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1963 Kay Model 703 "Widowmaker" Amp Made Safer
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- čas přidán 6. 08. 2015
- The Kay 703 is probably one of the more famous "widow maker" vintage amplifiers. "Widowmakers" are called such because they utilize a series filament design using the AC directly from the wall without a power transformer. There seems to be a lot of confusion out there about these simple, cheap, vintage amps. The hysteria is often so great that people think even looking at one will kill them. While the safety issues are often trumped up, there are still some ways amps such as the 703 can be made a bit safer.
The 703 has an isolation step up transformer with a single winding for the tube plates, but filaments are still not isolated from the current coming out of the wall. To resolve the slightly increased danger this poses, we will equip this amp with a 3-prong, grounded power cord and add a fuse in series with the power switch. We will also clean the tube sockets, input jacks, and check out the schematic. Finally, we'll plug her in and check out the tone with a Fender Telecaster.
NOTES: I no longer recommend installing a fuse in this version of the 703 because the filaments themselves act as fuses. Each filament has a rating of 150mA max current. Much beyond that and they will blow, cutting off the circuit. This version of the Kay 703, the original early version with 50L6 octal power tube and isolation transformer supplying the rectifier plate voltage should not be confused with the later 703B or 703C, which had a different set of tubes and isolation on the preamp tube only. On those versions, I recommend a power isolation transformer not only for safety purposes, but for noise mitigating purposes. Hum is greatly reduced in the later versions by isolating the 120VAC before feeding the tubes.
no..... !!!! A filament can short as well. Also the Filaments are in Parallel with the rest of the amp, meaning the rest of the amp could have a fault, that would take out the transformer. the extra current would not be seen on the filament side of the amp so they would not blow.
Do you have a link to they type of power isolation transformer you recommend using for the 793c. Want to be sure I get the right one. I picked up a 703c today without knowing of the safety issues. It sounds great but I don't want to take a risk with any safety issues.
Hi Brad, I rarely comment but I think it is very important from an electrician and electrical safe work practices trainer's standpoint. I certainly respect your electronics and technical abilities but think maybe since I do have such great respect for you and your abilities I need to clarify some details about shock hazards.
First of all CEI 1984 established hand to hand resistance dry for an average human is 1000 ohms. Additionally, an often misquoted context of Kirchoff's law is electricity takes the path of least resistance, and by the conclusion of most is the equipment ground. Current will pass through ANY electrical contact path in varying amounts based on resistance yielding a 120 Ma shock based on 120V at 1000 Ohms hand to hand.
However, any charge present while circuit protection works due the ground creating a dead short WILL pass through the human body which on 120V will yield a 120 Ma charge based on OHMs law passing through the body.
Since the let-go threshold is about 1 mA per 10 pounds of human body weight if you weigh 160 the let-go threshold for you is 16 MA That means at 16 MA for a 160-pound person you will not be able to let go and will die at 16 MA
Now as for the tube filaments being an electrical circuit interrupter, not even remotely true in the eyes of UL, the people that list the contemporaries. And even at 100-150 MA that is not even remotely shock protection, it merely shuts down the amp as a function of the current limiting of the components. And bear in mind not UL-approved circuit protection.
So to wrap this up, only a Class A UL943 GFCI is human shock reduction protection under UL943 Class A. The equipment ground or any other method (tube filament burn out, inline fuse circuit breakers, or even equipment ground) is NOT shock avoidance or shock reduction to prevent death. These precautions help and contribute to safety, but a 20 AMP breaker, 5 amp fuses, 150 MA filament burnout or the equipment ground can help minimize the opportunity to be electrocuted but not eliminate it.
All the options outside of GFCI provide fire protection, equipment catastrophic failure and may as slight benefit help the user by turning the power off. Only a GFCI can deal with a 16 or 30 or 150 MA charge flowing through the human body turning it OFF in 1/30 of a second protecting an average healthy person from electrocution. 20 amp breaker takes 6-8 times the full load current (20 amps) to trip in a short.
And yes do NOT minimize low current shock hazards. Just because you got zapped and live to tell about it you are blessed and missed the bullet. But I promised you with reams of empirical data and 46 years of electrical experience and an SME rating by Toyota for teaching Electrical Safe Work Practices minimizing any concern for any shock 50 V 50MA or more AC or DC can be fatal!
Thank you for possibly saving someone's life - both whomever is going to use this amp - and also all of the CZcamsrs now enlightened about the widow-maker amp circuits out there. Seriously man I've been bit by this type of amp and so have 1000s of others. And now with the rise in popularity of old tube amps again the risk is huge.
Wow!! My first amplifier!! It was a great amp for learning Louie Louie, I'm not your stepping stone Wipeout and Last time.
I had this amp as my first amp. Had a killer tone.
Thanks Brad, love Kay amps. killer kitchen jam amp. keep em commin, we ampoholics cant get enough!
Thank you so much for doing this video! I have a Kay 803 is amazing condition, and recently had a three prong adapter added to it for safety. I learned a lot from this video about this interesting little amp.
Thanks, Thad. Glad you enjoyed it. Hope you enjoy your amp even more.
I don't know anything about capacitors or transformers or roaches but I find your videos fascinating
We used to use a shure pa with the two prong .If I got too close to the mic I could actually see the arching to my lips .I was 16 and didn't know any better.It had a really good reverb .
sounds fantastic!
and love your playing in this one especially
I found one of those things at a yard sale in Atlanta in the 70's for the princely sum of $5. it was just about the best apartment dweller's amp I had ever heard. Along with my little MXR Dyna Comp compressor, I was able to wail without goin' to jail! do we miss the old free America yet?
Very interesting vid, as always. Lovely sounding little amp and great playing. Thanks.
Brad, ya just saved my ass with this vid! I watch your channel pretty religiously as a gear-head anyhow. If I'm wrenching on gear I'm usually a guitar and sometimes pedal tech out in the PNW. Due to COVID BS I've had some out-of-my-league work plopped into my lap the last few months which I've tried to be as guaranteed and dutiful about as I can be. Just dealt with one of these which was a total barn find- had to deal with a pot fusion/rust-stoppage on the volume pot (automotive contact cleaner works wonders for everything) but the schematic was missing and after I pulled the tubes and de-soldered the pot I lost my placement notes and the damn thing didn't have the schematics. Remembered this vid and didn't have to dig for the schema somewhere else. Happy to see you putting out new material again! Your work does not go unnoticed. Got the little amp purring like a kitten again. Might put in a 3-prong if the owner wants it and I will cite this reference and take note of your update.
I have a 1960 Danelectro widow maker. Did a complete cap job on it. I should Do the cord also. Great video!
You might want to watch my other vid if you have the other version of this amp: czcams.com/video/k0ympuTdwwY/video.html
Also, yes, in either case, change power cord to a 3-prong.
Nice playing - even with the wound. I love your channel. I'm a new subscriber and I enjoy the detail and thorough explanations as you work through the repairs and upgrades.
Bachs cello suites in the background! Nice :) btw the rest of the video is pretty cool to.
Sweet sounds from a classic amp!!
it was my firdt amp in 1967.. Wow what memories
Very tweed like , especially with that tele ..sounds like a champ
I took a charge from one these amps my buddy bought at a garage sale. Knocked me on my ass and took 2 inches of my manhood. Well, that's what I told my wife anyway.
This fella caught 450 volts DC from a 68' super reverb, while barefoot on a damp garage floor, 2 prong cord, and lived to tell about it. Both arms were locked up solid for 15 minutes and I could *not* move them. Be safe.
Holy shit, you could have your own documentary.
Love your playing at the close
Such a simple schematic, and sounds nice!
you took the fun out it. they were getting electrocuted in the 60's. thats part of the experience
I doubt anyone was getting electrocuted by this amp. It has an isolation transformer from the factory isolating the input.
they called this amp the widow maker. the transformers must have failed. so after a performance you would get shocked while taking a bow bzzzzzzzzzzzzz
You are correct. It's often cited as a 'widowmaker' because of the series filaments, and you'll see people all the time recommending people get isolation transformers for this model, but it does already have one. The main danger in series filament amps is if the preamp tube shorts between the filament and cathode or filament and grid somehow, then you got full 120VAC on your finger tips at your guitar's pickups. That's bad. But the transformer between the other filaments and the preamp tube filament means that can't happen.
whooooooaa......I remember those amps.....I had forgotten all about them....
+The Guitologist wouldn't a torroid transformer be more safe and produce less noise? thats what I have in my amps
Sounds real good. Nice work on the amp and video. Subscribed!
+Tuomas Raatikainen Thanks for subscribing! Keep up with my new videos and let me know how I'm doing!
With a name like Guitologist, he gotta Rock. My SG is calling me.
Brad, I think that I watch your videos for your playing as much as the tube amp education lol
Ah man, you just made my evening. I was feeling kinda low before that. Thanks.
It is the cost-cutting of the makers that left the amplifiers to be dangerous. All it needed was 3 pin plug & a double-pole mains switch.
So the chassis could never be live, whether the amp was on or off if the mains plug was reversed.
The nasty part is that the guitar is grounded to the amp chassis via the shielded cable. That means the strings could be 120V above true mains ground.
Is that enough to be guaranteed death? Maybe if the current path crosses your heart.
You are holding the guitar by the neck in one hand and touch anything else that is at true mains ground.
There were far more risks that were generally accepted back in the day. Cars weren't so safe in crashes. Brakes and handling were poor. Many people smoked. Houses had lead based paint and asbestos.
That amplifier is nothing out of the ordinary for anyone used to dealing with shoals of All-American 5 tube radios. It is just the high B+ voltage that is eyebrow raising in a guitar amp.
I like to look up these old addresses on street view. I'll bet it's the same building but now it's an entertainment rental place :) (I'm weird)
Not weird at all. You'll find in the comments of my videos a whole subculture of people who enjoy doing exactly that, myself included. Several of my videos have had addresses and phone numbers. etc. and people will call them and share google earth links.
Do R/C! I look up old addresses as well it's not weird
I just picked one up today. From what I can see on the speaker the date stamped is from the same week as yours! Has the iso transformer like yours but definitely needs a 3 prong cord. Have you had any issues since making this video?
I have just the chassis of one of these and I would like to rebuild it and make it safer and better and build a nice cabinet for it!
toy caldwell, mark knopfler, jeff beck all would give this video a thumbs up. Me too!
***** To get a "thumbs up" from any of those guys would be an endorsement, indeed! I never realized how close my picking style is to Jeff Beck until a couple years ago. Completely by accident.
My thumb is just fine and I can't play that good!☺
I just picked up an old Kay K2 SG copy. All original. For $40.
Very nice AmpPorn and review!!! Thanx and keep´em commin´:) :)
TubiCal Thanks for watching!
There's nothing hysterical about electrical safety. The filaments do not have "rating" as a fuse at all. They may draw 150 ma in normal operation, but a fuse is there for when things go abnormally.
On this one, we fundamentally disagree, and that's ok. And they do have a max current rating of 150mA each. They are for all intents and purposes, a fuse. That's like triple redundancy. The weakest one will pop first when the max is exceeded, and luckily, it's well below what most health professionals cite as the 300mA needed to cause a heart arrhythmia in humans. At the end of the day, Underwriter's Laboratories signed off on the original design of these and ass far as I'm concerned, there is little need for fear being electrocuted by one, especially with a 3-prong cord installed. As long as the house is wired right, there's no real issue.
Brad, you mentioned that the isolation transformer inside the chassis actually steps up the voltage. Do you remember what that stepped up voltage was? I ask because I tried to convert a old transformer-less amp with the same tube compliment to the Kay 703 schematic and tried to used a 120v to 120v isolation transformer. Well it worked but the voltages at the plates was so low it just seemed like something was wrong. It sounded very anemic. After rewatching your video and hearing you mention the stepped up voltage, I believe I'll need to step up the voltage similarly to my converted amp. Thanks.
Some isolation transformers are slightly mismatched on the windings, so they do step voltage up or down. Don't recall the voltage stepped up in the other video you mentioned, but it was too high for comfort. Maybe swapping your transformer around would solve your low voltage issue.
the tube filament loop makes no electrical connection to chassis ground as in an old AA5 radio, or series filament tv did back about that same time
Thanks for this video! The amp sounds great! What were the settings (volume and tone) while you were playing? If you said, I must have missed it.
Sounds sweet.
Tiny obscure old amps,sound great in the studio for recording albums,forget about gigs!
What are those "series filaments" purpose? because they aren't even being used to drop the voltage so I'm not sure if those are being used as fuses.
Talking about safety, when I built my small project studio, i used GFCI outlets, cool looking brown ones. Well, anyway I was working on a shure vocal master and boom, lights out. I guess I touched something, but I wouldn't know it, my outlet saved me from a shock! So how about it guitologist, do you use a GFCI? Couldn't it save your life? Or are they problematic in some way I'm not seeing? Thanks for the great content!
Great video, sick little amp. I have a question; how might I hard wire a foot switch on an amp with a button on it that does the same? Would I have to exchange the button for the foot switch? Or might I be able to use a relay attached to the button? Help
Would only running an amp like this through a GFI outlet make it a little safer?
crazy glue a finger or thumb after cleaning it works great!!!!!!!!!!!!!!
What brand of spray d o you use on your input jacks etc.? How much would an amp just like this one cost someone interested? I really enjoy your videos. Thank you!
I have a little tube radio I just keep for decoration and it uses the same rectifier and output tubes and I believe a 12sq7 and one other tube. In researching the radio I saw a few tutorials about how to make a simple cheap tube amp using those same tubes and I really hope no one followed those tutorials 😨 one looked like a baking tin someone used for the chassis and no isolation transformer... I don't think anyone should attempt to make an amp of this type, preserving vintage examples is one thing, making a new one and then showing it as a tutorial just seems like a bad idea. Someone getting into this hobby could see it and try to do it because it's shown as being cheap and simple and not being equipped with the proper knowledge it could be their first and last tube project...
I have always been told that wall voltage was less in the 60s than it is now (maybe 110 or 115 then, and 120+ now). If you have a 35, 50 and 12 series filament, don't you have 97 volts required with 120 supplied? I wonder if that affects the tube characteristics? Have you ever put it on a variac and experiemented with supply voltage?
It has a resistor to drop the rest.....up to about 115 volts. Of course that leaves about 5 to 10 volts unaccounted for.
Often on repair videos I see a two prong cord being replaced with a three prong cord for grounding. Why don't you replace the two prong cord with a a three prong socket ? I have three prong cords of different lengths all over my studio for various equipment. I take a bundle of them to gigs of different lengths. If supplied a socket then the user could use any of the cords they already have and there wouldn't be a cord stuffed in the back of the amp bumping into the tubes. Hmmmm. Also, does it effect the tubes when, like this Kay amp, they are mounted in close proximity to the speaker magnet and a transformer ? Thanks, I like your show.
Your filament string is still unfused in the arrangement you have used. You need to put the fuse ahead of the switch so in the event of a fault all power will be disconnected and the chassis will be safe.
Thanks for the comment. I used to think this too, and used to include fuses, but the fact is, the tube filaments themselves ARE the fuses. The max current rating of a filament on these tubes is like 100mA or something. Anything above that and they'll pop in short order.
Filaments are not fuses. I'm sorry but this type of installation is negligent.
All it would take is to move the fuse to the line side of the switch. Tubes are not fuses and you know it.
Yes they are. In a series filament amp, adding a fuse, you're just adding another filament. In a 3-tube series filament amp, you have THREE fuses on the string rated at 150mA each. They're basically a 150mA slo-blo fuse EACH. Sorry, but you're wrong on this one, and calling it "negligent" is hysterical.
I think I would swap out the power transformer and rewire the filaments
brad- love the video's,am a fan of all you do! i also have a home workshop and lots of guitars and amps of all kinds. just picked up a magnavox 1963 twin 12's with 2-fives,want to rebuild my hm 60 lafayette tube receiver to make a guitar amp,would you be interested in helping with hand drawn diagram yes a little new at this but want to give this a try!
I wouldn't have time to do the actual drawing right now, but if you want to draw something up, I can take a look and offer suggestions. Actually, this gives me an idea for a video I should make sometime.
Thumbs-up! .
Man, I remember well that nasty little snap to the lips when they came too close to that "quality" Philmore Mic plugged into an ungrounded Bogen PA amp while my fingers were wrapped around the neck of my Fender Duo-Sonic... Good times!
Yep, that'll do it. Amps like this were not meant for jamming with a full band and PA. If the plug is in the wrong way, there could be fireworks on your teeth.
Guitologist, I bought the exact same amp with the same tube configuration. It plays great until about 8 minutes into playing I lose the sound. It becomes a whisper. However, when I pull out the cable from the guitar and plug it back in it starts up like noting ever happened. Could it need new resistors or even a new caps? Let me know what you think. Thanks! Greg
Very likely caps. But it needs more than that really. To be a viable recording amp and be its safest, you really should install an isolation transformer.
@@TheGuitologistThank you Guitologist for your help. As I can see there is a small transformer. My tube configuration, as well as the general wiring layout is "exactly" like the one you are presenting in this video. As far as the amp goes, I will just use it as a practice amp. My 1963 Fender Tweed Champ with 5F1circuitry is my holy grail for everything else. Can I get the caps and resistors at Antique Radio Supply. The first thing I will do is change out the cord this weekend. Found this thing on Saturday for a $100 bucks.
@@TheGuitologist I installed a three prong cord as well as spraying the volume, tone pots, and the tube socket prongs. Yet, no difference. I turned the volume knob back and forth, and starting hearing the volume come back. I found the sweet spot on the knob and kept it there. The volume was about three quarters up and with the guitar it was great. Good thing was the volume never faltered. Sounds like the volume knob. I guess I could get a new pot or spray the heck out of old one and hope it breaks free.
@@TheGuitologist Thank's man! I changed out the caps, resistors, and installed a three prong cord. The old caps were starting to crumble and the ends, and a bit of what looked like brown black tar was evident. I now plays like butta! Thanks again!
I know nothing about amps, but Im surprised at how little is in there. Im sure its a simple amp, but how can that cost much at all to make? Any modern "handwired" amp like this would cost a lot. Where does the price come from?
Largely human labor.In the case of robots, energy.
Here in 2024 (via reddit)
So where (to what) did you attach the ground wire?
+jbfisher1 To the leg of the old cap can. Grounded to chassis.
I am curious because I really dont know a whole lot about tubes, but, Why is a series-string set dangerous? or a widowmaker? Just about every single AA5 AM Radio (stuff I am familiar with) are all series string.
Same animal. The fear of series filament guitar amplifiers is overblown, in my opinion. But there are some ways one could electrocute you in theory. If there is no isolation transformer for at least the first amplifier stage, if that tube shorts from filament to grid (not likely), you could get wall AC on your strings, although probably not enough to kill anyone bigger than Tinkerbell because the tubes already dissipate power in the rest of the series string. The tubes in a series filament amp each have a maximun filament rating of, like, 100mA. It takes something like 300mA at least to kill someone, so even if you got shocked, a filament would blow from excess current before your heart. The chassis isolation cap could also blow and put AC on the chassis. If the plug is in backwards, meaning if the hot side is on the side of the cap and it's shorted and you're standing in a bucket of water on a basement floor, you're dead. A grounded power cable ensures there's no way your body will find a better ground than the wall ground, provided the house is properly wired.
Well I was just wondering because I have been hit with full stored voltage from tube amplifier caps when working on them in my early days, and then of course the 30KV of high voltage when trying to reach into a rear projection TV to adjust the yoke rings, and I am still here.... So I just couldnt really see how a tube string could hurt anything... When I was a kid just learning repair, I got a monitor working and answered a phone call, not paying attention I laid my arm down on the anode cap and ZAP... I saw stars for a second, hurt like hell, and the cordless phone in my other hand was dead. then I got grounded from my mother for killing her phone...
Amp is older than me!!!!
That moment your cut gets torn open on a string and you start bleeding all over your neck...
I assume this it the infamous "death capacitor" issue? if so, there is a fabulous video by a guy named Uncle Doug on CZcams that explains this very well and other ways you can get the piss shocked out of you with vintage amps, and it has nothing to do with the old fashioned two prong plugs. It is a circuit that tries to eliminate hum. If you have a vintage amp, *WATCH THAT VIDEO*
Brad do not shave your thumb. Hairy thumbs are still in. lol
btw how's that amp in a gfci circuit?
***** I was going for that hipster "shaved knuckle" look.
***** Never plugged into one with it, but GFCI should save your life with any kind of fault. Safety on all guitar equipment starts with the house wiring. If it's up to code with good grounding, will have a good start. Any amp can be unsafe if the house wiring isn't correct. Singing into microphones makes the risk greater. Anyone playing live should always take a multi-meter with them to test the outlets and also to test whether there is a potential between your guitar strings and the microphones you'll be using onstage.
do you need a special ground tester or just a multimeter? how would you do a test to ensure a good ground? My house was built in 63 and I'm pretty sure it's Kosher but you never know until you check, ya dig?
Why not install that absolutely brilliant South African invention the earth leakage circuit breaker?
I thought 'Widowmaker" referred to the punch some capacitors deliver when removed & discharged into human flesh ?
This whole shock hazard with series filament amps is soo overblown. I've had transformer amps zap the devil out of me when the filter caps went bad. An amp with the worst filter caps in the world won't zap ya if it's got a 3 prong. Just put a 3 prong on it and chill out. I can't believe the mods people do to these poor amps... a new cord and replacing out of tolerance parts is all they need.
I can understand the paranoia to an extent, but I agree, it's overblown. There are a few things you can do, including putting a very small fuse in series with the filaments, moving the switch to the hot side, and installing a 3-prong cord, that mitigate most of the issues. Also, not standing in a bucket of water on basement concrete while playing also helps.
Indeed. If they could sell the "All American 5" type radios by the millions for 30+ years... they must not have killed many people. I didn't know squat about the AA5 radio until I started looking at Series Filament guitar amps... These guitar amps are a straight up extension of that technology... The technology of cheapness. Where else would you put tubes with a 50v, and 35v heater... It's pretty innovative, 50v + 35v+ 12v+ 12v+ 12v= 121v... the whole wall current negating the power Xformer requirement. Our lil guitar amps use that 1500ohm resistor to substitute for the 2 missing radio tubes. Simple, but brilliant. The 50C4, and the 50L6 are not known for beautiful sound... just being loud with a puny transformer. 18$ buys your output transformer for one of these critters. LOL. Amps like the Mag 107 are the "First Act" amps of their day... but somehow suck way less.
What you say about the death toll from AA5 radios is true, however, ALL those radios had plastic cases and plastic knobs. Once you have a guitar cord with a shield, connected to a grounded phone input jack and all your metal guitar parts and strings are thus grounded, the odds change some. Now, let us add in the idea that one of the ways you are going to use your guitar is to kind of steady it in one hand and reach to adjust a microphone with the other hand; and the case of that mic is likely grounded. Now, if there is any voltage differential from amp chassis to PA amp chassis, it's going right across your heart. Or, you and you buddy are going to swap guitars and you are going to switch from (say) guitar to bass and vice versa. When you hand over the guitar, you are probably going to touch the strings on both and once again.......cross your heart and hope to not die.
Oh Lord! This thing has a Transformer in it. It would´nt have cost much more to have put a decent transformer in it. Totally isolating it from the mains supply. This AC DC tech is absolute shit.
If you're gonna make instructional videos, please turn down your distracting background music...what'''s the amps power rating, speaker size, ohm & power rating?