I'm not saying it may change anything, but it can be instructive to cut the cable tie and sort through all the flying cables to work out whether they should be separated, or twisted, or routed away from certain sections.
Appreciating where the noisy wiring is, and where the high signal level wiring is, and trying to lay out the wiring according to their risk of causing problems (capacitive or magnetic coupling) is a good learning curve.
By the way, is the speaker wiring grounded?
Appreciating where the noisy wiring is, and where the high signal level wiring is, and trying to lay out the wiring according to their risk of causing problems (capacitive or magnetic coupling) is a good learning curve.
By the way, is the speaker wiring grounded?
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I thought of doing that too. I tried moving things around in the bundle and it didn't seem to make much difference. But as it gets quieter things that didn't matter before may start to make a difference.
Here's whats in the wiring bundle:
Red/Red/Green: LV secondary and center tap
Orange/Orange/Yellow: Heater secondary and center tap
Red/Red: HV secondary
Thinner red wired: standby switch (B+) wiring
Black: Power switch, channel select switch and channel select/reverb footswitch wiring
Gray: reverb tank send/return (shielded)
The speaker is not grounded.
I'm ready to attack the problem at it's apparent source (the diodes), so it looks like UF4007s are called for, and maybe snubbers on the secondary. I have nothing to go on for that and would just install a 'typical' value, 5k-10k R 2w, and 10nF, of suitable voltage rating.
Red/Red/Green: LV secondary and center tap
Orange/Orange/Yellow: Heater secondary and center tap
Red/Red: HV secondary
Thinner red wired: standby switch (B+) wiring
Black: Power switch, channel select switch and channel select/reverb footswitch wiring
Gray: reverb tank send/return (shielded)
The speaker is not grounded.
I'm ready to attack the problem at it's apparent source (the diodes), so it looks like UF4007s are called for, and maybe snubbers on the secondary. I have nothing to go on for that and would just install a 'typical' value, 5k-10k R 2w, and 10nF, of suitable voltage rating.
Not the diodes
I tried the UF4007 diodes with 0.01uF caps across each one. It made no difference. I made a scope capture of the noise at R17 which is the output of the V2 tube.
[/url]image by Dennis Kelley, on Flickr\ }[/IMG]
I also installed the 250 ohm pot between the heater wires and ground and adjusted it for minimum hum. That helped quite a bit. I also got a screen shot of the signal at R17 with the pot installed.
[/url]image by Dennis Kelley, on Flickr\ }[/IMG]
It is interesting that what this seems to have done is lower the spikes, but not the 60Hz hum. I adjusted it to get the best sounding hum, which was more 60Hz and less buzz. It's interesting that the pot minimizes the buzz at a different point than it does the 60 Hz hum?
One other thing I noticed. The sharp spikes in the waveform before i put the pot in, completely disappear immediately when I put the amp into stand by mode, ie remove the load from the hv supply. The amp is still working and the 60Hz hum is still there, but the spikes and the buzz drops off the waveform as soon as I flip the stand by switch. Just another clue.
Today I plan to implement the alternate heater wiring mod on V1 and V2. I determined that V1 also has the same issue, but normally the volume control is down so you don't hear it. But by looking at the output of V1 before the volume control I could see similar noise.
I'd still like to go after the root of the problem, which I still think is just the diodes turning off quickly causing a flyback ignition coil type pulse to be generated by the transformer.
I tried the UF4007 diodes with 0.01uF caps across each one. It made no difference. I made a scope capture of the noise at R17 which is the output of the V2 tube.
I also installed the 250 ohm pot between the heater wires and ground and adjusted it for minimum hum. That helped quite a bit. I also got a screen shot of the signal at R17 with the pot installed.
It is interesting that what this seems to have done is lower the spikes, but not the 60Hz hum. I adjusted it to get the best sounding hum, which was more 60Hz and less buzz. It's interesting that the pot minimizes the buzz at a different point than it does the 60 Hz hum?
One other thing I noticed. The sharp spikes in the waveform before i put the pot in, completely disappear immediately when I put the amp into stand by mode, ie remove the load from the hv supply. The amp is still working and the 60Hz hum is still there, but the spikes and the buzz drops off the waveform as soon as I flip the stand by switch. Just another clue.
Today I plan to implement the alternate heater wiring mod on V1 and V2. I determined that V1 also has the same issue, but normally the volume control is down so you don't hear it. But by looking at the output of V1 before the volume control I could see similar noise.
I'd still like to go after the root of the problem, which I still think is just the diodes turning off quickly causing a flyback ignition coil type pulse to be generated by the transformer.
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Yeah, I was thinking I'd change the diodes, too bad that didn't cure it for you.
I went ahead and took my boards out, because I had a second problem with a loose solder joint somewhere in the power supply board (I think on one of the B+ caps), so I had to get to that side and not just tack new diodes onto the cut off leads of the old.
This means I will probably make all my planned changes at once, it's too much work to remove and hook this back up, especially with my modified output stage. So I won't know any results probably for a week or so.
I'm going to suggest again that you try a heater lift, this has helped hum every time I've tried it, and is widely recommended. Two resistors and a capacitor, about 300k total, sized appropriately for voltage and wattage. Run the chain from a filtered B+ voltage to ground, bypassing the lower resistor with a capacitor. I don't remember what's an appropriate sized capacitor, maybe 10 or 22uF? I usually have to look up this recommendation. I'm guessing putting the heater center tap at +30 or +40v or so would do the trick.
I went ahead and took my boards out, because I had a second problem with a loose solder joint somewhere in the power supply board (I think on one of the B+ caps), so I had to get to that side and not just tack new diodes onto the cut off leads of the old.
This means I will probably make all my planned changes at once, it's too much work to remove and hook this back up, especially with my modified output stage. So I won't know any results probably for a week or so.
I'm going to suggest again that you try a heater lift, this has helped hum every time I've tried it, and is widely recommended. Two resistors and a capacitor, about 300k total, sized appropriately for voltage and wattage. Run the chain from a filtered B+ voltage to ground, bypassing the lower resistor with a capacitor. I don't remember what's an appropriate sized capacitor, maybe 10 or 22uF? I usually have to look up this recommendation. I'm guessing putting the heater center tap at +30 or +40v or so would do the trick.
I just read through this thread, and thought I'd try to offer some thoughts. Keep in mind that I am not a licensed tube amp tech, although I do have a good amount of electronic repair experience.
As with any noise reducing or eliminating measure, it is imperative that you figure out how much of what is getting in where. Seeing the scope waveforms, they are pretty well useless at this point. We are seeing a few millivolts on a signal that should be in the tens of volts peak to peak range. If it is not probe-induced chatter, it's not enough signal to cause alarm. Due to the subjective nature, it is more productive to just probe somewhere far downstream that does what the speaker does. That way, what you hear can be quantified objectively. If you want to stay close to V2, tie to IC1 pin 1, or just tie straight to V2 pin 6. If the probe won't handle the DC, tack a coupling cap to pin 6 and the probe to the free leg.
Some really quick checks can be done to figure out how much noise is coming from where. This assumes a ground point has been found that eliminates detectable noise:
-Stick a can shield over V2, ground the can (directly radiated noise).
-Jump R14 (kills input to V2-grid)
-Remove C48 (lowers stage gain)
-Jump R13 with something smaller (220K or so - increases input signal)
-Jump R15 (ties V2 output to B+)
While trying these tests, separate the zip-tied jumper wires, and move them around somewhat with a chopstick or Popsicle stick or something. The nature of the problem will make itself evident if the noise is indeed finding its way in at the suspected stage.
Feel free to dismiss my suggestions as the ravings of a heretic. It won't hurt my feelings.
As with any noise reducing or eliminating measure, it is imperative that you figure out how much of what is getting in where. Seeing the scope waveforms, they are pretty well useless at this point. We are seeing a few millivolts on a signal that should be in the tens of volts peak to peak range. If it is not probe-induced chatter, it's not enough signal to cause alarm. Due to the subjective nature, it is more productive to just probe somewhere far downstream that does what the speaker does. That way, what you hear can be quantified objectively. If you want to stay close to V2, tie to IC1 pin 1, or just tie straight to V2 pin 6. If the probe won't handle the DC, tack a coupling cap to pin 6 and the probe to the free leg.
Some really quick checks can be done to figure out how much noise is coming from where. This assumes a ground point has been found that eliminates detectable noise:
-Stick a can shield over V2, ground the can (directly radiated noise).
-Jump R14 (kills input to V2-grid)
-Remove C48 (lowers stage gain)
-Jump R13 with something smaller (220K or so - increases input signal)
-Jump R15 (ties V2 output to B+)
While trying these tests, separate the zip-tied jumper wires, and move them around somewhat with a chopstick or Popsicle stick or something. The nature of the problem will make itself evident if the noise is indeed finding its way in at the suspected stage.
Feel free to dismiss my suggestions as the ravings of a heretic. It won't hurt my feelings.
You heretic you!
Just kidding. Actually thank you for the suggestions. Some of these I have tried already, some not.
Actually R17 is the output of the last V2 stage just before it goes into the op amp. So in terms of testing for noise in V1 and V2 it is a good place to measure. It is a low level signal, but that is the nature of the problem I am chasing at this point. I think I am just about done at this point. Not that it is perfect, but I don't really know where to go next?
I completed the new jumpers for the heater circuit for V1 and V2.
IMG_0825 by Dennis Kelley, on Flickr
I also added the trimpot across the heater voltage with the center to ground.
IMG_0826 by Dennis Kelley, on Flickr
That was a convenient spot to tack it in, it had ground and both heater voltages right there.
I tested it after these mods and the hum from the node on the grid input at the junction of R13 and R14 is fixed now. Shorting out R14 does not make any difference. Just for reference here is the noise where it was when I started.
https://soundcloud.com/capndenny1/reverberockect-noise-before-recap-gain-at-300-mic-at-a-under-ampeg-11db
And here is where I am today.
https://soundcloud.com/capndenny1/ampeg-with-all-fixes-hrtr-v1-and-v2
Both these recordings where mad with a mic in fron tof the cap in the same spot against the front of the grill cloth. I also had the gain on the USB recording interface set up pretty high, and after I recorded it I boosted both signals inside Reaper (DAW) by +11dB. So this is way louder than it sounds in the room.
There is no hum left from V1 or V2. I can turn the Volume all the way up and I have no additional hum. Just whatever is there in the background with all the knobs at zero. In fact that same hum is now there with a shorting jumper in the Return input just ahead of the power amp section.
V1 and V2 and V3 all have shields.
I did cut the zip tie, and moved the wires around and it made no difference.
So I think I need to start thinking in terms of power amp hum. I can actually hear the transformer hum on it's own a little.
I might play it a while. I played it for about an hour and it got real quiet. But it might have been the chord or the guitar. I turned it off and tried it later, and it was fine. So I will see if it happens again?
Just kidding. Actually thank you for the suggestions. Some of these I have tried already, some not.
Actually R17 is the output of the last V2 stage just before it goes into the op amp. So in terms of testing for noise in V1 and V2 it is a good place to measure. It is a low level signal, but that is the nature of the problem I am chasing at this point. I think I am just about done at this point. Not that it is perfect, but I don't really know where to go next?
I completed the new jumpers for the heater circuit for V1 and V2.
IMG_0825 by Dennis Kelley, on FlickrI also added the trimpot across the heater voltage with the center to ground.
IMG_0826 by Dennis Kelley, on FlickrThat was a convenient spot to tack it in, it had ground and both heater voltages right there.
I tested it after these mods and the hum from the node on the grid input at the junction of R13 and R14 is fixed now. Shorting out R14 does not make any difference. Just for reference here is the noise where it was when I started.
https://soundcloud.com/capndenny1/reverberockect-noise-before-recap-gain-at-300-mic-at-a-under-ampeg-11db
And here is where I am today.
https://soundcloud.com/capndenny1/ampeg-with-all-fixes-hrtr-v1-and-v2
Both these recordings where mad with a mic in fron tof the cap in the same spot against the front of the grill cloth. I also had the gain on the USB recording interface set up pretty high, and after I recorded it I boosted both signals inside Reaper (DAW) by +11dB. So this is way louder than it sounds in the room.
There is no hum left from V1 or V2. I can turn the Volume all the way up and I have no additional hum. Just whatever is there in the background with all the knobs at zero. In fact that same hum is now there with a shorting jumper in the Return input just ahead of the power amp section.
V1 and V2 and V3 all have shields.
I did cut the zip tie, and moved the wires around and it made no difference.
So I think I need to start thinking in terms of power amp hum. I can actually hear the transformer hum on it's own a little.
I might play it a while. I played it for about an hour and it got real quiet. But it might have been the chord or the guitar. I turned it off and tried it later, and it was fine. So I will see if it happens again?
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Next step.
I watched a long series of youtube videos titled "chasing amp buzz or hum " or something like that. He had a homebuilt 18 watt amp with the same issues as this Ampeg. He saw a very similar glitch from the rectifiers, but he was able to make it go away by adding snubbers across the diodes.
He then went on to try and external heater transformer, a DC heater supply, and elevating the heater supply to 75Vdc, and finally he added a 7H choke to the hv supply.
His snubber's actually worked for him and removed the glitch. But he still had a loud 120Hz and 60Hz hum. The difference was that he had I think 10K in series with his caps. I just added caps, and in my research most people said that just changed the frequency of the ringing, but did not eliminate it. That agrees with my Pspice runs. i tried to add resistors but they didn't seem to do much in Pspice. However I never tried anything like 10K? So I added the higher values of resistance to my Pspice sims and it does reduce the spikes by a lot. Exactly what value cap and what resistor is best will depend on the inductance and capacitances in te circuit that is ringing. So I just ordered a range of parts. Caps I ordered are 0.022uf, 0.0068uF, 0.0033uF. I ordered 1/4 resistors in 500, 1K, 2K, 5K, 10K, 15K, 20K. From the Pspice sims there is a sweet spot where you get a minimum spike. Raise or lower the resistance and it gets higher. Likewise with the cap values. So I will start with the 0.01uF caps I have in the circuit now and try to add the resistors to that and see what it does. Then I will go from there.
He did not get much benefit from using a DC heater voltage, nor from raising the heater circuit to a 75Vdc offset voltage. I don't think his problem was related to heater noise coupling into the signal. I think it was his hv supply just was not filtering the 120Hz enough. That was where his hum was coming from.
When he added his choke to the hv supply his hum issue all but disappeared. I did some looking and they are not that much, so I will probably end up buying a choke to try that in the hv supply.
The amp is quiet enough that if there is anything else running in the room like the furnace, or the kegerator, or the TV, you basically can't hear it. And it doesn't get bad when you turn it up, even at full volume. There is more hiss than hum at high volume. So this is just to experiment and understand how to actually fix this problem. I have other amps that hum/buzz issues, so perhaps this will help with those?
My Fender Champion 600 single ended amp is a humming fool. But I put a better speaker in it and it sounds really nice now, except for the hum. I even have a SS Peavey Vypyr 15W amp that hums pretty badly.
I watched a long series of youtube videos titled "chasing amp buzz or hum " or something like that. He had a homebuilt 18 watt amp with the same issues as this Ampeg. He saw a very similar glitch from the rectifiers, but he was able to make it go away by adding snubbers across the diodes.
He then went on to try and external heater transformer, a DC heater supply, and elevating the heater supply to 75Vdc, and finally he added a 7H choke to the hv supply.
His snubber's actually worked for him and removed the glitch. But he still had a loud 120Hz and 60Hz hum. The difference was that he had I think 10K in series with his caps. I just added caps, and in my research most people said that just changed the frequency of the ringing, but did not eliminate it. That agrees with my Pspice runs. i tried to add resistors but they didn't seem to do much in Pspice. However I never tried anything like 10K? So I added the higher values of resistance to my Pspice sims and it does reduce the spikes by a lot. Exactly what value cap and what resistor is best will depend on the inductance and capacitances in te circuit that is ringing. So I just ordered a range of parts. Caps I ordered are 0.022uf, 0.0068uF, 0.0033uF. I ordered 1/4 resistors in 500, 1K, 2K, 5K, 10K, 15K, 20K. From the Pspice sims there is a sweet spot where you get a minimum spike. Raise or lower the resistance and it gets higher. Likewise with the cap values. So I will start with the 0.01uF caps I have in the circuit now and try to add the resistors to that and see what it does. Then I will go from there.
He did not get much benefit from using a DC heater voltage, nor from raising the heater circuit to a 75Vdc offset voltage. I don't think his problem was related to heater noise coupling into the signal. I think it was his hv supply just was not filtering the 120Hz enough. That was where his hum was coming from.
When he added his choke to the hv supply his hum issue all but disappeared. I did some looking and they are not that much, so I will probably end up buying a choke to try that in the hv supply.
The amp is quiet enough that if there is anything else running in the room like the furnace, or the kegerator, or the TV, you basically can't hear it. And it doesn't get bad when you turn it up, even at full volume. There is more hiss than hum at high volume. So this is just to experiment and understand how to actually fix this problem. I have other amps that hum/buzz issues, so perhaps this will help with those?
My Fender Champion 600 single ended amp is a humming fool. But I put a better speaker in it and it sounds really nice now, except for the hum. I even have a SS Peavey Vypyr 15W amp that hums pretty badly.
I once had a Fender in repair, was it a Champ or an Excelsior (I believe it was the Excelsior) with plenty of hum.
Nothing could drive it away and I tried a choke I had laying around (I believe it was a 15H).
No hum any more; at least it was very very very faint.
Worth a try since you tried almost everything.
Nothing could drive it away and I tried a choke I had laying around (I believe it was a 15H).
No hum any more; at least it was very very very faint.
Worth a try since you tried almost everything.
Choke
I ordered one of these.
Not real sure about the current, but I think that's plenty. I run my amps pretty cool in terms of idle current.
https://www.amplifiedparts.com/products/P-T158Q
It's the only amp I have that when I went back to playing my Egnater Rebel 30, it felt like something was missing? So it's worth the effort for sure. But I won't be getting rid of the Rebel 30 anytime soon either. That thing just has so much range, and it is dead silent.
I ordered one of these.
Not real sure about the current, but I think that's plenty. I run my amps pretty cool in terms of idle current.
https://www.amplifiedparts.com/products/P-T158Q
It's the only amp I have that when I went back to playing my Egnater Rebel 30, it felt like something was missing? So it's worth the effort for sure. But I won't be getting rid of the Rebel 30 anytime soon either. That thing just has so much range, and it is dead silent.
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CapnDenny1, did you have any luck with the choke?
I've completed my changes including changing to UF4007s. I tried a heater lift and temporarily jumpering in a snubber (about 3.8k R and .01uF) on each rectified winding. No luck. At least I fixed the loose part and the added grid stoppers on the preamp tubes reduced the RFI noise.
I'll run through it stage by stage again, but my current theory is that the unshielded bottom of the PT is radiating interference into the chassis.
I've completed my changes including changing to UF4007s. I tried a heater lift and temporarily jumpering in a snubber (about 3.8k R and .01uF) on each rectified winding. No luck. At least I fixed the loose part and the added grid stoppers on the preamp tubes reduced the RFI noise.
I'll run through it stage by stage again, but my current theory is that the unshielded bottom of the PT is radiating interference into the chassis.
I didn't try the choke yet.
I have been enjoying the amp.
Moving that heater wire next to the signal at the R14-R13 junction got rid of about 90% of my problem. I can hear the noise, but it's so low I can live with it.
The heater lift or even a DC heater won't fix the coupling issue I found. I tried the DC heater and it made no difference. But that was before I moved the heater wire.
The remaining hum I have is now in the power amp. Before I thought the power amp was silent. So either it changed, or the other hum issue was so loud the power amp hum was insignificant.
The push pull output is supposed to cancel ps ripple, and the choke goes past the power tube plate connections. So I don't think it will help much.
The hum is low enough now that it's not driving me up the wall. So working on it more makes me think I am going overboard. If this were a hi-fi amp it would not be acceptable, but for a guitar amp it's OK.
I will give the choke a quick try.
The biggest helps were the moving the heater wire, and the humdinger pot. The pot had to be a little off center for minimum hum. So the grounded center tap would not have been the minimum hum.
I have been enjoying the amp.
Moving that heater wire next to the signal at the R14-R13 junction got rid of about 90% of my problem. I can hear the noise, but it's so low I can live with it.
The heater lift or even a DC heater won't fix the coupling issue I found. I tried the DC heater and it made no difference. But that was before I moved the heater wire.
The remaining hum I have is now in the power amp. Before I thought the power amp was silent. So either it changed, or the other hum issue was so loud the power amp hum was insignificant.
The push pull output is supposed to cancel ps ripple, and the choke goes past the power tube plate connections. So I don't think it will help much.
The hum is low enough now that it's not driving me up the wall. So working on it more makes me think I am going overboard. If this were a hi-fi amp it would not be acceptable, but for a guitar amp it's OK.
I will give the choke a quick try.
The biggest helps were the moving the heater wire, and the humdinger pot. The pot had to be a little off center for minimum hum. So the grounded center tap would not have been the minimum hum.
Thanks for replying. Glad to hear you've got it to a good state.
You mentioned the heater jumper, that fired a few synapses... I did the same re-routing on mine, but in my rebuild last week I mashed that jumper back down close to the board with the coupling cap. I thought I heard a hum increase, I'll have to correct that. I recorded the amp before making my latest changes, but I think I may have deleted the file...
Actually I think the choke between the plate supply and the screen supply may be useful as the screen supposedly has a greater effect on the tube's operation. Plus it will clean up the supply of all the preamp tubes downstream. If I get tempted to go to extremes I would add a bias balance control, and maybe an AC balance control.
You mentioned the heater jumper, that fired a few synapses... I did the same re-routing on mine, but in my rebuild last week I mashed that jumper back down close to the board with the coupling cap. I thought I heard a hum increase, I'll have to correct that. I recorded the amp before making my latest changes, but I think I may have deleted the file...
Actually I think the choke between the plate supply and the screen supply may be useful as the screen supposedly has a greater effect on the tube's operation. Plus it will clean up the supply of all the preamp tubes downstream. If I get tempted to go to extremes I would add a bias balance control, and maybe an AC balance control.
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I went ahead and tried the choke. It either made no difference or the noise level increased. Pretty sure it just stayed the same.
It's not anything to do with the preamp tubes. With nothing plugged in I can turn the clean volume all the way up and all I get to increase is a small amount of hiss.
The humdinger is a balance pot between the heater wires to ground. It did help.
It sounds like the same rectifier hash that I have been fighting.
I am working on the VOX AC4 C1 BL basket case for my son. So once I get that off the bench I may spend some more time on this one..
I played it for about an hour today and it sure sounds nice. I put the long decay reverb tank in place of the medium decay that sounded too tight. This one sounds pretty loose? Just never happy I guess.
Next to my Egnater Rebel 30 this is my next best sounding amp. If it didn't have this noise issue it might even surpass the Rebel 30 in my favorites. The Rebel 30 has a lot variation available, this is just 2 sounds.
It's not anything to do with the preamp tubes. With nothing plugged in I can turn the clean volume all the way up and all I get to increase is a small amount of hiss.
The humdinger is a balance pot between the heater wires to ground. It did help.
It sounds like the same rectifier hash that I have been fighting.
I am working on the VOX AC4 C1 BL basket case for my son. So once I get that off the bench I may spend some more time on this one..
I played it for about an hour today and it sure sounds nice. I put the long decay reverb tank in place of the medium decay that sounded too tight. This one sounds pretty loose? Just never happy I guess.
Next to my Egnater Rebel 30 this is my next best sounding amp. If it didn't have this noise issue it might even surpass the Rebel 30 in my favorites. The Rebel 30 has a lot variation available, this is just 2 sounds.
I've finally tried the heater hum balance pot, and I think that is the key, now I can null out the 120hz and harmonics buzz. Referencing the pot wiper to +15v (conveniently available on R64) reduced the 60hz hum also. I'm going to call this a success.
Yes, this Ampeg is a nice sounding amp.
Yes, this Ampeg is a nice sounding amp.
I will give that a try. For capacitive coupling, like what I fixed by moving the heater wires, the DC offset doesn't matter. But if what is left is coupling from the heater to the cathode, then it might help? I will give it a try and report back on my results.
My initial test was with a 100k pot, then I used a more typical for the application 100 ohm pot. I only get the increased 60hz with the 100k to ground, with the 100ohm the hum is reduced equally whether referenced to ground to +15v, and is as good as the 100k referenced to 15v. So I'd say don't bother, assuming you have a pot in the range of 100 ohms.
I spent a little time on the amp today, troubleshooting.
I pulled V3, the phase splitter tune and the hum went way down. I actually put my multimeter on the speaker terminals. With V3 in I measured 4mV of noise.
I also repeated my test of shorting the receive input. With the receive input shorted the signal at the speaker went to 1mV and the hum was almost imperceptible.
So I looked at the layout and decided to try the re-routing of the heater wires to V3. Unfortunately I still had 4mV of noise after that change.
I also readjusted the idle current to around 34 mA, it was down around 24mA. That really didn't have much effect, except maybe the amp has a tad more sparkle now?
So by removing V3 and getting no hum I know my hum is NOT coming from the output stage. Likewise the shorted Receive input also reducing the hum shows it is ahead of there.
When in the Gain mode, the master volume controls the signal going to the following stages. Turning this down does not have any effect on the hum level.
The only thing between the Master Volume and the Receive input, is an op-amp circuit that drives the Send Output, and the rest of the amp. I wouldn't expect an op-amp circuit to have hum, but those signals are routed to the front panel for the Send/Receive jacks. That also has the power indicator and the Stand-By switch, both what I would call noisy sources.
I am going to repeat these tests, because I'm not 100% sure about the results. I can use the speaker voltage to give a quantitative measurement as well.
A signal of 4mV of hum works out to be about -40dB down from a 1V signal. Looking at a typical Eminence speaker's response curve it would have a 90dB spl at 60Hz with 1W applied. So that means I have a hum level from the speaker at 1ft of 90-40 = 50dB. That level is usually what is referred to as a "Quiet Office". Normal conversation is about 60dB, and bedroom level is 40dB. If I could get the voltage at the speaker down to 1mV, that would reduce the sound level another 6dB.
I pulled V3, the phase splitter tune and the hum went way down. I actually put my multimeter on the speaker terminals. With V3 in I measured 4mV of noise.
I also repeated my test of shorting the receive input. With the receive input shorted the signal at the speaker went to 1mV and the hum was almost imperceptible.
So I looked at the layout and decided to try the re-routing of the heater wires to V3. Unfortunately I still had 4mV of noise after that change.
I also readjusted the idle current to around 34 mA, it was down around 24mA. That really didn't have much effect, except maybe the amp has a tad more sparkle now?
So by removing V3 and getting no hum I know my hum is NOT coming from the output stage. Likewise the shorted Receive input also reducing the hum shows it is ahead of there.
When in the Gain mode, the master volume controls the signal going to the following stages. Turning this down does not have any effect on the hum level.
The only thing between the Master Volume and the Receive input, is an op-amp circuit that drives the Send Output, and the rest of the amp. I wouldn't expect an op-amp circuit to have hum, but those signals are routed to the front panel for the Send/Receive jacks. That also has the power indicator and the Stand-By switch, both what I would call noisy sources.
I am going to repeat these tests, because I'm not 100% sure about the results. I can use the speaker voltage to give a quantitative measurement as well.
A signal of 4mV of hum works out to be about -40dB down from a 1V signal. Looking at a typical Eminence speaker's response curve it would have a 90dB spl at 60Hz with 1W applied. So that means I have a hum level from the speaker at 1ft of 90-40 = 50dB. That level is usually what is referred to as a "Quiet Office". Normal conversation is about 60dB, and bedroom level is 40dB. If I could get the voltage at the speaker down to 1mV, that would reduce the sound level another 6dB.
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Ok, I have been busy with other stuff, one project was rebuilding a Stratocaster. So tonight I was playing the guitar through the Ampeg. It sounded really nice, except the hum was bugging me.
I repeated the last test. If I plug my guitar cable into the Receive input of the effects loop and turn the guitar down to zero, it is silent.
I don't use an effects loop so I could just jumper the whole thing out. But I suspect it wouldn't help. It has to be getting into the Send side of the opamp. I can't see how though?
I think I will do a little more testing to try and determine how it is getting in.
There are two inputs to the opamp. The signal and a signal blanking circuit to blank the sound when switching between the gain channel and the clean channel. So I can remove the transistors, at least one leg to see if they are the source. Otherwise it may be the resistors feeding the signal. Perhaps that signal path is close to a heater circuit. That was the cause of the worst of the noise I fixed already.
If I get this last one this amp will be as silent as my Fender Princeton,which is silent.
I'm gonna get this sucker or else!
I repeated the last test. If I plug my guitar cable into the Receive input of the effects loop and turn the guitar down to zero, it is silent.
I don't use an effects loop so I could just jumper the whole thing out. But I suspect it wouldn't help. It has to be getting into the Send side of the opamp. I can't see how though?
I think I will do a little more testing to try and determine how it is getting in.
There are two inputs to the opamp. The signal and a signal blanking circuit to blank the sound when switching between the gain channel and the clean channel. So I can remove the transistors, at least one leg to see if they are the source. Otherwise it may be the resistors feeding the signal. Perhaps that signal path is close to a heater circuit. That was the cause of the worst of the noise I fixed already.
If I get this last one this amp will be as silent as my Fender Princeton,which is silent.
I'm gonna get this sucker or else!