1979 Fender Twin (very slow) restoration

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laurie
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Re: 1979 Fender Twin (very slow) restoration

Post by laurie » Mon Jan 02, 2023 10:29 pm

The behemoth Twin is finished.

Who needs paint-stripper? Crank this thing to 10 and the paint will fall off the walls :)

Pics to follow.

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Pepe
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Re: 1979 Fender Twin (very slow) restoration

Post by Pepe » Tue Jan 03, 2023 8:56 am

laurie wrote:
Mon Jan 02, 2023 10:29 pm
Who needs paint-stripper? Crank this thing to 10 and the paint will fall off the walls :)
:lol:

Congratulations for the repair! So sad that a tube failed.
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laurie
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Re: 1979 Fender Twin (very slow) restoration

Post by laurie » Wed Jan 04, 2023 1:11 am

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laurie
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Re: 1979 Fender Twin (very slow) restoration

Post by laurie » Sat Jan 14, 2023 1:14 am

Now that it has been a couple of weeks since the initial rebuild I've had time to play the 135W twin a bit more and do a bunch more research.

This model twin does not have a bias adjustment on it - the bias is fixed, and the only adjustment is a "tube matching" potentiometer, which doesn't add any value when you buy a matched quad of tubes.

The tubes are running "cold" - a measured bias current of 20mA. This amp needs about 40mA bias current, so there is a lot of cross-over distortion. It is good sounding distortion, but the amp is not giving the traditional clean Fender tone - it is always a little "dirty".

When I get some time over the next few weeks I will rebuild the tube matching circuit into a proper "blackface" bias circuit. Just needs 3 resistors changed and rewiring the hum balance potentiometer.

I use a Eurotubes "Pro One" bias meter:
https://www.eurotubes.com/store/pc/bias%20probes.htm

I'm still not thinking I'll keep it (it weighs north of 80 pounds) and 135W is ridiculous, even if they are "Fender Watts".

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laurie
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Re: 1979 Fender Twin (very slow) restoration

Post by laurie » Wed Jun 28, 2023 6:59 pm

UPDATE:

The output tube sockets were having some issues. The connectors for some of the pins were losing tension and could not be fixed. Plus I did some more work on it. It is now completely finished. This is the full list of stuff done:
  • All new electrolytic capacitors (a “recap”).
  • Brand new Apex matched quad of JJ 6L6GC output tubes installed.
  • Old tired output tube sockets replaced with new ceramic sockets.
  • Output tube control-grid and screen-grid resistors upgraded to resistance-matched 2W resistors.
  • There is no bias control in the original circuit. The original “output tubes matching” circuit has been upgraded to a Blackface Twin (AB763) bias circuit. The amp can now actually be biased.
  • All preamp tubes (12AX7, 7025, 12AT7) tested on a tube-tester and weak ones replaced.
  • Vibrato opto “roach” replaced with a custom-made LED roach. The 0.01MFD vibrato grounding cap replaced. Vibrato is now dead silent (no ticking or thumping).
  • The ground switch and associated capacitor removed (the so-called “death cap”).
  • Damaged original power cord upgraded to a modern 12 foot long cord.
  • Broken fuse-holder replaced.
  • Dodgy power and stand-by switches replaced with Fender factory parts.
  • All potentiometers and switches cleaned and lubricated. The contacts in all switched jacks cleaned, retensioned, and lubricated.
  • New Fender factory footswitch and dust cover.

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laurie
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Re: 1979 Fender Twin (very slow) restoration

Post by laurie » Wed Jun 28, 2023 7:05 pm

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Pepe
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Re: 1979 Fender Twin (very slow) restoration

Post by Pepe » Wed Jun 28, 2023 9:15 pm

Nice! Very clean job.
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Dirk
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Re: 1979 Fender Twin (very slow) restoration

Post by Dirk » Fri Jun 30, 2023 6:19 pm

That looks great, Laurie

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bigtone23
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Re: 1979 Fender Twin (very slow) restoration

Post by bigtone23 » Mon Jul 03, 2023 7:03 am

So cool! The amp thanks you for bringing it back to full health!

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