Reissue '65 Twin preamp mod

[JonnyBoy]

Hey fella's

I want to preamp tap my 65 twin reissue but the board is preprinted not like the old school stuff. one good thing is that the caps, resistors and such are all numbered and well organized, or clean. Does anyone have a direction for information or know which and what to tap by number?

[JonnyBoy]

For further reference it is resistor (R35) at the top end (closest to the tone/vol pots). It took me a while to follow all those lines then double check, then triple check, then "well here's now or never", then viola! it was right. Thank God. I answered my own question out of being impatient.

[JonnyBoy]

I am hoping I did this right, this preamp is HOT. I mean, why on gods earth would you need an amp more than 50 watts SS with this preamp? This sucker is shaking the house amp 1/2 way and pretwin on 2 going through the effects loop(on 5 too) of my amp. The loudest I have pushed my 2x12 cab! I LOVE IT, I LOVE THIS SITE. I can only imagine what your wives, roomates, life partners, whatever think of some of your MC2300's-MC2100's. That's got to be against the law in a few states.
And yes I am talking to myself in this thread and not ashamed to say it.

[tigerstrat]

For further reference it is resistor (R35) at the top end (closest to the tone/vol pots). It took me a while to follow all those lines then double check, then triple check, then "well here's now or never", then viola! it was right. Thank God. I answered my own question out of being impatient.

That was probably a fairly useful mod you posted- you see a lot of people posting similar questions about RI's, and I haven't noticed a lot of answers besides "get a p2p model".So even though I happen to agree with them ... nice post!

[waldo041]

agreed, nice work. i do have some altered schem and layouts for this. it is actually the c18/r35 junction. that c18 should be r34 if if it were the original ab763 layout. wonder why they have c18/r34 swapped.

did you use the switching jack and lift r35's leg and also shunt the power section?

if your at all into modding that amp, i'd swap the cathode bypass caps to atom spragues and put some orange drop caps on the tone stack. then some rn65 100k's on the plates. you'll enjoy that improvement also!

peace,
waldo

[deadguise]

Is there a link to this mod somewhere?
Thanks.

[JonnyBoy]

I pulled the tubes, I started to pull that resistor to shunt, but I wasn't really sure it was the right one since the shape,color and placement are completely different than the older versions i was going by, and I didn't want to be sorry some how, so I am storing the tubes as of right now. Plus, if it is lifted, does it mean you cant run it as a combo? As long as the tubes are out i'm fine,right? Now that I am sure of the circuit, I'll go back and do it later if need be. I was thinking it was a CAP from your drawing not a Res?
I have played it now two days straight without fail, sparks, electric shock, flames, smoke, you know, the usual sign you fucked up. The tone sounds great too, super HOT preamp, but the hiss is there since the rest of the parts are stock still. It's not bad at all, but it could be better. I WILL be doing the rest of the mods, and I will post pics for the further future research at the RUKIND Institute of Technology.
As for the question about where to get the info to start, Waldo's site is the most up to date and comprehensive. Dozin's site works too and has helpful live pics, but know it is not the 65 twin Blackface circuit, I believe it is a silver face?? that may not matter. Thanks for all your help. I couldn't have done it without y'all.

[JonnyBoy]

I am sorry you asked for the link:
Waldo:
http://www.wald-electronics.com/preampmods.html
Dozin:
http://www.dozin.com/gear/preamp/mod.html

This should get you where you want to be -Jboy

[waldo041]

jonnyboy,

which end of r35 has continuity with c18? that is the shunt point, and the otherside of r35 should have continuity with c19? that is the tap point, but i would tap it off of c19 leg that has continuity with r35.

let me know if you get a chance and i will draw them out on the layout.



here are the power shunt and tap points for the alembicization preamp taps. note healy's tap would require lifting that c19 leg and configuring a 13b switching jack to achieve that properly. healy's tap completely disengages the vibrato, and gives a higher voltage signal. it also isolates just the vibrato channel(or channel tapped) from the normal.


peace,
waldo

[waldo041]

here is the healy tap for the vibrato channel on the 65 reissue schematic.



peace,
waldo

[JonnyBoy]

Sweet man. I will tackle it tomorrow now that I have this. You posted one of the most important and informative threads here. I am making a new head cab to ditch the speakers anyway. The easiest way to say where I tapped it, I didn't lift anything, only soldered a strand yanked out from a cutting of a belden blue to the back switch vib/revb pedal in to the exposed portion but still intact resistor r35 north end. The solder point was on the "north" side closest the vol/tone pots. seeing that the current will go the path of least resistance, it has to go out since the tubes are gone. If I lift the cap leg and break the circuit the amp won't run as a combo right?

[waldo041]

here are some other "jerry/healy/alembic" mods i would take on if i owned a 65 resissue twin reverb.

Plates:
100k plate resistors (r4,r11,r15,r22,r28,r33) - Mouser part # - 71-RN65D-F-100K

Tonestack:
.1uF tone stack bass cap (c3,c8) - Mouser part # - 75-225P400V0.1
.047uF tone stack mid cap (c4,c9)- Mouser part # - 75-225P400V0.047
(note c3,c4 are the normal channel & c8,c9 are the vibrato channel)

Preamp Output:
.1uF vibrato preamp output cap (c19) - Mouser part # - 75-225P400V0.1
.047uF normal preamp output cap (c18) - 75-225P400V0.047 (see note 1 below)

Preamp Taps:
you'll need to use shielded cable and only ground the shield on the one end at the jack(s). only expose a small portion of the inner conductor wire at the signal tap and trim away the shield on that end. you'll also want to heat shrink the trimmed shielded portion to keep it from contacting anything. this will provide a shield all the way back to the jack.

you'll need 2 of each if you are going to tap both the normal and vibrato channels.
Healy Preamp Tap vibrato unuseable
healy preamp tap jack switchcraft 13b - Mouser part # - 502-13B
healy preamp tap pulldown resistor - Mouser part # - 588-OF156JE

Alembic preamp tap - vibrato still useable
alembic preamp tap jack switchcraft 13 - Mouser part # - 502-13
no pulldown resistor used

Input:
the input mod can be achieved by removing the 68k input resistor(s) and jumpering their position(s). but the use of both input simultaneously will be unachievable as you will be removing the mixing resistors needed. swapping them to 68k metal films is also an option to reduce hiss and with this amps parts construction may be the best option.

Notes:
1.)i don't have a 65 reissue to verify, but according to the schematic c18 & r34 in the normal channel are flip flopped from the original ab763 circuit. if they are actually like this on the circuit board, i would swap there position by removing c18 & r34 and placing c18 where r34 was, and placing r34 where c18 was.

2.) with the power tubes removed for preamp tap use, the voltages will rise and it is advised to either a.) keep the power tubes in so the voltage doesn't rise, or b.) swap the filter caps for higher voltages - 500 - 600 volts is recommended. this goes for the old twin reverbs also!



peace,
waldo

[JonnyBoy]

Thanks Waldo, I know that took you a minute to write on my account, but hopefully this info will serve many others in the future in the archives. You are right about one thing, Fender does NOT have the EXACT 65 twin circuit here. They have switched a few things up. As I figure them out, I will add it to the thread for future reference!!! Thanks for the Help!! Jboy

[JonnyBoy]

Ok, got the vibrato and reverb wires that were at the pedal jack grounded and the vibrato works!! as a preamp, and the reverb doesn't. I have swapped tubes I think to be the reverb tube and the reverb still doesn't work. This may have something to do with the switch in the caps and resistors Waldo talked about? (Remember we are talking about the PCB 65 reissue fender twin.) When you shake the reverb box you get a reverb crash through the amp, so it is in the line. I may have switched the wrong tube. Which tube specifically is the "Reverb" tube? I am tapped at the C19 leg on the north end or the other side of the cap connected to pin 6 of the tube. its hard to see what is connected to what in there..... If I can solve this I will be very happy! almost there...

[waldo041]

don't know if this will help, but you should actually be tied in on the otherside of that capacitor, and not the leg that goes to pin 6.

what does the tube chart state is the reverb 12at7?

peace,
waldo