Hi all. I'm building a prototype 5C1 Tweed Champ. For parts I've salvaged a Silvertone tape recorder for the transformers. Single ended 6V6 amp with a 6X5 rectifier. The power transformer is pretty straightforward, but the OT has two output taps, a green wire and a yellow wire ( along with the black ground). The yellow wire reads 0.3 ohms and the green wire reads 0.6 ohms WRTG. Is this a 4/8 ohm or 8/16 ohm arrangement, and which is which? Thanks!
Is there anything written on the transformer at all? Brand, model? In the end it may not matter much, but yea it would be nice to know.
As an aside are you working from a design/schematic? If so I would be fun to see.
As an aside are you working from a design/schematic? If so I would be fun to see.
The code on the OT is 700671 over ATC-5509. The amp was already pulled from the recorder when I got it. The green wire went to a 1/4" jack in the front of the chassis, presumably for an extension speaker, while the yellow wire was loose, which I presume had been connected to an internal speaker. The circuit is the classic 5C1 Champ, with the 6SJ7 preamp.
OK, so it probably is two impedance taps. This is just a tiny amp. Build the thing, then take the two transformer tap wires, and try them one at a time to your speaker. The one that sounds best? Use it, tape off the other.
I do this even with known transformers. Building a guitar amp is all about getting the tone you want.....there is no right or wrong way, although some ways are more right than others.
If you like the sound on one wire when the amp is fed with a pedal and cranked, and the other wire when trying to play clean, then add a switch! Just don't flip the switch while playing at full crank since there will be an instant when the amp has no load.
I build about a dozen "Turbo Champs" that used Hammond 125CSE OPT's. I had a 4 position rotary switch to select one of the 4 taps on the secondary. The user chooses whichever one he likes at the time.
To misquote an old TV show "I blow stuff up so that you don't have too."
I started out making tube guitar amps when I was about 12 years old. Back then tubes, transformers, and everything else you needed were free. You just needed to visit the local trash dump and pick from the dead TV's, radios, and HiFi sets that had been thrown out. An old ham radio guy showed me how to hook stuff up. I really didn't understand what I was doing, and blew up a lot of tubes, but did manage to make some real screamers!
High power OPT's were rather hard to get for free, so I used power transformers. Use the CT HV winding for the primary, and wire all the heater windings in series for the secondary. I didn't understand why the system got louder and sounded better as you connected more speakers in parallel, but it became obvious once I began to understand that "impedance" stuff.
Purposley msimatching the speaker load does create some unusual tones though. A light load (high plate impedance) tends to work well for clean tones, while a heavy load goes toward grunge. Really want "wild man" stick a tone stack in the feedback loop. Caution...some amps really don't like this, oscillation is possible if there is enough phase shift.
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