Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: sixs929 on April 21, 2010, 02:41:05 pm
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Hello all -
Yep, another newbie blackfacing a Fender :embarrassed:
One of the parts of the process (that you guys could probably do in your sleep) is converting the balance pot to a bias pot.
And part of that process involves moving the capacitor lead and the 3.3k resistor (see photo) to the terminal on the other side of the balance pot.
But I haven't seen anywhere (and I have The Tube Amp Book and Weber's desktop reference) that mentions the white wire that also shares the terminal with the other two components. This white wire comes from the 2.2meg resistor that's off the center post of the vibrato pedal jack.
Judging by all 5 variations on the schematic for these amps, as well as the one for the 100 watt Silverface in question, it appears that the white wire has to move over with the other two leads so as to maintain its connection to ground through the capacitor.
Am I anywhere near correct on this?
Thanks for your help
Mark
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Connect that white wire directly to the anode of the bias diode. Make it look like this layout...
http://www.schematicheaven.com/fenderamps/twin_reverb_ab763_schem.pdf
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Thanks for the fast reply, sluckey - I'll hook it up as you indicated.
Good to have a site like this to ask these questions :headbang:
Out of curiosity, what's the advantage of this modified hookup? Am I correct in understanding that it basically eliminates the wire I hilited in red in the attached snapshot from the AB763 schematic?
Thanks, man.
Mark
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No. You must have the red highlighted wire too. Look at the layout. The white wire that goes to the tremolo circuit is shown connected directly to the diode. Your red highlighted wire connects to the diode also (via a jumper on the board) and goes to the pot.
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Yep, I see it. Thanks again>
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> the white wire .... off the center post of the vibrato pedal jack.
Nothing to do with bias.
The trem will kick-start faster if you throw a big pulse into it just when you hit the trem switch. It isn't essential: a lot of trems get a small kick or just start slow. But on this chassis there was that handy 60V supply, already paid-for. When trem is off, part of the CRCRCR network is held down to -60V and one of the speed caps holds a 60V charge. When trem is started (short the trem jack), that point pops-up 60V, gooses the trem oscillator grid, gets the vibe started real quick.
In theory there is an optimum value of trem kick-start (and in fact about 60V for this circuit), but any similar voltage will work about the same.
In practice there's just one good place to tie this trem-kick: where Sluckey said, the raw negative supply (the "bias" diode and bias filter cap). Same place you go to the bias pot.
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Very cool, appreciate the additional explanation.
Things are going OK with this project - re-capped with orange drops, built a full replacement bias board and a filter cap board (with blackface values). I'm pretty much following the suggestions in the library on this site.
Then I busted off a terminal on one of the power tube sockets while replacing the 470 ohm resistors - but that gave me the push I needed to do what I should have done in the first place, that being replace all 4 sockets, along with all the associated resistors and heater wires.
This is (or was) a middle-seventies silverface that was in rough shape, so it'll be getting Tolex, a new grill, and I'll probably put the original speakers in storage and throw in some Eminence Legends or something, unless you guys have a better suggestion. I threw a 12 inch Legend in a '64 Princeton Reverb I repaired (had the original Oxford reconed by Weber and put it and the original 10" soundboard and grill in storage) and that amp sounds like a 50-watt now. They seem like pretty decent speakers.
Thanks again