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Thread: 5F6A Bassman total custom build - super low output

  1. #71
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by eschertron View Post
    I see the resonant freq (fs) is much lower than on any guitar speaker. What I didn't see on their website is a graph to show the speaker's loudness over a frequency range. Typical guitar speakers have a peak around 2k. I'll wager the woofer does not.
    The stats regarding efficiency aren't too impressive either. Maybe OK for a hi fi woofer where you have 100+ watts to drive it. Most guitar speakers fall into the 95 to 103 dB/Watt range.

    It's a good idea to keep ears open, when you hear something outstanding coming out of somebody's guitar rig in a live setting, find out what it is & put it on your tentative shopping list. "In a live setting" is important, on recordings you have no idea what was done to the signal once it came out of the speaker as far as mic, preamp, compression/limiting, eq etc. Half the people I give this advice to land on Celestion Vintage 30, which is a fine guitar speaker. But it's definitely not for everybody. So listen, audition & pick one you like. Or more - nobody says you have to use multiples of the same speaker.

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    Quote Originally Posted by Leo_Gnardo View Post
    The stats regarding efficiency aren't too impressive either. Maybe OK for a hi fi woofer where you have 100+ watts to drive it. Most guitar speakers fall into the 95 to 103 dB/Watt range.

    It's a good idea to keep ears open, when you hear something outstanding coming out of somebody's guitar rig in a live setting, find out what it is & put it on your tentative shopping list. "In a live setting" is important, on recordings you have no idea what was done to the signal once it came out of the speaker as far as mic, preamp, compression/limiting, eq etc. Half the people I give this advice to land on Celestion Vintage 30, which is a fine guitar speaker. But it's definitely not for everybody. So listen, audition & pick one you like. Or more - nobody says you have to use multiples of the same speaker.
    Yeah, once I saw a show and the guy had a 1x12 tweed that sounded fantastic. So many people play giant loud rigs in tiny venues. I don' t play out, so efficiency and loudness are not really important to me. That's why I have the big soak resistor in there. Even with that, this thing is blowing my head off at volume 4.

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  3. #73
    Supporting Member The Dude's Avatar
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    It's difficult to recommend a speaker. Everyone has different tastes and a speaker that sounds great with one amp may not sound so good in another. That said, I just put one of these in a little Fender amp I worked on for a customer. Both he and I were very happy with how it sounded.

    https://www.ebay.com/itm/Celestion-T...QAAOSwp-RaX-hc

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    Bent Member Chuck H's Avatar
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    Ok, this ones for Tom (just good natured ribbing). Speculating on the inspiration for his design:
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

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    Quote Originally Posted by Chuck H View Post
    Ok, this ones for Tom (just good natured ribbing). Speculating on the inspiration for his design:
    LOL!

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    OK, so I reversed the leads from the OT. Bad idea, that was definitely positive feedback, completely unplayable. All scratch and nastiness. I put it back and reconnected the negative feedback.

    I cut my zip ties and separated the groups of wires. The guitar inputs are very sensitive. Even when I put my fingers near them the hum goes waaay up, theremin-like. I think a key issue may be that I put the grid stoppers at the jacks. I did this because if I didn't, then I would need four pairs instead of two to get the input signals over to the first preamp stage. I am rethinking this now. Sage advice (read, Chuck) says put grid stoppers right at the tube socket. That's how the second stage CF and third stage PI are set up. Thoughts?

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  7. #77
    Supporting Member The Dude's Avatar
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    I can't really tell from the pics or layout diagrams. Did you use switching jacks on the inputs so that the jack is shorted when nothing is plugged in?

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  8. #78
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    Quote Originally Posted by Chuck H View Post
    Ok, this ones for Tom (just good natured ribbing). Speculating on the inspiration for his design:
    A cat should never think outside the box.

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  9. #79
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    Quote Originally Posted by tomgilmartin View Post
    I cut my zip ties and separated the groups of wires. The guitar inputs are very sensitive. Even when I put my fingers near them the hum goes waaay up, theremin-like.
    Good move on the zip ties, There are a lot of wires bundled together which we normally try the keep separate. I also see a lot of long wires looping between the input, preamp and power amp panels but I don't see any screened cable. I think twisted pairs work best on low impedance differential signals. I'd use them for the PI outputs and OT inputs but I'd be thinking of using screened cable for the high impedance single ended signal wires. The most sensitive wires in the whole amp are the guitar inputs and they look to be the longest. I'd definitely use screened wire for those and the preamp tube grid wires. How well would a twisted pair work for a guitar lead?

    Is that a multi-section can cap on the power amp panel? It's surrounded by the power and rectifier tubes and then inserted in a tube. How hot does it get? The pre-amp sections of the cap are a long way from the pre-amp they are trying to de-couple. Perhaps some extra caps mounted right on the pre-amp would help with the oscillation problem?

    Good luck and keep us posted.

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    Quote Originally Posted by The Dude View Post
    I can't really tell from the pics or layout diagrams. Did you use switching jacks on the inputs so that the jack is shorted when nothing is plugged in?
    Yes, they are switching type, Switchcraft MN112A. That part, at least, I wired in the standard way. Well, except that I used isolated jacks to keep with my star ground scheme. Reinvent the wheel and all that. That input circuit is so simple, elegant and functional that I call it genius.

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  11. #81
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    Quote Originally Posted by Dave H View Post
    Good move on the zip ties, There are a lot of wires bundled together which we normally try the keep separate. I also see a lot of long wires looping between the input, preamp and power amp panels but I don't see any screened cable. I think twisted pairs work best on low impedance differential signals. I'd use them for the PI outputs and OT inputs but I'd be thinking of using screened cable for the high impedance single ended signal wires. The most sensitive wires in the whole amp are the guitar inputs and they look to be the longest. I'd definitely use screened wire for those and the preamp tube grid wires. How well would a twisted pair work for a guitar lead?

    Is that a multi-section can cap on the power amp panel? It's surrounded by the power and rectifier tubes and then inserted in a tube. How hot does it get? The pre-amp sections of the cap are a long way from the pre-amp they are trying to de-couple. Perhaps some extra caps mounted right on the pre-amp would help with the oscillation problem?

    Good luck and keep us posted.
    I believe you mean Coax when you refer to screened cable. I have always considered Coax as being for Mhz signals. I haven't considered it as keeping OUT Mhz. I have to think about this some more.

    Maybe I will have to try an unshielded twisted pair for a guitar cord. It works for 100 meters on an ethernet cable without picking up too much noise.

    Have to go to work, more later...

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  12. #82
    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by tomgilmartin View Post
    ...
    Maybe I will have to try an unshielded twisted pair for a guitar cord. It works for 100 meters on an ethernet cable without picking up too much noise...
    The answer to that is a strong no. You need to shield the low level signal from the guitar pickups.

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    Quote Originally Posted by Tom Phillips View Post
    The answer to that is a strong no. You need to shield the low level signal from the guitar pickups.
    That was sarcasm. You can tell by the ellipsis...
    So hard to convey without vocal tone.

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    Last edited by tomgilmartin; 01-20-2018 at 01:49 PM.

  14. #84
    Supporting Member eschertron's Avatar
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    I don't think a 100m guitar cable is a good idea, screened or not... (note the elipsis)

    but seriously, on my first build I put all the PSU caps together, doghouse style. On power up I got serious motorboating - flub, flub, flub - which was fixed by moving the input stage de-coupling cap right onto the plate resistors for that node (you know what I mean, positive lead where the plate resistor sees B+ and neg lead to where the cathode resistor sees signal ground). Now as a matter of design I place the de-coupling caps for the preamp right on location. Never have looked back

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    Quote Originally Posted by eschertron View Post
    I don't think a 100m guitar cable is a good idea, screened or not... (note the elipsis)

    but seriously, on my first build I put all the PSU caps together, doghouse style. On power up I got serious motorboating - flub, flub, flub - which was fixed by moving the input stage de-coupling cap right onto the plate resistors for that node (you know what I mean, positive lead where the plate resistor sees B+ and neg lead to where the cathode resistor sees signal ground). Now as a matter of design I place the de-coupling caps for the preamp right on location. Never have looked back
    This I believe I got right. I have a lovely Sprague Atom for my clean stage filtering connected directly to the pins of V2 CF.

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    Last edited by tomgilmartin; 01-20-2018 at 05:59 PM.

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    OK back to weekend warrior mode. I now have to go full-on scientist here. I know it sounds nuts, but actually I considered myself lazy for using the twisted pairs of 600V PVC insulated wire for cables inside my amp. I had intended to use shielded-twisted-pair (STP), but it just takes FOREVER to do it. Below is a pic of lead dress for the STP military cable. This one pair of cables took me over an hour. It is exponentially more difficult to do it this way. Just untwisting the damn braid seems like forever.

    With this particular cable, the silicone insulation gives a 600V rating with VERY thin insulation. Thinner insulation has advantages because the conductors (actual metal) are closer together. The self-capacitance and self-inductance of the cable are reduced and less electro-magnetic field can get between the conductors from outside to couple to the cable. No matter how tight I twist the PVC insulated 20AWG 600V wire from McMaster, there is always a gap between the wires of 2 times the insulation thickness. Bigger gap = more crosstalk.

    So now I will have a separate wire from each input. The pic shows 2 resistors soldered to the red (+) wires, and connected together. These are the 68K grid stops into the preamp. With this scheme I'm NOT dropping my already-low guitar voltage across these resistors and putting it on a wire in the noisy environment inside the amp until the absolute last possible point.

    The blue (-) wires will be tied together with another group of 2 and connected to the star-ground point at the preamp tube socket.

    The cable shields will all be tied together and then go to chassis ground, only touching the blue (-) wires through the point where they tie to the chassis. The shield is like an antenna - charge is pushed and pulled into and out of the ground point, but doesn't travel on the blue (-) wire.

    Let's see if it works as advertised.

    PLEASE DO CALL BULLSHIT ON ME IF YOU THINK I AM GETTING IT WRONG. I LOVE TALKING ABOUT THIS CRAP!
    I know it's obsessive overkill, no need to tell me that! :-)

    Click image for larger version. 

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    Quote Originally Posted by tomgilmartin View Post
    This I believe I got right I have a lovely Sprague Atom for my clean stage filtering connected directly to the pins of V2 CF.
    Click image for larger version. 

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    Last edited by tomgilmartin; 01-20-2018 at 05:58 PM.

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by tomgilmartin View Post
    ...Just untwisting the damn braid seems like forever...
    Tom M,
    There is a really efficient technique to extract the conductor wires from a braided shield. Rather than write a narrative trying to describe it, I found a video demonstration at https://www.youtube.com/watch?v=JW2esD1NIa8 The extraction process is shown starting at the 2:15 time mark. In the demonstration shown a pigtail wire is attached to the shield but that step is not always necessary if the shield is to be terminated close to the conductor connection in the build.
    Check it out.
    Cheers,
    Tom P

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    Quote Originally Posted by Tom Phillips View Post
    Tom M,
    There is a really efficient technique to extract the conductor wires from a braided shield. Rather than write a narrative trying to describe it, I found a video demonstration at https://www.youtube.com/watch?v=JW2esD1NIa8 The extraction process is shown starting at the 2:15 time mark. In the demonstration shown a pigtail wire is attached to the shield but that step is not always necessary if the shield is to be terminated close to the conductor connection in the build.
    Check it out.
    Cheers,
    Tom P
    Awesome, thanks!!

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    UPDATE:

    Installed the SHIELDED twisted pairs from the inputs to the preamp. 4 wires, one each for each input. Moved the grid stopper 68K resistors to the other end of the cables, right at pins 2 and 7 of V1.

    What incredible noise!!!! GOOD noise!

    The whistling feed back is completely gone. I still get some whup-whup-whup at volume 10 with nothing plugged in. I can't crank it with the guitar on because it's literally waking up the children at volume 3.

    New pics below. Thanks again for all the great advice!!

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    At the back of preamp V1.
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  21. #91
    Supporting Member The Dude's Avatar
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    When do we get to hear it? Some sound samples would be great!

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by Tom Phillips View Post
    Tom M,
    There is a really efficient technique to extract the conductor wires from a braided shield. Rather than write a narrative trying to describe it, I found a video demonstration at https://www.youtube.com/watch?v=JW2esD1NIa8 The extraction process is shown starting at the 2:15 time mark. In the demonstration shown a pigtail wire is attached to the shield but that step is not always necessary if the shield is to be terminated close to the conductor connection in the build.
    Check it out.
    Cheers,
    Tom P
    That's a good video, Tom. But,......it's so much easier with a helping hand!

    Click image for larger version. 

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  23. #93
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by The Dude View Post
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    I have one. My wife bought it for me Used it twice for some tricky stuff and it was eminently helpful. It sits on the tool shelf waiting for a job. But when you need it...

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    Welcome back Dude and Chuck, I thought you guys were mad at me or something. I will get a sound sample up soon. I want to tweak a bit more.

    I am going to re-do the feedback wire shielded, and also my leads for the "Boost" switch. Boost is a Robinette mod which is a switch on the cathode of V2A which places a 0.68uF cap in parallel with the cathode resistor. Supposed to increase the gain of the CF for earlier breakup, but really I don't notice much difference on or off. The boost wires are sensitive and are picking up hum.

    The tone bypass switch, on the contrary, leads to serious crunch. The heavy metal tone is EXCELLENT.

    Soak resistor seems to be working well, I can get crunch without blowing out the opposite wall. I think the resistor limits my ability to have clean tones at reasonable volume, so I am considering replacing it with a variable pot like this: DIY Workshop: How To Build Your Own Attenuator - The Guitar Magazine | The Guitar Magazine I don't have multiple output taps on the OT so it's 4 ohms only. Speakers are 8 combined so need 8 ohm L-Pad.

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by tomgilmartin View Post
    Welcome back Dude and Chuck, I thought you guys were mad at me or something......
    Oh, not at all. Just waiting for the results of your tweaking.

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    Quickie video. Lemme know how it sounds, my crappy computer seems to have the sound going in and out.

    https://youtu.be/T3IlU4JBC6U

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    Last edited by tomgilmartin; 01-25-2018 at 02:34 AM.

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    Supporting Member The Dude's Avatar
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    I tried to watch it, Tom. It says, "This video is unavailable."

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    Try again, I posted the link while it was still uploading.

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  29. #99
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by The Dude View Post
    I tried to watch it, Tom. It says, "This video is unavailable."
    I get that too

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    If it still won't get loud enough, it's probably broken. - Steve Conner
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    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


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    I'm a dummy. Set the video to private on YouTube. It's public now, for those who are still interested. Sorry to waste your time there.

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  31. #101
    Supporting Member The Dude's Avatar
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    I was hearing a sort of high pitch whistle type oscillation with the guitar. Is that the amp, or could it just be that the mic in the recording device is being overdriven with the high SPL and reacting strangely?

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  32. #102
    Bent Member Chuck H's Avatar
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    It sounds really cool in a lot of ways. I really like the tone stack bypass tone. And the percussive qualities you mention. That's a signature of the Bassman circuit But that "sizzle" you mention... Hear the way it's there and then abruptly cuts off? That's actually a parasitic artifact. When the radiant field of whatever leads are interacting are large enough there is sizzle. When the fields shrink it cuts off. The reason it cuts off abruptly is because it's a positive feedback affect. So it's self propagating and obvious until conditions change and it's not. It's probably a reaction to the LF in the circuit. Maybe just see if adjusting the bass doesn't change the affect in any way.

    I, personally, don't like that artifact. Yours isn't the only amp that does it. But it's usually a mild sort of crackling and not a on/off sizzle. I actually see it as a flaw that should be corrected. Unfortunately because your design is unusual and a parasitic can manifest in many ways I don't have a "Change this to that" suggestion.

    EDIT: Also, the left/right cut in/out and similar effects on ambient EQ and volume throughout the video were seriously getting on my nerves and making it very hard to hear what was happening or even focus at all.

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    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

  33. #103
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    Yeah, not sure why the left/right thing was happening, I'm going to guess my son was blocking all or part of the mic with his hand while recording. He was trying to get cute zooming in and out. Maybe next one I will just put the phone on a stand. The SPL really wasn't that loud. Loud, but not ear-splitting. Or maybe I'm going deaf, LOL! Did you notice Finn being funny with his ear protectors?

    I'm going to shield some more wires, it just takes a long time to dress them properly and do it up. Did up the neg feedback line just now.

    I agree, that little sizzle drives me nuts too and MUST be rooted out.

    I'll get a better video up soon, didn't have time to really do it justice. It was HD from my Galaxy S6, 344MB to upload!!

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  34. #104
    Supporting Member The Dude's Avatar
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    There are inevitably bugs to work out on a build, so don't get discouraged. Your experience is not singular. Actually, given the unconventional layout of that amp I'm quite surprised there aren't more bugs.

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  35. #105
    Bent Member Chuck H's Avatar
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    Yes. That^^^^^^^^^^^^^^

    I attribute it to Tom's engineering experience. For the most part his ideologies are solid, there just may be some things specific to the genre that escaped. My worry has been that taking this build all the way to fully functional may require a virtual tare down of the wiring.

    Remember you mentioned that out of phase cross talk was also happening within an amp? Well... Long leads, twisted pairs bundled in gobs from one aluminum pipe to another,.. There are a lot of tiny capacitances in that amp, both positive and negative feedback interactions, that are not part of the stock 5f6a circuit. And always will be. Tuning THIS amp as it's own entity is the only option.

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    Last edited by Chuck H; 01-25-2018 at 06:07 AM.
    "In fact when I run into problems working on electronic circuirts, there are so many times that when I finally track it down, the source of the problem is located between my soldering iron and my seat." SoulFetish

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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