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Old August 8th, 2007, 09:47 AM   #1 (permalink)
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More Amp Build Questions??

Currently building Firefly amp . Have my chassis drilled out and switches ,jacks,pots etc. mounted. I'm starting to wire chassis components. I have all my resistors and caps lined up and here's the question. 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? Resistors are 1.8K ohm and 440 ohms. Can I use a 1.5K and a 430 ohm in place of these?

Unrelated question:
Can someone explain what an Octal Type Tube is ?

Thanks,
martini

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Old August 8th, 2007, 10:44 AM   #2 (permalink)
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Martini,

The voltage rating of a capacitor is the maximum voltage that can be applied to that capacitor. So in short - if they specified 400V, you should substitute with one rated at least 400V. This is the only thing you have to watch in the changes you mentioned.

As far as the values you want to substitute, none will break the amp at all, but may slightly change the sound of it (i.e. the gain of the 12AX7's, freq response).

An octal type tube is a tube that has a particular eight-pin base. Many power tubes you'll see in guitar amps will have an octal socket (i.e. 6V6, 6L6, EL34).
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Old August 8th, 2007, 12:58 PM   #3 (permalink)
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Quote:
Originally Posted by tele-martini View Post
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?

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.



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Originally Posted by tele-martini View Post
Resistors are 1.8K ohm and 440 ohms. Can I use a 1.5K and a 430 ohm in place of these?


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."



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Originally Posted by tele-martini View Post
Unrelated question:
Can someone explain what an Octal Type Tube is ?

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.
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Old August 8th, 2007, 02:10 PM   #4 (permalink)
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Thankyou both for the in depth answers. I did meter the caps and resistors that are close and they seem to be pretty much the value stated. I had an idea the slight variance would alter the sound a little. Just wanted to make sure I wasn't risking frying anything.

Just finished a wood shop project for a customer so now I can devote my free time to amp building. I think in a week or so I may be ready to plug in and flip the switch. HAHA Really hoping my main breaker dosen't flip with it.
Seriously though I will triple check every wire and connection before powering her up.

On another note I will be starting work on a couple cabs soon. One will be for the Firefly head I building. The other will be for a Tweed Champ Clone combo I want to build in the near future. I want to wrap both in tweed cloth and will need to buy tweed ,handles ,corners, etc. So far I have found ampwares to have the most competitve pricing. If someone has other suggestions I would
love to hear them.
Thanks again,
martini
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Old August 8th, 2007, 04:08 PM   #5 (permalink)
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Quote:
Originally Posted by tele-martini View Post
On another note I will be starting work on a couple cabs soon. One will be for the Firefly head I building. The other will be for a Tweed Champ Clone combo I want to build in the near future. I want to wrap both in tweed cloth and will need to buy tweed ,handles ,corners, etc. So far I have found ampwares to have the most competitve pricing. If someone has other suggestions I would
love to hear them.
Thanks again,
martini
Do NOT go with Ampwares. It may take months, yes I said months, to get your stuff from him.

TubesAndMore.com have been great for a long time now (although I have a business account with their wholesale wing, CEDist). As have Hoffmanamps.com, Ken at Turretboards.com, Nik at Ceriatone.com, and a few others.

Also, tweed may look weird with corners, as the original tweed Fenders did not have them... for an idea if corners are for your tweed or not, check out Carvin and Peavey's stuff.
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Old August 9th, 2007, 08:20 AM   #6 (permalink)
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JC,
Thanks for the heads up on ampwares. I've done buisness with TubesAndMore and have had good results. On the tweed for the 2 cabs I was thinking corners for the Firefly (head) cab and the more trad. look for the champ clone. I really haven't thought about what the tweed head would look like w/corners.
will have to check out the Peavy and Carvin look.
thanks,
martini
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