Quote:
Originally Posted by tele-martini
On my layout there is a capacitor and 2 resistors I'm having trouble finding. The cap in question is a 1uf at 400V. i have found a 1uf at 250V and a 1.2uf at 630V. Can I substitute one of these if so which one?
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I am assuming this is the cap on the first 12AX7's pin2 (grid) or pin8 (cathode)?
In a Firefly voltages are pretty low. In these particular locations of the circuit the 1uF-250v should be OK.
Quote:
Originally Posted by tele-martini
Resistors are 1.8K ohm and 440 ohms. Can I use a 1.5K and a 430 ohm in place of these?
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Did you measure them to be 1.5k and 440? Sometimes they aren't 100% the value it says. Some components have 5-10% tolerances, or even more. you may find a 430 or a 1.5k that strays into nearing these values. Time to pull out the MMeter :)
The 440 is the cathode resistor on your 12AU7 power tube (for both triode halves' cathodes). The 1.8k is your cathode resistor for one half of each triode - it is also wired with the 1uF cathode bypass cap on one of them (pin 8 on the 1st 12AX7, pin 3 on the 2nd 12AX7).
For the 12AU7 it will determine your bias. You can go a little higher or lower. It will make the 12AU7 run hotter or colder.
For the 12AX7 it will also determine some bass cutoff frequencies. A 1.8k cathode resistor with a 1uF bypass cap rolls off 3dB below 187.3 Hz, whereas a 1.5k will start roll off at 205 Hz. What this may mean in the preamp stages is a thinner/thicker sound overall.
You definitely have elbow room for a 1.2k, a 1.5k, or even a 2.2k. you can even try other value caps with other resistors - for instance a 470ohm with 2.2uF cap will get you a similar result of a 199 Hz bass "shelf."
Quote:
Originally Posted by tele-martini
Unrelated question:
Can someone explain what an Octal Type Tube is ?
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Octal means "eight."
Any eight pin tube is called an Octal. Noval means "nine," tubes like 12A*7 dual triodes, EL84 power tubes, etc are considered Noval tubes.
This generally only covers how many pins a tube has. Some tube sockets are different or unusual, regardless of pins.
Then another nomenclature for tubes is things like "Pentode," "Triode," double or dual triode, diode, "Beam Tetrode," etc. This usually indicates how many electrodes a tube has.
"Penta" means "five" so with a Pentode tube you will have 5 internal components (called electrodes). For instance an EL34 will have 3 grids (screen, control, supressor), a Cathode, and an Anode (AKA the plate). It is an Octal base (8), but the heaters on pins 2 and 7 are not electrodes (heater filament for heating the cathode) and pin 6 is a non-connection. "Triode" = 3 "Tetrode" = 4, etc.