5f1 tweed champ questions.

[Pinvektor]

Hello,

Hope someone can put me straight on a few things regarding the 5F1 champ circuit. I aim to build one very soon for my first amp build.
I have recently acquired Dave Hunters Guitar Amp handbook and have some older books on tubes from the fiftys. I am making an attempt to work things out for myself using these books and researching the net, it's just my wife is getting fed up with me spending so much time doing it.
At this stage I would just like to get my head around what exactly is going on in the 5F1 schematic, then progress to more sophisticated gear later on.
At first I knew nothing on amp circuits or electronics, I am a lot more familar now, its been fun learning but there is still some grey areas I would like to cover.

Here's a few questions I am just going to put out there. I have never used a forum of any kind before today, so I will be interested to see the response I get.

i) Could someone please explain what the purpose is of the 220k resistor connected to ground just before the 6v6 grid?

ii) How exactly does the 5y3 tube work to rectify AC to DC?

iii) Do I need the .05uF Cap if im using a 3 pin plug? I ve heard its not needed, but I would like to know why, if some one could explain?

iv) How does the second input lose 6 db? Is it via the 1 meg resistor to ground? How does that work?

v) How does the negative feedback loop work exactly, why is it connected to the cathode of the 12ax7 driver?

vi) Does any one know the original transformer specification's?

Thank's in advance to any one who help's me out, I know the answers are probably a bit detailed so I am very grateful to anyone who take the time to respond.
I would love to find a copy of Dave Funks Fender workbook. I wonder if it would help me much in understanding the theory behind the Fender Champ circuit in more depth. I would love to de-mystify this circuit fully.

I am confident that the actual build will go well because I have found plenty of info' regarding putting together this amp.

Thanks for reading.

Take it easy.

[MWJB]

i) Could someone please explain what the purpose is of the 220k resistor connected to ground just before the 6v6 grid? - It provides a load for the 6v6 grid, if the cap feeding the 220K was going straight to ground there would be no load for the grid & all signal prior to this point would go straight to ground & not make it to the tube. If the resistor was too large, or not there at all (infinite resistance) the power tube life would be shortened and the amp wouldn't work properly.

ii) Google "full wave rectifier"...you don't need to know the hows & whys to build the amp, even tube rectifiers are pretty simple devices, concentrate on connecting the right secondaries to the correct pins.

iii) AC wall voltage is 240VAC in Australia, don't fit a 0.5uf 400/600v cap...it will just go up in smoke. It is not necessary.

iv) When you plug into the hi gain input the input load is 1Meg, but when you plug into the low gain input the 1Meg is shorted by the input jack "switch" terminal and the signal passes through a 50% voltage divider made by the 2x68K resistors - one 68K is in series with the signal, dropping the voltage, the other 68K is the input load. When you drop a signal voltage by 50% you actually reduce the amplitude by 75%, or make the amp a quarter as loud. Half as loud is -3dB, so a quarter as loud is -6dB. Obviously if you then turn the volume up to compensate the volume the amp makes increases again, but the input stage sees a smaller signal voltage and therefore may give you better clean headroom with pick ups that have a big output. The ony deciding factor as to which input you should use is deciding which one sounds best with your guitar.

v) Negative feedback injects some of the output signal back into the preamp/driver at a point that is 180degrees out of phase, this smooths frequency resonse and increases headroom. The 2nd stage cathode (pin 8) of the 12AX7 happens to be 180degrees out of phase with the output at the speaker, so is the obvious place to tap the NFB. The loop ratio is (22000+1500)/1500, or 15.7:1...so roughly a sixteenth of the output signal is fed back to the preamp. (Google "negative feedback loops valve amps").

vi) There is no single specification as transformer manufacturers work from differeing datums. PT might be 320-0-320VAC up to 360-0-360VAC and still be "correct". Plate voltage at the 6V6 might be anywhere from 350vdc to 390vdc. B+ winding is rated at 70mA dc, Heaters are rated at 2A @ 6.3VAC, rectifier winding is 2A @ 5VAC. There's not much call to analyse this too heavily as there are numerous Champ transformers on the market...unless you are considering possible upgrades to much bigger power tubes? The output transformer matches a 6V6 to 4ohms output, it might have to handle 60mA of plate current at idle and have a primary impedance of 5000ohms up to 7500ohms, again these are widely available.

Go to Onlineshop Startseite and check out their photofile 5F1 build in the kits section. Follow the component placing & wire routing very carefully.

Break the circuit down - signal in on the grid, supported by a load resistor. Out on the plate, supported by a plate load resistor, then passes through a coupling cap to the next stage/tube grid. Cathode sets the bias via the grounded cathode resistor, which may/may not be bypassed by an electrolytic cap (usually 25uf) to bump up gain (most 5F1 built had this cap at V1, pin 3). Same for the power tube, but the power tube plate connects to a transformer rather than a resistor, which supplies the dc it needs to work. Power tube has an additional high voltage supply at the "screen grid", that comes from the junction of the 5K & 22K power supply resistors. Now you know everything that I do! ;-)

Do yourself a favour, add a 470ohm 5W resistor to the tube socket running from pin 6 to pin 4, connect the screen supply to pin 6. Adda 1500ohm grid stopper to pin 5 of the 6V6...see a Super Reverb AB763 socket wiring for an example. Use 50v rated cap at the 6V6 cathode, preferably 100v.