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EF86 Vox AC-30 Mod

9/2/2003 4:18 PM
Howard EF86 Vox AC-30 Mod
Has anyone tried to mod a Vox AC30 Reissue Normal channel using a EF86? (Kinda like the original?)  
9/5/2003 12:33 PM
In an AC-30, the Normal channel shares a 12ax7 pre with the Top Boost channel. Since there are two pres in an AC-30(Normal/TB and Vib/trem) one preamp socket would have to be sacrificed or one must be added to accomodate an EF-86. This means trading your TopBoost and Normal channels for the EF-86 or trading your Vib/Trem channel for the EF-86 or using the Normal input hole for an added-on EF-86 somewhere. The later will give you an unused triode, but you could parallel it with the TB triode pre like Matchless did in the HC-30 if you don't want to waste it.  
I think someone in England is actually making an EF-86 add-on module for a fourth channel.
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9/5/2003 5:23 PM

"The later will give you an unused triode, but you could parallel it with the TB triode pre like Matchless did in the HC-30 if you don't want to waste it."
Or, you could use it as a direct-coupled cathode follower with the EF86 and gain the ability to drive lower impedance loads, like a tone stack or what-have-you, with no loss of gain. Heck, even with just the usual 500k volume pot, you'd regain some gain due to not loading the pentode down.... :D  
C ya,  
9/14/2003 9:25 PM
Yeah, Duch I hear you! I was thinking on even experimenting with an ECF80 (pentode/triode in one envelope) to accomplish that very circuit. Of course it will be replacing the 9 pin EF-86.  
Ever try one of those?
9/16/2003 2:29 PM

"Ever try one of those?"
No, but I do have a 6AN8 lying about that I did this with at one time. Worked like a charm!  
C ya,  
9/25/2003 8:21 PM
Ian Anderson

I modded my AC30 to have the EF86 frontend and like it alot, it's got more mids than the topboost version, but it's hard to get non-microphonic EF86s (unless you pay alot for them)  
One thing I've been meaning to try is to add the topboost tonestack after the EF86 for mega-gain, might be fun.  
... Ian :)
9/26/2003 5:37 PM

Hey Ian--  
Howzitgoin? How's that 6V6 amp with the EF86 working out? :)  
I finally broke down and got an EF86 from Lord Valve to pay with, but have been busier than a one-armed paper hanger since it arrived, and so it sits.... I've been using my little Marshall 18 clone in the meantime, slaved with a dummy load in the FX loop of my Hafler HellRazor preamp, and have been having a blast with it. It sounds really good through my stereo power amp and pair of AlNiCo Blue Dogs--I finally got a second Blue Dog so I could use 'em with my stereo rig and retire the **heavy** 150W Mesa Black Shadows I'd been using. Hopefully soon I'll get some time to build that EF86/6V6 thingie that you so heartily recommended in the Knight KM15 chassis I have hanging around....  
If it works out as good as your glowing reports suggest it should, I'm going to do a ground-up build in a 2U rack-mount chassis with two switchable channels: an AC15 channel and a Dumble ODS channel, feeding opposite sides of an AC15 style PI and running into a pair of cathode-biased 7591As in UL through a Heyboer 18W Hygrade clone OT I picked up in January or thereabouts (for a ground-up Marshall 18 build I was contemplating at the time), to replace the HellRazor and Marshall 18 clone. If I end up not liking the EF86 AC15 pre, I'll use the Marshall 18 pre instead, since I already know what it sounds like and that I like it.  
C ya,  
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