How do you reduce white noise in a high gain channel?

[Mike_J]

I eliminated the #124 overdrive and added the S.I.R. #39 overdrive to my Sluckey inspired Plexi/800 with overdrive, active effects, etc.  Like the sound, don't like the white noise that comes with it.  The #124 overdrive squealed at high volumes whereas the #39 overdrive makes the amp go from quiet to lots of white noise.  I have it on a relay so I can switch it out when I am not using it but I would sure like to see it get quieter.  Does anyone have a solution to the white noise issue?

Thanks
Mike

[Mike_J]

On both sides of the amp I placed the bright caps on a pull out pot.  Just noticed when pulling out the pot on the Plexi side there is a small amount of white noise.  Have been trying to research this and the answer appears to be get a noise gate.  Any other suggestions?  I have the treble pot turned all the way off.  Going to change the caps in the PI from .02 to .1 and see if that helps at all.  Can anyone think of anything else that can be done to reduce the noise?  I have some carbon comps in the amp.  Considering replacing them with metal films or at least carbon films.

Thanks
Mike

[SILVERGUN]

Hey Mike,
It might be helpful to others if you repost your updated schematic on this thread.

It has helped me in the past to remove any late stage cathode bypass caps, because they seem to amplify any noise that occurs earlier in the circuit.

If you have enough gain (or signal amplitude) that late in the amp, there are other ways to add 'clipping' without adding volume.

Look at one of the last stages in a Soldano SLO preamp for a technique known as a 'soft-clipper', where a higher than usual cathode resistor (39K - unbypassed) is used to manufacture distortion. It quiets down the stage, as compared to a fully bypassed high gain stage, but adds a smooth dist. quality that some guys love.

http://el34world.com/charts/Schematics/files/soldano/soldano_slo100.pdf

[Mike_J]

Hey Mike,
It might be helpful to others if you repost your updated schematic on this thread.

It has helped me in the past to remove any late stage cathode bypass caps, because they seem to amplify any noise that occurs earlier in the circuit.

If you have enough gain (or signal amplitude) that late in the amp, there are other ways to add 'clipping' without adding volume.

Look at one of the last stages in a Soldano SLO preamp for a technique known as a 'soft-clipper', where a higher than usual cathode resistor (39K - unbypassed) is used to manufacture distortion. It quiets down the stage, as compared to a fully bypassed high gain stage, but adds a smooth dist. quality that some guys love.

http://el34world.com/charts/Schematics/files/soldano/soldano_slo100.pdf

Silvergun

Thank you for the response.  I have attached the area of the amp where the noise is present.  Part of the noise is contributed by the addition of the .68 cathode bypass cap.  I will have to study the Soldano schematic.  May take some time to figure it out. 

I was looking at the ISP Technologies Decimator ProRack G Noise Reduction System  (costs half as much as it did for me to build the amp).  I am considering it because I really don't care much for noise and am learning that some noise comes along with high gain unless a noise gate is used.  Is anyone familiar with it or another unit that they think works sufficiently?

The noise level I am getting is probably normal for a Marshall amp, especially one with an extra triode of gain but I find it annoying.

Thanks
Mike 

[shooter]

are you talking high freq noise or white noise?  cuz white noise, I believe is all frequencies at once, kinda a ssshh sotra sound, if you have a tone generator, play white noise, or pink, then something like 4khz triangle to hear the diff.

[Mike_J]

are you talking high freq noise or white noise?  cuz white noise, I believe is all frequencies at once, kinda a ssshh sotra sound, if you have a tone generator, play white noise, or pink, then something like 4khz triangle to hear the diff.

I am not sure.  I have the bright cap on a pull out pot and notice more noise when the bright cap is engaged.  Is that high freq noise?  Sorry, I don't have a tone generator.

Thanks
Mike

[AZJimC]

I think you may be on the right track with resistors, particularly the first stage(s). Any amp is going to have a noise floor, the more gain involved, the higher that floor gets raised with every added stage. Carbon comps have been said to induce more noise than other types. Also, increasing the wattage of those plate resistors has been said to help with hiss. I think you'd notice a reduction in noise by swapping out the first couple stage's plate resistors with a different type, or at lease bigger wattage.

By changing the coupling caps, you would be changing the frequency response curve at that stage, perhaps filtering out the hiss somewhat, and that would be more effective earlier in the preamp as well. IMO it's best to knock it down early in the chain, so it's not being amplified in later stages.

The noise floor is addressed in some amps by paralleling the input stage. Also, you could drop the gain in your added gain stage a bit with cathode resistor/bypass cap changes.

[DummyLoad]

what noise is it?

link below for samples of white, pink, etc..

http://playnoise.com/

--pete

[Mike_J]

what noise is it?

link below for samples of white, pink, etc..

http://playnoise.com/

--pete

Pete

I am pretty sure it is white noise.  Seems to be increased with higher treble and gain.

Do you have any suggestions for reducing white noise?  I can't believe they are selling a unit so people can be soothed by the different colors of noise.  This white noise is definitely not soothing at all to me.  Considering a Decimator II G String to knock down the noise level.  If I add pedals in front of the amp I am sure it would make the noise more pronounced.  Probably not very noticeable in a band setting but very noticeable on a stand alone basis.

Thanks
Mike

[sluckey]

Noise is directly proportional to gain. Get a handful of 47pF and 100pF caps and start tacking them across plate resistors, or plate to cathode (easy right on the tube socket), or grid to ground. Put one between the plates of a LTP PI also. This will definitely decrease the white noise but it will take some of the twang out of your Tele also. You have to find a balance between your tone and how much white noise you can tolerate.

[Mike_J]

Noise is directly proportional to gain. Get a handful of 47pF and 100pF caps and start tacking them across plate resistors, or plate to cathode (easy right on the tube socket), or grid to ground. Put one between the plates of a LTP PI also. This will definitely decrease the white noise but it will take some of the twang out of your Tele also. You have to find a balance between your tone and how much white noise you can tolerate.

Sluckey

I don't have a Tele but if I did I would want it to be twangy.  Have you ever used a noise gate to reduce the noise at high gain?  That Decimator II G String sure seems like a good unit and I think I could probably remove the 100pF cap across the plate and grid of the 800 channel if I used one.  I know in years past noise gates killed sustain but this looks like a technological advance and I don't know why anyone wouldn't want it on their pedal board.  (Other than $185)

From what I have read you plug the guitar directly into it, then it goes out into your amp or pedal board if you use one.  You also connect it to the effects loop with the delay and reverb effects after it and it some how takes the signal coming directly from the guitar and preserves it and eliminates unwanted noise. Do you think that pedal would take the twang out of my Tele if I had one? 

You are right about the noise.  As I think about it there is not an amp, no matter how quiet at low gain that can't be made to hiss when enough high gain pedals are added.  Just trying to find a way to make the noise a little more tolerable as my tolerance level for white noise is pretty low.  In the meantime I am going to replace the CCs and see how much that helps.

Thanks
Mike

[sluckey]

I thought you wanted to do something INSIDE the amp to reduce white noise (let's call it thermal noise). Getting rid of those CCs will help reduce the amount of thermal noise generated. Using low noise tubes will also reduce the amount of noise generated. Good layout helps too. The caps I suggested will remove more thermal noise but the caps don't differentiate between noise and wanted sounds.

[tubenit]

Sluckey gave you great ideas with smoothing caps!  I like 5751 and 12AY7 tubes. 

Also look up the "enhance cap" for a cap across the plate resistor of the LTPI. It has been very useful to me.

With respect, Tubenit

[SILVERGUN]

Back in the days of long hair and big rack mounted gear effects, I would TRY to use a noise gate that was built into my Rocktron 300G compressor.

You have to set it up so that a minimal signal will "open the gate", and then the gate closes when you come back down to that minimum signal level.

I hated the fact the I could hear the noise kick in just by placing my hand on the guitar, but if I set the gate threshold any higher it would inhibit the beginning of light volume swells and the end of some notes where you're just holding onto the guitar as everything fades out, and it comes down naturally.
The gated "effect" can be just as annoying as just dealing with a little background noise, which is what I wound up doing by finding other ways to reduce the noise in the signal chain.

Sluckey and tubenit's approach are commonly used by commercial amp mfg's to tame high gain amps, and for the pennies it will cost for those caps,,,it's worth a try.
If there's that much gain and noise in that amp then it would behoove you to reduce it through circuit tweaks rather than cover it up with a gate.

Reducing the presence control setting or just reducing the NFB voltage will also help.

Also, I've found that out of the modern production preamp tubes that I have tried, the TAD 7025 has consistently been the quietest in high gain use. (as advertised)
Remember, you are amplifying the noise through each successive stage so if you've got hiss in V1, it's only gonna get worse as it goes through all of that wiring and the rest of your preamp tubes.

The amount of twang that your gonna lose by adding a couple caps is marginal....sluckey was just trying to warn you that there will be some tradeoff. The amount is controllable by how far you choose to go with the tweaks.

[2deaf]

The number of components in front of the first stage grid affects the noise level, but I think you already have your component count at a minimum.  Component quality and count only matter at the first stage and the difference is minimal in relation to the thermal noise inherent to the tube.

Noise is directly proportional to gain.

The obvious thing to do is to employ non-linear gain after the first stage so that lower amplitude signals (such as noise) have lower gain than higher amplitude signals (such as audio).     

[Mike_J]

Back in the days of long hair and big rack mounted gear effects, I would TRY to use a noise gate that was built into my Rocktron 300G compressor.

You have to set it up so that a minimal signal will "open the gate", and then the gate closes when you come back down to that minimum signal level.

I hated the fact the I could hear the noise kick in just by placing my hand on the guitar, but if I set the gate threshold any higher it would inhibit the beginning of light volume swells and the end of some notes where you're just holding onto the guitar as everything fades out, and it comes down naturally.
The gated "effect" can be just as annoying as just dealing with a little background noise, which is what I wound up doing by finding other ways to reduce the noise in the signal chain.

Sluckey and tubenit's approach are commonly used by commercial amp mfg's to tame high gain amps, and for the pennies it will cost for those caps,,,it's worth a try.
If there's that much gain and noise in that amp then it would behoove you to reduce it through circuit tweaks rather than cover it up with a gate.

Reducing the presence control setting or just reducing the NFB voltage will also help.

Also, I've found that out of the modern production preamp tubes that I have tried, the TAD 7025 has consistently been the quietest in high gain use. (as advertised)
Remember, you are amplifying the noise through each successive stage so if you've got hiss in V1, it's only gonna get worse as it goes through all of that wiring and the rest of your preamp tubes.

The amount of twang that your gonna lose by adding a couple caps is marginal....sluckey was just trying to warn you that there will be some tradeoff. The amount is controllable by how far you choose to go with the tweaks.

Silvergun

After reading you post I noticed the presence control was maxed.  Might be contributing to the noise.  I am in the process of changing out all the CCs on V1 of the Plexi channel to see if that helps.  I also noticed the heater wires on V2 (the cathode follower tone stack tube) is sitting flush against the 100K plate resistor that goes across the tube socket.  My guess is they probably would be better off not touching.  Does anyone agree?

I had a Pod when they first came out.  Seemed great except for what you are saying.  Had some noise gate in there that just shut off the signal unnaturally.  I was hoping that technology had improved and my wife was happy because she thought she had my Christmas present bought with the Decimator II G String unit.  Wish someone we trust had already tried one to see how well it works.

Thanks
Mike

[Mike_J]

Sluckey gave you great ideas with smoothing caps!  I like 5751 and 12AY7 tubes. 

Also look up the "enhance cap" for a cap across the plate resistor of the LTPI. It has been very useful to me.

With respect, Tubenit

Thank you for the reply Tubenit.

If I am trying to get a tone.  Let's say ZZ Top type of overdrive.  Don't you need a certain gain level to get there?  If I start out with a 5751 or a 12AY7 then it should be quieter because I have less gain.  However, won't I have to manufacture gain to get where I want to be through a pedal or additional gain stage?  Wouldn't I be at the same noise level then once I get the gain level to ZZ Top territory?  Guess the advantage is I can turn the pedal off and the noise is no longer there so I see the logic in what you are saying I think.  I have a lot of NOS 12AY7s, suppose I will give it a try.

I was really hoping to remove the enhance cap across one of the 800 tubes instead of adding more.  Especially if I ever get a Tele and want to get twangy with this amp.  The Tele twang tone is actually one of my favorite tones.

Thanks
Mike

[Mike_J]

The number of components in front of the first stage grid affects the noise level, but I think you already have your component count at a minimum.  Component quality and count only matter at the first stage and the difference is minimal in relation to the thermal noise inherent to the tube.

Noise is directly proportional to gain.

The obvious thing to do is to employ non-linear gain after the first stage so that lower amplitude signals (such as noise) have lower gain than higher amplitude signals (such as audio).     

2 deaf

How do I go about employing non-linear gain after the first stage so that noise has lower gain than the audio signal other than by purchasing a Decimator II G String unit?  I apologize in advance for the lack of understanding of your question but I would certainly like to know how to achieve what you are talking about other than with better layout, quieter type resistors, lower gain tubes, enhancement caps and lowering the presence pot from its present setting of ten (no idea how it go there or why I didn't notice it) is there something else I can be doing?

Thanks
Mike

[sluckey]

I also noticed the heater wires on V2 (the cathode follower tone stack tube) is sitting flush against the 100K plate resistor that goes across the tube socket.  My guess is they probably would be better off not touching.  Does anyone agree?

I agree. In cases like this, wrap the filament wires around the socket, not across the socket. This ain't part of your hissy noise though.

[Mike_J]

I thought you wanted to do something INSIDE the amp to reduce white noise (let's call it thermal noise). Getting rid of those CCs will help reduce the amount of thermal noise generated. Using low noise tubes will also reduce the amount of noise generated. Good layout helps too. The caps I suggested will remove more thermal noise but the caps don't differentiate between noise and wanted sounds.

Thanks Sluckey

I do want to make the amp as quiet as I possibly can but it is pretty quiet until I get into high gain territory.  Your suggestion of using the enhancement cap is a good idea.  Placing an enhancement cap on the extra gain triode only makes sense.

Thanks
Mike

[Mike_J]

I also noticed the heater wires on V2 (the cathode follower tone stack tube) is sitting flush against the 100K plate resistor that goes across the tube socket.  My guess is they probably would be better off not touching.  Does anyone agree?

I agree. In cases like this, wrap the filament wires around the socket, not across the socket. This ain't part of your hissy noise though.

But it could cause a little hum and maybe part of the hissy noise is a tiny bit of hum.  I am changing the socket to the way you have it shown, can't be anything but an improvement.

Thanks
Mike

[SILVERGUN]

I had a Pod when they first came out.  Seemed great except for what you are saying.  Had some noise gate in there that just shut off the signal unnaturally.  I was hoping that technology had improved and my wife was happy because she thought she had my Christmas present bought with the Decimator II G String unit.  Wish someone we trust had already tried one to see how well it works.

I'm sure technology has improved, and that unit might work fine.
I'd just try to fix the amp first, before I spent the money....
There's plenty of other good stuff to suggest for Christmas 

If that amp will never see a stage then I'd live with more hiss than if I had to explain it to a soundman.

That amp probably has enough high end to battle a train whistle....you can calm it down and not lose the Twang

Also, while we're talking about presence and NFB,,,make sure you have the NFB tap off of the "correct" OT secondary tap for the resistor value you are using.
To get an idea of how much hiss the NFB adds, just disconnect the tap temporarily....that'll tell you if it needs adjustment or if you'd care to adjust it.

But it all leads back to the preamp....if you're preamp is hissy, you'll always have hiss.

[Mike_J]

I had a Pod when they first came out.  Seemed great except for what you are saying.  Had some noise gate in there that just shut off the signal unnaturally.  I was hoping that technology had improved and my wife was happy because she thought she had my Christmas present bought with the Decimator II G String unit.  Wish someone we trust had already tried one to see how well it works.

I'm sure technology has improved, and that unit might work fine.
I'd just try to fix the amp first, before I spent the money....
There's plenty of other good stuff to suggest for Christmas 

If that amp will never see a stage then I'd live with more hiss than if I had to explain it to a soundman.

That amp probably has enough high end to battle a train whistle....you can calm it down and not lose the Twang

Also, while we're talking about presence and NFB,,,make sure you have the NFB tap off of the "correct" OT secondary tap for the resistor value you are using.
To get an idea of how much hiss the NFB adds, just disconnect the tap temporarily....that'll tell you if it needs adjustment or if you'd care to adjust it.

But it all leads back to the preamp....if you're preamp is hissy, you'll always have hiss.

Your suggestion regarding the presence pot level was spot on.  Turned the presence level down to 2 and changed all the resistors on tube one to metal films.  Turned both volumes up to ten and pulled out the knob on the bright channel to engage the bright cap and now I am noticing a slight hum but no white noise.  I think it may be that cathode follower tube and the wires being on top of the plate resistor.  Off to see if I can get that fixed and get it real quiet. 

My presence goes to the 8 ohm secondary tap.  There must be something wrong with it because it sure injected a lot of hiss into the amp when it was on ten.  Any ideas on how to fix that?  Other than the obvious one of not putting it on ten any more.

Thanks
Mike

[SILVERGUN]

Your suggestion regarding the presence pot level was spot on.  Turned the presence level down to 2 and changed all the resistors on tube one to metal films.  Turned both volumes up to ten and pulled out the knob on the bright channel to engage the bright cap and now I am noticing a slight hum but no white noise.  I think it may be that cathode follower tube and the wires being on top of the plate resistor.  Off to see if I can get that fixed and get it real quiet. 

My presence goes to the 8 ohm secondary tap.  There must be something wrong with it because it sure injected a lot of hiss into the amp when it was on ten.  Any ideas on how to fix that?  Other than the obvious one of not putting it on ten any more.

A well designed NFB circuit takes multiple things into account, that's why you can't just copy one from one amp and paste it on another. (and expect it to be correct)
If you can picture your OT taps as being a voltage divider, in the sense that you will get different voltages off of each tap, you'll see that resistor values become critical based on what tap your using and how much voltage is on the OT secondary.

I've seen 100 watt 'Silver Jubilee's" with 100K off of the 4ohm tap, and JCM 800 100 watters with 100K off of the 8 ohm tap.

What value is your NFB resistor?,,,,pot,,,cap?

Schematic?

[tubeswell]

White noise is a sign from the universe that you should try 2-3W MF resistors on all AC-load points.

[Mike_J]

Your suggestion regarding the presence pot level was spot on.  Turned the presence level down to 2 and changed all the resistors on tube one to metal films.  Turned both volumes up to ten and pulled out the knob on the bright channel to engage the bright cap and now I am noticing a slight hum but no white noise.  I think it may be that cathode follower tube and the wires being on top of the plate resistor.  Off to see if I can get that fixed and get it real quiet. 

My presence goes to the 8 ohm secondary tap.  There must be something wrong with it because it sure injected a lot of hiss into the amp when it was on ten.  Any ideas on how to fix that?  Other than the obvious one of not putting it on ten any more.

A well designed NFB circuit takes multiple things into account, that's why you can't just copy one from one amp and paste it on another. (and expect it to be correct)
If you can picture your OT taps as being a voltage divider, in the sense that you will get different voltages off of each tap, you'll see that resistor values become critical based on what tap your using and how much voltage is on the OT secondary.

I've seen 100 watt 'Silver Jubilee's" with 100K off of the 4ohm tap, and JCM 800 100 watters with 100K off of the 8 ohm tap.

What value is your NFB resistor?,,,,pot,,,cap?

Schematic?

The NFB resistor is 47K, pot is 5K and the cap is .1uF.  As mentioned earlier it is tied to the 8 ohm tap.  I have attached the schematic.  The overdrive channel has changed and there are a few resistor values that were changed in order to get the voltages to spec.

Thanks
Mike

[Mike_J]

White noise is a sign from the universe that you should try 2-3W MF resistors on all AC-load points.

Thanks Tubeswell

Are AC load points also called plate resistors or do they live by another name?

Thanks
Mike

[tubeswell]

White noise is a sign from the universe that you should try 2-3W MF resistors on all AC-load points.

Thanks Tubeswell

Are AC load points also called plate resistors or do they live by another name?

Thanks
Mike

Anywhere there is an AC load on the signal path - (incl plate resistors and all load resistors in cathodyne stages, tail resistor in LTP, grid leak resistors, resistors in the tone stack)

[SILVERGUN]

The NFB resistor is 47K, pot is 5K and the cap is .1uF.  As mentioned earlier it is tied to the 8 ohm tap.  I have attached the schematic.  The overdrive channel has changed and there are a few resistor values that were changed in order to get the voltages to spec.

Based on the issue you are having I wouldn't hesitate to raise that to 100K.

You 'could' put in a temporary 250K pot and try to dial in a perfect amount, but 100K with the presence control set at a moderate level should take care of it.

[SILVERGUN]

I have attached the schematic

I don't see it Mike....just 1/4 of a hand drawn layout in Reply #3

[2deaf]

How do I go about employing non-linear gain after the first stage so that noise has lower gain than the audio signal

It's easy to get a stage to do the opposite -- that is to have less gain as the amplitude increases.  So use a stage like this to generate NFB so that the stage it is modifying does the opposite.

I can elaborate when I get home if you are interested.

[Mike_J]

I have attached the schematic

I don't see it Mike....just 1/4 of a hand drawn layout in Reply #3

Sorry.  I am getting a little feeble at times it seems.

Thanks
Mike

[Mike_J]

How do I go about employing non-linear gain after the first stage so that noise has lower gain than the audio signal

It's easy to get a stage to do the opposite -- that is to have less gain as the amplitude increases.  So use a stage like this to generate NFB so that the stage it is modifying does the opposite.

I can elaborate when I get home if you are interested.

I am very interested.  This is how I learn and hopefully others will benefit as well.

Thanks
Mike

[Mike_J]

The NFB resistor is 47K, pot is 5K and the cap is .1uF.  As mentioned earlier it is tied to the 8 ohm tap.  I have attached the schematic.  The overdrive channel has changed and there are a few resistor values that were changed in order to get the voltages to spec.

Based on the issue you are having I wouldn't hesitate to raise that to 100K.

You 'could' put in a temporary 250K pot and try to dial in a perfect amount, but 100K with the presence control set at a moderate level should take care of it.

Thanks Silvergun

Can you or anyone explain why increasing the NFB resistor would reduce hiss?  Does it make the pot more useful in that the resistances that don't cause the hiss are available on a larger swing of the pot?

Thanks
Mike

[Mike_J]

Well I have been turning knobs to try to find out where the noise is coming from.  The Plexi side is quiet with the presence pot on two on the bright volume control all the way up to ten.  No noise at all.  The normal volume control is quiet until it gets to eight on the dial and then a hum sets in from eight to ten.  I redid the wiring on V2's heater wires with no change.  Then I took a bamboo skewer and moved it around and found a straggler on the normal pot.  Thinking surely this will fix the offending noise I yanked it out like a bad tooth but to no avail, the hum is still there.  I already changed the tube with no change in the hum level.  I don't see any loose grounds.

There are only two possibilities I have left that I can think of.  First, replace the coupling capacitor as it might be passing dc but I don't hear any dc on the pot so I am not optimistic there.  The second choice is to put a shielded wire on the wire entering the pot.  It is running by the cathode resistor and cap and may be picking up enough noise there to cause hum at high volume.  Can anyone think of anything else to try?

Now on the 800 side.  The normal volume is quiet but the master volume has hiss at full volume.  I will try the enhancement cap on V3b and V5a and see if that will tame it.  The enhancement cap on the normal volume side is yielding zero hum or hiss so I think I will try to remove or at least reduce its value.  Is there a preference silver mica or ceramic for the enhancement cap? 

Thank you to everyone for your help.  I can see light at the end of the tunnel now.  Just a few more tweaks and we will get there I hope.

Thanks
Mike

[SILVERGUN]

Thanks Silvergun

Can you or anyone explain why increasing the NFB resistor would reduce hiss?  Does it make the pot more useful in that the resistances that don't cause the hiss are available on a larger swing of the pot?

I can give you the 5 cent answer.

Because you are lowering the voltage of the signal that you are "feeding back" into the P.I.

The NFB wire carries a small amount of (potentially noisy) signal from the OT secondary and injects it into half of the PI.
Lowering that signal voltage lowers the noise.

[2deaf]

I am very interested.  This is how I learn and hopefully others will benefit as well.

Attached is a schematic with NFB in the clipping stages.  For low amplitude signals, V2A and V2B make a relatively low gain linear amplifier due to the NFB.  When V2B clips, the amount of NFB it sends to V2A becomes a fixed amount while the incoming signal continues to rise.  Now the gain and frequency response is no longer restrained by the NFB.  This is the same thing as what is going on in a PA with NFB when overdriven unless the NFB is compromised by something like a PPIMV.

There are three main things I like about this arrangement.

1.)  The amplified noise from V1A is less relative to the amount of clip in V2A & V2b as compared to the typical four stage preamp.

2.)  The preamp is now way more touch sensitive because the gain really takes off when you hit it harder.

3.)  That annoying ring from the unused strings is reduced as compared to the typical four-stager because the ring is usually in the low-gain region.

[Mike_J]

I am very interested.  This is how I learn and hopefully others will benefit as well.

Attached is a schematic with NFB in the clipping stages.  For low amplitude signals, V2A and V2B make a relatively low gain linear amplifier due to the NFB.  When V2B clips, the amount of NFB it sends to V2A becomes a fixed amount while the incoming signal continues to rise.  Now the gain and frequency response is no longer restrained by the NFB.  This is the same thing as what is going on in a PA with NFB when overdriven unless the NFB is compromised by something like a PPIMV.

There are three main things I like about this arrangement.

1.)  The amplified noise from V1A is less relative to the amount of clip in V2A & V2b as compared to the typical four stage preamp.

2.)  The preamp is now way more touch sensitive because the gain really takes off when you hit it harder.

3.)  That annoying ring from the unused strings is reduced as compared to the typical four-stager because the ring is usually in the low-gain region.

2 deaf

Thank you for the schematic.  Do you have the power amp schematic as well? 

This is completely different that anything I have ever built.  The bass is on V3 while the treble and mid pots are on V1.  Has to be a good reason for that.  I like the American/British switch.  Don't understand the dual JFET in the reverb section but as I recall tubenit uses a JFET as a replacement for a triode in a cathode follower tone stack circuit so I am guessing that a dual JFET must be similar to a whole 12A_7 tube.  As I recall tubenit doesn't believe there is a noticeable difference between the JFET on the back end of a cathode follower circuit verses what a tube provides.  This builder must think the same in the reverb section.

I can't see a PI section in the schematic (unfortunately doesn't mean it isn't there).  Must be on the power section page.  Curious to see how that is handled.

Thanks
Mike

[2deaf]

Don't understand the dual JFET in the reverb section

Had me scared that I had posted the wrong schematic because I frequently use JFET's for reverb recovery, but this time it is a very common, very cheap dual IC.

I have attached the other two sheets.  It's all pretty standard stuff.

[Mike_J]

Don't understand the dual JFET in the reverb section

Had me scared that I had posted the wrong schematic because I frequently use JFET's for reverb recovery, but this time it is a very common, very cheap dual IC.

I have attached the other two sheets.  It's all pretty standard stuff.

2 deaf

Thank you for the other two schematics.  It looks to me like you are going to great lengths to isolate V1's heater.  Could you tell me what a 25D600 is and what it does in the V1 heater circuit?  So can we use JFETs for reverb recovery and in the back end of a cathode follower tone circuit and save three triodes without significant loss of tube tone?

The big question is how does the amp sound and is there anything that it can be compared to?

Thanks
Mike

[Mike_J]

This hum in the Plexi side is driving me nuts.  Unplugged the refrigerator and turned off the lights and ceiling fans and took a dB reading.  The supposedly quiet room rating was 38dB.  It is surprising how much noise the lap top fan makes when there is no other noise in the room.  With the amp on and the normal and bright volumes maxed the dB rating goes up to 47dB with the dB meter one foot from the speaker which to me is way too high.  Plugged the refrigerator back in it is 67dB.  Don't know how we sleep at night.  Might as well live next to the train tracks as live in a home with that noisy refrigerator.

I have done everything I can think of to find the source of the hum.  Changing to a 12AY7 in V1 seems to reduce the hum level but it is still reading 47dB.  The hum level seems to all be coming from the normal channel as the hum shows up at 8 on the normal channel with the bright side maxed.  One observation I have made is the hum shows up on 16mm mini 1MA volume pots.  The two full sized CTS pots are not causing any problems.  The mini pots seem to work fine for the tone controls.  May not be as good for the volume pots.  Next thing on my list is to change out the 16mm volume pots to CTS 24mm. 

If that doesn't work I am going to never turn the normal control above 7 or buy a Decimator II G String unless someone has another suggestion.

Thanks
Mike.

[Mike_J]

Had to order the CTS 1MA pot so I will be waiting for them to come in the mail.  In the meantime I can try and tackle the hiss level problem on the 800 side.  Will try to implement sluckey and tubenit's suggestions.  Another thing I might try is some higher pF wire.

I use the George L cable Doug sells because it has the lowest pF rating of any shielded cable I have measured.  Since the more pF the more high-end you lose it usually matters.  However, in the case of the 800 high-end is in abundance and appears to be contributing greatly to the hiss level.  I have some shielded cable that measures 78pF per foot versus 21pF per foot for the George L.

How do you think using the higher pF cable would compare to the smoothing cap idea?  I would think it would at a minimum reduce the size of the cap needed to do the job.  However, if I end up in the same place at the end of the day there is no sense in doing the work of replacing the cable that is already in the amp if it is no different than using a slightly larger smoothing cap.

Thanks
Mike

[2deaf]

It looks to me like you are going to great lengths to isolate V1's heater.  Could you tell me what a 25D600 is and what it does in the V1 heater circuit?

It's a Sovtek thing and this is a Sovtek chassis with Sovtek transformers.  I liked the DC heater for V1 so I kept it.  The 2SD600 is used as a voltage regulator in the little regulated DC power supply.  I wouldn't put it in my amp if it wasn't already there.

So can we use JFETs for reverb recovery and in the back end of a cathode follower tone circuit and save three triodes without significant loss of tube tone?

I don't use SS in the main signal path so I don't know about hybrid CF's to drive a tone stack.  I personally couldn't tell what recovered the reverb since it is a small signal that is being amplified very cleanly.  A real sweet twin-style reverb doesn't seem necessary to me in an amp that roars, so I used op amps.

The big question is how does the amp sound and is there anything that it can be compared to?

It's hard to describe sounds in words and I don't know enough commercial amp sounds to compare it to one.  It doesn't have as much maximum overdrive as most amps I build.  It is capable of of a wide array of sounds from smooth overdrive to downright obnoxious.  I think it sounds really good with moderate pre-amp overdrive and the Master at 10.   

[2deaf]

The bass is on V3 while the treble and mid pots are on V1.  Has to be a good reason for that.

I sometimes split the tone controls over two stages, but there is more to it here.  Too much bass in a four-stage preamp gives it a muddy tone that I don't like.  If I give someone playing my amp the option of turning the bass up and making it sound like crap, they will do it.  A lot of the time I'll only give them a 25K bass pot so they can't do much harm.  With this amp, I just took it out altogether. 

Four stage preamps come out kinda heavy on the bass side, so I always cut the bass in various ways.  Here I used the Bass control to vary the cut so that when someone turns it up it sounds like more bass without the problems that bass causes in the preamp.