I am making a small guitar buffer / preamp using a 12ax7 in cathode follower mode. The high voltage from the cathode is kept out of the signal output by a large capacitor , but there is always a possibility that capacitors will at some point fail , so I'm wondering if there is a simple way to protect the input of whatever the preamp gets plugged into . I suppose two caps in parallel would be one way provided there was some indication of one cap' failure , are there any other simple fuse / trip bits of circuitry that could be applied?
Two caps in parallell will double the risk !!
Two caps in series will reduce the risk.
But
a series resistance (suggest 1k ) at the output will protect the following amp, also
have a resistor to ground after the cap.
Don't make the cap too large, 0.47 uF will fit most cases.
Two caps in series will reduce the risk.
But
a series resistance (suggest 1k ) at the output will protect the following amp, also
have a resistor to ground after the cap.
Don't make the cap too large, 0.47 uF will fit most cases.
A schematic of the proposed buffer/preamp might be helpful to answering your question.
The cathode quiescent DC voltage, voltage swing during power-up, and the maximum swing of the cathode with maximum signal would be helpful.
And the maximum capacitance of cable between the buffer / preamp and the power amp needs to be known to, as well as the input resistance of the power amp.
Since this is for guitar amp systems, and not Hi Fi, that gives a little more flexibility of the protection circuit.
Remember, a capacitor acts like a dead short until it is charged to its final voltage.
A short time constant that has enough voltage might blow out a solid state circuit, or a woofer or tweeter.
The cathode quiescent DC voltage, voltage swing during power-up, and the maximum swing of the cathode with maximum signal would be helpful.
And the maximum capacitance of cable between the buffer / preamp and the power amp needs to be known to, as well as the input resistance of the power amp.
Since this is for guitar amp systems, and not Hi Fi, that gives a little more flexibility of the protection circuit.
Remember, a capacitor acts like a dead short until it is charged to its final voltage.
A short time constant that has enough voltage might blow out a solid state circuit, or a woofer or tweeter.
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JonSnell,
Good way to do it.
One question to ask is how much transient voltage can the power amp take?
Some Zeners draw a fair amount of current, long before they get to the specified Zener voltage.
That can be a problem for a 12AX7 cathode follower that tries to drive the Zener leakage current (the 12AX7 cathode is not a very low impedance out).
But often a little third harmonic distortion that may produce is OK.
This is a Guitar application.
For Hi Fi, the Zeners must be low leakage, low reverse capacitance (or use two steering diodes, one in series with each Zener, and a steering diode and Zener is connected across the signal, and the 2nd steering diode and Zener is connected across signal, but in the other polarity.
And that allows a low Ohm series resistor before the steering diodes and Zener pairs, to limit the transient current of the cap charging and or discharging.
These are always System problems, not isolated simple circuits.
Good way to do it.
One question to ask is how much transient voltage can the power amp take?
Some Zeners draw a fair amount of current, long before they get to the specified Zener voltage.
That can be a problem for a 12AX7 cathode follower that tries to drive the Zener leakage current (the 12AX7 cathode is not a very low impedance out).
But often a little third harmonic distortion that may produce is OK.
This is a Guitar application.
For Hi Fi, the Zeners must be low leakage, low reverse capacitance (or use two steering diodes, one in series with each Zener, and a steering diode and Zener is connected across the signal, and the 2nd steering diode and Zener is connected across signal, but in the other polarity.
And that allows a low Ohm series resistor before the steering diodes and Zener pairs, to limit the transient current of the cap charging and or discharging.
These are always System problems, not isolated simple circuits.
Sorry if I wasn't clear enough , I'd be using the preamp/buffer before the amp , so it would be going into the preamp stage of an amplifier. And maybe not be me using it into my amplifier , which is why I'd want something that will protect whatever it's plugged into , amp , preamp , fx box etc. I did look at values for the cap and (from a thread on this forum I think , but it was quite a long time ago) I got the impression it should be quite large , of the order of several uf.
The final cap should be just large enough to transmit 20hz in whatever load
the amp has.
For a amp with 100k input impedance 0.068uF will do just fine.
The risks associated in the first post in minimized if the cap is not oversized.
A suitable resistor that grounds the output will protect from "pop" when the
cathode follower is connected if done "live". 220k might be a suitable value.
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Merlin, I’m sure you wrote a nice book and perhaps I will buy one. However this is a forum where people help each other. So if you have good suggestions, please respond to the OP or be specific in your reply.
Regards, Gerrit
I gave the OP a direct link to relevant material and pointed out nonsense material that might mislead him. I think those are good suggestions.
Hi Merlin, why 16 Volt zeners (according to your book). What make you choose this value?
Regards, Gerrit
Regards, Gerrit
Petertub,
Do you want his guitar buffer to be -3dB @ 20 Hz, or - 1 dB @ 20 Hz.
Now, where did I put my super duper 20Hz bass guitar?
Do you want his guitar buffer to be -3dB @ 20 Hz, or - 1 dB @ 20 Hz.
Now, where did I put my super duper 20Hz bass guitar?
Anybody that needs several uF coupling cap from a 12AX7 cathode follower, is probably driving to low of an impedance at the next stage.
12AX7 Gm 1,600 uMhos.
12AX7 cathode follower output impedance = ~ 625 Ohms.
10uF @ ~ 40 Hz bass guitar frequency = 398 Ohms.
Yes, maybe you do need several uF, but only for a Guitar preamp.
A 12AX7 cathode follower that has to drive 398 Ohms will not work for well for Hi Fi . . .
Way too much distortion.
It seems that we are back to having the Guitar electronics in the Tubes / Valves threads;
Instead of Guitar electronics in the Instruments & Amps threads.
12AX7 Gm 1,600 uMhos.
12AX7 cathode follower output impedance = ~ 625 Ohms.
10uF @ ~ 40 Hz bass guitar frequency = 398 Ohms.
Yes, maybe you do need several uF, but only for a Guitar preamp.
A 12AX7 cathode follower that has to drive 398 Ohms will not work for well for Hi Fi . . .
Way too much distortion.
It seems that we are back to having the Guitar electronics in the Tubes / Valves threads;
Instead of Guitar electronics in the Instruments & Amps threads.
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petertub,
You mentioned setting the response down to 20Hz in your post #8.
I did a little joke on that, I hope that is OK to do that.
This cathode follower is for a Guitar.
That is why I "invented" the super duper 20Hz bass guitar.
So the cathode follower could work with that 20Hz product that is still unobtanium.
Get it?
The problem I see here, is that Guitar amp questions keep getting asked in the Tubes / Valves threads.
Then, people answer the question as if it was for a Hi Fi or stereo amplifier.
No, the question belongs in the Instruments & Amps threads.
A lot of us answer the Tubes / Valve threads as if the product is a Hi Fi one.
I often make that mistake too.
And then I find out that the question posted in the Tubes / Valve thread actually belongs in the Instruments & Amps thread.
If nobody sees a problem with this, we will continue to give wrong answers to the question.
Wrong thread category, wrong answer. Does that make sense?
Hello, is this microphone on?
Can you hear me, Moderator?
You mentioned setting the response down to 20Hz in your post #8.
I did a little joke on that, I hope that is OK to do that.
This cathode follower is for a Guitar.
That is why I "invented" the super duper 20Hz bass guitar.
So the cathode follower could work with that 20Hz product that is still unobtanium.
Get it?
The problem I see here, is that Guitar amp questions keep getting asked in the Tubes / Valves threads.
Then, people answer the question as if it was for a Hi Fi or stereo amplifier.
No, the question belongs in the Instruments & Amps threads.
A lot of us answer the Tubes / Valve threads as if the product is a Hi Fi one.
I often make that mistake too.
And then I find out that the question posted in the Tubes / Valve thread actually belongs in the Instruments & Amps thread.
If nobody sees a problem with this, we will continue to give wrong answers to the question.
Wrong thread category, wrong answer. Does that make sense?
Hello, is this microphone on?
Can you hear me, Moderator?
Only because that's what I had in my drawer, and most SS preamps run off +/-15 to +/-18V. You could use whatever voltage suits your application.