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Vintage threads from the first ten years

EF86 preamp

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12/16/1997 4:41 AM
E Lion	EF86 preamp Hi, Steve K, Mike B., M. Romasco, Marcus T., R.G. - and others! A while ago you had a enthusiastic thread here regarding EF86 preamp circuits. I'd like to give that EF86 a try in a second channel in my DIY AC50 amp. Could someone tell me where to find the schematics for the AC30/4 or AC15 with the EF86. Also useful tips or any advice will be appreciated. Thanks E Lion

12/16/1997 8:51 AM
Preben Hansen	Hi ! Try svetlana.com Technical Bulletin No. 23 3 different configurations of a single stage EF86. With explanations !!!!! There are also several sites, se links page, that have complite schematics, where EF86 are involved. God - luck. Preben.

12/17/1997 4:32 AM
JR	If you've got chance to get your hands any of the 50's tape recorder schematics ,usually they are equipped with the EF86 as the first gain stage for the microphone, these might contain some useful tips. Jukka

12/17/1997 7:33 AM

The Gerald Weber book has a few schematics of the EF86 early Vox amps. I can give you the pinout but I will have to e-mail you ( I'm at work -cant remember all the connections).
I've been using my sf vibrochamp as a platform for experimenting w/ some mods. I just recently replaced the 12ax7 preamp w/ an EF86. It sounds good but I'm still experimenting w/ different components to tailor the sound I'm looking for. Just playing it by ear!

Matt.

12/17/1997 12:11 PM
E Lion	Hey Matt! Thanks, I'd like that very much! Are you using a Vox EF86 layout in your Vibrochamp? Sounds good - you say, but still not right. What are you looking for? Do you know if there's any differences in the EF86 preamps circuits in the AC4 - AC15 - AC30. If my memory serves me right you'r the guy with the old AC30/4 head. Do you still have it - and play it. Just curious! Erik.

12/18/1997 12:01 PM
M.Romasco	Erik, There are a few differences in the Vox preamps you mentioned. The AC-4's cathode resistor is 1.5K and the AC-15 & 30's are 2.2K. Also the AC-15 has a brilliant switch (like my old AC-30) that switches in & out a 250pf cap. Do I still play my AC-30? Every day! I'll e-mail you more details. Matt

12/18/1997 1:02 PM
Doc	Matt: I'd be interested to know at what voltages do they run the plate & screen of the EF86 in these amps? Pentodes have a lot more signal voltage gain than triodes, so can be run at lower values like 90v plate/ 65v screen and still get a V.G. around 60. I don't have any VOX amps to perform measurements on myself. Thanks, Doc

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