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Re: 1979 Fender Twin (very slow) restoration
Posted: Mon Jan 02, 2023 10:29 pm
by laurie
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.
Re: 1979 Fender Twin (very slow) restoration
Posted: Tue Jan 03, 2023 8:56 am
by Pepe
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
Congratulations for the repair! So sad that a tube failed.
Re: 1979 Fender Twin (very slow) restoration
Posted: Wed Jan 04, 2023 1:11 am
by laurie
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Re: 1979 Fender Twin (very slow) restoration
Posted: Sat Jan 14, 2023 1:14 am
by laurie
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".
Re: 1979 Fender Twin (very slow) restoration
Posted: Wed Jun 28, 2023 6:59 pm
by laurie
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.
Re: 1979 Fender Twin (very slow) restoration
Posted: Wed Jun 28, 2023 7:05 pm
by laurie
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Re: 1979 Fender Twin (very slow) restoration
Posted: Wed Jun 28, 2023 9:15 pm
by Pepe
Nice! Very clean job.
Re: 1979 Fender Twin (very slow) restoration
Posted: Fri Jun 30, 2023 6:19 pm
by Dirk
That looks great, Laurie
Re: 1979 Fender Twin (very slow) restoration
Posted: Mon Jul 03, 2023 7:03 am
by bigtone23
So cool! The amp thanks you for bringing it back to full health!