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Old 24th August 2003, 07:50 PM   #1
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Default Troubleshooting a single ended EL84

Does anyone see anything wrong with this schematic...??

This guy over at AA is trying to fix it for a friend. Problem is that is distorts heavily even at very very low volumes..

I told the guy that that it might be clipping because of the 89db speaker...but the very very low volume it should not clip..! Am I right, or am I right?

I noticed that the driver 12ax7 does not have a cathode resistor...but see no fixed bias...so would that be the problem..???


Here is what he mailed :


Quote:
Anyway here is the schematics. Some correction though, the 12ax7 cathode is
connected to ground. The plate of the el84 is connected to the transformer
and not the B+. The other end of the transformer is connected to B+.

The pin9 of the EL84 is not connected to the plate directly but actually
goes through 1K/1W resistor and then to B+. I think it is configured as
Pentode mode


What I have done:

I have tap out from C3 output and make it as pre-out. Then I took the
pre-out and attached to my other amp. It produces nice sounding though some
clipping/distortion can be heard when the music is getting loud. I take it
as the driver is doing ok.

I am thinking on how to get to test the el84 now.
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Old 24th August 2003, 08:22 PM   #2
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Hi,

Bas,

With the cathode of the ECC83 grounded and a 100K volumepot as a variable gridleak this section MUST distort.

I don't understand the two caps in series at the input either.

Do you know what the B+ on the EL84 is? I can't read it...

Cheers,
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Old 24th August 2003, 08:56 PM   #3
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Hi Bas. It looks to me like you need to do away with the input caps since most source components use an output cap these days. Also do away with R1. P1 will suffice there. In order to drop equal voltages across the plate resistor and the 12ax7 change the plate resistor to 220K and add a 2.4K cathode resistor. I would bypass the cathode resistor with a 100 uF cap also since you are going to lose a lot of gain in the filters following the gain stage. I would personally want a higher B+ on the 12ax7 as 225 volts doesn't leave a lot of room for biasing. The values I've given will only give about a -1.2 volt bias on the grid with about .5 mA though the tube. That's alot better than no bias though. HTH.
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Old 24th August 2003, 09:06 PM   #4
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Or, he could remove P1 and increase R1 to 10M in order to get grid current biasing. Not that I see any point in that, though.

Gs suggestions is alot better.
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Old 24th August 2003, 09:07 PM   #5
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Hi Bas

Where did your friend get this circuit from? His grandad's radio? The net is full with working and sensible circuits, what's the need to use something dysfunctional?
Anyway, can't quite get how the EL84 is getting power - the primary of the transformer seems grounded.

peter
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Old 24th August 2003, 09:29 PM   #6
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Hi Peter. I think that Bas's friend made a correction on the EL84 in the email that he sent Bas.

" The plate of the el84 is connected to the transformer
and not the B+. The other end of the transformer is connected to B+"

Here is the statement I don't get:

"I have tap out from C3 output and make it as pre-out. Then I took the pre-out and attached to my other amp. It produces nice sounding though some clipping/distortion can be heard when the music is getting loud. I take it as the driver is doing ok"

I don't understand how he is getting anything but distortion if the schematic he sent is correct. My bet is that there is a cathode resistor but the input signal is making the stage clip. If the correction he made in the letter about the EL84 is true and the values given on the schematic are correct I doubt the output stage is the problem.
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Old 25th August 2003, 12:38 AM   #7
EC8010 is offline EC8010  United Kingdom
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100R seems terribly low for an EL84 cathode resistor. I would have expected something nearer to 270R
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Old 25th August 2003, 12:53 AM   #8
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Hi,

Quote:
I don't understand how he is getting anything but distortion if the schematic he sent is correct.
The schematic isn't correct...Neither is his correction IMHO.

As EC8010 points out, for the EL84 you often have a 270R cathode R and about 270V B+.

It's "working" in pseudo triode mode as far as I can tell, not penthode.

This is one circuit in need of a complete redraw and it still wouldn't fit the speakers anyway.

Cheers,
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Old 25th August 2003, 01:04 AM   #9
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"As EC8010 points out, for the EL84 you often have a 270R cathode R and about 270V B+"

That's true. They may have used one cathode resistor for both EL84 tubes though. I've seen SE EL84 amps from old magnavox consoles that used that arrangement. It does seem like a 120 ohm resistor would be better if that is the case. Those EL84's are running hot. As you said Frank, the schematic for the gain stage cannot be correct.
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Old 25th August 2003, 01:40 AM   #10
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Something else to consider too. The 12AX7 is adequate to drive an EL84 in pentode, but not really in triode where the 84 is a load of about 250pF.
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