Hello to all,
About 2 years ago, I built a train wreck clone. Worked ok to me, but there was this hum. Not present when no guitar is plugged in, but noticeable when a guitar is plugged in. Anyway, I decided to re-visit the amp to see if I could reduce the hum further. So I plugged the guitar in and I noticed that when I adjust the volume on the guitar, I could hear some scratchiness. This is usually a sign of DC present so I decided to measure if there was any DC present at the grid of the 12ax7, and sure enough there was, but it was -137mVolts.
Is this normal? I thought that the 1 meg resistor would keep the grid at ground potentia? And negative?
Can some of the gurus here make sense of this and help me figure this out?
Some notes: it seems there some oscillation going on as every once in a while there would be some crackling/popping sound from the speaker. It does go away and I noticed that if I moved the tube around the socket, this sound also goes away.
Everyone's input is greatly appreciated.
Thanks
About 2 years ago, I built a train wreck clone. Worked ok to me, but there was this hum. Not present when no guitar is plugged in, but noticeable when a guitar is plugged in. Anyway, I decided to re-visit the amp to see if I could reduce the hum further. So I plugged the guitar in and I noticed that when I adjust the volume on the guitar, I could hear some scratchiness. This is usually a sign of DC present so I decided to measure if there was any DC present at the grid of the 12ax7, and sure enough there was, but it was -137mVolts.
Is this normal? I thought that the 1 meg resistor would keep the grid at ground potentia? And negative?
Can some of the gurus here make sense of this and help me figure this out?
Some notes: it seems there some oscillation going on as every once in a while there would be some crackling/popping sound from the speaker. It does go away and I noticed that if I moved the tube around the socket, this sound also goes away.
Everyone's input is greatly appreciated.
Thanks
137mV DC there is important, compare it to typical guitar signal around 50/100mV .
It's hard to diagnose without being present, and it still *might* be some instability after all, but to define which one it is, add a series capacitor between the 1M/grid node and the actual jack contact , anything will do, say .02uF any type .
If it stops, it's DC; if not, it's instability and you'll need to check/modify grounding.
Of course, a scope would help a lot.
It's hard to diagnose without being present, and it still *might* be some instability after all, but to define which one it is, add a series capacitor between the 1M/grid node and the actual jack contact , anything will do, say .02uF any type .
If it stops, it's DC; if not, it's instability and you'll need to check/modify grounding.
Of course, a scope would help a lot.
A small negative voltage is to be expected (that is how grid leak bias works, although usually with a 10M resistor). -137mV seems a bit high, though. Fortunately it is in the right direction - a gassy valve can push it positive.
However, even this small voltage should be kept away from pot sliders so use coupling caps as necessary.
Add grid stoppers if they are missing, especially if internal wires are long! A negative grid voltage can also be a sign of parasitic oscillation.
However, even this small voltage should be kept away from pot sliders so use coupling caps as necessary.
Add grid stoppers if they are missing, especially if internal wires are long! A negative grid voltage can also be a sign of parasitic oscillation.
Thanks for your reply.
I added a .02 cap in series with the input and yes no more scratchy sound when turning the volume pot on the guitar. I'm still at a loss as how this voltage appears there. This input is configured like a typical preamp setup; 100K on the plate, 1K on the cathode to ground and 1 Meg from grid to ground. When I plug in a guitar cord and measure the resistance on the other end of the cord, I measure .980 Meg, between chassis and tip, so it is "seeing" the 1 Meg between grid and ground. If it is parasitic oscillation, what can I do to mitigate it?
Hi,
Thank you for your input, however, this is not a grid leak type biasing. This is a typical guitar preamp setup; 100K on the plate, 1K cathode bias resistor and a 1 Meg grid to ground resistor. The other half of the 12aX7 is pretty much setup the same way (1 Meg volume pot) and I don't measure any voltage there. If I put the 68K right at the grid, this would constitute a 68K grid stopper, no?
Regards
A little grid current still happens, whether it is intended to use it for bias or not.
I can't see your circuit, so I don't know what 68k you refer to. If the circuit diagram shows a 68k directly connected to the grid then this is probably a grid stopper and must be wired right at the grid tag on the valveholder. A few inches away will not always work.
I can't see your circuit, so I don't know what 68k you refer to. If the circuit diagram shows a 68k directly connected to the grid then this is probably a grid stopper and must be wired right at the grid tag on the valveholder. A few inches away will not always work.
Ah, it's not the main biasing there , but it still exists, as DF96 wrote
Yes, the typical 68K (47K or 33K are also perfectly fine) is a grid stopper and it's best if soldered straight to the grid socket pin.
Quite certain now you have no oscillation (although Trainwreck preamps are tricky) but it never hurts and will certainly help avoid your local radio intruding uninvited.
Thanks for chiming in Nigel. This is the part where I scratch my head. I metered the ground point where the 1 meg is grounded to the power supply ground and it read 0. All system grounds are via the chassis. Power supply ground is on the one side and preamp ground is on the opposite side, as far as I put it. Any suggestions on how I should rework the grounding?
Have you tried a different 12ax7, e.g. by swapping with another stage in the amp?
The hum means there is a small AC voltage where there shouldn't be, as well as the DC you have found.
I don't know if this adds up, but I am just wondering whether some leakage current between the heater and the cathode could be involved.
I'm presuming you have AC heaters. How is your heater circuit referenced to ground?
The hum means there is a small AC voltage where there shouldn't be, as well as the DC you have found.
I don't know if this adds up, but I am just wondering whether some leakage current between the heater and the cathode could be involved.
I'm presuming you have AC heaters. How is your heater circuit referenced to ground?
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Hi Malcolm,
Thanks for your thoughts. Yes I have tried other 12ax7 and the hum is still there. There is a center tap on the heater wiring that is connected to chassis. I could try lifting the heater wiring to DC by connecting this center tap to the output stage bias. You think this might work? Or, I also have some 12 volt DC regulated wall wart power supplies that I can try. I will have to connect them to pins 4 and 5 to see if that gets rid of the hum. I will post results when I get home after work.
Thank you.
I think that would be worth trying. If it makes no difference then we are probably looking elsewhere.
But if it does make an improvement, you could go further and elevate the heater reference to 35V or so, by using a suitable potential divider across the B+. (Also with a smoothing cap. across the 35V.)
Eureka! I'm happy to report that I found the problem. Malcolm had asked earlier if I had swapped tubes, and yes I did, but the problem was still there, however I was swapping tubes of the same brand ( labeled Baldwin with green label), they were pulls from an organ. I tried about 5 of them and they all had this -137 or so millivolts. So I tried a JJ and the voltage was down to 7 millivolts. I tried an old Japanese 12ax7 and now I have 0 volt reading. Wunderbar, but the hum/buzzing was still there. Malcolm had mentioned some "leakage" current that was interacting between the heater and the cathode, so that got me thinking about using a DC heater supply. I got a bunch of wall wart power supplies and soldered them on to the heater pins 4 and 5. Turned the amp on and buzzing was gone but hum was still there. I then decided to create a pseudo centre tap by putting two 150 ohm resistors for the DC supply. Turned the amp on and voila, hum and buzzing is gone, just hiss. Tried it with the guitar and now that the noise is gone, the amp sound much better.
I suppose with a high gain amp, we have to beware of these noise/hum sources. And don't use these Baldwin 12ax7s for the first stage. Strange though, as the other 2 12ax7s down the chain are still Baldwins but do not have this weird offset voltage. Go figure?
Thank you all for chiming in.
I suppose with a high gain amp, we have to beware of these noise/hum sources. And don't use these Baldwin 12ax7s for the first stage. Strange though, as the other 2 12ax7s down the chain are still Baldwins but do not have this weird offset voltage. Go figure?
Thank you all for chiming in.
That's good news. If the hiss is unacceptable, you can try the following:
Replace the resistors around V1 with metal film type (if they are carbon at present). Reduce the value of the V1 grid-stopper (you can go down to 20k, or even 10k if radio pickup is not a problem in your area).
If hiss is still a problem when the volume control is set to zero, then repeat the above for the first gain stage after the volume control.
I think you should be OK with AC heaters, if you elevate the DC reference on them to 35V or so.
Replace the resistors around V1 with metal film type (if they are carbon at present). Reduce the value of the V1 grid-stopper (you can go down to 20k, or even 10k if radio pickup is not a problem in your area).
If hiss is still a problem when the volume control is set to zero, then repeat the above for the first gain stage after the volume control.
I think you should be OK with AC heaters, if you elevate the DC reference on them to 35V or so.
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