How much boost before you blow up the amp?

[rockgardenlove]

So, how much boost can be applied before you blow up the amp?  And how would you measure "too much."

Thanks!

[brett]

Hi.
I'm no expert, but here goes:
Generally, you can put a seriously large signal into the pre-amp before it gets stressed.  A volt or two doesn't destroy a 12AX7, but it might sound quite bad, especially if it is biased close to 0V and goes into grid conduction.  So I guess a 10dB to 15dB boost is ok, but more than that might sound fairly bad.  I wouldn't stick 10V into it (20dB gain).
Power tubes can also handle a lot of abuse, but it may shorten their life, especially if the heat can't get away.
cheers

[rockgardenlove]

So what if I did something like fed a Ruby(or other LM386 system) into an amp input?  Safe?

[JimRayden]

Hi.
I'm no expert, but here goes:
Generally, you can put a seriously large signal into the pre-amp before it gets stressed.  A volt or two doesn't destroy a 12AX7, but it might sound quite bad, especially if it is biased close to 0V and goes into grid conduction.  So I guess a 10dB to 15dB boost is ok, but more than that might sound fairly bad.  I wouldn't stick 10V into it (20dB gain).
Power tubes can also handle a lot of abuse, but it may shorten their life, especially if the heat can't get away.
cheers

Actually, 10V is quite small for 'em. A previous 12AX7 stage will give out aLOT more than that. 'Cause with 300V of supply, you have quite large of a headroom. And when you have a distorted 12AX7 running straight into another one... well, let's say I've tried and it won't kill the tube. Maybe give it a helluva headache but definately won't kill. 10V into a single triode stage will perhaps mildly overdrive it.

As to transistor amps, I'll have a guess, it might warm up and melt down at a few tens of volts.

----------
Jimbo

[rockgardenlove]

So, how would something like this fare?

[trevize]

i would not try this for a booster cause:

_it's probably too much boost
_it distort easily
_without an input buffer your guitar signal will be heavily loaded.

If you want a lot of gain and clean tone try the mosfet booster or the
keeley fet/mosfet booster http://www.musicianshotline.com/issue/2006/0406/46.pdf

[d95err]

So, how would something like this fare?

This design will give no more boost than a regular 9V booster. The maximum amplitude is still 9V peak-to-peak. To push your amp harder (than 9V), you need more amplitude, not more power. Try a booster with an 18V supply instead.

The LM386 has a very low output impedance, meaning it has power enough to drive a speaker. This power could potentially destroy your amp, since amps are designed to expect high impedance sources. If you want to use LM386 based distortion as a booster, I suggest you look at the Grace and Big Daddy overdrives instead.

[JimRayden]

Yup, you only have headroom as much as your power supply. A power amp will only give out a whole bucketful of useless current, voltage still pans from zero to nine.

-----------
Jimbo

[R.G.]

There is no single answer. It depends on the amp. If the amp designer was good and expected little contretemps like large voltages on the input, it can withstand a lot. If the desiger was a beginner, small overvoltages may kill it.

In tube  amps, The damage is limited to the first tube, and there may be no damage at all. Tubes are generally robust against overvoltages because they can conduct so little current. In the case of a 12AX7 input, the grid is proof against negative voltages up to a significant fraction of 100, to the point that you get an arc between plate and grid. The grid starts conducting like a 5K-6K resistor when it goes positive, but there is usually a 68K resistor in series with the grid to limit current. For positive voltages with a 68K limit resistor, the voltage will be similar to the negative one, maybe up to 100V at a guess. An the damage will be limited to the input tube if it occurs.

Solid state amps with no protection may be quite fragile. The typical input to a solid state amp is a differential amplifier pair of bipolar transistors in the input to an opamp or the input to a power amp. The bases of a bipolar diffamp can only be pulled twice the reverse breakdown voltage of the transistors apart before you break over the junctions. That's 10-14V for most bipolars. If they happen to be JFETs, you get a similar number before you forward bias one of the two JFET junctions. If the input has no current limiting, then death occurs when the input current burns out the bonding wires connecting the chip to the pins. That can be as low as 10ma, although it's usually more these days of better packaging.

But you can protect an input against truly egregious overvoltages if you know how and try. For some of that, go to GEO and read "When Good Opamps Go Bad" and "What are all those parts for???" in Circuit Sweepings. A well designed guitar amp can be designed to survive connection of its input to the full AC power line. Not a good idea, not safe to do, but the amp will survive. A poorly designed one can be killed by an effect putting out 20V peak to peak, although 9V powered stuff generally has too low a voltage to kill them.