I’m still tinkering and I noticed something today:
Nothing plugged into input jack.
Bass, Mid, and Treble controls on zero.
Volume control on maximum.
No hiss comes thru speaker.
The surprising thing is when I bring the volume control down, hiss develops around “8” or so. It then diminishes around “5” and goes to silent at “0.”
I assumed the ground terminal on the volume pot might need to be moved away from the Mid’s ground. I tried it at other locations on the chassis with no change in the phenomenon. The hiss level, during this experiment, is never more than when the BMT controls are in normal use.
I don’t know if this situation is adding any hiss or if it’s merely “revealing” hiss that’s gonna come thru in regular “mode” anyway. I don’t think there’s any ultrasonic oscillation or phase cancellation of the hiss (if that’s even possible) when the volume is maxed. My scope revealed no surprises or hints. I have no hum (60hz) and no buzz (120hz.) The amp is fully functional. I checked the shielding and the pot-body’s grounding.
Is this phenomenon normal? Does it happen on your Express type amp? Why does it happen? Something with the “sharing” of the hiss that’s being grounded out by the mid control’s ground?
Express Volume control phenomenon -strange or normal?
Moderators: pompeiisneaks, Colossal
Express Volume control phenomenon -strange or normal?
If it says "Vintage" on it, -it isn't.
Re: Express Volume control phenomenon -strange or normal?
I don't have an Express but from looking at the schematic, I can see how this is possible. With all the tone controls at zero, the "hot" side of your volume pot is grounded (or very close to it). So, with the volume all the way up, the grid of the stage that the volume wiper goes to is at ground. As you turn the volume down, you begin to introduce resistance between the wiper and ground which gets larger until you reach a point (about 5 you said) where the normally grounded side of the pot has more influence and then the resistance from wiper to ground begins to get smaller again. Follow me?
Re: Express Volume control phenomenon -strange or normal?
Yes, I think that's what's happening but I'm still wondering "why/how." What would the signal path of the hiss be in this case?
My assumption was that the hiss is from the 1st stage. In that case it's strange that the source side of the hiss, on the pot, would pass no hiss to the wiper, while moving away from the source-side of the hiss increases the hiss.
I know that playing guitar with the BMT all at zero lets no guitar sound thru. Maybe the hiss is getting onto the next grid via another means. (I checked it's shield.) I think I'll try disconnecting the input to the pot and re-check.
My assumption was that the hiss is from the 1st stage. In that case it's strange that the source side of the hiss, on the pot, would pass no hiss to the wiper, while moving away from the source-side of the hiss increases the hiss.
I know that playing guitar with the BMT all at zero lets no guitar sound thru. Maybe the hiss is getting onto the next grid via another means. (I checked it's shield.) I think I'll try disconnecting the input to the pot and re-check.
If it says "Vintage" on it, -it isn't.
Re: Express Volume control phenomenon -strange or normal?
The hiss doesn't seem to be "getting in there" but rather the second stage is the source of the hiss. With a higher impedance from its grid to ground (high resistance) that stage is able to generate more hiss. I don't think it is anything you did wrong, I think it is the way it is.
For your test, not only disconnect the input to the pot, but also ground it.
edited typo
For your test, not only disconnect the input to the pot, but also ground it.
edited typo
Re: Express Volume control phenomenon -strange or normal?
The test revealed the same level of hiss while the input to the pot it disconnected and grounded. So the hiss that I’m hearing during the “phenomenon” is from the 2nd stage and is grounded off when the pot is at either end of it’s rotation. (Both ends are essentially grounded with the BMT at zero.)
So my new project is why is hiss getting onto the grid wire to the 2nd stage. It’s shielded properly and I tried moving it away from all else. It seems strange that hiss, rather than hum or buzz would get to it.
.
So my new project is why is hiss getting onto the grid wire to the 2nd stage. It’s shielded properly and I tried moving it away from all else. It seems strange that hiss, rather than hum or buzz would get to it.
.
If it says "Vintage" on it, -it isn't.
Re: Express Volume control phenomenon -strange or normal?
I don't think hiss is "getting onto the grid wire." I think you are missing the point of what I am saying, the hiss is coming from the second stage tube, not getting into it, it is creating it. You already have the answer as to how to lower the hiss-- lower the impedance of the grid input. But that will require a significant deviation from the wreck circuit.
bottom line is, tubes hiss, some more, some less. High gain amps reveal more of it.
bottom line is, tubes hiss, some more, some less. High gain amps reveal more of it.
Re: Express Volume control phenomenon -strange or normal?
On a logarithmic pot you'll be at half of the overall track resistance somewhere around the 8 mark.
At 0 or 10 you have a comparatively low impedance source (very low at 0) but at the mid point of the resistance track you have a high impedance source (500K plus the original source on a 1M pot). This is why you can expect to have hiss, hum and other externally introduced noise at its highest at about this point.
Paul
At 0 or 10 you have a comparatively low impedance source (very low at 0) but at the mid point of the resistance track you have a high impedance source (500K plus the original source on a 1M pot). This is why you can expect to have hiss, hum and other externally introduced noise at its highest at about this point.
Paul
Re: Express Volume control phenomenon -strange or normal?
Thanx Jana! Now I see. I had the same problem with a friend’s amp that uses 6sl7's. Only it was hum/buzz. Grounding the grid would stop it and only changing to NOS tubes would fix it.
I did another test. I connected the wire coming off the wiper/output of the treble pot ONLY to my scope. The scope provides a 1meg to ground load as would the volume pot. Turning all 3 tone controls up does not increase the noise on the wiper/output. Same result if I plug a guitar in with it’s volume turned up (hum bucker and no strumming the strings.) I did that step so the #1 grid would not be fully shorted to ground, like when playing. So the 1st stage appears to not be generating hiss. I have a MF on it’s plate from my previous hiss-reduction quest. Changing to it from a CF (and CC) did reduce overall hiss somewhat.
So I’m thinking that most all the hiss I’m currently hearing in the amp is related "only" to the 2nd stage’s grid position from ground. I had always assumed it was all coming from the 1st plate resistor and 1st triode. This now explains why the hiss could be increased by moving any of the 3 tone controls up since they all change the load-to-ground on the grid of the 2nd stage, yes? This is a major revelation for me!
I’ve already selected V1 for the lowest overall hiss with the BMT control and volume at 50%. Am I correct that the plate resistor type in stage 2 is not related to this stage 2 hiss? The amp is nice and quiet with the volume control down. The stage 2 plate resistor is CF.
I did another test. I connected the wire coming off the wiper/output of the treble pot ONLY to my scope. The scope provides a 1meg to ground load as would the volume pot. Turning all 3 tone controls up does not increase the noise on the wiper/output. Same result if I plug a guitar in with it’s volume turned up (hum bucker and no strumming the strings.) I did that step so the #1 grid would not be fully shorted to ground, like when playing. So the 1st stage appears to not be generating hiss. I have a MF on it’s plate from my previous hiss-reduction quest. Changing to it from a CF (and CC) did reduce overall hiss somewhat.
So I’m thinking that most all the hiss I’m currently hearing in the amp is related "only" to the 2nd stage’s grid position from ground. I had always assumed it was all coming from the 1st plate resistor and 1st triode. This now explains why the hiss could be increased by moving any of the 3 tone controls up since they all change the load-to-ground on the grid of the 2nd stage, yes? This is a major revelation for me!
I’ve already selected V1 for the lowest overall hiss with the BMT control and volume at 50%. Am I correct that the plate resistor type in stage 2 is not related to this stage 2 hiss? The amp is nice and quiet with the volume control down. The stage 2 plate resistor is CF.
If it says "Vintage" on it, -it isn't.
Re: Express Volume control phenomenon -strange or normal?
jana - Strange to see nobody chiming in here, eh? Anyway, you are correct here in your thinking here except I wonder now if jjman hasn't skewed the results?
It seems to me that you were originally saying V2 was the cause of the constant hiss. Since the second half of V1 can be effectively shut down by turning the tone controls to zero, and the first half of V1 can be shut down by turning the volume pot to zero, there is no place else to look but the single stage of V2. ........Isn't this what you were saying jana?
So, jjman, if you are suggesting the 100K at V2 be changed to MF, then you might reduce a little noise. I do notice this noise you refer to but short of putting a pot where that 150K resistor is, you will always notice something. Why bother? It is what it is, you know? It isn't a Boogie or a Soldano, or something else - its a TW Express, they all have this hiss.
Now, if yours is out of control, find a better tube. As jana says, this is the path you need to follow. Either that or change the circuit.
It seems to me that you were originally saying V2 was the cause of the constant hiss. Since the second half of V1 can be effectively shut down by turning the tone controls to zero, and the first half of V1 can be shut down by turning the volume pot to zero, there is no place else to look but the single stage of V2. ........Isn't this what you were saying jana?
So, jjman, if you are suggesting the 100K at V2 be changed to MF, then you might reduce a little noise. I do notice this noise you refer to but short of putting a pot where that 150K resistor is, you will always notice something. Why bother? It is what it is, you know? It isn't a Boogie or a Soldano, or something else - its a TW Express, they all have this hiss.
Now, if yours is out of control, find a better tube. As jana says, this is the path you need to follow. Either that or change the circuit.
Last edited by rooster on Sun Dec 28, 2008 8:27 pm, edited 1 time in total.
Most people stall out when fixing a mistake that they've made. Why?
Re: Express Volume control phenomenon -strange or normal?
Hi Rooster
To clarify, there is no constant hiss. With the tone controls at min, there is hiss only with the volume set "in the middle." My understanding, with the BMT at min, is that with the volume control at either 0 or at 10, the stage 2 grid is at ground. Looking at the scheme, this makes sense. (At 10 the stage 2 (not V2) grid it is grounded thru the tone controls since they are at min.) Setting the volume control away from either end keeps the #2 grid away from full ground by creating resistance between the grounded ends and the wiper.
This is explaining much of what I've been seeing.
To clarify, there is no constant hiss. With the tone controls at min, there is hiss only with the volume set "in the middle." My understanding, with the BMT at min, is that with the volume control at either 0 or at 10, the stage 2 grid is at ground. Looking at the scheme, this makes sense. (At 10 the stage 2 (not V2) grid it is grounded thru the tone controls since they are at min.) Setting the volume control away from either end keeps the #2 grid away from full ground by creating resistance between the grounded ends and the wiper.
This is explaining much of what I've been seeing.
If it says "Vintage" on it, -it isn't.
Re: Express Volume control phenomenon -strange or normal?
jjman - OK, I get this. I was trying to see if jana was saying something different.
FWIW, to my way of thinking, and I may be too simple minded here, anything that is amplified in V1 exists at the PI because of V2 - and V2 has a constant gain. I see V1 as a network of sorts then, like a noise generator/antenna, and it has a basic character to it, depending on where you set its controls. I think you agree with this in principal. Since I cannot remove this 'antenna' - short of shutting down/removing V1 - I live with the results - which are really not a problem to me.
However, since you brought it up..................
Because I think this hiss issue will always come up with these TWs, wouldn't it be very cool if you or someone could figure out a way to create a file or device that could somehow compare homebrew TWs with known good ones (clones or otherwise)? Then everyone would feel they were either in need of some help or could rest easy knowing that they had done the work correctly. Eh?? Maybe we should be busying ourselves not with sound clips of music, but instead hiss. What do you think? Let me know.
FWIW, to my way of thinking, and I may be too simple minded here, anything that is amplified in V1 exists at the PI because of V2 - and V2 has a constant gain. I see V1 as a network of sorts then, like a noise generator/antenna, and it has a basic character to it, depending on where you set its controls. I think you agree with this in principal. Since I cannot remove this 'antenna' - short of shutting down/removing V1 - I live with the results - which are really not a problem to me.
However, since you brought it up..................
Because I think this hiss issue will always come up with these TWs, wouldn't it be very cool if you or someone could figure out a way to create a file or device that could somehow compare homebrew TWs with known good ones (clones or otherwise)? Then everyone would feel they were either in need of some help or could rest easy knowing that they had done the work correctly. Eh?? Maybe we should be busying ourselves not with sound clips of music, but instead hiss. What do you think?
Let me know.Most people stall out when fixing a mistake that they've made. Why?
Re: Express Volume control phenomenon -strange or normal?
Rooster,
JJman has what I was trying to say, I think you do too, just in a different fashion.
Tubes have an inherent hiss to them, it is the nature of the beast. I think what we hear is the electrons moving across the tube. Maybe a physicist could explain it better, with space charges and the lot, lol.
Some tubes hiss more than others, some a lot more. A few years ago, well, maybe like ten years ago, I got a "Russian" tube from groove tubes, wow did that thing hiss and pop. It was unusable even in a low gain spot.
I don't know how you could record and give audio references that would have any meaning. Speaker choice, mic placement, the list goes on and on for the variables that would affect this. It is an interesting thought to ponder though. I'm sure someone much smarter than I am could figure out a way to do this.
JJman has what I was trying to say, I think you do too, just in a different fashion.
Tubes have an inherent hiss to them, it is the nature of the beast. I think what we hear is the electrons moving across the tube. Maybe a physicist could explain it better, with space charges and the lot, lol.
Some tubes hiss more than others, some a lot more. A few years ago, well, maybe like ten years ago, I got a "Russian" tube from groove tubes, wow did that thing hiss and pop. It was unusable even in a low gain spot.
I don't know how you could record and give audio references that would have any meaning. Speaker choice, mic placement, the list goes on and on for the variables that would affect this. It is an interesting thought to ponder though. I'm sure someone much smarter than I am could figure out a way to do this.