Trouble working on a Twin Reverb Silverface

[PJBayer]

Hi all. I'm new to this board, but I'm following Ray's suggestion to post this here.

HELP!

Here's my problem: I have a silverface Fender Twin Reverb Amp in excellent condition. It even has the ever-rare JBL speakers. However, it's gotten a little noisy. Reading this board for some time, I know the ~40 year old electrolytic caps are my #1 target. So far, so good. All I need is a schematic. Plus, I'd also like to "blackface" it.

Other data: My chassis is embossed with "A 38 204". Inside, there is an ink stamp "Richard Hand  Jan 6 1972". Tube chart is stamped 047944.

On line I can find three schematics that appear viable, but non really nails my unit. Here's the particulars: My amp does have a Master Volume, but it's not a push-pull. The rectifier circuit has the 6 diodes (importance of this in a minute). In the "dog house" (DH) I have 3 20mFd 500VDC and 2 70mFd 350VDC caps. Using this data as touch points, I started eliminating schematics.

AA769: No master volume, DH caps match, 6 diodes in rectifier, no date on dwg.
AA270: same as AA769.
CBS Dwg # 010182: DH caps aren't 70s but are 100's, does have a Mst Vol, but not push-pull, dwg date is 11/17/71. Promising
So I contact Fender Customer Service. Sent them all the details. They promptly send me a...
Fender/Rogers/Rhodes dwg (013694) dated 2/24/77 (at least five years after my amp was built) with a 4 diode diamond shaped rectifier (remember mine has six diodes).
2nd contact, they say that's all they have.

Is there a correct dwg I should follow, or should I use the 1971 CBS dwg

Also, none of the write-ups describe how, or even if, I could "Blackface" my amp.

HELP!!

[FYL]

Other data: My chassis is embossed with "A 38 204". Inside, there is an ink stamp "Richard Hand  Jan 6 1972".

An early '72 model, as staped and confirmed by the A38204 manufacturing code.
 

On line I can find three schematics that appear viable, but non really nails my unit.

Fender shipped a lot of non-standard models between the late '60s and the '80s : they used whatever was available and didn't properly update schematics or documentations.

CBS Dwg # 010182: DH caps aren't 70s but are 100's, does have a Mst Vol, but not push-pull, dwg date is 11/17/71. Promising

The CBS 100 used a PP master volume from November '72 on.

Is there a correct dwg I should follow, or should I use the 1971 CBS dwg

Can you supply some quality pix of the main board ? Or check the R values in the PI : the AA270 uses 22K/270R, the CBS 100 33K/470R. Look for 1200 to 2000p caps between the 6L6GCs grids and ground. If there => AA270 or derivative; if not CBS100.

Also, none of the write-ups describe how, or even if, I could "Blackface" my amp.

Blackfacing is quite easy.

[bigdaddy]

Blackface does not equate to better. Having those JBLs makes the amp a different animal. The blackface was rated at 85 or 88 watts not 100. It's a bit different amp, I know I have owned a few BFs and some early SFs. The transformers are different.

Before you BF the amp make it work like it should by replacing all and I mean ALL electrolytic caps. Then start checking for drifted resistors and any burnt resistors, they can hide under things, burnt especially around the power tube sockets. I think those amps had the 1 watt plate resistors on the preamps. But still those carbon resistors drift badly so check your voltages and resistor values. If they are way off replace them. Even so I would replace all the resistors in the B+ rail with 3 watt metal oxide like Doug sells and use them too for your screen resistors. If you are experiencing noise there's a million reasons for that. Once all checked out you might want to start re-twisting the heater wires and rerouting some other wires in there, the SF amps are poor compared to the earlier Fender were lead dress is concerned.

And of course retube it, for a true Fender sound I feel that the EH 12AX7 sounds the best, IMO. As for power tubes I'm a fan of the 6L6GC JJs they are built like a tank. 12AT7s are less about tone but you might try a few different ones in the PI but both Eh and JJ seem to be good. I'm pretty sure that model used the 12AT7 in the PI.

Once you have the amp working the way it should you can start modding it. You might find you like the sound the way it is or with some simple mods like a post PI master volume and some coupling cap value changes. I've had SF amps from the 70s that sounded great stock. That twin is going to be very loud and if you turn it up those nice JBLs will go puff. Once they go unless you get a NOS recone kit forget about having them reconed, the kits being made now ruin the speaker. So be careful, those are old speakers and cannot handle more then 50 watts for both, it's the distortion that kills speakers, lows too will eat a speaker. Think about replacing them if you are going to be using the amp a lot, there are some really nice higher powered speakers that sound good both clean and distorted.

[jeff_free69]

For anyone else treading down the same ancient path, I found the correct drawing for this.
I have a very similar Twin reverb:  Early 1972 , master volume / no push pull.
It would match what was referred to above as "CBS 100"

Schematic : CBS Dwg # 010182  Rev C  

( by the RevE  I also found, it had the pushpull added) 
quality look like its been copied a bunch of times, and I couldn't find a layout, but the values match my amp exactly ( I marked up a clean BF layout (physically very similar until you get to the MV/PI and  Bias areas

[sluckey]

For anyone else treading down the same ancient path, I found the correct drawing for this.
Schematic : CBS Dwg # 010182  Rev C

That same drawing can be found in Hoffman's schematic library under these file names...

https://el34world.com/charts/Schematics/files/Fender/Fender_quadreverb_100w_nomstrvol.pdf

https://el34world.com/charts/Schematics/files/Fender/Fender_twinreverb_sf_100_mv_noboost.pdf

[jeff_free69]

Yep - thats the one!
But I still haven't seen the matching layout.  I started marking up my own, so I think I'll start my own Trouble thread for my rescue project...