Help requested for EL84 SET amp

dikarner · 14th May 2011, 07:51 AM

Hi there,
my home-built EL84 SET Amp is way to low when I feed it with new model JJ EL84 or Russian EL84 equivalents (6N14N-EB), and I'd like to figure out why.
With old-stock EL84/E84L of European manufacture there are no problems. I get around 1.2 watt driving my AER-drivers in a back-loaded horn to more then satisfying levels. pic on flicr
Since my electronics knowledge is quite basic I am looking once again for help from this marvelous community of experts.
Can anybody figure out why this amp is playing (guessed) 18db lower with those 6n14n-eb valves?
Schematics are attached.
thanks, Dieter

DF96 · 14th May 2011, 09:07 AM

What is the EL84 cathode voltage with the various different valves?

oldhifiguy · 14th May 2011, 12:44 PM

I think you mean SEP don't you? EL84 is a pentode.
try a different driver tube.

quikie22 · 14th May 2011, 01:38 PM

Quote:

Originally Posted by oldhifiguy

I think you mean SEP don't you? EL84 is a pentode.
try a different driver tube.

His schematic shows EL84 wired as a triode :-)

rvrazvan · 14th May 2011, 01:42 PM

try using 6p15p. I have a Zen triode amp clone running 6p14p and had some issues with it. I was designed to be used with 6n15p not 6n14p. Don't know the details of the problem.

dikarner · 15th May 2011, 01:50 PM

with "old stock" el84 the cathode shows 9 volts.
with a new JJ EL84 it shows 16! volts. how come?

DF96 · 15th May 2011, 02:19 PM

The new ones are not really EL84 but something only vaguely similar?

Or, the new ones are very gassy so have high grid current? What is the DC voltage at the control grid? This should be around zero - anything more than about +0.5V means a problem. As a temporary workaround you could reduce R8 to, say, 330K.

dikarner · 17th May 2011, 06:47 AM

I also guessed that there's a inherent difference between these tubes and that this circuit "suffers" from that difference.

they are not gassy.

Why would changing R8 make a difference, would you care to explain?

thx

chrish · 17th May 2011, 07:20 AM

R8 is the grid leak resistor. 1M is the max specified for cathode bias for an EL84. This resistor leaks away any charge on the control grid. The temporary fix above can be done with a couple of clip leads and another resistor, say 500k-1M, in parallel. As mentioned above, measure the DC voltage on the control grid to see if this is the issue. If it is not 0 volts, it may be a grid leak problem.

dikarner · 17th May 2011, 08:30 AM

thx.
I think I found the problem. As I am not at home I looked at the pictures very closely. It looks like I soldered the grid resistor onto Pin 1 and not Pin 2 as it is supposed to be.
I made that mistake when replacing the original run-of-the-mill parts with naked foil and mills resitors.

Funny that it works at all with old-stock EL84s still some with the newer ones.

17th May 2011, 06:47 AM	#8
dikarner diyAudio Member Join Date: Oct 2003 Location: lower Austria	Different EL84 I also guessed that there's a inherent difference between these tubes and that this circuit "suffers" from that difference. they are not gassy. Why would changing R8 make a difference, would you care to explain? thx

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14th May 2011, 09:07 AM	#2
DF96 diyAudio Member Join Date: May 2007	What is the EL84 cathode voltage with the various different valves?

14th May 2011, 12:44 PM	#3
oldhifiguy diyAudio Member Join Date: Apr 2009	I think you mean SEP don't you? EL84 is a pentode. try a different driver tube.

14th May 2011, 01:42 PM	#5
rvrazvan diyAudio Member Join Date: Jul 2010	try using 6p15p. I have a Zen triode amp clone running 6p14p and had some issues with it. I was designed to be used with 6n15p not 6n14p. Don't know the details of the problem.

15th May 2011, 01:50 PM	#6
dikarner diyAudio Member Join Date: Oct 2003 Location: lower Austria	with "old stock" el84 the cathode shows 9 volts. with a new JJ EL84 it shows 16! volts. how come?

15th May 2011, 02:19 PM	#7
DF96 diyAudio Member Join Date: May 2007	The new ones are not really EL84 but something only vaguely similar? Or, the new ones are very gassy so have high grid current? What is the DC voltage at the control grid? This should be around zero - anything more than about +0.5V means a problem. As a temporary workaround you could reduce R8 to, say, 330K.

17th May 2011, 07:20 AM	#9
chrish diyAudio Member Join Date: Oct 2003 Location: Sydney	R8 is the grid leak resistor. 1M is the max specified for cathode bias for an EL84. This resistor leaks away any charge on the control grid. The temporary fix above can be done with a couple of clip leads and another resistor, say 500k-1M, in parallel. As mentioned above, measure the DC voltage on the control grid to see if this is the issue. If it is not 0 volts, it may be a grid leak problem.

17th May 2011, 08:30 AM	#10
dikarner diyAudio Member Join Date: Oct 2003 Location: lower Austria	thx. I think I found the problem. As I am not at home I looked at the pictures very closely. It looks like I soldered the grid resistor onto Pin 1 and not Pin 2 as it is supposed to be. I made that mistake when replacing the original run-of-the-mill parts with naked foil and mills resitors. Funny that it works at all with old-stock EL84s still some with the newer ones.