what are these caps doing?

[JammyBStard]

Purely out of interest really; I just built this guitar amp from an Ampmaker kit, it all works fine, but I was wondering what is the function of the 1uF caps C1 and C2?

[rotaspec]

C1 is the cathode bypass. It makes sure that the cathode is at ground potential as seen by ac signal.
C2 is the coupling capacitor for the first two stages. It's only actual purpose is to stop the high DC voltage on the anode of the first valve reaching the grid of the second while allowing the signal to pass.
You will see the same ideas used in most common amplifier stages, including the rest of your amp.

[JammyBStard]

Sorry, I ment C1 and C3; So why do you want the cathod to be at gnd potential for ac and not dc?

[badman]

Originally Posted by JammyBStard

Sorry, I ment C1 and C3; So why do you want the cathod to be at gnd potential for ac and not dc?

Because if it's grounded for DC there will be no idle current. That's the bias voltage, is how far above ground (assuming an unbiased grid) the cathode is. The grid must be negative WRT the cathode for the triode to operate.

[rotaspec]

Originally Posted by JammyBStard

Sorry, I ment C1 and C3; So why do you want the cathod to be at gnd potential for ac and not dc?

If the capacitor is left out, the voltage on the cathode will vary according to the amount of signal through the valve. As the grid sees this voltage via R1, the grid voltage will try to follow the signal, but the cathode signal is reverse phase (polarity) so the grid will actually be getting signal that cancels out the signal coming from the input. This means the volume (gain) goes right down.
The capacitor prevents this negative feedback by shunting signal off the cathode, keeping the gain up.
The stage is called a common cathode or grounded cathode stage, because the cathode is effectively shorted to ground to ac. It's the most widely used voltage gain stage you will see.

Gary

[Eli Duttman]

JB,

The RC network self bias technique is extremely common in small signal stages and quite common in power O/P stages. The other posters have indicated what goes on, when the bypass cap. is omitted. The technical name for the phenomenon is degeneration. It's a form of local current NFB. As such, it is sometimes intentionally used (particularly combined with CCS loading) for linearity reasons.

BTW, while parts values and voltages are different, a JFET voltage amplifier is set up the same way.

[gingertube]

C1 is maximizing gain (unbypassed cathode bias resistor leads to negative feedback). It is increasing gain above about 100Hz. This means below 100Hz (Bottom E on a 6 string guitar is 81Hz) the gain is reduced. That helps keeps subsequent stages from being overdriven with a lot of bass which leads to a lot of intermodulation distortion and a fat flabby sound. C3 does the same thing for the 2nd gain stage but the rolls off the gain below about 200Hz.
This has to do with "voicing" - getting the sound you want. Personally I would try C3 of 470nF (based upon experience with amp building and voicing).
Choosing the right cathode bypass capacitor value becomes more important as more gain stages are added. You want to roll off just enough bass energy to keep the distortion (overdrive) sounds pleasant. They don't really affect clean sounds (undistorted) that much.
Cheers,
Ian

[JammyBStard]

Thanks or the replys so far they've been very helpful; I'm looking at potential mods to the circuit, to try once I have a combo cab built for it (by the weekend hopfully) another mod I've come accross is canging the the slope resistor in the tone stack, though I havent found a decent explenation of what that does, can anyone shed any light on that for me?

[gingertube]

The FVM (Fender/Vox/Marshall) tone stack is a fairly interactive beast. Basically the Treble Control acts like a volume control BUT also determines how much effect the Bass and Mid controls have. At high Treble settings the Bass and Mid Controls have little effect. Conversely at low Treble settings the Bass and Middle Controls have much more effect. The slope resistor affects this proportioning of control.
For clean channel 100K is fairly typical, for overdrive channels 56K is more typical.
If you want to see some examples of tone stack tweaks go to the Schematic Heaven webpages and download the Hughes and Kettner TriAmp service manual. This amp is 3 Channel Amp with each Channel having 2 "voices". It is instructive to note the tweaks done to the tone stacks.
Cheers,
Ian