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Guitar amps are really pretty forgiving to build. There are plenty of ways to do it wrong, but those ways are rarely self-destructive, just kinda crappy-sounding. Figure out the crap, change it. That's the great thing about point-to-point valve amps: they're easy to fiddle with.
This is going to be the first in a series on Building a Guitar Tube Amp From Scratch. The series will demonstrate the creation of a guitar amp from a random set of transformers.
I've been volunteering at a recycling/reusing facility that takes donations of appliances (computers in particular), refurbishes and remixes them, then sells them. The facility pursues the noble goal of providing computers and internet access to people who otherwise wouldn't be able to afford them. This effort is subsidized by rendering broken donations down to their base elements and selling these raw materials to other recycling facilities.
This is going to be the first in a series on Building a Guitar Tube Amp From Scratch. The series will demonstrate the creation of a guitar amp from a random set of transformers.
I've been volunteering at a recycling/reusing facility that takes donations of appliances (computers in particular), refurbishes and remixes them, then sells them. The facility pursues the noble goal of providing computers and internet access to people who otherwise wouldn't be able to afford them. This effort is subsidized by rendering broken donations down to their base elements and selling these raw materials to other recycling facilities.
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To business! At three dollars a pound I get to take home the shiny stuff. Exhibit A: three transformers taken from a tube amp of some sort. When reusing parts there are three useful circumstances to take note of:
I: Model numbers of the amp you are dismantling:
A: This lets you look up schematics. Just about any tube amp is halfway to being a guitar amp; usually fiddling with the gain structure will get you something musical so the chassis, sockets, and components can be reused.
B: In this case the amp had at some point made someone feel sad and impulsive, so most of the chassis was no longer usable or recognizable. Remember kids: the gear's not the @#%#, you're the @#%#.
II: Model number on the transformers:
A: Lacking a schematic, one can always search for the transformers. The internet is just one big pile of information. And cat pictures . Even a scan of an eighty year-old electronics catalog could have the info you need. Why is there a scan of an eighty year-old electronics catalog online? Who knows? It's the internet.
B: My Google-fu was no match for this iron. No matter: I have one final refuge.
III: What valves the chassis was populated with. This lets you know:
A: Roughly how many watts the Output Transformer can handle
B: How much heater current the Power Transformer can supply
C: Roughly how much high voltage (B+) the Power Transformer can supply. Different configurations and biases will use up different amounts of current, but this should get you quite close. On Hi-Fis the Choke will likely be able to handle whatever B+ the Power Transformer can dish, but not always. For our little demonstration the Choke will be downstream of the output stage, so if they came from the same amp the Choke should be hardy enough.
D: It can get complicated. Ask questions and check out some of the great forums and libraries available online . Keep in mind that all forums have search functions.
E: I'm not trying to scare you. Ask questions but GET TO WORK.
I: Model numbers of the amp you are dismantling:
A: This lets you look up schematics. Just about any tube amp is halfway to being a guitar amp; usually fiddling with the gain structure will get you something musical so the chassis, sockets, and components can be reused.
B: In this case the amp had at some point made someone feel sad and impulsive, so most of the chassis was no longer usable or recognizable. Remember kids: the gear's not the @#%#, you're the @#%#.
II: Model number on the transformers:
A: Lacking a schematic, one can always search for the transformers. The internet is just one big pile of information. And cat pictures . Even a scan of an eighty year-old electronics catalog could have the info you need. Why is there a scan of an eighty year-old electronics catalog online? Who knows? It's the internet.
B: My Google-fu was no match for this iron. No matter: I have one final refuge.
III: What valves the chassis was populated with. This lets you know:
A: Roughly how many watts the Output Transformer can handle
B: How much heater current the Power Transformer can supply
C: Roughly how much high voltage (B+) the Power Transformer can supply. Different configurations and biases will use up different amounts of current, but this should get you quite close. On Hi-Fis the Choke will likely be able to handle whatever B+ the Power Transformer can dish, but not always. For our little demonstration the Choke will be downstream of the output stage, so if they came from the same amp the Choke should be hardy enough.
D: It can get complicated. Ask questions and check out some of the great forums and libraries available online . Keep in mind that all forums have search functions.
E: I'm not trying to scare you. Ask questions but GET TO WORK.
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-- For a tube power transformer (which it definitely is), that looks plenty large enough for a 6V6 PP amp. No problem. Probably big enough for 6L6's.
-- Taking an educated guess, I'd say the AC mains primary is the Black and the Black/Yellow wires.
Once you know the AC primary, it's simple to test the other wires for voltage. When "recycling" an old PT, note the tube compliment of the equipment it came from. That'll give you a ballpark as to it's current capabilities, for both output and heater.
Somewhere in the 275-350V is the "classic" voltage range for a 6V6 push-pull amp.
-- 99% of tube PTs had a center-tapped secondary, so the center tap is the GND, and only two diodes are necessary for full-wave rectification. Most rectifier tubes are dual diode, and it was cheaper (then) to double up the transformer secondary than to use two rectifier tubes (and power two different tube filaments).
That's important--if you use a SS bridge like your PS schematic, the HV secondary will be 2X the original voltage, and can only supply half the original current.
-- You can certainly substitute SS diodes for the original tube rectifier. Usually two 1N4007 diodes in series will replace a 5Y3 (or three 1N4007 diodes for larger rectifiers). Of course, that's two 1N4007s per diode of the 5y3, for a total of four. That would also leave you an unused 5V secondary you could use for switching relays or an indicator lamp, or whatever.
The power supply schematic at the bottom of the step one is more of a general PSU. Mostly I wanted to draw attention to how different B+ voltages are tapped before and after the choke. This (perhaps wordy) instructable was focusing on finding winding ratios, and PSU topographies will be discussed in the next. Yours would be the concise version :) Me, I like to ramble :P
You said 99% of tube PTs had a center-tap-- are you familiar with the remaining 1%? The PT I have does not have a center-tap, and to compound things the heater supply is two taps in the middle balanced around the center of the HT winding. Thus:
_HT
(
(
(_Heat
(
(_Heat
(
(
(_HT
I haven't found any reference to a PSU like this-- can you offer any insight?
Could this be a later tube PT, after SS or selenium diodes were introduced? It certainly saves wire to have the heater wiring like that, if you use a bridge, and that makes sense...
There are two ways to "share" the secondary windings: in the middle, and on the end. There's good reason, electronically, to use the middle--if the end of the coil were used instead, the low voltage portion would be an "elevated" voltage, referenced to ground. If the "shared" part is in the middle, that lower voltage isn't elevated.
For the "end" coil winding, if the total secondary voltage was 0-300V, and the filament voltage a typical 6V, then that 6V would be offset from the maximum voltage. 294V to 300V, rather than 0 to 6V.
That does actually work--except for HV there is a "Maximum Heater-Cathode Voltage" for tubes. A 12AX7, for instance, that maximum difference is 180V. This could easily be exceeded with a 300V (our chosen "example" voltage) power transformer...
I have used a PT with a shared end coil successfully, and it did work. But that was a lower voltage (145V) secondary.