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Thread: Marshall VS100R hum problem

  1. #36
    "Thermionic Apocalypse" -JT nickb's Avatar
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    ***sigh*** I know it was a long shot.

    The slight pop is the power on/off muting circuit dropping out I would guess. That sounds about normal.

    For now, I'm out of good ideas.

    The 30 second delay has to be a clue. Also, we know the tube is involved. Let's try to eliminate the heater current. Disconnect the transformer heater winding (blue & brown I think) and feed the board heater supply 12.6V or so from a power supply or batteries. Turn the amp on then connect the heater supply and observe what happens to the hum over a minute or so. Leave the pots down and the guitar unplugged as before. That's just confusing us with more noise. Switch off and disconnect the heater supply.
    Experience is something you get, just after you really needed it.

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    Quote Originally Posted by nickb View Post
    ***sigh*** I know it was a long shot.

    The slight pop is the power on/off muting circuit dropping out I would guess. That sounds about normal.

    For now, I'm out of good ideas.

    The 30 second delay has to be a clue. Also, we know the tube is involved. Let's try to eliminate the heater current. Disconnect the transformer heater winding (blue & brown I think) and feed the board heater supply 12.6V or so from a power supply or batteries. Turn the amp on then connect the heater supply and observe what happens to the hum over a minute or so. Leave the pots down and the guitar unplugged as before. That's just confusing us with more noise. Switch off and disconnect the heater supply.

    Ok. I have a laptop power supply that has 12 and 14 volts DC. The filament winding of the transformer pushes 13.99v AC so Im going to use the 14VDC on the power supply.

    1) No guitar plugged in, clean channel, all pots on zero, heater filament circuit disconnected from transformer and connected to 14VDC power supply: I turned on the amp, and the hum is there; increases just a tiny bit in time. If I disconnect the power supply the hum doesn't change at all. The overdrive channel same behavior.

    Comparing the loudness of the hum between the one produced with the transformer and the one produced with the power supply, the one with the transformer is louder

  3. #38
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Can you please repeat but measure the VAC across the speaker so we have some numbers to work with.

    You are really good at following directions It makes it so much easier
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    Quote Originally Posted by nickb View Post
    Can you please repeat but measure the VAC across the speaker so we have some numbers to work with.

    You are really good at following directions It makes it so much easier
    Here we go:
    Everything on zero, no guitar plugged in, filament circuit connected to transformer:
    Clean Channel, AC Voltage across the speaker= 0.25 (silent for about 5 seconds, then hum increases in time; it takes about 3 minutes to reach 0.22v but varies up to 0.26 even 0.27v after 5 minutes
    After this I switch to overdrive channel and voltage across speaker= 0.205v

    Everything on zero, no guitar plugged in, filament circuit connected to 14vdc power supply:
    Clean Channel, AC Voltage across the speaker= 0.015v (similar behavior but jumps in values around 0.015v, 0.010,0.012 it doesn't take that long to get those values)

    Same situation, overdrive channel, ac voltage across speaker= 0.001v stable!

    I hope this helps. Thanks a lot

  5. #40
    Old Timer J M Fahey's Avatar
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    It sure does.
    Clearly hum is getting in through the tube,jumping from filament to cathode.

    Didnīt even look at the schematic yet, but you need to turn that 12VAC filament supply you curently have into a 12V DC clean one ... no big deal
    Juan Manuel Fahey

  6. #41
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    Quote Originally Posted by avoverdu View Post
    Here we go:
    Everything on zero, no guitar plugged in, filament circuit connected to transformer:
    Clean Channel, AC Voltage across the speaker= 0.25 (silent for about 5 seconds, then hum increases in time; it takes about 3 minutes to reach 0.22v but varies up to 0.26 even 0.27v after 5 minutes
    After this I switch to overdrive channel and voltage across speaker= 0.205v

    Everything on zero, no guitar plugged in, filament circuit connected to 14vdc power supply:
    Clean Channel, AC Voltage across the speaker= 0.015v (similar behavior but jumps in values around 0.015v, 0.010,0.012 it doesn't take that long to get those values)

    Same situation, overdrive channel, ac voltage across speaker= 0.001v stable!

    I hope this helps. Thanks a lot
    Well that proves it's coming from the heater. Have you acutally tried a new tube? I thought you had.
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  7. #42
    Old Timer J M Fahey's Avatar
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    Iīm somewhat worried that on schematic kindly supplied in post #6 , filament wires leaving btransformer (page 4) are both Blue, while those reaching the tube on page 1 , one of them through fuse F2, are one Blue and one Brown, yet I see no point showing the colour change.
    Maybe younger sharper eyes can find that point.

    I was about to suggest a bridge + 2 capacitors and 1 resistor simple and clean DC supply but hesitate to do so until the doubt is cleared.
    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post
    Iīm somewhat worried that on schematic kindly supplied in post #6 , filament wires leaving btransformer (page 4) are both Blue, while those reaching the tube on page 1 , one of them through fuse F2, are one Blue and one Brown, yet I see no point showing the colour change.
    Maybe younger sharper eyes can find that point.

    I was about to suggest a bridge + 2 capacitors and 1 resistor simple and clean DC supply but hesitate to do so until the doubt is cleared.
    The heater supply cables are brown and measure 13.9 vAC between the two terminals. They go across a fuse (F2) and to the tube socket. I checked all the connections there and they are fine.

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    Quote Originally Posted by nickb View Post
    Well that proves it's coming from the heater. Have you acutally tried a new tube? I thought you had.
    I have tried several tubes: Marshall original ones, EH, Chinese and nothing.
    Last thing I did I placed the best tube I have (sovtek 12ax7 LPS which I have on V1 on another amp), clean channel and the AC voltage across the speaker = around 1.8 vAC. and overdrive channel = around 1v. Clearly the hum is louder with this tube. Why?? why is it always louder on the clean channel when you compare it with the overdrive channel?

  10. #45
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    Quote Originally Posted by avoverdu View Post
    I have tried several tubes: Marshall original ones, EH, Chinese and nothing.
    Last thing I did I placed the best tube I have (sovtek 12ax7 LPS which I have on V1 on another amp), clean channel and the AC voltage across the speaker = around 1.8 vAC. and overdrive channel = around 1v. Clearly the hum is louder with this tube. Why?? why is it always louder on the clean channel when you compare it with the overdrive channel?
    OK one more test. Disconnect your DC supply and measure the DC resistance between pin 9 of the tube socket and ground.
    Experience is something you get, just after you really needed it.

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    Quote Originally Posted by nickb View Post
    OK one more test. Disconnect your DC supply and measure the DC resistance between pin 9 of the tube socket and ground.
    Ok, I didn't get that. Last test I did with the other tube was using the transformer. Can you give me more detail?

  12. #47
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    Quote Originally Posted by avoverdu View Post
    Ok, I didn't get that. Last test I did with the other tube was using the transformer. Can you give me more detail?
    Remove the tube. Locate pin 9 on the socket and put one probe in to make contact with pin. Put the other probe on a ground on the preamp board and measure the DC resistance. It should be very low.


    I just thought of something else to check once it's all hooked up again. Measure the DCV on pin 3 and pin 8 to ground of thetube with it in place.

    These cathode follower stages have a pretty high heater to cathode voltage applied. Often too much for some tubes. This design is bad as there is no standby switch IMHO so when cold the full plate voltage is applied to the grid and that exceeds the tube limits. Sometimee I fit an LED or neon between the grid and cathode of the follower stage. But we're straying ...
    Last edited by nickb; 12-06-2017 at 07:58 PM.
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    Quote Originally Posted by nickb View Post
    Remove the tube. Locate pin 9 on the socket and put one probe in to make contact with pin. Put the other probe on a ground on the preamp board and meauer the Dc resistance it shuldbe very low.


    I just thought of something else to check once it's all hooked up again. Measure the DCV on pin 3 and pin 8 to ground of thetube with it in place.

    These cathode follower stages have a pretty high heater to cathode voltage applied. Often too much for some tubes. This design is bad as there is no standby switch IMHO so when cold the full plate voltage is applied to the grid and that exceeds the tube limits. Sometimee I fit an LED or neon between the grid and cathode of the follower stage. But we're straying ...
    Ok, tube is out, amp is off: Resistance between pin 9 and a nearby ground= 0.3 Ohm, from pin 9 to the main ground connection to chassis= 10.1 Ohm

    Now I put the tube in place, turned on the amp and read: DCV from pin 3 to ground = 126v (it starts low but increases as the hum does)
    DCV from pin 8 to ground = 0.912v (and i hear some radio interference)

    Let me know

  14. #49
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    Quote Originally Posted by avoverdu View Post
    Ok, tube is out, amp is off: Resistance between pin 9 and a nearby ground= 0.3 Ohm, from pin 9 to the main ground connection to chassis= 10.1 Ohm

    Now I put the tube in place, turned on the amp and read: DCV from pin 3 to ground = 126v (it starts low but increases as the hum does)
    DCV from pin 8 to ground = 0.912v (and i hear some radio interference)

    Let me know
    I was thinking that perhaps the heater circuit is floating due to a bad pin 9 connection or the cathode voltage was way too high but everythng checks out as it should. There are thousaunds of these amps out there that work happily without resorting to a DC supply. Somehow, heater current is getting into the ground. We know that from an earlier test where we shorted out the signal path with the tube in place and still got the hum.

    I'm stumped.
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  15. #50
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    Maybe some H-K leakage measurements on the board?
    The comments about H-K leakage by nick correspond to problems with using the LPS type tubes in cathode followers, which were noted in the measurements with the LPS tube installed.
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  16. #51
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    Also, if JM's suggestion of converting to DC heater implies he thinks the noise level is normal for this model, I'm good with that too.
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  17. #52
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by g1 View Post
    Also, if JM's suggestion of converting to DC heater implies he thinks the noise level is normal for this model, I'm good with that too.
    250mV rms is a lot of hum. I don't recall these being especially bad in that respect. Am I mistaken?

    For certain HK leakage is a component but how is it getting into the ground? Unless of course the test in post 19 is wrong. I wonder if there has been a misunderstanding? I took it that with R48 and R77 shorted and with the tube in the hum was there. On a re-read maybe that is not the what was meant. Perhaps the OP could clarify?

    A cheap alternative to a DC supply would be to elevate the heater supply up to 100V or so with a divider* off the plate supply and isolating pin 9 from ground.

    *PS
    i.e. isolate pin 9 from ground by cutting track. Add 100K 0.5W from ground to pin, add 390K 1W from pin 9 to to pin 1, add 2.2uF 100V ( bigger uF and bigger volts would be OK). Check voltage across 100K after rework. Aim for 75V to 100V.
    Last edited by nickb; 12-06-2017 at 11:18 PM. Reason: Added PS
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  18. #53
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    Quote Originally Posted by g1 View Post
    Also, if JM's suggestion of converting to DC heater implies he thinks the noise level is normal for this model, I'm good with that too.
    No, I donīt think that such high hum is normal, by any means.

    But I also know there is a large batch of 12AX7 out there with higher than normal cathode to filament loss/coupling which lets hum jump where it should not.

    Iīm quite certain Marshall did not meet that problem while designing and making this amplifier definitely not with the tubes they were using at the time, and one does not solve nonexistent problems.

    But since this amp and tube combination hums a lot, it does not hurt at all to improve filament supply, specially because itīs easy and inexpensive.

    What if you get a similar problem in the future?
    Then and only IF so, repeat the solution.
    Last edited by J M Fahey; 12-07-2017 at 02:35 AM.
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    Quote Originally Posted by nickb View Post
    I was thinking that perhaps the heater circuit is floating due to a bad pin 9 connection or the cathode voltage was way too high but everythng checks out as it should. There are thousaunds of these amps out there that work happily without resorting to a DC supply. Somehow, heater current is getting into the ground. We know that from an earlier test where we shorted out the signal path with the tube in place and still got the hum.

    I'm stumped.
    Well, heater current goes to ground through the tube filaments center tap. Is there any chance that any of the ICs are getting the hum in the signal?

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    Quote Originally Posted by nickb View Post
    250mV rms is a lot of hum. I don't recall these being especially bad in that respect. Am I mistaken?

    For certain HK leakage is a component but how is it getting into the ground? Unless of course the test in post 19 is wrong. I wonder if there has been a misunderstanding? I took it that with R48 and R77 shorted and with the tube in the hum was there. On a re-read maybe that is not the what was meant. Perhaps the OP could clarify?

    A cheap alternative to a DC supply would be to elevate the heater supply up to 100V or so with a divider* off the plate supply and isolating pin 9 from ground.

    *PS
    i.e. isolate pin 9 from ground by cutting track. Add 100K 0.5W from ground to pin, add 390K 1W from pin 9 to to pin 1, add 2.2uF 100V ( bigger uF and bigger volts would be OK). Check voltage across 100K after rework. Aim for 75V to 100V.
    I lost track. What you suggest I can do it but it seems complicated; do you want me to run some tests again??? If we want to be sure just tell me what to do and I will

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    Quote Originally Posted by J M Fahey View Post
    No, I donīt think that such high hum is normal, by any means.

    But I also know there is a large batch of 12AX7 out there with higher than normal cathode to filament loss/coupling which lets hum jump where it should not.

    Iīm quite certain Marshall did not meet that problem while designing and making this amplifier definitely not with the tubes they were using at bthe time, and one does not solve nonexistent problems.

    But since this amp and tube combination hums a lot, it does not hurt at all to improve filament supply, specially because itīs easy and inexpensive.

    Whay if you get a similar problem in the future?
    Then and only IF so, repeat the solution.
    What components should I buy to convert the filament supply (around 14vAC) to an equivalent DC heater supply?

  22. #57
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    Quote Originally Posted by avoverdu View Post
    Well, heater current goes to ground through the tube filaments center tap. Is there any chance that any of the ICs are getting the hum in the signal?
    No heater current flows in the center tap. It starts at the transformer, through the filaments and back to the transformer. The center tap serves only to define the average potential of the heater. For current to flow in the center tap there would have to be another connection between the transformer heater winding and ground.

    Please read post 52 and post 19. Is it true that when R48 and R77 were shorted with the tube in place there was still hum?
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  23. #58
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    I'm not sure it would be enough, but have you tried a spiral filament tube?
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  24. #59
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    That was the LPS tube he got the much worse reading with. (post #44)
    In my experience they have issues with heater to cathode voltage which is why I thought it gave the worse reading.

    I'm not keen on the 14V heaters, be they AC or DC.
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    Thanks g1. I missed that. Do you know why they have issues with H to K voltages? I've not encountered that problem.
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    Not sure why, it only seems to be with high cathode voltages. I had problems with them mostly in Marshall cathode follower circuits.

    edit: Only JJ 12AX7 buzzing in Cathode follower position, Why?
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    Quote Originally Posted by avoverdu View Post
    What components should I buy to convert the filament supply (around 14vAC) to an equivalent DC heater supply?
    Letīs go step by step, I hate giving blanket answers, prefer to explain the method so it can be applied somewhere else if needed:

    1) 14VAC will give you 1.41 times that peak, but in a bridge rectifier we have 2 diodes in series in the path, so :
    14VAC*1.4142=17.8v .
    each diode drops about 0.7V so we end with 17.8-1.4=18.4VDC .
    We can use a 2200uFx25V electrolytic as first capacitor.

    2) we need to drop from 18.4V to 12.6V , and filament current is 0.15A so we need a series resistor of:
    (18.4-12.6)/0.15=5.8V/0.15A=38.7 ohms.

    Power dissipation will be 5.8V*0.15A=0.87W

    Nice numbers but we live in the Real World and suppliers can supply what they have on their shelves so, letīs pick "standard" values, close enough.

    To begin with, raw voltage will drop a little respect to the ideal value, so I think a 33 ohm resistor will be fine ; IF available a 36 ohm one *might* be closer ... or not, as said above transformer winding might drop a little more than expected under load.

    To begin with, I doubt Marshall wanted to cook te tube, what for? , so I bet they "expected" that filament winding to give around 12.6V ... under load that is, so .... I have a hunch that the 33 ohm one will be fine.

    As of dissipation, a 1W one WILL work, no doubt, but for peace of mind specify 2W .

    And after that resistor, add a second 2200*25 capacitor, this combination is called CRC filtering, because.... weeeeellll ..... you can imagine why
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  28. #63
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    Quote Originally Posted by avoverdu View Post
    I lost track. What you suggest I can do it but it seems complicated; do you want me to run some tests again??? If we want to be sure just tell me what to do and I will
    Have you abandoned this discussion? I was waiting for a response to this post 52

    I don't think it's any more complicated than adding a DC supply - cut one track, add two resistors and one capacitor. On the other hand the DC supply needs wiring on the input and output, maybe add one resistor. It's more expensive but is guaranteed to work.
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    Quote Originally Posted by J M Fahey View Post
    Letīs go step by step, I hate giving blanket answers, prefer to explain the method so it can be applied somewhere else if needed:

    1) 14VAC will give you 1.41 times that peak, but in a bridge rectifier we have 2 diodes in series in the path, so :
    14VAC*1.4142=17.8v .
    each diode drops about 0.7V so we end with 17.8-1.4=18.4VDC .
    We can use a 2200uFx25V electrolytic as first capacitor.

    2) we need to drop from 18.4V to 12.6V , and filament current is 0.15A so we need a series resistor of:
    (18.4-12.6)/0.15=5.8V/0.15A=38.7 ohms.

    Power dissipation will be 5.8V*0.15A=0.87W

    Nice numbers but we live in the Real World and suppliers can supply what they have on their shelves so, letīs pick "standard" values, close enough.

    To begin with, raw voltage will drop a little respect to the ideal value, so I think a 33 ohm resistor will be fine ; IF available a 36 ohm one *might* be closer ... or not, as said above transformer winding might drop a little more than expected under load.

    To begin with, I doubt Marshall wanted to cook te tube, what for? , so I bet they "expected" that filament winding to give around 12.6V ... under load that is, so .... I have a hunch that the 33 ohm one will be fine.

    As of dissipation, a 1W one WILL work, no doubt, but for peace of mind specify 2W .

    And after that resistor, add a second 2200*25 capacitor, this combination is called CRC filtering, because.... weeeeellll ..... you can imagine why
    Ok, we are on holidays here and I canīt get the parts until Monday. But I have a question: I connected the filament heater to my 14VDC supply, turned the amp on and connected my guitar and there is still hum. The hum on the clean channel is loud!!! so I don't see the point on convert the AC to DC if the hum will stay after the conversion. Am I wrong?

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    Quote Originally Posted by nickb View Post
    Have you abandoned this discussion? I was waiting for a response to this post 52

    I don't think it's any more complicated than adding a DC supply - cut one track, add two resistors and one capacitor. On the other hand the DC supply needs wiring on the input and output, maybe add one resistor. It's more expensive but is guaranteed to work.
    Sorry mate I'm still here but we are on holidays and the stores are closed until Monday. Ill let you know when I do those changes. I appreciate it!!!

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    Quote Originally Posted by avoverdu View Post
    Ok, we are on holidays here and I canīt get the parts until Monday. But I have a question: I connected the filament heater to my 14VDC supply, turned the amp on and connected my guitar and there is still hum. The hum on the clean channel is loud!!! so I don't see the point on convert the AC to DC if the hum will stay after the conversion. Am I wrong?
    Just asking the obvious. You did disconnect the heaters from the transformer when you clipped in the 14VDC?

    nosaj

  32. #67
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    Quote Originally Posted by avoverdu View Post
    Ok, we are on holidays here and I canīt get the parts until Monday. But I have a question: I connected the filament heater to my 14VDC supply, turned the amp on and connected my guitar and there is still hum. The hum on the clean channel is loud!!! so I don't see the point on convert the AC to DC if the hum will stay after the conversion. Am I wrong?
    In post 39 you stated there was no hum using the DC supply. All the tests and answers since that point have been based on that. You are now saying the opposite?
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    Quote Originally Posted by nickb View Post
    In post 39 you stated there was no hum using the DC supply. All the tests and answers since that point have been based on that. You are now saying the opposite?
    The post 39 was without the guitar. Today I plugged the guitar with the DC supply hooked to the filament just to be sure that it worth to make the conversion and it hums.

    ANYWAY I FOUND SOMETHING ELSE THAT MAYBE IS MESSING WITH ALL THE THEORIES: THE TIP OF THE JACK WAS SUPPOSED TO GROUND THE SIGNAL WHEN NOTHING IS PLUGGED IN AND IT WAS NOT MAKING CONNECTION. I GUESS THIS CHANGES EVERYTHING. PLEASE LET ME KNOW

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    Quote Originally Posted by nosaj View Post
    Just asking the obvious. You did disconnect the heaters from the transformer when you clipped in the 14VDC?

    nosaj
    Yes I did

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    Quote Originally Posted by avoverdu View Post
    The post 39 was without the guitar. Today I plugged the guitar with the DC supply hooked to the filament just to be sure that it worth to make the conversion and it hums.

    ANYWAY I FOUND SOMETHING ELSE THAT MAYBE IS MESSING WITH ALL THE THEORIES: THE TIP OF THE JACK WAS SUPPOSED TO GROUND THE SIGNAL WHEN NOTHING IS PLUGGED IN AND IT WAS NOT MAKING CONNECTION. I GUESS THIS CHANGES EVERYTHING. PLEASE LET ME KNOW
    It should only make a difference if the second shorting contact were broken also as that one mutes the signal input to the tube. Earlier you said that it hummed with all the knobs at zero and no guitar plugged in. All the tests were supposed to done that way. If you changed them at any time it invalidates everything.

    Right now I have zero confidence in any of the results - there are too many contradictions. I suggest you fix the jack and carefully check the operation of both switching contacts and we'll re-evaluate the situation.
    Experience is something you get, just after you really needed it.

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