Step 13: Three-prong Plug Safety Mod

If your amp doesn't have a grounded plug, then this mod will make it both safer and quieter. I reused a computer cord. This hasn't completely isolated the amp from line (the heater circuit mentioned in previous references), but it should be safe enough to use.

A grounded AC plug (in the USA) has the following color codes:

--BLACK : Hot
--WHITE : Neutral
--GREEN : Ground

Route the black wire through a fuse holder to the power switch. The white wire is connected directly to the transformer. Connect the green GRD to the chassis ground. This means screwed or riveted directly to the chassis--not just soldered to an existing grounded wire.

The fuse holder (and fuse) was added, not being OEM for this amp. So a 1/2 hole was drilled for mounting.

A larger rubber grommet replaced the original cord grommet. The 'stress relief' consists of two zip ties, and could be better...

Here's an in-depth explaination of the mod: http://www.rru.com/~meo/Guitar/Amps/Kalamazoo/Mods/safe.html

For folks outside the US, the international power cord color codes:

--BROWN : Hot
--BLUE : Neutral
<p>I just happen to have one of those! I did some basic 40 year maintenance work on it, but I put in an old jensen speaker, and in moving the transformer from the speaker to the chassis, something went wrong and it's dead now. I think it may need a new output transformer. I benched it and figured I'd get back to it later. I'll read through this carefully when that time comes. I've fixed a lot of vintage high-fi gear. Looks good, lots of safety warnings! I've had some close calls, but never got bit. My friend was working on a tube radio at his desk when he was a kid. He touched the power supply and woke up on top of his bunkbed! No kidding folks, every time you open one of these you take your life in your hands.</p><p>That thing sure is clean. Usually those are in terrible shape. Mine is pretty beat. The frosting drizzle grill is hilarious! Psychedelic man...</p>
<p>Thanks, man! They are pretty cool...</p><p>Always check the tubes before blaming the OT. You can check the OT too, with a VOM, just make sure neither coil is &quot;open&quot; or a dead short. Any reasonable low resistance value is probably OK...</p>
<p>When I removed the OPT from the speaker, I had to rewire it and solder to those tiny magnet wires the coil is wrapped from. I may have failed in that, endeavor somehow but it is obvious that it was very hot at some point as the wax has oozed out of the transformer a little bit. It will probably be a while till I get back to that project. I'm much more partial to a little Gibson ranger I have anyway as far as actually playing something goes. That thing is awesome. Like a lot of those small old amps, the Kay seems more like something someone slapped together to sell than something that was created for great tone and versatility.</p>
<p>Got it. Good luck.</p><p>Yeah, most small amps don't have the tone (for me). The best small SE I've played is a Silvertone DM10. These days I prefer something with reverb, great cleans and enough wattage to project (12 to 30W is good).</p>
Thank u
Hey gmoon. Do u have any info on an Ampeg vt60 combo. I'm trying to bring it back to life and I'm having a hard time. Thanks.
<p>No direct experience. I know there's a VT120 schematic on Schematic Heaven, which is probably identical to the VT60, other than having double the output tubes...</p>
Hey gmoon. Ur helping me a lot.
<p>Cool. Be safe with this stuff...</p>
hi! I'm thinking of modding my amp and found this very helpfull. only one question: what you mean with high-end? I thought lowering those caps would increase the bass... thanks for all
Hey, good question. I assume by saying &quot;lowering those caps,&quot; you mean make smaller values, right?<br/><br/>General rule is that the smaller the cap value, the more it blocks low frequencies.<br/><br/>(jeeze, I see that I have several duplicate component names-- two C3, two C2, etc. Bear with me, I hope this makes sense with the lousy schematic..)<br/><br/>How that works to filter frequencies depends on how it's oriented in the circuit:<br/><br/>-- caps <em>inline with the signal</em>, like the 0.01uF (upper most cap labeled C3), are &quot;high pass&quot; filters. They block low frequencies from passing forward. These are called &quot;coupling caps&quot;, and also block any DC voltage offsets from messing with the next stage.<br/><br/>Changing from .05 to .01 blocks more lows from passing.<br/><br/>-- caps that <em>shunt or bleed signal to ground</em>, like the 0.001 that's attached to the tone POT (lower C2) are &quot;low pass&quot; filters. Those caps block low frequencies from passing to the ground--which means the the high frequencies are now escaping away from the signal path.<br/><br/>That's the way a passive guitar tone control works--the cap cuts the high frequencies by letting them pass through to the ground.<br/><br/>So in each case, small caps block more lower frequencies. It's just a matter of how they are used. <a rel="nofollow" href="http://www.muzique.com/schem/filter.htm">Here's a simple calculator and illustration of high vs. low pass.</a><br/><br/>The other caps are cathode bypass and control grid caps for the tubes, and they're a little more tricky to explain...<br/>
Thanks for you very good explanation, now the hole thing is more clear for me! Now comes the hard question. I have an amp that's almost the same as yours, it's a 803c, and because I'm a harp player I'd like it to be more "warm" (less bright). I found a lot of mods to warm the sound but no one tells you why the amp warms up. So, here is the question: what could be the frequency response of a "warm" amp? I know that the asymmetric distortion of the tube is also important obtaining "that sound", thats why I bought that type of amp, but I'd like to understand better the other variables. thanks for all
It really difficult to quantify something like &quot;warmth,&quot; since everyone has a different idea of what that means.<br/><br/>One thing tubes do is color sound with even-order (mostly second-order) distortion. It's a type of distortion closest to the fundamental sound itself, and is generally found to be pleasing to the ear. Mosfets can do this too, but somewhat differently. <br/><br/>But a good guitar tube amps also have odd-order distortion (third, fifth, etc.), which is the &quot;bite&quot; or &quot;crunch&quot; we love.<br/><br/>Frequency response itself is interesting, since a guitar (or harp) amp doesn't reproduce sound faithfully--as a stereo system they sound like crap. Plus a guitar speaker normally doesn't have a frequency response above 5kHz or so. But you will hear higher frequencies--they come from the speaker cone and the cabinet / baffle board vibrating at higher harmonics.<br/><br/><hr/>Just to clarify, when I wrote: <em>small caps block more lower frequencies</em>, that's a little misleading. <br/><br/>Caps block frequencies below the cutoff freq. Smaller caps don't block <strong>more</strong> lower frequencies--the cutoff frequency moves higher, so more mid-range cut is added to the already blocked frequencies below.<br/><br/>In the <a rel="nofollow" href="http://www.play-hookey.com/ac_theory/images/hi_pass_1st_order_freq_response.gif">chart here</a> of a high-pass filter, the cutoff would move to the right, as the cap value gets smaller.<br/>
It will become clear if we look at the formula for the REACTIVE resistance of a capacitor the formula is Xc=1/2*3.142*frequency* copacity IN FARAD Or as we say it one over two pie eff see Here you can see that the higher the frequency the lower the effective resistance and the larger the capacitor the lower the effective resistance
Hi.&nbsp; I was wondering if you could help me.&nbsp; I have an old Lindell three tube amp and it's not working properly.&nbsp; When I turn it on, as soon as the tubes warm up there is a VERY LOUD hum , and it's unaffected by the volume dial.&nbsp; Could this be a bad filtering cap?
Yes bad filtering caps are usually the cause.
Hi. Older amps almost always need new filter caps at some point. Loud hum regardless of volume does seem to point in that direction.<br /> <br /> It could be other things, too, though. Sometime the tubes themselves develop hum and have to be replaced.<br /> <br /> You could try pulling the preamp tube and then turning on the amp--unless it's a &quot;radio tube&quot; type amp that has all the filaments wired in series (like the amp in this instructable.) In that case the amp won't work with one tube pulled.<br /> <br /> Always nice to have a few backup tubes anyway, so new tubes are good investment...<br />
Thanks. I'm actually on my way out to Radio Shack for new caps.&nbsp; As for the tube&nbsp; configuration, I think it is how you said.&nbsp; It's a very simple circuit.&nbsp; And don't worry, I have plenty of tubes around.&nbsp; I bought a bin of four tube radios and a box full of tubes at a swappers meet for $10.&nbsp; Thats where I got the 12AU7 for my tube boost pedal.&nbsp; I have like seven.
Ok, cool, you've eliminated tubes as a problem.<br /> <br /> Good luck finding the caps at RadioShack, though. The store near me doesn't carry electrolytics at the capacitance &amp; voltage needed for a tube amp...(even for the relatively low voltage of &quot;radio tube&quot; amps-- 120V-150V.)<br />
Unfortunately I found out it's the same story here.&nbsp; I biked a few miles to RadioShack only to find they didn't have the caps I needed.&nbsp; Not even the right capacitance.&nbsp; It wasn't a total loss, though.&nbsp; I got some electrical tape and solder that I needed badly.&nbsp; And I got a ride home with a friend I ran into.
Oh, and&nbsp;earlier&nbsp;today&nbsp;I found a fuse holder with a mount for free.&nbsp; I'll have to do the fuse/three-prong-plug mod sometime.
Good news!&nbsp;&nbsp; Last week I bought some new caps at a local amp repair shop called &quot;Mike Tech&quot;.&nbsp; I soldered them in and took it back there and he formed them with a variac for free.&nbsp; I brought it home, hooked it up,&nbsp;and the results were great.&nbsp; It sounds beautiful!&nbsp; It's got a great tube sound when I crank up the volume.&nbsp; Very happy with it, and glad my dad's old amp is still in the family and finally working and being played again.
my grandpa was move out to a smaller house and geting rip of most of his stuff a few years back and i took some thing that look like a ham raido <br> <br>i hadnt realy look at it since then in till last weekend and found out in was a 300w tube amp by eico <br> <br>im trying to get speakers for it now <br>
Pretty cool. Should be worth fixing up.<br><br>(don't run it without speakers...)
im sry i get all execited and grously over-estamate what some things can do.... <br>its only 14 watts <br> <br>buts its the famed hf-81 <br> <br>every review i saw was like this <br> <br>http://www.stereophile.com/integratedamps/606eico/
Definitely worth fixing up.<br> <br> 12-14 watts of tube power is LOUD. Loudness is logarithmic--subjectively, twice as loud as 14 watts is 140 watts.<br>
on the back of it it has screws for the speakers marked g 4 8 16 and 32 <br>what dose this mean(i think it the ohms but i dont know...) i do know what g is... <br>1st is what im talking about 2nd is a bouns
That is your speaker impedance terminal strip. Use them to connect your speakers to the corresponding impedance screws. For example if you have a set of 8 OHM speakers. Connect the positive (+) terminal of your speaker to the 8 OHM screw and the negative (-) terminal of your speaker to the 0 screw to the amp. Sometimes that 0 terminal screw on your amp is also known as C or common. Hope this helps. Let me know if anything. Thanks.
Yeah, that's ohms. For an 8 ohm speaker, connect one wire to &quot;8&quot; and the other wire to &quot;g&quot; (ground).<br> <br> Must be stereo, so two speakers (at least) are required--one per channel.<br> <br> <em>1st is what im talking about 2nd is a bouns</em><br> <br> Sorry, I didn't get that...<br>
ok im looking to buy some speakers for this what would be best 2 8 16 or 32?
None is inherently better; all the better speakers come in different ohms loads. Just work with what's available.<br> <br> You can also wire speakers together. Two (identical) speakers in series doubles the resistance; Two in parallel halves the resistance...
cool thanks for that <br> <br>&quot;1st is what im talking about 2nd is a bouns&quot; i was talking about the pics <br>the 2nd one is the uncoverd bottom of my amp no real need to show but its still fun to look at.
i was wondering, what kind of output you could get from just one tube? I've been thinking about making a tube headphone amp but haven't seen any schematics anywhere (maybe i haven't looked hard enough), and i wasn't sure whether it would be plausible...maybe just a tube preamp and if the signal is still too weak a little bit of solid state amplification (i heard FET's sound more &quot;tubie&quot; then most opamps)...i should probably just look at some schematics and see how they manage the headphone jack
A single tube is a little problematic, especially if you're looking for a classic tube sound.. A hybrid approach has been done--tube preamp with an FET or a LM386 to drive the headphones would work.<br> <br> The other possible approach would be to use triode/pentode paired in a single tube. There are quite a few out there. You might even try one of the low-voltage tubes intended for cars and portable radios. <a href="http://www.junkbox.com/electronics/lowvoltagetubes.shtml">This page has some possible combo tubes</a>... These are safer to use than normal high-voltage tubes.<br>
I just got this exact same amp. I thought it would be a good starter project since its a simple circuit, but it seems over my head, since I have very little experience with electronics, wiring diagrams, etc. Now I'm considering some options: 1. Have it serviced by a pro. It would be safer and the work would be done right. Low risk, but higher cost. 2. tearing out what's there and installing a kit in the body. Higher cost, but doing the work myself. 3. Trying your mods and risk messing it up irrevocably. Low cost, but high risk. <br><br>When you said &quot;I'd tear the amp down to the bare chassis and rebuild from there.&quot; did you mean rebuild from a kit or design a circuit yourself? Thanks for a great instructable!
Hey, that looks good.<br> <br> By &quot;tear it down and rebuild,&quot; I meant that a less &quot;hacked&quot; approach would probably lead to a cleaner, more organized, less cluttered circuit. As-is, it's gonna be <em>more</em> difficult to replace the caps in 30 years. The tangled wiring is more fragile than a well-planned strip-down / rebuild, too.<br> <br> However, it's extremely quiet and sounds great, so I'm not too disappointed.<br> <br> I'm not sure what approach is right for you. If you try rebuilding it yourself (and fail), you can always take it in to a profession to get it sorted. Just be REALLY CAREFUL if you do.<br> <br> If you're not confident you can do this safely, take it to an amp tech...<br>
Man, it's crazy that you posted this because I was just wondering why my old Gibson Falcon amp shocked me when I was barefoot! How long do the filter caps hold a charge?
Caps can hold a charge for a loooooong time (although the older ones usually don't, 'cause they're failing.) Were you pokin' around inside or just playing? <em>Isolation</em> can be a problem with these oldies, too.<br/><br/>I've been dumb enough to play my old Ampeg in the basement with barefeet....wasn't a painful shock, I could just feel the current flowing from my left hand to the floor. One lesson learned.<br/>
I was just playing. Man, I didn't think an amp could be all lethal and junk! Thanks for the info.
Yeah, actually quite a few people have died from this stuff. Very likely your guitar is wired to use you as a ground (through the strings.) On some old amps, the chassis ground might instead be a lethal voltage in respect to another 'ground' (you in your bare feet, or a mic stand, for instance.) There's a little info on Step 3 about that, if it's an amp without an isolation transformer. But even the amps with isolation can be dangerous--that's why I added the three-prong cord here....
Here's an illustration of an older amp with an isolation transformer that still dangerous-- This is part of a schematic of my Ampeg Gemini II.<br/><br/>Because AC line plugs were all two prong, the amp has a two-position standby switch. This switch not only works as a standby, but <em>it also connects the AC line to the chassis through a single capacitor.</em><br/><br/>Since the plugs weren't polarized, you could use this to connect the chassis ground indirectly to the line 'neutral' wire, and your amp would run quieter. But if the cap degrades and fails, and your guitar strings are grounded (as most are)--it's like poking your finger in the wall socket...<br/><br/>On cheaper amps without a standby switch, the cap was still there, but you'd change the plug direction to ditch the hum...<br/><br/>(picture: red square shows the switch and the 'death cap.')<br/>
&nbsp;Shocking to say the least, especially when I'd be playing my guitar then would touch my friends at the same time...YIKES!!!! &nbsp;that GeminiII....
That Gemini cured me of <em>ever</em> playing guitar in bare feet...<br />
i have an ampeg, its the reverb rocket. its busted tho. my dads friend said that a guy gave it 2 him and then it sat in his basement and he gave it to me. when i plug a guitar into it the light turns on but there is no sound. could it be the tubes?
a short in a output tube can cause the fuse to blow replace the fuse 250ma pull out the output tubes the larger ones and and look for voltage form chassis ground for g1 g2 and plate b+ should be about 350 to 450 volts g1 look for bias and make sure amp has speaker load bias differs on the type of tube el34 is about -42 6l6 -52 el84 -15 6550 -62
do i need to discharge the amp to do that?
i find it so satisfying to build an amp from tubes the art work of human is awesome and money is the other drag parts are kinda pricey i can feel sick or stressed out but being in my work bench figuring out am i going to use 6aq5s el38s or how high of voltages will be best at this currant and making someones jaw drop after they see your work is cool after a week drilling the holes bolting up transformers and sockets at the end everyone is jamming out loud and sweet.
well i will say the voltage is high but you need to make these test with the amp on it is critical to measure the bias if not you can burn a set of good tubes the pin out on a 6l6gc or 5881 is 1 neutral 2 filament 3 plate or b+ 4 g2 screen b+ slightly lower than 3 5 control grid g1 bias is preasant 6 neutral 7 filament 8 ground notch clock wise upside down chassis between 1 and 8 i have 7591 tubes but they have the tall bases cant look to see how it works but you can get the pin out on google please be aware of the high voltage preasant and unplug the amp before you discharge the caps replace any resistors that are out of tolorance if it is self bias look at the 400 ohm resistor 7 watt or maybe 10 on a 6l6 it would be 220 ohms g1 grids are about 300k ohm to chassis 1/2watt resistors also look for proportion in the center tap primarys to the plates maybe around 30 to 60 ohms from center tap on output transformer and look at the preamp tubes and look at the voltages there to pins 1 and 6 are b+ on a 12ax t or u7
It could be the tubes...but it could be other things too, like bad connections. Is there any sound at all? (humming etc.)

About This Instructable




Bio: Go sit in the Faraday cage and think about what you've done...
More by gmoon:Isolation transformer upgrade for old guitar amps The ValveLiTzer: Low-voltage Tube Booster Guitar Tube Amp 
Add instructable to: