass cap on the low value gridleak.

My noise starts at or before V1B, since hiss is attenuated with 6.8n at the anode. That means the input and/or the valve or its wiring itself. I will start in that area first, since it affects everything down the chain.

Also noteworthy are the massive values of the cathode resistor bypass caps. I tried inreasing those to no avail. My research found higher values than 1uF simply allow more bass through; a somewhat moot point in a lead amp, in my opinion. I have a 10uF film cap and it is physically huge.

This problem is bothering me so much I refuse to play!

gingertube 5th August 2013 06:28 AM

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.

Just some random shots at the barn door.

Cheers,
Ian


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