'74 Fender Silverface Twin Reverb Project (w/MV)

[Basslice]

Getting closer. Goodness gracious! The wiring and soldering takes a leck of a lot longer than populating an Ampeg board! I no longer wonder why hand-wired amps cost so much!

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Somehow I am supposed to believe that this pile of wires is going to make some musical sounds at some point....

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[Chris Brown]

Hey fun project! Extra cool points for sticking to the original circuit! I have had a few of these Twins with that circuit and they do sound fantastic as is (aside from the pull boost). I sold the best specimens and kept the one that needed work lol

The one I kept had some noise issues that could not be defeated. At least not at my skill level. Eventually I started thinking it was a conductive board. Really, who knows. I ended up replacing it with a new board, the standard AB763 circuit, and a black panel face plate.. no regrets.. it's one of the best amps I own, but it's cool that you stuck to the original MV circuit setup! Those things really sing!

[Basslice]

Been a while since my last update. The new Jensen C12Ks came in along with the reverb tank. I mounted the speakers.

By the Geezum this thing is LOUD AF!!!!

Still debugging some annoying glitches in the tremolo circuit and the Volume pot in the clean channel works, but needs replacing. I hate to have to do it, but it looks like I will need to remove the board to flip it and look at things around the tremolo circuit. Likely a solder glob or stray wire is the problem.

Other than the tremolo and the pot problem this thing sounds nice!

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[pompeiisneaks]

This seems like a loose wire? Maybe this photo was a bit older before you got it working?

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~Phil

[Basslice]

p.s. Among other things I did over the last month was change the bias resistor from 15K to 11K which brought the bias current up to 30mA on the power tubes. It was hovering around 21mA.

I put in the new JJ 6L6GC set and installed the Jensen C12Ks and brought it to life. Like I said in my previous post, this thing is stupid loud. The hum is not so bad, but I think some of it is coming from the ailing tremolo circuit. The MV works and the reverb works. I hate to have to take it apart again, but I think if I can get the tremolo right this thing will be as good or better than new and safe & stable.

[Basslice]

@Phil. Ha ha ha! Great eye my friend!

I had everything together when I noticed that. It was removed a week or so ago. I don't think it needed an antenna.

[Basslice]

And while I have the board off again, I might as well "float" it on some stand-offs. I think someone's uncle suggested that and it probably is a good idea.

[Basslice]

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It is done! It works. It sounds great. It is LOUD. BTW. From all the codes I am now pretty sure it is a 1973, not 1974. Circuit-wise it makes no difference.

Here's the skinny:

• It came to me as a rebuild. Rusted badly inside.
  Good thing is all the iron was operational
  Had no power tubes and questionable preamp tubes
  It had no speakers or reverb tank
  The front grill had been a cat's scratch post
  The cab and Tolex were pretty good (no wheels)
  Had all the original caps
  Everything inside had a very thick coating of parafin wax that was degrading
  Just about every solder junction was corroded

Decision: Scrap or Spare?

• Spare it from the dump. It was my first Fender rebuild and I was intrigued.
  Decided to keep the MV circuit and not mod or "Blackface'

Process:

• Replace all electrolytic caps
  Test all pots and connectors
  Remove and replace all boards
  Grind off rust and repaint for cosmetics and sealing
  Replace 95% of board components (caps, wires, resistors)
  Remove deathcap and replace plug
  Replace all 4 of the power tube sockets.
  Clean and replace other hardware as needed (normal channel pot, RCA jacks)
  Install 2 X Jensen C12k
  New reverb tank and wiring
  A matched quad of JJ 6L6Gcs and one 12At7 and one 12AX7 (TubeDepot.com - bless you)
  New Foot switch
  New wheels

Work Synopsis:
It was a lot of work and time - mostly the wiring and replacing oxidized and weak wiring. The new board and components would have been easier if I had been starting a fresh new build. Connecting new stuff to old reliably is a pain in the butt and time-consuming. Maybe the most frustrating thing was to have to order parts and wait for them to arrive. Mojotone, AmplifiedParts.com, TubeDepot.com, and Mouser were nothing short of miricle makers, but still the 2 days wait here and 3 wait here days here was a bit numbing.

Probably the worst part of this was the problems I was having with the reverb and tremolo circuits. A combination of bad work and bad tubes made finding the problem hard. I finally broke out my tone generator and scope and found that the 0.022uF cap in the tremolo chain was bad. Replaced that and VOILA!

The amp sounds great. All the MV push-pull circuit works as designed. Both the Normal and Vibrato channels. The reverb is a little noisey, but lacking familiarity with these amps I don't have a benchmark for normal.

Good amp though! Little under 3 months of bench time. Most of that was for checking and restoration of rusted components.

I will be uploading and sharing a complete start-to-finish video and commentary soon. Shwew...

[martin manning]

Looks good, and quite the resurection! That 0.02 phase shift cap in the LFO is a common failure point.

[Basslice]

My plan was to sell it, but it really sounds fantastic. I can't stop playing through it!