. I am hoping for someone with bench experience to help me with this.
I am the original owner of a Gibson GA75. Great amp. Just dusting it off after more than 30 years idle, and it seems to work, mostly.
One thing i want to upgrade right away is the power cord. I feel competent to do this, but not competent to judge whether the 'death capacitor' has to be removed. From the schematic it looks like a simple matter of grounding the 3d wire to the chassis.
Have I got that right? Do I leave the 'death cap' in place? Is this even a good idea?
Last edited by flatfoot; 01/08/1812:12 PM.
Flatfoot sez: Call me when 'Talent-in-a-Box' is ready to ship! -- [8{>
This is NOT ADVICE. I would NEVER tell you how to do line voltage (power mains) mods - if you are a EE, a certified tech, or have been doing amp mods, building radios, radars, etc., most of your life you'd NOT be asking in the first place and I don't want the liability.
HOWEVER if it were ME and I was (re)doing this on my '62 GA-19RVT
REMOVE THE CAP completely (removes any possibility of accidental shorts, arcs, or any dumba$$ery by user) - don't get lazy thinking you can "lift a leg" or wire around it.
AND make sure my NEW GREEN wire (ground) is NOT just soldered on somewhere but is solidly and physically connected to chassis through existing chassis hole (or a new one) using an eye-let - staked/connected with nut and bolt/screw also using both a lock washer and an internal (or external) star washer both on nut side (helps keep nut from working loose due to vibrations) e.g.,
Is that overkill? not really - you won't be the one KILLED
Larry
I would also ensure and DOUBLE CHECK that the black (aka HOT) wire is FIRST to FUSE (as shown) then to switch; that way if fuse blows this keeps ANY voltage out of amp "side."
Last edited by Larry Kehl; 01/08/1806:53 PM.
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With the first turn of the switch the capacitor doesn't actually connect. But it does if you turn it some more. So you have the option.
In an ac circuit a capacitor allows frequencies to pass but can greatly reduce others. So its main purpose was to reduce the mains humming at 60hz. (The tuner of an analog radio is basically a range of adjustable capacitors).
There were many badly designed and built amps and radios in the early days of sound engineering. It meant that they became live if the input polarity was changed. Easily done if you plugged in the mains the wrong way round. Added to that the venue might well have faulty wiring, causing reversed polarity and a poor earth. Typically an old church or such.
The so called death capacitor picked up a bad name because in some cases it failed and caused a short circuit in an already bad circuit (or one of reversed polarity) which lead to shocks and death. But it was never intended to be there as a safety device, its purpose was to reduce noise.
So the Gibbo amp may need a new capacitor to function as best as it was intended. For the purposes of authentic sound and that elusive feel. But probably best left to a professional to make that decision.
Older model amps can be converted to safer electrical connections. But this might mean changing quite a lot more in the power supply stage.
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. Hi Larry - and thanks. You are reinforcing my paranoia, which is probably a good thing. Better to pay somebody and not take a chance.
So... is it a good idea? Should I go to 3-prong or leave it as it is?
Take it to a pro and have him change it to a three prong. If something goes wrong it is his/her responsibility to repair it. He/she would also guaranty their work.
I remember the old days when you have to grab your guitar strings then quickly touch the mic to make sure you wouldn't get shocked or killed if you grabbed the mic while holding the guitar. Thanx God for three prong plugs.
I get most of my exercise these days from shaking my head in disbelief.
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I remember the old days when you have to grab your guitar strings then quickly touch the mic to make sure you wouldn't get shocked or killed if you grabbed the mic while holding the guitar.
That made me laugh from an incident a band I was in had years ago..... The two guitarists in the band touched their guitars together during a show. Little did they know they were each plugged into an outlet that was reverse wired from the other. ****SPARKS**** everywhere. No one was hurt luckily. Although some strings were killed.
If it’s an older AMP (one like yours) and it has 2 prong plug THEN CHANGE IT TO 3 PRONG!
YOU WILL NOT HURT VALUE! If paranoid KEEP old cord, the OLD cap (and make notes on where and how it was connected) and don’t drill any new holes in chassis or make any other UN-reversible or cosmetic changes – it can be restored to original configuration in about 30 minutes (and that includes a coffee break while soldering iron heats up).
I think you already know this but that particular cap has ZERO impact on audio path – so removing it has no, zip, zero, nada, nein, audible impact. It CANNOT affect audio because it is NOT in signal path.
MarioD has best suggestion (I should have stressed this last night):
TAKE IT TO A PRO. They can make it SAFE and in a way that allows it to be completely reversible (but I doubt that will ever be an issue or even come up if you try to sell it later).
The plus side of taking it to a Pro they can evaluate and, if it has not been done in last 30+ years, its probably also time for a re-cap job anyway.
Good Luck Larry
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This circuit has three possible conditions. Where the cap is unconnected, or switchable to either live or common wires in the event that incorrect polarity is applied to power the amp. So logic dictates that if the cap can be switched out it was never intended as a safety device.
Some famous products simply connected the circuitry directly to the mains and caused many a shock and death. But the Gibson has incorporated wiser circuitry.
The first point at which safety is introduced is with the fuse, which will blow if it senses current beyond its designed use. This will work in both polarities.
The ac works at 60hz, a resonant frequency that can become a dominant sound in the amplifier. So the cap helped reduce its presence. Canny engineering can achieve a marked improvement.
Also the cap is not a stand alone thing. The cap and the transformer form a circuit. Its ability to filter is integral to the value of the coil in the transformer, which is acting as an inductor. So it is an LC filter.
It would need a lot more maths and physics to understand more about it, and I didn't achieve the headpiece for that.
But if its a fundamental thing to the balance of the whole system, it would need to be retained.
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Be sure you have the hot and neutral in the right place..the hot side should be going to the fuse......and then connect the ground with a crimp style ring connector to the chassis.
If the so called "death cap" ever shorts, the ground should prevent dangerous voltages where they shouldn't be. It will cause the fuse to blow on a chassis short.
You can find my music at: www.herbhartley.com Add nothing that adds nothing to the music. You can make excuses or you can make progress but not both.
The magic you are looking for is in the work you are avoiding.
The reason they call it a death cap is because if it shorts out it results in the amp chassis and your guitar being directly connected to the mains. ...
BlueAttitude has it EXACTLY right!!! AGAIN TAKE IT TO A PRO
BTW: ANY Pro who would mod this amp from a 2 prong to 3 prong will REMOVE it (at least electrically remove it - I prefer physically removing it, it's not needed get it out)
Your amps polarity reversal switch (whatever label calls it as I can't quite find a clear GA-75 panel pic for this "newer" version GA-75 - the OLDER GA-75 only had an ON-OFF switch) will no longer do anything (but yes they can do mod and keep/use the original switch)
Larry
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Okay I have read through some theory again. The power supply to the transformer is in fact a tunable parallel rlc circuit (resistor, inductor and capacitor). the capacitor we know about. The inductor is the coil in the transformer and there is a resistor in the neon bulb.
The combination of these in an ac circuit will have a complex performance but they can be selected to produce a bandwidth of frequencies. If the frequency to be desired is set at say 16hz that would not have been achievable in that era of amplification and would not be reproduced suitably loud enough. Thereby reducing the noisy 60hz.
A crude but reasonably effective solution, where HiFi isn't required.
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Yes, I see. Point well explained Dave. Also, while I don't know the inductance of the transformer primary, I calculate the capacitive reactance at 60Hz at something approximating 120k ohm (120,571 ohms). With the amp in a completely un-grounded situation, I imagine to be able to feel something if I was grounded and touched the amp chassis itself while it was switched on.
Better to get that 3 pin connection happening.
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I see a lot of advice to take it to a pro which is certainly advice I would heed. Steve wrote about an incident where two guys on stage touched guitars and sparks flew. This tells me it is important that all the gear goes back to one common point. This is particularly true of earth or ground points. Some usually older buildings may have electrical circuits going back to different earths this can create issues such as earth loops, tingling and/or sparks and also increased buzz or we would say 50 cycle hum. For you 60 cycle hum.
Just a thought
Tony
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A few days ago, I remembered that I have a local friend who is my age, plus. I found out he made a living working on tube circuits before moving on to computers in the 80s. He says the thing to do is test and replace all the capacitors. Upgrading to 3-prong is a no-brainer when doing a caps job.
It will be fun, since he will talk to me while he works. I am sure to learn some important stuff.
Thanks for all your help.
Flatfoot sez: Call me when 'Talent-in-a-Box' is ready to ship! -- [8{>
Not to beat this horse to death ("the horse is never so dead it can't be beat some more") since you are taking it to a pro but consider this a "Oh by the way:"
Here is another Gibson amp and note this vid was posted AFTER we discussed what to do with a “death cap”
Go to about 8:37 and he talks about changing it to a three prong and the “death cap” – he doesn’t SAY he will remove the cap BUT you can see it there BEFORE he mods and it’s GONE after he mods.
You can plainly see it in circuit on the left (big black one) connected to ground next to transformer (go to around 8:57) and goes to power switch and by time 10:23 it has been REMOVED completely.
Larry
PS I had a Gibson GA-5 (I was first original owner, along with a original first owner of a ES-125) the model that preceded the Crestlines. They were physically smaller and covered in an almost white tolex I TRULY MISS that thing.
Last edited by Larry Kehl; 01/19/1805:32 PM.
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