Hey guys,
I bought an echolette m40 from my local ebay site and did some work on it, but I can't get rid of the hum.
What I did so far:
- Set the voltage selector so that the heaters now get 6.3VAC
- Replaced all of the old electrolytes
- Replaces some tube sockets
- Inspected the grounding
A shematic can be found here:
http://music-electronics-forum.com/atta ... -input.jpg
Some measurements of the power rails (that look fine to me):
c37/A2 154VDC 0.6VAC
C36/A1 232VDC 0.8VAC
C35 366VDC 0.9VAC
C34 379VDC 4.2VAC
H1 6.31VAC
H2 6.34VAC
The hum is also there when all of the channels are on 0 volume, and is effected by the main 2 tone controls. Could you give my any pointers on what to look at next?
Echolette M40 hum
Moderators: pompeiisneaks, Colossal
Re: Echolette M40 hum
This is all useful information. Help us out a bit more. Is it 60Hz or 120Hz hum?
Not sure what's what? The 5th string A on the guitar is 110Hz, so a little sharp of that, or tune the low E down to A (55Hz), so 60Hz is a bit sharp of that, but not as high as B.
Sometimes you have both and one will mask the other. In that case, you'll definitely hear the 60Hz and we can start with that. The frequency gives us a clear hint where to look and more importantly, where not to look.
If you are in Europe, your mains is 50Hz, so we are talking 50/100Hz, but the same method applies. Your reference note is between G and Ab.
Not sure what's what? The 5th string A on the guitar is 110Hz, so a little sharp of that, or tune the low E down to A (55Hz), so 60Hz is a bit sharp of that, but not as high as B.
Sometimes you have both and one will mask the other. In that case, you'll definitely hear the 60Hz and we can start with that. The frequency gives us a clear hint where to look and more importantly, where not to look.
If you are in Europe, your mains is 50Hz, so we are talking 50/100Hz, but the same method applies. Your reference note is between G and Ab.
Re: Echolette M40 hum
I measured it on my scope and got a 50 hertz signal.
I also noticed that when I turn my main volume up beyond 8 there suddenly is a loud popping sound (from the speaker), like some kind of feedback maybe. I don't think that this was there a couple of months ago when I put the amp away.
I also noticed that when I turn my main volume up beyond 8 there suddenly is a loud popping sound (from the speaker), like some kind of feedback maybe. I don't think that this was there a couple of months ago when I put the amp away.
Re: Echolette M40 hum
50Hz indicates the hum is from an AC source. In general, make sure any wire carrying AC is away from the audio circuit.
I'm no expert on ripple, but I'm thinking from your posted voltage readings that ripple is not excessive. You might consider swapping C34 if nothing else shows promise. I wouldn't be in a hurry to do this. It would be helpful if someone with a better understanding of this would comment.
The first I'd suspect for 50Hz hum is the audio circuit picking up something from the heaters. Are you able to post some pictures? Someone might spot something.
You might try:
1) using a chopstick or other non-conductive probe to move the heater wires away from the audio wires while the amp is powered so you can listen for a change. You might get lucky.
2) elevate the heaters by using the power tube cathode as if it is the ground point for either the center tap or the faux CT/pair of 100Ω resistors.
Try one thing at a time, otherwise you won't know what gave you a result and what didn't. There is also a chance you will make it worse, so you want to know what to reverse. Actually, making it worse might also be a good hint at the problem.
I'm no expert on ripple, but I'm thinking from your posted voltage readings that ripple is not excessive. You might consider swapping C34 if nothing else shows promise. I wouldn't be in a hurry to do this. It would be helpful if someone with a better understanding of this would comment.
The first I'd suspect for 50Hz hum is the audio circuit picking up something from the heaters. Are you able to post some pictures? Someone might spot something.
You might try:
1) using a chopstick or other non-conductive probe to move the heater wires away from the audio wires while the amp is powered so you can listen for a change. You might get lucky.
2) elevate the heaters by using the power tube cathode as if it is the ground point for either the center tap or the faux CT/pair of 100Ω resistors.
Try one thing at a time, otherwise you won't know what gave you a result and what didn't. There is also a chance you will make it worse, so you want to know what to reverse. Actually, making it worse might also be a good hint at the problem.
Re: Echolette M40 hum
Poking around didn't help, here's an image of the amp:
http://i.imgur.com/ztFANLU.jpg
I don't get your cathode tip, the cathodes of the power tubes are directly grounded themselves right? So how would this elevate the voltage of the heaters? I believe that this would only work in a cathode biased setup.
http://i.imgur.com/ztFANLU.jpg
I don't get your cathode tip, the cathodes of the power tubes are directly grounded themselves right? So how would this elevate the voltage of the heaters? I believe that this would only work in a cathode biased setup.
Re: Echolette M40 hum
I raised the H1 heaters to about 46V above ground like so (the top one):
http://www.valvewizard.co.uk/heater9.jpg
But this didn't help.
http://www.valvewizard.co.uk/heater9.jpg
But this didn't help.
Re: Echolette M40 hum
Ground loop?
Re: Echolette M40 hum
Mains frequency hum is always either heaters coupling to a signal wire or ground hum: almost always ground hum. Everyplace you go is filled with 60Hz or 50Hz hum (depending on where you live). In some designs the hum is self-cancelling; in others, you have to isolate inputs and sometimes outputs to prevent hum signal developing across small intra-chassis resistances. If the amp was once hum-free, verifying and remaking ground connections should do it.
Re: Echolette M40 hum
Heaters are neatly wired and sting tied professionally. I am thinking this isn't where to look. I'll cast my lot with Firestorm and say it's ground related.
You did a cap job -- all the e-caps. I wonder if you rearranged the grounds while you were in there and didn't realize it. Lead placement counts when space is so tight. Did you happen to take a picture of it before you touched it? That might tell you what changed. Otherwise, you might be looking around for a while.
You did a cap job -- all the e-caps. I wonder if you rearranged the grounds while you were in there and didn't realize it. Lead placement counts when space is so tight. Did you happen to take a picture of it before you touched it? That might tell you what changed. Otherwise, you might be looking around for a while.
Re: Echolette M40 hum
Well, I spend a lot of time on the amp, but the hum still remains. It was and is actually both 50 hertz and 100 hertz noise, and the 100Hz is adjustable by the master volume knob.
I replaced all of the single core ground wires (but kept the layout) hoping there was a break somewhere, but this didn't help. I also disconnected the signal from the first tubes and replaced it by a 250K resistor to ground and removed the connections to the "output III", to make the shematic easier for me and to make sure that the inputes are fully muted. I now have this schematic:
http://i.imgur.com/Qos2W2G.png
The hum still remains, and so does the weird feedback like sounding effect that occurs when the master volume is on 10 and the treble is also at a high posistion.
I already ordered 4 new el84's, because I can't be sure that any of the tubes are good since I bought it with these tubes and it never worked withoud hum while I have had it so I'm hoping that they are bad.
I also made sure that the elytes are now at the same position as they originally where.
Any more ideas?
I replaced all of the single core ground wires (but kept the layout) hoping there was a break somewhere, but this didn't help. I also disconnected the signal from the first tubes and replaced it by a 250K resistor to ground and removed the connections to the "output III", to make the shematic easier for me and to make sure that the inputes are fully muted. I now have this schematic:
http://i.imgur.com/Qos2W2G.png
The hum still remains, and so does the weird feedback like sounding effect that occurs when the master volume is on 10 and the treble is also at a high posistion.
I already ordered 4 new el84's, because I can't be sure that any of the tubes are good since I bought it with these tubes and it never worked withoud hum while I have had it so I'm hoping that they are bad.
I also made sure that the elytes are now at the same position as they originally where.
Any more ideas?
Re: Echolette M40 hum
Here is a list of possibilities for hum:
Loose connections or missed solder
Loose hardware -- tighten all jacks and pots
Output not properly biased; tubes seriously mismatched
Bad tubes -- if the new ones are badly matched, it might not help
Bad output transformer (IMO, seems doubtful)
Bad power transformer (IMO, seems doubtful)
Open filament resistors (faux CT)
Bad input jack
Arc on or loose tube socket
Open resistor
Shorted capacitor
Unintended ground (magnifying glass to look for whiskers)
You've got both kinds, so you might be looking for a while. I'd have a hard look at the whole ground scheme. It may not be optimal. It is really hard to say without the amp present.
For 100Hz, one thing that you can do that's simple is pull one tube at a time and see what kills the hum. Start with pulliing only V1. Next, remove V2 (2 tubes pulled), and then pull V3 (the PI tube). When the hum stops, you know the general location in the circuit. If it still hums with the 3 preamp tubes out, the power section is making the hum. If the hum is still present with no tubes, it is likely one of the transformers or maybe an output jack.
Do you know if the rectifiers are good? You can pull the EZ81's and tack some 1n4007 or similar cheap diodes onto the sockets in place of the tubes and see if that makes a difference. EZ81 only use 5 pins, so you've got 4 spares on each socket to play with. I don't believe it will hurt your amp to pull one rectifier tube at a time and power up with no signal briefly to see if one or the other is contributing hum.
One EZ81 is rated for 150mA. 4 x EL84 requires 152mA plus no more than about 8mA for the 3 preamp tubes. Your about 10mA over spec. So, as long as you aren't driving a signal, you really should be OK for a quick test.
Loose connections or missed solder
Loose hardware -- tighten all jacks and pots
Output not properly biased; tubes seriously mismatched
Bad tubes -- if the new ones are badly matched, it might not help
Bad output transformer (IMO, seems doubtful)
Bad power transformer (IMO, seems doubtful)
Open filament resistors (faux CT)
Bad input jack
Arc on or loose tube socket
Open resistor
Shorted capacitor
Unintended ground (magnifying glass to look for whiskers)
You've got both kinds, so you might be looking for a while. I'd have a hard look at the whole ground scheme. It may not be optimal. It is really hard to say without the amp present.
For 100Hz, one thing that you can do that's simple is pull one tube at a time and see what kills the hum. Start with pulliing only V1. Next, remove V2 (2 tubes pulled), and then pull V3 (the PI tube). When the hum stops, you know the general location in the circuit. If it still hums with the 3 preamp tubes out, the power section is making the hum. If the hum is still present with no tubes, it is likely one of the transformers or maybe an output jack.
Do you know if the rectifiers are good? You can pull the EZ81's and tack some 1n4007 or similar cheap diodes onto the sockets in place of the tubes and see if that makes a difference. EZ81 only use 5 pins, so you've got 4 spares on each socket to play with. I don't believe it will hurt your amp to pull one rectifier tube at a time and power up with no signal briefly to see if one or the other is contributing hum.
One EZ81 is rated for 150mA. 4 x EL84 requires 152mA plus no more than about 8mA for the 3 preamp tubes. Your about 10mA over spec. So, as long as you aren't driving a signal, you really should be OK for a quick test.
Re: Echolette M40 hum
I've spend some more time on the amp without real succes. The new tubes didn't fix the hum, so I decided to just disconnect the first stages and first get the powerstage quiet. These are the results:
- No tubes is no sound
- Only powertubes gives my a bit of 50Hz hum, but I think thats normal
- With the phaseinverter added only a bit more 50Hz hum, still normal
- With tube number 4 added the hum increases and 100Hz hum is added, to much hum now
I should add that for this test I connected the grid of the first triode to ground using a 15K resistor (very close to the pin, random value resistor), so that that triode shouldn't produce any sound, but when I disconnect C13 (leading from the first triode to the tone control) the hum decreases (not gone though but a lot less). So somehow the first triode of tube 4 still adds to the hum when its grid is grounded, any thought on that?
I also couldn't find any of the other problems you mentioned
Below is no longer true
To simplify the grounding scheme I disconnected the ground to the first (non present) tubes, so the grounding sheme is now as follows:
- There are two ground points on the chassis, using the bolts from the rectifiers, about 3 cm appart
- one of those grounding points is only used only for the grounding of the powertransformer and the first elyte (C34).
- the other grounding point has 4 intertwined ground wires connected to it:
1. goes to the potentiometer for the heaters
2. goes to the ground for C36 and C37 as well as tube 4 and the tone control grounds
3. goes only to C35
4. goes to the power tubes and the paseinverter and is used to ground one side of the H2 heaters
- No tubes is no sound
- Only powertubes gives my a bit of 50Hz hum, but I think thats normal
- With the phaseinverter added only a bit more 50Hz hum, still normal
- With tube number 4 added the hum increases and 100Hz hum is added, to much hum now
I should add that for this test I connected the grid of the first triode to ground using a 15K resistor (very close to the pin, random value resistor), so that that triode shouldn't produce any sound, but when I disconnect C13 (leading from the first triode to the tone control) the hum decreases (not gone though but a lot less). So somehow the first triode of tube 4 still adds to the hum when its grid is grounded, any thought on that?
I also couldn't find any of the other problems you mentioned
Below is no longer true
To simplify the grounding scheme I disconnected the ground to the first (non present) tubes, so the grounding sheme is now as follows:
- There are two ground points on the chassis, using the bolts from the rectifiers, about 3 cm appart
- one of those grounding points is only used only for the grounding of the powertransformer and the first elyte (C34).
- the other grounding point has 4 intertwined ground wires connected to it:
1. goes to the potentiometer for the heaters
2. goes to the ground for C36 and C37 as well as tube 4 and the tone control grounds
3. goes only to C35
4. goes to the power tubes and the paseinverter and is used to ground one side of the H2 heaters