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EL84 in 5E3 circuit?


 
7/18/2000 11:45 AM
gt
EL84 in 5E3 circuit?
I need to convert 5E3 circuit to EL84. It's my first DIY and I arranged an existing chassis.  
I built the amp but it sounds too much distorted.  
Tried .022 caps followed by 330k before grid resistors (in place of .1) but it doesn't clean up things.  
Volume knob is almost useless because breaks up very soon.  
Cathode resistor is 120/5W.  
 
What mods are needed to arrange 5E3 to EL84?  
 
Thanks in advance!  
Giacomo
 
7/18/2000 1:50 PM
Dave H.

The volume does come up quickly on that circuit because the pots are wired back to front. Also EL84s are more sensitive than 6V6s. Try removing the 25u cap across the 1k5 cathode resistor of the second gain stage.  
 
Dave
 
7/19/2000 5:27 PM
gt

Thak you everybody!  
 
quote:
"The volume does come up quickly on that circuit because the pots are wired back to front"
 
 
What do you mean with "back to front"?  
Excuse me but can't understand this point.  
 
Thanks
 
7/20/2000 10:28 AM
Dave H.

A pot is usually wired up with the input signal going to the cw terminal and the output is taken from the wiper. The 5E3 has the input signal going into the wiper and the output is taken from the cw terminal. It messes up the taper of the pot when itís wired this way.  
 
Dave
 
7/21/2000 1:23 AM
Steve A.

Dave:  
 
    I just checked the 5E3 drawings from FFG and they don't specify the taper on the pots in the 5E3. Back in the 50's weren't most of the pots linear? If anybody knows the specs on these pots, let me know!  
 
--Thanks!  
 
Steve Ahola  
 
P.S. I think I've asked this a dozen times but what is the value of the unmarked cap linking the two triodes of the 12AX7 (presumably V2)? I seem to remember someone telling me .02uF- is that correct?
 
7/21/2000 7:52 AM
gt

quote:
"what is the value of the unmarked cap linking the two triodes of the 12AX7 (presumably V2)?"
 
 
Hoffman's schematic says .02  
 
Regards  
Giacomo
 
7/19/2000 4:42 AM
Steve A.

Giacomo:  
 
    I was told that EL84's get ugly sounding if you put more than 17vac on their grids. You might try adding something like a 150k "cross-line" resistor between the outputs of the power amp coupling caps (right where they meet the 220k grid load resistors). If the sound is too bright you could add something like a 500pF to 1200pF cap in parallel with the 150k resistor. YMMV so you might need to experiment with these values...  
 
--Good luck!  
 
Steve Ahola  
 
P.S. Take a look at this picture from the 2/97 issue of GP. See the "V" with the twisted blue and green wires (which go to the grids of the EL84's)? To the right of the big PS filter caps...  
 
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