# Champ OT Primaries question



## bcmatt (Aug 25, 2007)

So, I got a good deal on a kit for a BF Vibro Champ Kit, when I got me JCM800 kit and figured I would do the Champ first. I really want to understand things as I go, and am realizing that I have lots to learn, but constantly reading stuff online seems to leave huge gaps in my understanding.
There's lots of info, but sometimes I wish there were more pictures online with an arrow that says "this is the part I'm talking about" when referring to a bias pot or which pin to touch with a dmm to measure plate voltage or whatever. Also, I was just thinking that if someone ever made a website that had tonnes of pics or video and sound clips with instructions on certain stuff, they would be my hero. For instance; Sometimes I wonder if I am hearing crossover distortion in my amp, but who would ever tell me what that certain sound I dislike is. Maybe I just have bad preamp tubes or just generally prefer power tube distortion. Maybe I just need other friends around that are into amps.
Anyways, I'm grateful for you guys and all you do to help. Ok, I'll stop my rant now so I can ask my stupid question.

I see that on my Champ OT there are 2 wires on the primary lead side; one red (marked 'B') and one blue (marked 'P'). What do those letters represent and does it matter which one is used for what? Obviously, the secondaries are way easier to recognize the purposes for because they are marked according to impedance (0, 4, 8, 16). 
So, about these primaries, one goes to pin 3 on the 6V6 tube, and the other goes to the end of the turret board by a big cap and then to pin 8 of the 5Y3 tube. Does it matter which one is the red (B) wire and which is the blue (P) wire? Feel free to explain why if you feel like it.
Thanks


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## Wild Bill (May 3, 2006)

bcmatt said:


> I see that on my Champ OT there are 2 wires on the primary lead side; one red (marked 'B') and one blue (marked 'P'). What do those letters represent and does it matter which one is used for what? Obviously, the secondaries are way easier to recognize the purposes for because they are marked according to impedance (0, 4, 8, 16).
> So, about these primaries, one goes to pin 3 on the 6V6 tube, and the other goes to the end of the turret board by a big cap and then to pin 8 of the 5Y3 tube. Does it matter which one is the red (B) wire and which is the blue (P) wire? Feel free to explain why if you feel like it.
> Thanks


The blue wire is for the "Plate" of the 6V6, which is pin 3. The red wire is marked "B" for "B+", which is the power supply 1st filter cap connection. It is called "B+" because in the days before AC power supplies running from a cord in the wall radios and amps ran on batteries. The high voltage battery was called the B battery. + of course means positive.

It's important to connect these wires properly. The reason has to do with the phasing of the signal.

Picture a sine wave ~. It swings from zero to a positive peak, and then back to zero, down to a negative peak and back again. If we had another signal that followed the same path ~ then the waves would be said to be in phase. 

Waves in phase reinforce each other. Like waves on the seashore, if they all build up and hit together they can cause a lot of damage.

Waves that are out of phase tend to cancel each other. When one wave goes positive the other is going negative. An example of positive phase action would be if you put your microphone in front of the PA speaker. You get instant squealing. The output signal wave is in phase with the input from the microphone. It just keeps reinforcing itself until it breaks into a self-sustaining feedback. This is called oscillation.

Some live action mixing boards will have a button on each channel to flip the phase. This allows the sound guy to use 2 microphones, with one out of phase. If you see old films of vintage concerts you may see dual mics for the vocals or maybe on the drums. One mic will be angled a bit away. The vocalist sings into one mic. With the other mic out of phase all the stage wash that is heard in both mics is cancelled out. When he sings into just the one mic that comes through just fine! The out of phase cancelling can eliminate feedback squealing and also keep that vocal channel "pure", with just the singer's voice and none of the other instruments in the line.

In the Glory Days the Grateful Dead had Electrovoice design a mic housing with two mic cartridges inside, wired out of phase. This just killed feedback! If you watch old footage from those times you'll see that the Dead played with a wall of PA speaker bins behind them and NO stage monitors! With no feedback worries they could just hear themselves from the out-front PA sound.

Many if not most amps including your Champ will tap a bit of the speaker out signal and feed it back to an earlier stage in the circuit. It is fed back out of phase and is referred to as "negative feedback". This nfb "loop" uses a bit of phase cancellation to reduce and smooth out the midrange peakiness of many speakers and also cancel out some distortion/hiss/hum, making the amp "cleaner".

Getting the wires wrong with the OT will make the phase of the feedback wrong as well. That nfb loop becomes a POSITIVE feedback loop! Instant squealing!

Some amps like Voxes and VibroKings don't use nfb. It gives them a distinctive sound. Most guys like it except for jazz cats, who tend to like everything as clean as possible.

Kinda long but I hope this makes things more clear!:smile:

:food-smiley-004:


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## bcmatt (Aug 25, 2007)

Hey Ya! I really appreciate that. Thanks Wild Bill!
I appreciate both the direct answer to help me carry on, as well as the extra education to help me understand better what is going on.
Cheers!:food-smiley-004:
Thanks for the speedy reply too.

I took a break from wiring to spray paint the face of the chassis gold (and eat supper) I'm being pretty daring on this build since the kit didn't come with any kind of face plates. I'll see later whether I want to keep the face gold, or perhaps change it to black; after-all, it is supposed to be a "BF" Champ. But it couldn't stay white.


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