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Discussion in 'Amp Tech Center' started by lonewalk78, Feb 18, 2020.
No need to "add" a bias pot. Its there. Just move a few wires. Jeeze.
A real hi fi amp does work w a guitar.... I know a guy who uses a Macintosh 30 at his regular club gigs playing pop and jazz to midi backing.
You can have bias AND the balance pot with the recommend changes, if you want Best of all options.
True dat. Hate to Sarah Palin that chassis tho.
Matched tubes are usually the order of the day _and_ the balance effects banks not individual tubes.... So with matching the tubes and _level_ control, you're better off. I see no great usefulness on a 4 tube amp. But thats me
Or you can do what this guy did and run two Twins through a McIntosh!
Is there a diagram for the conversion to the Blackface specs? That would be nice so I can be adjusted easier.
My nuts are in hiding.
Like Chief Brody told Quint - “You’re Gonna Need A Bigger Boat.”
Forget about blackfacing this amp! The ultra linear is different enough that doing a full on "blackface mod" is a pretty deep job.
All you need to do is get a bias setup you can adjust easier than the balance setup it came with. As is you either find tubes close enough or swap resisters to get it in the "zone".
You can rewire the pot that's there or add a pot ahead of the balance pot. I prefer to add a pot, actually a trim pot, takes a minute or two.
The only draw back is you gotta pull the chassis to set bias, like every Marshall. Unless you're swapping power tubes all time in search of some tonal nirvana, you won't need to bias very often. Power tubes last a long time in a Twin.
Leaving the balance pot can be nice if your tubes aren't exactly matched. Or maybe you like a little mismatch. It's there so I just leave it. Otherwise you just set it in the "middle" same voltage on both sides and forget about it. Maybe down the road you got two matched pairs of 6L6 you want to use. That's when you'll be glad you left it.
You won't find a layout of this amp, apparently Fender stopped printing those about 1970 or so. I've never seen one for any of the amps with master volumes and those started like '73 or '74 or so. But you can find a schmatic and those are a whole lot more useful anyway. Once you learn how to read the damned things of course.
The layouts show you where the parts go, which is nice. But a schmatic shows you how the electricity flows, if you're tracking down a problem that information really helps.
Do all the electrolytics, not just the ones that "look bad". Those pesky cathode bypass cap cause trouble. Like noises, no trem or a rather annoying squeal when you try to use the reverb. And trust me, you're gonna want that reverb!
Its VERY complex. All new iron needed. Much revision and frankly a bit if improv too. Better to sell that jewel, and find an earlier one.
This should be your schematic.
Good luck with changes.
Pay very close attention to polarity in the filter caps.
The first two filter caps--the 220uf--and the bias supply cap are the ones that usually fool people.
Be triple sure the polarity is correct.
Ok, thanks for the info...I'm gonna do all caps and set bias, the new tubes are matched..everything on this amp works great but just needs alittle TLC.
A really easy one is attached. It's not "Blackface", but it will most likely get you to where you want to go.
This next one is more like a Blackface. This amp has a completely different power supply and it's not going work with an exact AB763 drop-in bias circuit.
If that means you don't want to drill for a new pot... no need. Hot glue and a trimpot, mounted on the back of the balance pot.
If you are up to adding an additional pot, you can have bias level and bias balance.
The bias circuit has a shunt resistor that is 15 times larger than the dropping resistor and the dropping resistor is only 2.2K. This makes it difficult to get a cold bias that is significantly colder than the stock bias. It also makes replacing the 33K resistor with a pot somewhat impractical because the pot will be working against a 2.2K resistor in the voltage divider.
I don't like the Fender schematic.
In case you forget how the doghouse cap's go:
Yeah.... I know. I'd never do that on a customers amp, just me.