Hi,
I'm a new comer here and hope someone can help me solve a few Issues.
I recently built the Ef 86 preamp channel from a 2008 Vox AC 15 Hand wired amp (TV).
I connected it to Pin 2 of the 12ax7 inverter tube on my 1998 Vox AC 15 and it sounds great but I have 2 issues.
1 - I connected the preamp output directly to pin 2 - the volume and tone are good but it cuts the volume on the amp's top boost channel down a whole lot.
This amp comes with 1 channel which has reverb, tremelo & master volume.
My question is - Is there a way by adding caps, resistors, etc. to avoid this from happening?
2 - The preamp design comes with a Triode/Pentode switch and when I switch from one mod to the other it pops. I'm using a DPDT mini switch - the original amp comes with a small switch like the ones on the silver face Bassman amps.
I attached the diagrams for your review.
If I could resolve these two issues I'd be super happy.
This mod sounds great and I didn't have to buy a Chinese Vox - I was able to put it in my British made Vox.
Thanks!
Tim
EF 86 Mod added to Korg Era Vox AC 15
Moderators: pompeiisneaks, Colossal
EF 86 Mod added to Korg Era Vox AC 15
You do not have the required permissions to view the files attached to this post.
Re: EF 86 Mod added to Korg Era Vox AC 15
Go on, then - I'll kick things off 
I think it would work to add a DC blocking capacitor (470n, say, rated for full B+) in the wire between the EF86 anode and the switch and then adding high value resistors (1M? 2M2? 4M7??) between the switch common and each 'way'. I don't know if this would make it pop free, or not, but it should help.
Without the blocking capacitor, I suspect the leakage through these resistors would upset the screen grid voltage in pentode mode. (Maybe someone could chip in and confirm the validity of this?)
In Merlin Blencowe's preamp book there is a 'Morph' control which uses a pot to blend triode and pentode behaviour which may be of interest.
HTH
Andy
Feed your signal to pin 7 of the PI (through your preamp coupling cap) but disconnect the existing 47n capacitor to ground from pin 7 (C25 on your amp schematic). Both channels will then be independent - it's how it was done on the original AC15 (AFAIK) - see attached.Timvox1 wrote:1 - I connected the preamp output directly to pin 2 - the volume and tone are good but it cuts the volume on the amp's top boost channel down a whole lot.
This amp comes with 1 channel which has reverb, tremelo & master volume.
My question is - Is there a way by adding caps, resistors, etc. to avoid this from happening?
Just to check I'm not missing something: You're only using one pole of the switch, right?2 - The preamp design comes with a Triode/Pentode switch and when I switch from one mod to the other it pops. I'm using a DPDT mini switch
I think it would work to add a DC blocking capacitor (470n, say, rated for full B+) in the wire between the EF86 anode and the switch and then adding high value resistors (1M? 2M2? 4M7??) between the switch common and each 'way'. I don't know if this would make it pop free, or not, but it should help.
Without the blocking capacitor, I suspect the leakage through these resistors would upset the screen grid voltage in pentode mode. (Maybe someone could chip in and confirm the validity of this?)
In Merlin Blencowe's preamp book there is a 'Morph' control which uses a pot to blend triode and pentode behaviour which may be of interest.
HTH
Andy
You do not have the required permissions to view the files attached to this post.
Re: EF 86 Mod added to Korg Era Vox AC 15
Agree w/ Tillydog, go into pin 7 and omit AC ground C-25. The EF86 needs it's own input and never sounds good mixed IMO. Don't like all the bells and whistles vox added, prefer the series cap(or two) w/ shunt switch for ultra fatness. I use the same values as the original schematic, 1M screen R instead of 4M7 and instead of 250pf for "Brilliance" I use 3 way 500p/1000p/shunt. And .01uF to pin 7.
Where in Md. are you?
Where in Md. are you?
Re: EF 86 Mod added to Korg Era Vox AC 15
Hi Masco & Tillydog,Masco wrote:Agree w/ Tillydog, go into pin 7 and omit AC ground C-25. The EF86 needs it's own input and never sounds good mixed IMO. Don't like all the bells and whistles vox added, prefer the series cap(or two) w/ shunt switch for ultra fatness. I use the same values as the original schematic, 1M screen R instead of 4M7 and instead of 250pf for "Brilliance" I use 3 way 500p/1000p/shunt. And .01uF to pin 7.
Where in Md. are you?
Dropping out that cap did the trick. Thanks.
I started out with the original early 60s circuit but after hearing a friend's reissue I kinda liked the bass shift & brilliance controls.
The brilliance on the reissue uses a 220p & 270p.
In what direction does the 500p & 1000p take the tone?
I'm not finished yet and might try a few different values now that I have the thing working.
That ef 86 channel sure sounds great.
No wonder so many amps companies have copied it over the years.
I still have to try Tillydog's suggestion to see if I can get the pop out of the Triode/Pentode switch.
I bought the suggested parts today but ran out of time - had to watch the Ravens beat Denver...
What a great game that was!
Masco - I live in Dundalk - where are you!
Thanks again!
Re: EF 86 Mod added to Korg Era Vox AC 15
Glad it worked out for you.
I'm in Gaithersburg. Know some cats around Balmer- Robert Frahm, Pete Kanaras, Matt Kelly...
-Matt Romasco
I'm in Gaithersburg. Know some cats around Balmer- Robert Frahm, Pete Kanaras, Matt Kelly...
-Matt Romasco