Hello,
I've constructed a simple 6EM7 amp based on Gary Kaufman's design (http://www.the-planet.org/6EM7.html) and I'm having a few problems with the voltages.
B+ is 325v as specified but I'm getting 170v after the 120K voltage divider (should be 115v) and 153v at the output stage cathode as opposed to the 140v stated (the resistors are also getting extremely hot). It plays music but only very faintly when the volume pot is turned up full.
The circuit is as Gary's design apart from a few changes to the power supply to get 325v and I've used 2x1.5K for the cathode resistors rather than 1.5k+1.3K as this is all I had.
A picture can be found at http://www.erpland.com/6em7.JPG
If anyone could provide any help it would be appreciated.
Thanks,
Chris
I've constructed a simple 6EM7 amp based on Gary Kaufman's design (http://www.the-planet.org/6EM7.html) and I'm having a few problems with the voltages.
B+ is 325v as specified but I'm getting 170v after the 120K voltage divider (should be 115v) and 153v at the output stage cathode as opposed to the 140v stated (the resistors are also getting extremely hot). It plays music but only very faintly when the volume pot is turned up full.
The circuit is as Gary's design apart from a few changes to the power supply to get 325v and I've used 2x1.5K for the cathode resistors rather than 1.5k+1.3K as this is all I had.
A picture can be found at http://www.erpland.com/6em7.JPG
If anyone could provide any help it would be appreciated.
Thanks,
Chris
Chris,
Direct connect circuits are difficult if you have a variac you may want to use it while you trouble shoot. Just given the voltages you’ve mentioned it is clear the grid on the triode output section is much more positive then intended, as you have may have guessed, but then so is the cathode voltage. The output triode grid bias in the schematic is -25V (115V- 140V) whereas in your circuit its -23V (153V-170V). The grid bias in both the schematic and your amp allows about 50ma of plate current to flow, which is okay. So your problem is in the driver section.
According to the schematic the voltage drop across the 120K resister (325V-115V) is 210V, which is 1.75ma of current flowing through the voltage amp plate. In your circuit the voltage drop is (325V-170V) is 155V for a plate current of 1.2ma, much lower then what the schematic calls for. So I would check that the cathode voltage to ground of the diver section is 1.4V or less. Make sure you do not have a DC off set on your input grid, a grid to cathode measurement should be no greater then 1.4V. It could also be that the 120K dropping resistor is off spec. Those are just some thoughts.
Matt
Direct connect circuits are difficult if you have a variac you may want to use it while you trouble shoot. Just given the voltages you’ve mentioned it is clear the grid on the triode output section is much more positive then intended, as you have may have guessed, but then so is the cathode voltage. The output triode grid bias in the schematic is -25V (115V- 140V) whereas in your circuit its -23V (153V-170V). The grid bias in both the schematic and your amp allows about 50ma of plate current to flow, which is okay. So your problem is in the driver section.
According to the schematic the voltage drop across the 120K resister (325V-115V) is 210V, which is 1.75ma of current flowing through the voltage amp plate. In your circuit the voltage drop is (325V-170V) is 155V for a plate current of 1.2ma, much lower then what the schematic calls for. So I would check that the cathode voltage to ground of the diver section is 1.4V or less. Make sure you do not have a DC off set on your input grid, a grid to cathode measurement should be no greater then 1.4V. It could also be that the 120K dropping resistor is off spec. Those are just some thoughts.
Matt
Chris,
Now that you’re getting the feel for the circuit, remember as you adjust the driver plate voltage and current you will be changing the operating point of the power triode. Shoot for about 80% of the plate dissipation of the power half, so 8W. Keep in mind the elevated cathode voltage on the power triode when you calculate its plate dissipation, the voltage for calculation is plate to cathode.
There is quite a bit to remember as you tune a direct-coupled circuit so it helps to have a few meters hooked up as you trouble shoot, especially the grid bias on the power triode and cathode voltage so you know where your plate current is. I’ve got a few direct-coupled 6EM7 amps and they are worth the work.
Matt
Now that you’re getting the feel for the circuit, remember as you adjust the driver plate voltage and current you will be changing the operating point of the power triode. Shoot for about 80% of the plate dissipation of the power half, so 8W. Keep in mind the elevated cathode voltage on the power triode when you calculate its plate dissipation, the voltage for calculation is plate to cathode.
There is quite a bit to remember as you tune a direct-coupled circuit so it helps to have a few meters hooked up as you trouble shoot, especially the grid bias on the power triode and cathode voltage so you know where your plate current is. I’ve got a few direct-coupled 6EM7 amps and they are worth the work.
Matt
Chris -
Matt's advise is on the money - sounds like the driver section is bias'd off.
You can pull the 6EM7 and measure the 120K resistors with a meter.
I can't spot any obvious errors in your wiring (which looks very nice btw). You will need to relocate the caps away from the 1.5K resistors eventually as they will be destroyed by the heat. With DC coupled amps the cathode resistors are tossing a lot of heat!
You might try substituting a resistor (somewhere between 750 and 1000 ohms) for the battery temporarily to see if that puts the voltages in line. If it does the NiMH's may be your problem. I have seen a few brands that just didn't work well (I don't know why).
If that doesn't fix things email me. It is a very sounding circuit once working!
- Gary
Matt's advise is on the money - sounds like the driver section is bias'd off.
You can pull the 6EM7 and measure the 120K resistors with a meter.
I can't spot any obvious errors in your wiring (which looks very nice btw). You will need to relocate the caps away from the 1.5K resistors eventually as they will be destroyed by the heat. With DC coupled amps the cathode resistors are tossing a lot of heat!
You might try substituting a resistor (somewhere between 750 and 1000 ohms) for the battery temporarily to see if that puts the voltages in line. If it does the NiMH's may be your problem. I have seen a few brands that just didn't work well (I don't know why).
If that doesn't fix things email me. It is a very sounding circuit once working!
- Gary
Hello,
Thanks for the replies.
I've tried replacing the 120K resistors without success. Replacing the batteries with 1K resistors also resulted in the same high voltages.
The only way I seem to be able to get the voltages right are when I disconnect the volume pot from the input stage grids.
BTW - the voltages I mentioned earlier are the wrong way round. They are actually 153v as opposed to 115v and 170v rather than 140v.
Thanks again.
Chris
Thanks for the replies.
I've tried replacing the 120K resistors without success. Replacing the batteries with 1K resistors also resulted in the same high voltages.
The only way I seem to be able to get the voltages right are when I disconnect the volume pot from the input stage grids.
BTW - the voltages I mentioned earlier are the wrong way round. They are actually 153v as opposed to 115v and 170v rather than 140v.
Thanks again.
Chris
Hello,
Input wiring and pot have been checked and are OK.
Along with the 100K resistor I was also advised to try a capacitor between input and grid - this is was what has fixed the problem.
The grid-cathode voltage is ~1.3v before the cap (0.22uF) and ~0.7v after. This is with the inputs disconnected from the source.
Could anyone offer any suggestions as to what is causing this, please?
Thanks,
Chris
Input wiring and pot have been checked and are OK.
Along with the 100K resistor I was also advised to try a capacitor between input and grid - this is was what has fixed the problem.
The grid-cathode voltage is ~1.3v before the cap (0.22uF) and ~0.7v after. This is with the inputs disconnected from the source.
Could anyone offer any suggestions as to what is causing this, please?
Thanks,
Chris
Ok re-reading your original post...the input stage doesn't seem to have enough plate current to pull the grid voltage of the output stage down to where it needs to be. That is making the output stage pass more current than it should, which is why you are toasting the output stage's cathode resistors, as Matt was saying.
As someone else mentioned, it seems like the battery bias isn't working correctly. You measured 2.4V from the cathode to ground. It should be more like 1.4V for a NiMH. Some of the batteries these days are fancier than others and this one seems to have a high internal resistance. Is it new? Maybe try a different brand...like a cheap one?
I'm guessing you have no grid leak resistor and the grid current is charging-up the cap causing the triode to turn on and bring the output triode into line. I don't think it is fixing anything.
As someone else mentioned, it seems like the battery bias isn't working correctly. You measured 2.4V from the cathode to ground. It should be more like 1.4V for a NiMH. Some of the batteries these days are fancier than others and this one seems to have a high internal resistance. Is it new? Maybe try a different brand...like a cheap one?
I'm guessing you have no grid leak resistor and the grid current is charging-up the cap causing the triode to turn on and bring the output triode into line. I don't think it is fixing anything.
Hi Russ,
Thanks for the reply.
2.4v was a typo - it was actually 1.4v. I've tried a couple of different batteries and have also tried replacing with a 1K resistor with the same result.
I've no grid-leak resistor as such but I've read somewhere that the volume pot would be acting in a similar fashion - is this the case?
Thanks,
Chris
Thanks for the reply.
2.4v was a typo - it was actually 1.4v. I've tried a couple of different batteries and have also tried replacing with a 1K resistor with the same result.
I've no grid-leak resistor as such but I've read somewhere that the volume pot would be acting in a similar fashion - is this the case?
Thanks,
Chris
That was true until you introduced the cap. Now there is no DC path to ground for the grid. So both the grid-to-cathode and cathode-to-ground are 1.4V? If so, it sounds like the high-mu section of this tube might be weak. The triode should conduct about 2mA at this operating point. You are using the correct section of the tube for the correct side of the amp, right?
Hello,
Right, makes sense.
What I'm getting is...
With Cap present or input grid left floating :
Cathode-Ground : 1.4v
Grid-Cathode : ~0.8v
Without Cap and input connected to grid :
Cathode-Ground : 1.4v
Grid-Cathode : 1.4v
New input wiring and pot have been tried with the same results.
Thanks,
Chris
Right, makes sense.
What I'm getting is...
With Cap present or input grid left floating :
Cathode-Ground : 1.4v
Grid-Cathode : ~0.8v
Without Cap and input connected to grid :
Cathode-Ground : 1.4v
Grid-Cathode : 1.4v
New input wiring and pot have been tried with the same results.
Thanks,
Chris
Yeah, what you see makes sense. Like I said, it seems like the high-mu section of that tube is weak. If it is all that you have, you can compensate for the low plate current by adding resistance to the 120k plate load. Add say 30k for 150k total. This is not ideal though, as it moves the intended operating point of the triode. You can also swap out the battery for something that will give you a lower grid-to-cathode voltage...say 1V. But this reduces the amount of signal the input section can amplify without distortion as the tube will go into cutoff at high volume.
Do you have a different tube to try?
Do you have a different tube to try?
Chris,
I don’t follow your measurements, so I assume you are making the measurements with the tube plugged in and the amp turned on. Did you have a source connected?
Turn the amp off, pull the tubes, pull any source components you have hooked up and pull the wall plug so you are back to a bare amp. If you don’t mind put a grid leak resistor (120K-200K) from pin number four to ground. The grid to cathode measurement should be identical to the cathode to ground measurement, though the polarity could be different depending on where you touch your probes.
If this is not the case try to figure out why not. The grid is grounded thru the grid leak so grid to cathode should read the battery voltage like cathode to ground. With no source component hooked up the presence or absence of a DC blocking cap on the input grid will make no difference. Now plug in the wall cord and see if anything changes, if it does you may have DC offset on your mains ground. Just for testing purposes try to disconnect the ground wire and see if your grid bias and cathode readings equalize.
If the grid bias and cathode to ground readings are similar with the amp turned off but plugged in, still with the tubes out, hook up your sources and turn them on, keep the amp off. See if your grid bias measurement or cathode to ground changes, switch the selector switch through its range measuring each setting. If anything changes then you will need the DC blocking cap, or figure out why your source has a DC offset, probably a bad output cap.
Once you get the amp so grid bias on the input is the same as cathode to ground with your sources turned on you are set to plug the tubes back in and begin trouble shooting all over again. Direct Coupled circuits are difficult and once you start adjusting things you will leave the schematic you are working off behind. Just keep plate current and plate to cathode voltage in places where the plates are dissipating about 80% of their power rating. You could have tired tubes, but you can just as easily compensate for this as you tune the circuit.
Matt
I don’t follow your measurements, so I assume you are making the measurements with the tube plugged in and the amp turned on. Did you have a source connected?
Turn the amp off, pull the tubes, pull any source components you have hooked up and pull the wall plug so you are back to a bare amp. If you don’t mind put a grid leak resistor (120K-200K) from pin number four to ground. The grid to cathode measurement should be identical to the cathode to ground measurement, though the polarity could be different depending on where you touch your probes.
If this is not the case try to figure out why not. The grid is grounded thru the grid leak so grid to cathode should read the battery voltage like cathode to ground. With no source component hooked up the presence or absence of a DC blocking cap on the input grid will make no difference. Now plug in the wall cord and see if anything changes, if it does you may have DC offset on your mains ground. Just for testing purposes try to disconnect the ground wire and see if your grid bias and cathode readings equalize.
If the grid bias and cathode to ground readings are similar with the amp turned off but plugged in, still with the tubes out, hook up your sources and turn them on, keep the amp off. See if your grid bias measurement or cathode to ground changes, switch the selector switch through its range measuring each setting. If anything changes then you will need the DC blocking cap, or figure out why your source has a DC offset, probably a bad output cap.
Once you get the amp so grid bias on the input is the same as cathode to ground with your sources turned on you are set to plug the tubes back in and begin trouble shooting all over again. Direct Coupled circuits are difficult and once you start adjusting things you will leave the schematic you are working off behind. Just keep plate current and plate to cathode voltage in places where the plates are dissipating about 80% of their power rating. You could have tired tubes, but you can just as easily compensate for this as you tune the circuit.
Matt
Hi Matt,
I was measuring the grid-cathode and cathode-ground voltages with and without a 0.22uf cap inline between the grid and pot. I've realised that this was incorrect as it interrupted the grid-ground dc path. However doing so did result in voltages more inline with what's specified within Gary's schematic.
Ok - 1.4v Grid-Cathode and 1.4v Grid-Ground. This and further subsequent checks have been done with a grid-leak in place and the 0.22uF cap removed.
Same results as above.
I've been trying this with the inputs disconnected from the source.
OK - I'm happy that the cathode-grid and cathode-ground voltages are the same (even with power applied) now that I understand that I'd positioned the cap in the wrong place.
Could it be that the characteristics of the tubes I'm using are way out compared to the ones refered to in Gary's design? I've read that tubes labelled as 6EM7/6EA7 (like the ones I'm using) result in voltages slightly higher than using 'real' 6EM7's with this circuit.
Do you think it's time to try alternative tubes?
Thanks for your help (and patience!).
Chris
I was measuring the grid-cathode and cathode-ground voltages with and without a 0.22uf cap inline between the grid and pot. I've realised that this was incorrect as it interrupted the grid-ground dc path. However doing so did result in voltages more inline with what's specified within Gary's schematic.
Ok - 1.4v Grid-Cathode and 1.4v Grid-Ground. This and further subsequent checks have been done with a grid-leak in place and the 0.22uF cap removed.
Same results as above.
I've been trying this with the inputs disconnected from the source.
OK - I'm happy that the cathode-grid and cathode-ground voltages are the same (even with power applied) now that I understand that I'd positioned the cap in the wrong place.
Could it be that the characteristics of the tubes I'm using are way out compared to the ones refered to in Gary's design? I've read that tubes labelled as 6EM7/6EA7 (like the ones I'm using) result in voltages slightly higher than using 'real' 6EM7's with this circuit.
Do you think it's time to try alternative tubes?
Thanks for your help (and patience!).
Chris
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