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Old 9th May 2009, 03:59 PM   #1
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Default Beginners attempt at SE 6EM7

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
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Old 9th May 2009, 05:17 PM   #2
mwiebe is offline mwiebe  United States
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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 youíve 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
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Old 9th May 2009, 09:32 PM   #3
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Hi Matt,

Thanks for the response.

Cathode to ground at the driver stage is 2.4v and grid-cathode is 1.4v so I'm going to try and replace the 120k resistors to see if that fixes things.

Chris
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Old 10th May 2009, 01:53 AM   #4
mwiebe is offline mwiebe  United States
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Chris,
Now that youíre 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. Iíve got a few direct-coupled 6EM7 amps and they are worth the work.
Matt
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Old 10th May 2009, 02:40 AM   #5
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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
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Old 10th May 2009, 04:49 PM   #6
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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
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Old 10th May 2009, 07:06 PM   #7
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Are you leaving the input stage grids floating when you take out the pot? I wonder if you have a bad pot. You might try shunting the wiper on the pot to ground with say a 500k resistor.
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Old 10th May 2009, 09:18 PM   #8
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Hello,

I've tried replacing the pot with a 100k resistor between input and ground (as per Gary's offline suggestion - thanks). The voltages are now closer so I'm going to look at my input wiring and maybe replacing the pot.

Thanks,

Chris
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Old 11th May 2009, 07:34 PM   #9
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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
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Old 11th May 2009, 07:51 PM   #10
rknize is offline rknize  United States
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Sounds like you have some DC coming from your audio source that is fooling with the operating point of the input stage. What are you using as a source?
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