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favorite 5f1 Mods?

8/24/2004 4:36 PM
Troy favorite 5f1 Mods?
Just built my first 5f1- I think I like it with the el34 so far. Anyway just looking for some mods to throw on this thing. I was thinking about doing the 'twang' cathode bypass switch. Has anyone done this before? Any other good mods to try?  
8/24/2004 10:54 PM
Dr. Photon

You can increase the gain (and change the sound character since the resistor in the cathode provides a bit of negative feedback) of the first stage by bypassing the cathode resistor. The resistor acts like an open circuit (not connected) to DC, so the 1.5 K resistor still biases the triode stage correctly. But it acts like a resistor somewhat (simplifying) to AC. The higher the frequency, the lower it's impedance, the formula is R = 1/(2 * pi * Hz * Farads). the standard 25 Uf bypass cap bypasses everything in the audio range. It boosts bass and trebble. a smaller cap such as a 1uf will boost midrange and trebble but not lower frequencies. An even smaller cap will bypass only trebble. Note that the high trebble is boosted by about the same amount with the 25 uF cap as with the 1 uf cap. The cap only effects the cutoff point of the boost. Remember, the coupling caps CUT OUT bass, while small bypass capacitors boost frequencies.  
As for the cathode bypassing of the second stage, the same as for the first stage applies about the gain and frequency response. But notice how the 22K NFB resistor applies negative feedback to the cathode of this stage. The bypass capacitor will also shunt the negative feedback signal to ground too. A 25 uF cap here will essentially be similar to disconnecting the feedback resistor. A small capacitor will raise the gain at high frequencies so the amplifier drives the output stage harder at these trebble frequencies, and it also lowers the negative feedback at said frequencies so the output stage will also have a higher gain at these frequencies (and of course a different tone/distortion). But for the lower frequencies, the amp remains the same. There is still feedback in place for bass frequencies (it's nice to help keep the amp together, this should help reduce fart-outs). If you like the frequency when the capacitor starts to kick in but you feel that it is too much, you can stick a resistor in series with it (like a 1 or 2-ish K pot).  
Annother thing you might try would be series grid stoppers on the power tube to prevent drawing grid current (results in farting).  
I have a homemade "Champ" amp that doesn't exactly follow any one schematic (I built it as I went, then drew the schematics). I have a .0047 capacitor accross the second cathode resistor (I have a non-trebbly speaker, I might stick a resistor in series with it if I change speakers) and I LOVE the tone (with an EL84).
10/20/2004 9:30 PM

You can also stick a switch after the NFB resistor to shut off the NFB. Very easy mod and very worth while, unless you really like NFB or really dislike NFB. Then either leave it in or take it out completely. A lot of folks just leave it out.  
You can also put in a tone pot and cap in place of one of the input jacks. I have the schematic for this at home.
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