Amp issue - JFETs going bad due to static electricity?

[Planet Boulder]

Here's the deal (and before I start, just assume that any stupid statements or questions below are simply the product of my own naivete, idiocy or whatever you wish to deem it. Simply put, if I knew the answer, I wouldn't be asking those who obviously DO know the answer.):

My G-K 1001RB-II has been in the shop twice this year, both times for the purpose of having the JFETs replaced. The guy at the shop said this is pretty common for amps in general in my area (Colorado) because it is so dry. According to him, the dryness lends itself to significant static electricity, which can easily cause the JFETs to fail.

I'm wondering if others here have had this problem and, if so, how did you rectify it? Some options:

- According to the tech, one way is to "ground" the cable before plugging it into the input by touching it against something metal, then plugging it in.

- My practice space is in my unfinished basement, where I've laid down a piece of carpet directly over concrete surface. Yesterday, I covered that carpet with connected squares of rubber, not unlike this: http://www.amazon.com/Rubber-Cal-Puz.../dp/B003BW2H00 . Not sure if that will do anything for the static, but I figured I'd try.

- My G-K head is housed in a rack with a compressor and a Furman power conditioner. I know that in the past, in order to mitigate a "hum" coming from an old racked Carvin power amp, I used an adapter on the amp plug, thereby removing the ground prong when plugging into the power conditioner (which, itself possess a three prong plug) so as not to "cancel out" the ground. Whether or not the terminology is accurate, it worked and the hum went vamanoose. I'm wondering if adding an adapter to the G-K's plug and plugging it in to the power conditioner will improve the ground, thereby reducing the danger of static electricity damage.

- I'm wondering if the power conditioner might have a role in this and whether or not I should change it out.

Looking for any thoughts, feedback, opinions, experiences which might help me to minimize the number of times I have to get my amp serviced for faulty JFETs.

For reference, I also own and use the following other amps:

- Eden WT800A
- SWR Workingman's 15 combo
- Old (early 80s) Trace elliot pre/power setup
- Hartke HA5000

The ONLY one to experience any issues whatsoever is my beloved G-K.

Thanks in advance and apologies in advance for my stupidity.

[--Vissinger --]

The rubber floor is a bad idea. It'll cause you to create more static charge.

Plug the cord into the bass, lay your fingers over the strings, then place your hand on the case of the amp before plugging in the cable. This will discharge you and the cable and the bass.

The problem is likely that you're building up a charge yourself and dumping into the input when you plug it in.

The power cord ground should not be removed. That's a bad idea.

[teemuk]

Well, the GK does have a JFET input, which is vulnerable to static electricity (AFAIK the other amps don't have such). On the other hand, the GK should also have a pair of back-to-back Zener diodes in input that should shunt any static voltage spikes, as well as a high-impedance input resistor that should work as a current limiter. All this should limit input voltage to about 12 volts and be a pretty effective protection against static electricity charges from input jacks. The effect loop and the direct out also have a similar protection.

[CnB77]

Originally Posted by teemuk

Well, the GK does have a JFET input, which is vulnerable to static electricity (AFAIK the other amps don't have such). On the other hand, the GK should also have a pair of back-to-back Zener diodes in input that should shunt any static voltage spikes, as well as a high-impedance input resistor that should work as a current limiter. All this should limit input voltage to about 12 volts and be a pretty effective protection against static electricity charges from input jacks. The effect loop and the direct out also have a similar protection.

I was just going to mention diodes. This is a step taken in JFET based pedals, which are much less expensive items, so I should hope that they're implemented in an amp.

[billfitzmaurice]

This is a new one on me. CMOS devices are fragile, but only until put into a circuit, otherwise computer failure rates would be horrid. JFETs are very robust. I'd be looking at another reason for their failure, such as faulty power supply regulation.

I'm wondering if the power conditioner might have a role in this and whether or not I should change it out

I would remove it, as they don't do anything. I wouldn't replace it, for the same reason.

[Planet Boulder]

Originally Posted by billfitzmaurice

This is a new one on me. CMOS devices are fragile, but only until put into a circuit, otherwise computer failure rates would be horrid. JFETs are very robust. I'd be looking at another reason for their failure, such as faulty power supply regulation.

I would remove it, as they don't do anything. I wouldn't replace it, for the same reason.

Thanks, Bill. So what would you recommend for powering multiple devices within the same rack? I've always considered these low-end power conditioners to be handy "power strips", if nothing else, and useful for only needing to use a single wall outlet. Is there something else I could/should use for this purpose or should I just plug the various rack items into the various wall outlets?

This particular rack has one other component: a rack mounted compressor which is run through the effects loop.

[billfitzmaurice]

Originally Posted by Planet Boulder

Thanks, Bill. So what would you recommend for powering multiple devices within the same rack? I've always considered these low-end power conditioners to be handy "power strips", if nothing else, and useful for only needing to use a single wall outlet..

A power strip with surge protection is fine for that job. That's not a power conditioner, though it may be called one. It's real power conditioners that go for well over $50 that are a waste of money.

[Jerrold Tiers]

Originally Posted by billfitzmaurice

This is a new one on me. CMOS devices are fragile, but only until put into a circuit, otherwise computer failure rates would be horrid. JFETs are very robust. I'd be looking at another reason for their failure, such as faulty power supply regulation.

I would remove it, as they don't do anything. I wouldn't replace it, for the same reason.

The JFET or even plain old bipolar input devices are fragile when exposed to 20,000 volt zaps on the input. Doesn't matter a lot if they are soldered in a circuit..*

It is quite common for the 'static electricity season" to have more amp failures. Summer (air conditioning can = dry air) or winter are the high spots, spring and fall tend to be less.

That said.....

1) input diodes, zeners, or diodes to the supply voltages, are protective IF they have a resistor of perhaps 1000 ohms in series with the input

2) Plugging in the bass to the amp (plug cord into bass first, then into amp) generally means teh cord touches the grounded 'sleeve" of teh jack, OR a ground contact for a "Marshall type" (Cliff) jack. That *should* pretty much fix it, unless the bass has a battery, in which case the bass electronics may get zapped.

* I experimented some years ago, and could not kill an old obsolete computer with static, but COULD kill analog electronics.

[PhiDeck]

Originally Posted by billfitzmaurice

A power strip with surge protection is fine for that job. That's not a power conditioner, though it may be called one. It's real power conditioners that go for well over $50 that are a waste of money.

Although it has nothing to do with the zapped JFETs problem, if it were my rack I'd include a Ground Fault Circuit Interrupter (GFIC). Not much money and it could save your life, at least under some circumstances.

Unfortunately, those circumstances do not include the one that killed Leslie Harvey (hot mic/stand). For that you need to measure voltage between the mic/stand and your ground.

[teemuk]

input diodes, zeners, or diodes to the supply voltages, are protective IF they have a resistor of perhaps 1000 ohms in series with the input

The G&K meets both clauses, at least according to schematics.

Maybe the protection's still ineffective, parts in it have failed rendering it ineffective, or maybe it's actually something entirely different than static killing the FETs. We may never know.

[jelwood]

The fact that none of your other amps are experiencing it makes me think it isn't the static or if it is the static then yes the protective measures of the amp have failed. Bob Gallien goes berserk putting defenses in his amps. I remember one time I made the guitard sweat big time by purposely connecting a short to my 2000RB. All right call me dumb but I got it for cheap...also I was a dumb*** one time and found out inadvertently that the short protect circuit was working great, so that when I did it in front of the guitard there was nothing to worry about...shoulda seen his face

You should send the member "B-String" a PM and see what he thinks of this, if he doesn't come on here and comment. He is the almighty amp tech that I know of and has been repairing GK stuff for a long time.