Help requested for EL84 SET amp

[dikarner]

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]

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

 

[oldhifiguy]

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

 

[quikie22]

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]

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]

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

 

[DF96]

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]

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

 

[chrish]

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]

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.

 

[DF96]

So it was a grid current problem, as I suspected! Your NOS valves have such low grid current that a bit of leakage (surface moisture?) is enough to do the job. Modern valves are not so good so need a real grid leak resistor.

 

[dikarner]

yep, that was it. on the NOS EL84 pin 1 and 2 seem to be connected, hence it worked.
on the NOS russian and the new jj's they r not connected.
just sloppiness on my part.
thx all anyway.

 

[DF96]

On the EL84 pin 1 is marked as IC - internal connection. This means that it should not be used externally. In practice it seems that many NOS EL84s connect this to pin 2 - no problem, but not to be relied on. You accidentally relied on this connection. It seems that some modern amp manufacturers rely on this connection not being there, and use pin 1 as a spare tag - they get problems when NOS are used.

My explanation of gas is thus incorrect. It was grid current, but only tiny amounts - you don't need much current to generate 16V across an open circuit!

 

[M Gregg]

Just a thought,


It's not linked to this fault, may be of interest!

I saw some where that some Chinese tubes have a different connection internally (Pins joined or not joined). Ill try and find the info.

Post 229#

http://www.diyaudio.com/forums/tubes-valves/155057-suppo-audio-new-chinese-el84-pp-amp-cheap-23.html




Regards
M. Gregg