#161797  by Incaroads77
 Wed Jul 04, 2018 11:25 am
After doing a tiger strat build and getting the twin and a couple JBL's together, it's time for me to do a couple things inside this twin to get it more "in the ballpark." The links are a few pics inside my 74 MV DSR. I suppose i'm also wondering if this twin has any modifications already, as it does have some newer caps. Filter caps look new as well. There is a wealth of info here about jerry twin mods including mouser parts numbers etc. Some of that info/links regarding parts is a tad bit outdated. So my MAIN question is; what minor stuff should i do? Tone stack and grid resistors? which one's and where from? new to all of this, and i will be using the power section for now. It has the master volume. Would it be a good idea to "blackface?"

Been meaning to sit down and make this post. Never gone inside an amp before, and i feel comfortable taking that dive now. Just curious about my amp and what some of you folks would do with it if it were yours! all help is VERY much appreciated.

-Vic



https://imgur.com/41O6IvM

https://imgur.com/Zvu7dIo

https://imgur.com/F59KuEX

https://imgur.com/3Z2U00R

https://imgur.com/AVuiLOX
 #161828  by lovetoboogie
 Sun Jul 08, 2018 3:04 pm
Since no one else responded I wanted to...

Looks all original with the exception of cathode bypass caps and the bias supply filter cap. I can only assume that under the doghouse the main B+ filter caps have also been replaced....

With that said, this amp is a pull boost master volume model. It would need an extensive rework and rewire to bring it into AB763 range. It is totally do-able so I don't want to seem discouraging. I would start with the following just to get some vibe back in the amp;

-Convert bias balance to bias adjust
-Backdate the Phase Inverter section to AB763 with new plate, grid and cathode resistors
-replace output coupling cap to .001uf
-remove pull boost matrix and convert reverb circuit back to AB763

Ironically the actual preamp tone caps and plate load resistors are fine as are. Garcia's amps had some upgrades there but he also played many a stock amp. You do not need to tweak that stuff now and can decide afterwards where you want to go. If you take care of the above and have it done correctly with period correct values this amp will jump right into Garcia-ish territory.

Hope this helps :wink:
 #161836  by pula58
 Mon Jul 09, 2018 8:48 am
Once Jerry went to using the Twins as a preamp-only wasn't his primary Twin his Silverface Twin, with the Blackface as a backup preamp?
That being the case, why not let it be Silverface for a while and see how you like it.
Not only that, but some of the mods that went into the SIlverface Twins were to stop oscillations caused by the sloppy wiring and layout of that time period. If you revert the circuit to AB763 you could quite possibly have the oscillation problems that led to some of the very design changes that created the SIlverface circuit. You'll then have to do a clean up of the wiring lead dress, grounding, etc. So, just an FYI that converting to AB763 isn't always a straight shot, and truthfully, there is no guarantee that you'll like the sound better even if it does not oscillate. Just my $0.02 based on my own experiences.
 #161841  by strumminsix
 Mon Jul 09, 2018 10:12 am
Had a 1968 Dual Showman Reverb and took it to a a tech blackface it. He said the best thing to do was remove caps and resistors over the poweramp tubes and start there. He said that's the biggest bang for the buck. He was right - amp came to life. I compared it to another buddy's 65 or 66 Twin Reverb and I couldn't tell a noticeable difference. Others have different ears than me. But I will say this: the poweramp section made a HUGE difference.
 #161853  by lovetoboogie
 Tue Jul 10, 2018 3:26 pm
pula58 wrote:
Mon Jul 09, 2018 8:48 am
Once Jerry went to using the Twins as a preamp-only wasn't his primary Twin his Silverface Twin, with the Blackface as a backup preamp?
That being the case, why not let it be Silverface for a while and see how you like it.
Not only that, but some of the mods that went into the SIlverface Twins were to stop oscillations caused by the sloppy wiring and layout of that time period. If you revert the circuit to AB763 you could quite possibly have the oscillation problems that led to some of the very design changes that created the SIlverface circuit. You'll then have to do a clean up of the wiring lead dress, grounding, etc. So, just an FYI that converting to AB763 isn't always a straight shot, and truthfully, there is no guarantee that you'll like the sound better even if it does not oscillate. Just my $0.02 based on my own experiences.

This is a great point, but for a few things...

Garcia's Silverface Twin is from late 67' which is basically a blackface circuit in new trim albeit a few lower quality caps that Fender started using. You have a great point about the messy lead dress showing up around 1970 and onward which prompted suppression caps on the power tube sockets. With that said i have blackfaced alot "those" silverface amps. By carefully cleaning up the excessive lead lengths and redundant grounds... any Twin Reverb up until the Ultra Linear models can carefully and successfully be backdated to a 64' Twin Reverb. There are some slight power transformer variables through the eras that can affect headroom(most notably the ill-conceived AC568 circuit). The overall effort put in yields great results when doing this kind of work.

:cool:
 #161857  by pula58
 Wed Jul 11, 2018 1:03 pm
Yep, the oscillation problems can be solved. It just takes time to clean-up the wiring. When I work on folks amps that time can mean more $$ which can be a bummer and a shock.

--------------------
This is a great point, but for a few things...

Garcia's Silverface Twin is from late 67' which is basically a blackface circuit in new trim albeit a few lower quality caps that Fender started using. You have a great point about the messy lead dress showing up around 1970 and onward which prompted suppression caps on the power tube sockets. With that said i have blackfaced alot "those" silverface amps. By carefully cleaning up the excessive lead lengths and redundant grounds... any Twin Reverb up until the Ultra Linear models can carefully and successfully be backdated to a 64' Twin Reverb. There are some slight power transformer variables through the eras that can affect headroom(most notably the ill-conceived AC568 circuit). The overall effort put in yields great results when doing this kind of work.

:cool:
[/quote]
 #161878  by Incaroads77
 Sat Jul 14, 2018 5:58 pm
Thanks for all the replies!

The more i look into all of the mods i'd need to do, the more i don't want to change around everything about this stock silverface while still wanting an SMS. I listed the DSR on reverb and it sold within the hour. Looks like i'm looking for a power amp now!
 #162385  by Pointclear
 Tue Aug 28, 2018 8:04 pm
Maybe you’ve moved on from your Silverface twin project, but I still want to chime in. I’ve got the same amp (late 73 twin reverb with all rca tubes. Mine doesn’t seem to have the power tube suppression caps either) and I’ve done a lot of the preamp mods mentioned in this forum and elsewhere about how to achieve Jerry tone. I’ll give you a little rundown of what I’ve done, and encourage you to dive in:

- changed the tone stack .022,.022, and 250pf - Sprague 225p and silver mica

- dale metal film plate resistors, RN series

- Sprague tantalum 150 series cathode bypass caps, 22uf

- black-faced the phase inverter and bias pot with dale metal film resistors, xicon metalized polypropylene caps - I like these in the phase inverter over the orange drops: responsive, 3-dimensional and textured

- changed the phase inverter input cap to .oo27 Sprague 225p - fattened up the tone just enough

- disassembled the master volume circuit, and disengaged the tremolo

- after lifting the wire to the Tremolo pot, I changed the cap on the right of the phase inverter from .1 to .047 (Sprague 715p) this tightened up the bottom end in a necessary way - I don’t see too many people talking about this but it seems important to my ears

- I changed out a few capacitors here and there to 225p orange. Drops

- AND just today I swapped out the reverb driver resistor on v4 from 850 ohm to 2200k metal film and found the sonic edge I’d been looking for. It added a bit of gain and breakup overall and a quality to the reverb that is just killer. I feel I’m almost there - think early spring ‘78

I run an e120 and k120 in the combo and an ext cab with the same speaker set up.
waldo041 liked this
 #162391  by Pointclear
 Thu Aug 30, 2018 1:14 pm
What is the purpose of disconnecting the vibrato?
The main reason I disconnect the vibrato is to achieve more gain. The vibrato circuit bogs down the signal a bit, and lifting the lead on the intensity pot gives each note more life. Apparently there is an impedance mismatch, or something, and the .1 capacitor on the right side of the phase inverter section needs to be dropped down some ( mine to .047 ) and the bass tightens up quite a bit. :thewave: