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Discussion Starter #1
Just picked up a little Garnet Tube practice amp. Russel is Gar's son so I guess this is like a Garnet Jr!







As you can see, it's a "widow maker" (no power transformer) so I may pick up a line transformer down the road.For now, what great fun: a tube amp you can actually crank up in an apartment! Champs etc are too loud to get dirt, this isn't nearly as loud although it hardly quiet (like a fairly loud stereo playing).

Anyway, some mods I thinking: replace speaker with a 10 or 12" speaker (depends on what fits). Even contemplated adding a master volume. In the pics you can see that the one large red cap is disconnected from the first input. Is this a bright cap?

Any suggestions or comments more than welcomed.

TG
 

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Just picked up a little Garnet Tube practice amp. Russel is Gar's son so I guess this is like a Garnet Jr!




As you can see, it's a "widow maker" (no power transformer) so I may pick up a line transformer down the road.

In the pics you can see that the one large red cap is disconnected from the first input. Is this a bright cap?


TG
Can you please explain how the "line transformer" concept works. I assume that it is a 120V AC to 120V AC transformer (but I'm going to bet that I'm wrong here).

Will it make the amp safe?

How do you wire it up to overcome the problem?

My friend has a KEIL amp. and I suspect that the circuit is similar. I'm trying to learn about electronics.

I am also curious as to the function of the cap. Hope somebody explains this.

Thanks for considering my basic noob questions.

Dave
 

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Discussion Starter #3
Hi Dave,

I'm a noob myself when it comes to electronics. AFAIK, the line or isolation transformer simply acts like an external power transformer.

The amp really isn't "unsafe" right now. Many amps and other electronics lacked a transformer. The only danger is if the main cap fails (affectionately known as the "death cap") then you are plugged directly into the wall with your strings as part of the circuit. But this would be extremely rare; how many thousands of people used these things over the years with no problems at all. I'm just as concerned with getting a three prong plug so I don't get zapped when touching other electronics.

I'm sure Wild Bill will chip in here. He knows this stuff inside and out.

Paging Wild Bill! :D

TG


Can you please explain how the "line transformer" concept works. I assume that it is a 120V AC to 120V AC transformer (but I'm going to bet that I'm wrong here).

Will it make the amp safe?

How do you wire it up to overcome the problem?

My friend has a KEIL amp. and I suspect that the circuit is similar. I'm trying to learn about electronics.

I am also curious as to the function of the cap. Hope somebody explains this.

Thanks for considering my basic noob questions.

Dave
 

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I have a couple of those that I have been waiting to find time to mess with

any of the 50L6 little tube amps are like that...they don't have a power transformer, but use the 117/120v from the wall in the circuit; you can get an isolation transformer & use it to play through them safely

they hum like crazy if you get the ground reversed ( flip the plug ) and I would suggest NOT to use any pedals with it that plug into the wall...also you can get a nice zap if you happen to touch another grounded amp/lamp/toaster etc while you're plugged in

they are fun but a bit dangerous...a few years ago a kid picked one up at a garage sale & his parents found him dead on the concrete floor of his garage with his gtr strapped on

there are some good circuits in the Jack Darr book, he talks about these in pretty good detail.....I think someone scanned it & it's online somewhere
 

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AFAIK an iso transformer is actually 2 transformers that take the wall voltage & "isolate" it because their windings don't physically touch



ah, here we go: highlighted the bold because it relates to the getting shocked bit :eek: sub in "amp" for "test device"

------------

From Wikipedia, the free encyclopedia



An isolation transformer is a transformer, often with symmetrical windings, which is used to decouple two circuits. An isolation transformer allows an AC signal or power to be taken from one device and fed into another without electrically connecting the two circuits. Isolation transformers block transmission of DC signals from one circuit to the other, but allow AC signals to pass. They also block interference caused by ground loops. Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers or laboratory instruments.

In electronics testing, troubleshooting and servicing, an isolation transformer is a 1:1 power transformer which is used as a safety precaution. Since the neutral wire of an outlet is directly connected to ground, grounded objects near the device under test (desk, lamp, concrete floor, oscilloscope ground lead, etc.) may be at a hazardous potential difference with respect to that device. By using an isolation transformer, the bonding is eliminated, and the shock hazard is entirely contained within the device.

In a pinch, a line-voltage isolation transformer may be made by determining the total load of the device under test and finding two identical line transformers each capable of handling the load. A power cord is attached to the primary of one transformer, an outlet to the primary of the other transformer. The secondaries are then connected to each other. An example with two 120 V:12 V transformers would yield 120 V → 12 V → 12 V → 120 V.

Isolation transformers are commonly designed with careful attention to capacitive coupling between the two windings. This is necessary because excessive capacitance could also couple AC current from the primary to the secondary. A grounded shield is commonly interposed between the primary and the secondary. Any remaining capacitive coupling between the secondary and ground simply causes the secondary to become balanced about the ground potential.
 

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Discussion Starter #6
 

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If you go to groupdiy, read up on the pultec eq. They discuss this matter and offer a solution......just got the inductors to build mine:rockon2:

(for those who hate searching, they use 2 220v-12V transfos (110 for here) back to back and tap the middle for heater voltage)
 

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you can get an isolation transformer & use it to play through them safely

they hum like crazy if you get the ground reversed ( flip the plug ) and I would suggest NOT to use any pedals with it that plug into the wall...also you can get a nice zap if you happen to touch another grounded amp/lamp/toaster etc while you're plugged in

they are fun but a bit dangerous...
I now understand about the isolation transformer ...it was as I had understood (just never heard the term "line" transformer used). I always thoought that they were quite expensive...the ones on Ebay seem very reasonable. I was consideing getting one for my the electronics workbench I am "developing", but I will need help as to what size I would have to get, given the total load that might be placed on it.

My question now remains as to how to make these amps as safe as possible if you do touch another grounded amp., etc. I expect that this will involve a cap placed somewhere...just wondering where in the circuit and what size of cap is typically used. If a cap is the solution, will it also reduce the hum?

Thanks for the detailed responses.

Dave
 

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I was consideing getting one for my the electronics workbench I am "developing", but I will need help as to what size I would have to get, given the total load that might be placed on it.
Dave
Dave, if you get an isolation transformer for the amp it has to handle the power load of your specific amp.

You have to deal with watts, or as electricians think of it: VA, or volt amps. It means the same thing since VxI=Watts, or volts x amps = watts of power.

So look at your amp and see if it tells you the amp rating drawn from the wall supply. If it doesn't then use the fuse value. If a big amp runs on 120 vac and is fused at 3 amps then the max power involved is 120 x 3 = 360 watts.

You will need an isolation transformer rated for at least 360 watts. In practice you would never find one of this oddball rating so you would have to go for the next up, likely a 500 watt rating.

This means the isolation transformer would have input and output windings both of 120 vac and rated at better than 3 amps or 360 watts.

Although I don't advise people to panic at these old amps it's still obviously a very good idea to use an isolation transformer. You usually have room to mount it right inside the amp.

If you've got a good junk box there's another solution that has already been suggested in this thread. You take two stepdown transformers of beefy enough ratings. One will run from the mains and step down, the other will work backwards and step it back up. You could use two 12 volt trannies, for example. For that big amp they would still have to handle 360 watts. That would be 12 vac @ 30 amps! Not very practical in such a case 'cuz the trannies would be huge. However, most of these transformerless amps are much lower power. You may be talking only a 1/2 amp fuse. This means the maximum draw will be 120 vac x .5 = 60 watts. That means a 12 v, 5 amp transformer. Much smaller!

Blown up car battery chargers are a good source!

There are some tube preamp projects that use this system. They take two "wall wart" plug in AC adapters. Since we're talking a preamp here the current draw will be only a fraction of a watt, plus the watts to run the heaters of a 12AX7 or two. It takes only 6 v at 300 ma to light up the tube so we've got
6x .3=1.8 watts of power to worry about for each tube! Call it 2 watts for rough figuring.

That means we need two 12 volt @ maybe 200 ma battery adapters. We can find them in junk stores! As was said by someone else, the 12 volts inbetween can be tapped to light up the tubes and the 120 volts at the end can be put into a fullwave diode bridge. Since a preamp tube draws very little plate current the voltage will rise in the filter caps right up to the peak voltage, which will be close to 170 vdc. This is more than enough plate voltage in a tube preamp.

The neat thing in this situation is that you can either scrounge suitable wall warts for nothing or next to nothing or buy two low current 12 volt regular trannies. This is still usually cheaper than an isolation transformer!

Let us know what you find in your junk box and we'll help you more then!

:food-smiley-004:
 

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I was consideing getting one for my the electronics workbench I am "developing", but I will need help as to what size I would have to get, given the total load that might be placed on it.

Dave
Damn! I spoke too quickly and didn't notice that you were specifically thinking of a big one for your bench, to be the main supply for testing.

Well, it still comes down to the same thing. What's the biggest load you're likely to ever plug into it?

The most you can draw from the wall is usually 15 amps. That's the size of the fuse or breaker on the line. That's also assuming that line doesn't also power your air conditioner.:eek: So 1800 watts is the maximum anyway.

I saw a neat article once in an electronic mag for a great "scrounger's" solution!

You hit the thrift stores and junk shops for two old microwaves of the same model. If you open them up you'll see a bigass transformer that runs the whole oven. Grab the trannie out of each oven and turf the rest of the stuff.

If you look at that transformer you'll see that you have a primary winding that connected to the wall power cord, a high voltage winding for the magnetron tube and another winding of a few turns of thick wire to supply the magnetron heaters.

You'll have to do some mechanical figuring to take each trannie apart but what you want to do is get rid of the HV and filament windings and put 2 primary windings on the same iron core! You then have an isolation transformer rated at the full microwave oven wattage rating!

If you're a good scrounger this is a VERY cheap solution! Just build a wooden box with an ordinary wall receptacle on the output and a power cord and on/off switch on the input side, for safety's sake.

Just FYI...

:food-smiley-004:
 

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Damn! I spoke too quickly and didn't notice that you were specifically thinking of a big one for your bench, to be the main supply for testing.

Well, it still comes down to the same thing. What's the biggest load you're likely to ever plug into it?

The most you can draw from the wall is usually 15 amps. That's the size of the fuse or breaker on the line. That's also assuming that line doesn't also power your air conditioner.:eek: So 1800 watts is the maximum anyway.

I saw a neat article once in an electronic mag for a great "scrounger's" solution!

You hit the thrift stores and junk shops for two old microwaves of the same model. If you open them up you'll see a bigass transformer that runs the whole oven. Grab the trannie out of each oven and turf the rest of the stuff.

If you look at that transformer you'll see that you have a primary winding that connected to the wall power cord, a high voltage winding for the magnetron tube and another winding of a few turns of thick wire to supply the magnetron heaters.

You'll have to do some mechanical figuring to take each trannie apart but what you want to do is get rid of the HV and filament windings and put 2 primary windings on the same iron core! You then have an isolation transformer rated at the full microwave oven wattage rating!

If you're a good scrounger this is a VERY cheap solution! Just build a wooden box with an ordinary wall receptacle on the output and a power cord and on/off switch on the input side, for safety's sake.

Just FYI...

:food-smiley-004:
Thanks Wild Bill. I was assuming that I would need something rated for about 1800 watts total for the bench. Just thought I might be able to get away with a bit less if I was very careful with the load I placed on the trannie(s) by not running some equipment at the same time as others. Not the best idea, I'm sure, as it is limiting and I can be forgetful.

The microwave concept is cool...until you got to the part about "taking each trannie apart" and "putting 2 primary windings on the same iron core"...I understand the concept...but that seems much too advanced for me to attempt in reality. Maybe someday.

I feel like I'm hijacking this thread (Sorry) but hopefully the information in all the responses will assist others to make these great little tube amps (and possibly their workbenches) safer.

Thanks again

Dave
 

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Discussion Starter #12
Hijacker!! :sport-smiley-002: :D (just kidding).

So Wild Bill, will either of the two isolation transformers I linked to work for my situation? I don't want to waste money but an isolation transformer looks like the easiest solution (don't have tech skills). I figured I would just mount it the bottom of the amp as you suggested.

Thanks
TG
 

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Hijacker!! :sport-smiley-002: :D (just kidding).

So Wild Bill, will either of the two isolation transformers I linked to work for my situation? I don't want to waste money but an isolation transformer looks like the easiest solution (don't have tech skills). I figured I would just mount it the bottom of the amp as you suggested.

Thanks
TG
Your idea will work fine, TG! As I posted, you just have to be sure the trannies can handle the wattage.

It's great to find ways to preserve these old amps. Think how many notes have gone through such amps over the years.

:food-smiley-004:
 

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I'm really enjoying this thread...guess that is quite obvious by the length and number of my posts and near conviction of hijackung.

I'm still curious about the "death cap" (size and placement in the circuit).

Is this the orange cap that isn't attached in TG's amp?

Does this cap make the amp 100% safe (in relative terms) if you touch another grounded amp, etc.?

Does it reduce hum also?

Do I ask too many questions ? (Don't answer that...LOL)

The curiosity about this cap is killin' me.

Long live these little amps and the notes that will continue to come from them (after some TLC)

Thanks

Dave
 

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if you're looking for an isolation transformer, drop me a pm, I think I can help.
I have a few of them I'd trade off for something.
 

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Discussion Starter #16
Well, I think my amp is about 3 watts so I', sure just about anything would be fine.

However, I just realized that one of the transformers I am looking at is a "step down' transformer. I'm not sure what this means. It's input is rated at 230 and its output is rated at 115. Is this made for running overseas?

Ugh.
TG


Your idea will work fine, TG! As I posted, you just have to be sure the trannies can handle the wattage.

It's great to find ways to preserve these old amps. Think how many notes have gone through such amps over the years.

:food-smiley-004:
 

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Discussion Starter #17
I'm still curious about the "death cap" (size and placement in the circuit).

Is this the orange cap that isn't attached in TG's amp?

Thanks
Dave
I think that orange cap is just a "bright cap" for input 1 on my amp. My guess is that it was way too bright so someone snipped it. This is just a guess though.

I think the "death cap" is one of those big silver caps; most likely the one on the left of the picture that is connected to the power cord. Again, I don't know just guessing as best I can. Don't touch anything based upon what I am saying.

TG
 

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I think the "death cap" is one of those big silver caps; most likely the one on the left of the picture that is connected to the power cord. Again, I don't know just guessing as best I can. Don't touch anything based upon what I am saying.

TG
The "death cap" is a little harder to find on small amps with no PT, TG!

With a "normal" amp that has a "ground reverse" switch you'll see that the switch has a film cap coming from its common terminal to ground. The other two terminals connect to either side of the power line wiring. The switch will toggle the film cap from one side of the power line to the other, bypassing hum currents on the "best" side.

In the other kind of amp they still use a film cap but you have to trace the circuit more carefully. There isn't any ground reverse switch! What you have is all the circuit grounds connected to one point and that one point has a cap that connects to chassis ground. The cap here connects the chassis to the circuit ground to pass hum currents and so allow the chassis metal to act as a shield against pickup. To the AC power it looks like a bigass resistor, limiting any unwanted ( meaning shocking!) current flow to a safe value.

I should add that even though a working death cap keeps things safe from electrocution it still allows enough current to happen that you can get annoying smaller shocks, like when you're holding the guitar and your lips touch a microphone. Also, when hum is coming in from the power lines it can get confusing trying to flip all the 2-wire plugs and ground switches to agree with each other!

So isolation trannies and 3-wire cords have more benefits than just safety.

What's more, I did once find a radio/record player combo from the 30's that DID have a hot chassis from poor design! How it ever passed electrical codes we can never know. Maybe things were more loosey-goosey in those early years. That thing could bite HARD!:eek:

My point is that with 2-wire cords you should always open up the amp and check it out for yourself. Make sure there IS a death cap! If there is and it's working then you can add an isolation trannie or whatever as soon as convenient and not immediately, if that's a hardship. If you're not sure how to check the amp then don't use it until you get someone more experienced to do it for you. Who needs a painful zap for an education?

Just don't forget about it!:smile: If you haven't done it within a few months then MAKE yourself do it!

:food-smiley-004:
 
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