(Apologies if you know all this already!)
SixtyFour wrote:
That's really original... which is both a good thing and a bad thing. The original electrolytic caps (the five large orange cardboard tube components on the circuit board) are almost fifty years old and far past their natural life expectancy of around twenty years now - even if they still work properly and the amp doesn't hum, they are a major failure risk and there's a small chance of them damaging something else if they do fail. Electrolytic caps degrade with time and can either simply stop working (partially or fully) in which case the amp will start to hum loudly, or occasionally short internally, which is much worse.
The three on the left are the power supply filter caps and should definitely be changed - because they are carrying a high voltage and store a lot of energy, if they fail they could literally explode and fill the amp with gunk which will be difficult to clean off. There's also a chance that a short in one could damage the rectifier tube or even if you're very unlucky, the power transformer. The one in the middle next to the large striped resistor is the power tube cathode cap, and if this shorts the power tube will fry itself, which might risk blowing the output transformer as well. The one on the far right is the first stage cathode cap (and is not present in all 5F1s, but roughly doubles the gain so amps with it are more desirable), and is the only one I would not change as a matter of routine, if the amp is sounding good - replacing it might alter the voicing slightly. Even so, I'd probably change it and see whether the tone changed in a bad way first, and put it back if so. If it fails the worst it will do is cook the 12AX7.
For what it's worth, a filter cap failed in my original '59 5F1 with no warning at all, it just made a loud bang and the amp started to hum at highly volume - luckily I turned it off immediately and there was no other damage.
Don't change any other non-electrolytic caps (the two yellow ones in this amp) unless they have started to leak measurable DC voltage - they are an important part of the tone of vintage amps. A routine 'cap job' should only mean changing electrolytics as these are the only type which have a definite age-related decay. The other types can and do fail but in a less predictable way.
You will also likely find that many of the resistors have drifted out of spec (almost always upwards) - many techs will change these as well, but I wouldn't unless it's so extreme that it's effectively a sign of likely failure and soon - anything above double the original value falls into this range, anything below 50% high doesn't, and in between you have to use your own judgement... some positions are more important than others. In the signal path, these old resistors contribute to that nice vintage tone, so you don't want to just change them unless you have to. Resistors always fail open-circuit so they don't cause other damage unless they actually catch fire in the process, and the only ones which are likely to do that in a Champ are the two below the filter caps and the power tube cathode resistor (and that only if the tube shorts), none of which are in the signal path or directly affect the tone, so I would apply a stricter standard to those. I would use modern metal-oxide or wirewound resistors for these, but if you need to replace the small ones in the signal path, make sure you use the original carbon-comp type.
If proper maintenance like this is done, there is no reason a vintage amp should be any less reliable than a modern one, by the way.
The original 5F1 is 3W (or sometimes rated at 3.5). However this can be varied very simply by changing the spec of the transformers without altering the circuit at all, up to about 6W (the limit for a 6V6), so a modern clone could be more powerful. You could go even higher with a bigger output tube (eg 6L6), if you wanted.
The circuit type is 'Single-Ended'. Whether it's 'Class A' or not is slightly debatable and actually not relevant, despite this term becoming so prevalent in the guitar amp world... Class A is a strict technical definition that almost no guitar amp meets - arguably not even most single-endeds - and in fact has no real meaning when the amp is distorted since the definition applies ONLY to an undistorted signal
. (Class A does have very real advantages in hi-fi for reducing harmonic distortion, which is why it was original perceived as better, but you can't just import the same ideas into guitar amplification! Harmonic distortion is DESIRABLE in guitar amps.)
The important characteristics which affect the type of sound the amp has are:
Single-Ended vs. Push-Pull. All one-output-tube amps are Single-Ended, and a tiny number of two- (or more)-tube amps are, but for practical purposes any amp with more than one output tube will be Push-Pull. This means that the output tubes operate 'back to back', each amplifying an opposite version of the waveform, which are then recombined the right way round by the output transformer. This gives a more symmetrical waveform (especially when distorted) and much higher power potential. But the asymmetric distortion is one of the desirable characteristics for those who prefer the sound of Single-Endeds, and has a sort of 'brassy' trumpet-like tone.
Cathode Bias vs. Fixed Bias. Cathode bias means the tube bias setting is determined by the tube current itself passing through a resistor. This makes it both self-regulating (it's also sometimes called 'self bias') so you don't need to adjust it when changing tubes, and also interactive with the signal level, so pushing the amp hard will change the dynamics. As the tube current rises, so does the bias voltage which then reduces the current, so the result is soft compression which is distinctive to this type of amp and sounds different from the 'sag' of a heavily loaded power supply. The really ironic thing is that in a TRUE Class A amp, the current rise does not occur (it's one of the characteristics of Class A that it remains constant throughout the wave cycle), so they do not have this compression. The classic "Class A compression" in fact only occurs in Class AB. Many Single-Ended amps are biased too hot for true Class A, and have an interesting characteristic which is that they clip at the forward end of the waveform first, which actually REDUCES the current demand on the power supply slightly - the result is a sort of 'bloom' to the notes as the distort which is also unique to this type of amp.
Fixed Bias means that the bias is set by an external circuit, not that it isn't adjustable - most are, with a trimmer. This makes the tube performance independent of the signal level, so there is no compression (unless the power supply can't keep up) and the result is a bolder, harder tone that remains clean to higher levels but then clips more suddenly. Almost all high-powered amps are fixed bias, but despite the higher actual power output the lack of compression can make them sound 'flatter' and less dynamic than lower-powered cathode-bias amps - cathode-biased amps are generally run hotter at idle (because the inherent rise in the bias regulates them and stops them overloading at full power) and so the initial note transient has more gain than in an equivalent cooler-biased fixed-bias amp, which increases the perceived volume. A fixed-bias amp generally can't be run hotter than about 70% of the maximum tube rating at idle (and usually lower), whereas a cathode-biased amp can usually be run as hot as 90% without shortening the tube life too much.
Negative Feedback. Some amps use this, which means that a small amount of the signal from the output transformer is fed back in reverse phase to an earlier point in the circuit, which is intended to reduce 'non-linearity' in the output stage. It also makes the amp cleaner to higher volumes, but makes the onset of distortion (when the output signal can no longer bear an accurate resemblance to the input and the NFB effectively stops working) harder. It isn't a 'bad' thing despite the name - but the type of distortion it produces is sharper and crunchier than in amps with no NFB.
Tube rectifier vs. Solid-state. In all amps, the power supply has 'internal resistance', which means that its voltage output will fall as current demand increases. This makes the output 'sag' and compress under heavy load. All amps do this to some extent, but a tube rectifier has much higher resistance than a solid-state one and shows the effect far more dramatically - both sag and bloom depending on how the tubes are biased.
The 5F1 is a Single-Ended, Cathode-Biased, Tube Rectified, Negative Feedback amp that is biased into the 'forward clip' range, so it has the characteristic brassy-sounding asymmetric distortion and bloom on the notes.
where did you acquire that knowledge 
.