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ef86 preamp...


 
4/30/2003 10:40 PM
dave mcculloch
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ef86 preamp...
got an idea this afternoon,  
 
how about an ef86 1st stage, with a second stage and cathode follower, sort of like a matchless lightning, but with an ef86 replacing paralell triode in the first position... follow it with a fmv tone stack and a pre PI master volume....  
 
gonna build it tonight and see how it goes.  
 
 
 
I'll keep you all updated  
 
 
Dave Mc
 
5/1/2003 9:20 AM
stephen conner
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The amp I'm playing just now has an ef86 first stage followed by both halves of a 12AU7. So it's basically a ef86 bolted onto the front of an ordinary clean channel. I originally built it with a 12AX7 but when I tried the 12AU7 it sounded much better.  
 
Steve C.
 
5/1/2003 12:12 PM
dave mcculloch
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cool, i'll probably try an au7 and a 5751 there as well, did you try it with a single stage with a cathode follower?  
 
 
Dave
 
5/2/2003 7:59 PM
stephen conner
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I tried a cathode follower after the EF86, driving the tonestack, that then drove the second section of the dual triode (an ax7 at that time) In the end I preferred the hookup where both halves of the a*7 were regular gain stages (with no cathode bypass caps)  
 
Steve C.
 
5/1/2003 1:14 PM
Matt
I built a Plexi type preamp but the Normal (dark) channel was an EF86 and the Bright channel was a paralleled 12AX7. So using only the Normal channel, it was the EF86 followed by the usual Plexi second stage then CF then tone stack. It's been a while so I don't remember the values/methods I used. I'm pretty sure I used some methods suggested by kg but if I try to elaborate on what I did, I'll probably get it wrong (IIRC it was voltage divider for the screen with a connection to the cathode thru a cap, or something like that :)). I'll check tonight and post it tomorrow. The PI in this amp was an attempt to "optimize" a 12AT7 (much lower plate loads and lower cathode resistor, again IIRC) so it's very different and probably wouldn't give a good idea of how it would sound with a "regular" PI. Plus the Bright channel bypass caps bleed off a lot of high end in the Normal channel so...apples and oranges probably.  
 
Matt
 
5/2/2003 11:42 AM
Matt
I checked what I had done (many moons ago) and this is what I did (some of these values are a "little" strange): 100k plate load, 2.2k/22uF cathode, screen supply was a 270k/470k divider (470k to gnd.) with a .01uF cap connected from the screen to the cathode. .0047uF Co and a 500k "volume" pot. With 325vdc B+ at that node I got 190v plate, 165v screen, and 3.5v cathode. I think I read somewhere here to try and keep the screen voltage up near the plate voltage (not like the 250v plate and 140v screen that my data sheet suggests). Last night I cascaded the paralleled 12AX7 into the EF86, then into the DC coupled pair but I raised the cathode resistor to 10k on the gain stage half of the pair. After removing several bypass caps, it sounded pretty good (not surprisingly very Marshally) but that CF has like 250v on the cathode so I'm a little apprehensive there (amps routinely exceed the 100v heater-to-cathode voltage rating but how high is too high?). This is probably all old hat for you but I figured I'd post it anyway. :)  
 
Matt
 
5/2/2003 7:04 PM
MattM
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How did you like the result?  
 
Here is a schematic of an amp i build using the same preamp idea, CF and tone stack are from the vox top boost channel.  
 
Sounds great clean and crunchy, a bit muddy at higher volume settings.  
 
Did you also find that the EF86 blasts the hell out of the second stage?  
 
;Matthias
 
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