1957 Fender Low-Power
Tweed Twin chassis restoration
Part 3
Transformers mounted
I went ahead and finally got the transformers in and mounted. Since this was a very special amp, I went on my intuition and paid a visit to good friend, mentor and amp guru Jon Bessent at Tonecraft Amp Repair. I wanted to get a 2nd opinion on a couple of things before firing it up.
Rewired power section
Jon also slightly altered my adjustable-bias design to get some more negative voltage. The way it was, there just wasn't enough adjustment in the pot.
Everything else looked good, and voltages checked out OK w/no tubes. Popped in rectifier tubes. Everything seemed OK. Pop in the power tubes. Turns out that the plate voltages were wacky. Nix another original Astron. Sad, but it was leaking like crazy. We were then having a problem with the PI tube drawing WAY too much current. Swapped tubes. Still drawing too much current.
Started backtracking. Nothing was changing the current draw. Eventually I popped in an NOS JAN 12AX7. Problem solved. Imagine TWO PI tubes having the EXACT same problem. RRRRRRR . . . OK. So, now the voltages are pushing hard, amp is responding on the scope. Then, starts losing power. Hmmm . . . worked fine, then not . . .
Turns out one of the 5U4s that tested good on the tester failed once there was some serious current draw. Oh well. Installed an NOS RCA one. The amp was functioning now, but not correctly. Only putting out about 12-watts of clean, with no crossover distortion. Not good.
Started tracking through the schematic and layout, fixing incorrect feedback resistor values, finding out that caps were missing in the tone stack (bass/treble cap wired together as one etc). Was getting better, but still not right. Started thinking that MAYBE the MM OT was bad . . . doubtful, but? So, we wired up a SF Bassman tranny, still same readings. Hooked it up to a cab, then BAM, amp was LOUD. Jon was spinning the knobs back and forth, then the power dropped again.
OK, big trouble in the pre-amp section. Jon bypassed the tone stack (learned another trick - learn something new all of the time), and then that amp was putting out like a BIG DOG! Really cranking out the power. So, went back through the stack. Eventually isolated the treble pot. That was the one causing all of the problems. Replaced the pot and a 220K resistor that was out of whack, and she started responding correctly. Have it biased at about 36-watts of clean before clipping. Jon really helped me out finalizing this one, and the chassis was finished. WOW what a day of sleuthing let me tell you!
On to the cabinet
Well, the homemade cabinet was held together with NAILS. Oh my, since Curley has been talking about possibly playing again, I knew that this would not do. It was pretty rickety, so I decided to reinforce the cab as best I could using Fender-style screws and a few L-brackets where screws could not be used. The baffleboard was literally loose all over. TALK ABOUT A FLOATING BAFFLEBOARD!!!!!
Reinforcement
Curleys original Jensen speakers were also to be used. But, unfortunately, the cones were extremely rotten. So, I gave him a set of my Eminence 12-BR-30s for now rewired with cloth connector wire. The Jensens are on the block for reconing soon. I also added a new Fender repro handle to replace his disintegrated one.
Cab finished
Cleaned, reinforced with the speakers installed. Looks like we are getting close here . . .
Now with the chassis installed. I redrilled the chassis mounting screws so that the chassis actually mates to the back panel securely now. The 1/4" gap between the panel and chassis is why there was SO much crud in that amp.
'57 Twin Restoration Page 4
Back to Homepage
DISCLAIMER: This website is not affiliated with FMIC, Ampeg or Marshall.