Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toxophilite on March 30, 2015, 02:44:22 am
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Sorry full of questions this eve
I'm checking out the ground lift switch on a silverface schematic (late 70s super)
It looks to me as if it switches the orientation of the leads going to the PT
Yes ? No?
Tried looking it up , Kept getting NFB mpd hits!
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Without seeing the schematic, I'm guessing yes. Not a ground lift, but a neutral swap; it performed the same function as flipping the 2 prong plug in the outlet. My Harmony had that. If you had a lot of hum with it in one position, you flipped the switch (mine was also the power switch, it went on-off-on) and the hum lessened.
I also found that if it was in the "wrong" position, I'd get nailed by the mike, and also couldn't play in my bare feet if it was humid. A 3 pronger fixed all that.
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+ 1 for John
I was thinking the same
K
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I was going to copy the 79 master volume supers ground switch which basically connects either the hot line or the neutral to the chassis via a .047mfd 125VAC cap
How crucial is it that I use an AC cap in that capacity??
Other than electrolytics I didn't know caps were AC/DC specific..Maybe Angus Young could tell me??
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I was going to copy the 79 master volume supers ground switch which basically connects either the hot line or the neutral to the chassis via a .047mfd 125VAC cap
why bother? this is the 'death cap' everyone removes when installing a 3-pronger.
How crucial is it that I use an AC cap in that capacity??
its not an AC cap, thats the voltage rating. a cap is a cap (nearly). you can use anything rated for 125VDC or higher, quite often you'll see 600VDC rated caps in this position. but,, you ought not replicate this part of the Fender schematic (for safety and health hazard reasons...).
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Well
i have a 1980 super reverb. it's the last of the silverfaces
It was designed with a three prong plug and the ground switch, which in some instances does make a significant difference as far as noise goes.
I've been using it for 25 years in a huge variety of venues up and down your west coast and I'm still here!! :icon_biggrin:
That what I thought about the cap.A cap is a cap is a cap! It is listed in AC on the schematic and I hadn't seen that before
Thanks!
I'm looking for ways to combat a ground loop problem between two amps
However i discovered that some little isolating transformers that i have from a Shure mic mixer (the one for the line out, 55 ohms to 55 ohms) seems to do a good job at uncoupling the grounds of the two amps when amp 1 is plugged into amp 2 to via it's line out
So I might just install the little transformer
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Well
i have a 1980 super reverb. it's the last of the silverfaces
It was designed with a three prong plug and the ground switch, which in some instances does make a significant difference as far as noise goes.
I've been using it for 25 years in a huge variety of venues up and down your west coast and I'm still here!! :icon_biggrin:
That what I thought about the cap.A cap is a cap is a cap! It is listed in AC on the schematic and I hadn't seen that before
Thanks!
I'm looking for ways to combat a ground loop problem between two amps
However i discovered that some little isolating transformers that i have from a Shure mic mixer (the one for the line out, 55 ohms to 55 ohms) seems to do a good job at uncoupling the grounds of the two amps when amp 1 is plugged into amp 2 to via it's line out
So I might just install the little transformer
Those polarity switches can make a difference in hum.
with new amp builds I always use a ground lift (see any modern Vox schematic), but with old amps it can be difficult since the chassis is often circuit ground and safety ground.
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oh!
Even better
I isolated the line out jack(which emulates the Silverface one) from the chassis and magic! hum is gone!
Yay!
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hmm
i'll have a look at the vox circuit just for interests sake evn though I might not be able to use it
My power amp, caps and PI are tied to the chassis near the rectifier, earth is on one leg of the PT and the preamps and input are grounded to the chassis near the input end of the amp
Still that sounds interesting
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> you can use anything rated for 125VDC
The peak voltage of a "120V AC" sine-wave is 170V. A part that broke-down at 125V would die on the first half-cycle.
Even "200V" caps die a lot in that application. Most audio caps take a steady DC which changes very little, from fairly saggy sources. Here the voltage is *forced* 0V to 170V 120 times a second. Significant current flows.
There's also voltage-spikes on power lines, which are much smaller after the PT and the filter caps.
And when the cap fails, it may go SHORT. This may be deadly.
Line-side caps *MUST* be rated for the purpose. These are now available for decreasing digital leakage from digital gear.
And in this day when 3-pin outlets are common, it is MUCH BETTER to get your Safety (and audio) ground from the Third Pin, not from some capacitor.
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> install the little transformer
Yes. Manipulate the signal, NOT the power and safety grounding.
If you still have a Radio Shack, if they still have car-sound parts, there is a can with two RCA plugs at both ends. Slit the label, pull it apart, and you have two medium-Z 1:1 transformers. If driven from low impedance (op-amp), the response is excellent; they will suck if driven from naked guitar pickup. I used them all the time to bridge ground-loops in recording setups.
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Thanks for that information!
My amp is grounded with a three prong plug and a straight to earth from the plug chassis ground
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Line-side caps *MUST* be rated for the purpose.
http://www.kemet.com/Lists/Filestore/EvoxRifaRFIandSMD.pdf (http://www.kemet.com/Lists/Filestore/EvoxRifaRFIandSMD.pdf)
K
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Crazy capacitors batman!
Neat
Hey does anyone have the full schematic of a modern vox with a ground lift circuit as shown in one of the previoud posts
Sounds interesting and I wouldn't mind trying it on a build
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On every amp I have owned or modified I replace the two-prong un-grounded A/C line cord with a three wire, three prong heavy duty replacement. I usually have several around from non-working appliances I have salvaged for parts. The green ground wire goes straight to the nearest transformer mounting bolt after I crimp and solder a ring lug to it. The black goes to the same primary transformer input as it did before and the white to the other. I change the now useless ground switch into a NFB selector (normal, 1/2 or no NFB). Sometimes it becomes a "standby" switch instead. The "death cap" is eliminated. Once I do this mod, I no longer have any problems with hum, shocks or noise from incorrect polarity. Easily worth the effort. The switchable NFB is one of my very favorite features of my amps. Clarity with it on normal, nice overtones and harmonics with it off and somewhere in between, depending on the resistor value I use for the 1/2 on loop. Jim
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Thanks!
I agree regarding the three prong plug
I always have them on my builds, and earth is always hooked to a solid chassis point, usually the PT leg
The ground switch on my 1980 super does have a positive effect in some situations (works with my space echo and some rehearsal spaces power setup)
I like a negative feedback control too
On the deluxes I built I'm going for clean tones so I didn't install a control but in many of my other amps I have. I usually use a 5 to 25 K pot so I can adjust it
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Hey does anyone have the full schematic of a modern vox with a ground lift circuit as shown in one of the previoud posts
Sounds interesting and I wouldn't mind trying it on a build
See http://el34world.com/charts/Schematics/files/Vox/Vox_AC15%20Reissue.pdf (http://el34world.com/charts/Schematics/files/Vox/Vox_AC15%20Reissue.pdf) or google 'Vox AC15C1' or AC15 handwired, etc.. They all have them. Look for the circuit ground going to a cap, a resistor, and two flip flopped diodes, all in parallel, and then connecting to the chassis.
ValveWizard writes about in in his free grounding chapter.
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The safety ground should always have its own bolt and nut to the chassis rather than be on one that shares something else such as a power transformer. It does require drilling a hole in a vintage amp to do this, but it is the safest way to do it, and it also meets code. The reason for this is if someone changes the power transformer and doesn't get all the nuts tight, then you have a safety problem, whereas if it is on its own bolt this will not occur.
Greg
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Good plan!
My amps are my own builds in Hammond AO-43 chassis and I've drilled all sorts of holes in them...most of them necessary! :icon_biggrin: , one more for safety isn't a big deal!
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Here the ground "lift" is evident (D1-D2-R13-C6)
(http://i.imgur.com/BSg9bjO.jpg)
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The safety ground should always have its own bolt and nut to the chassis rather than be on one that shares something else such as a power transformer. It does require drilling a hole in a vintage amp to do this, but it is the safest way to do it, and it also meets code.
I was convinced that there was a rule about that (A rule that imposes to use the earth connecton only for earth connection), but in an italian forum of professionals we longly discussed about this thing and seems that there isn't a rule or regulation about, so you can connect to the bolt of the earth connection also other things that have to be connected to ground (I was surprised but seems it is so)
Franco