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Thread: First build 5F2-A: low output

  1. #1
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    First build 5F2-A: low output

    I just finished my first amp build: a Princeton 5F2A from a Weber parts kit. No smoke, but the output is ridiculously low. It's barely detectable until you hit 9 o'clock, and even at full volume it isn't loud enough for a bedroom amp. I've gone through and tested for bad solder joints, shorts to ground, bad resistors, and the usual things, but now I'm going to have to approach it more logically. Any suggestions for troubleshooting this symptom?
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  2. #2
    Supporting Member loudthud's Avatar
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    First thing to look at is the Voltages at each tube socket terminal. This tells us things are hooked up and operating as they should. Errant Voltages can also point to wrong value resistors. Also measure the Voltage at the plus side of the large filter capacitors and post the readings.

    Another common problem is a short at the speaker jack. Because the output transformer winding is a low resistance, testing with a meter is tricky. The easiest thing to do is connect a meter across the speaker's terminals with the amp connected, but turned off. You should measure a low resistance, less than 1 Ohm. Now unsolder one of the output transformer wires at the speaker jack. The resistance reading at the speaker should go up to about 75% of the rated speaker impedance. If it doesn't, you have a short at the speaker jack.
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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Senior Member Enzo's Avatar
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    You built it, you have been looking at it for some time, it looks familiar to you. You might test all the parts, but you could have a part misplaced. When i work on amps, I usually have the controls facing me. Some guys work on them with the controls facing away. WHichever way you do it, turn the amp around 180 degrees. Turn your layout drawing around also. So they face the same way. Now your familiar amp is no longer so familiar. Now go through the layout drawing part by part, and verify each is indeed the value it is supposed to be. Very easy to install a 470 ohm resistor where a 470k resistor belongs. Also it is easy to have a cap and a resistor next to each other and you installed them in each other's places. They look like they belong, but are actually in the wrong places. I find by turning the amp around, I am less likely to skip something due to familiarity.

    Avoid the temptation to tell yourself, "Well I KNOW these three parts are good, I just tested them yesterday."

    NEVER think up reasons not to check something.


    Then in terms of circuits, I agree with LT^^^, go down the row of sockets and check voltages.
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    Supporting Member loudthud's Avatar
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    Another common first timer mistake is connecting a wire on the backside of the eyelet board to the wrong eyelet. You can check them easily with your Ohm-meter from the front side, just press the probes on the two eyelets that are supposed to be connected, the resistance should read less than one Ohm.
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    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.

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    Check each and every resistor to make sure the measured value is the same as the marked value... depending on what kind you used, colors are easy to confuse, and manufacturers DO make mistakes...

    Justin
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    Thanks for all the suggestions. I'll work through them all and update the post with what happens.

    Much appreciated!
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    Quote Originally Posted by Justin Thomas View Post
    Check each and every resistor to make sure the measured value is the same as the marked value... depending on what kind you used, colors are easy to confuse, and manufacturers DO make mistakes...

    Justin
    Oh man, and those awful blue background resistors, don't know about youall, but I can NOT read the band colors on them, even under a magnifying ring light. I would have never gotten anything right without measuring 3x before soldering.
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    Quote Originally Posted by JDodd View Post
    Thanks for all the suggestions. I'll work through them all and update the post with what happens.

    Much appreciated!
    Photos of your amps guts if you can, would help.
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    Everything seems fine across the speaker jack, as you described. Negligible resistance normally, then up to ~6 ohms with one of the transformer wires removed.

    I did add a pair of 100 ohm resistors to ground across the 6.3 VAC tap on the power transformer along with the filaments and the pilot light. This wasn't on one of my schematics, but was on another one and seems to be something they assumed I knew. It didn't seem to make a difference, though.

    Pin voltages are as follows:
    12AX7B
    1) 128.5V
    2) < 1mV
    3) 1.2 V
    6) 128.5 V
    7) < 1mV
    8) 1.18 V

    6V6GT
    3) 294 V
    4) 241 V
    5) 1.25 V
    8) 14.75 V

    All filaments are measured at 6.8 VAC.

    I have gone through and checked values for resistors by color code (I have trouble with orange and red), and metered in place (when possible) and disconnected (when not). They all seem correct, but I am continuing to double and triple check.

    Just as a sanity check, I played a 5W Fender Champ in a local store over the week and it is louder on 2 as mine is on 12 (max).

    Again, I appreciate everyone's input on this.
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  10. #10
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by JDodd View Post
    Everything seems fine across the speaker jack, as you described. Negligible resistance normally, then up to ~6 ohms with one of the transformer wires removed.

    I did add a pair of 100 ohm resistors to ground across the 6.3 VAC tap on the power transformer along with the filaments and the pilot light. This wasn't on one of my schematics, but was on another one and seems to be something they assumed I knew. It didn't seem to make a difference, though.

    Pin voltages are as follows:
    12AX7B
    1) 128.5V
    2) < 1mV
    3) 1.2 V
    6) 128.5 V
    7) < 1mV
    8) 1.18 V

    6V6GT
    3) 294 V
    4) 241 V
    5) 1.25 V
    8) 14.75 V

    All filaments are measured at 6.8 VAC.

    I have gone through and checked values for resistors by color code (I have trouble with orange and red), and metered in place (when possible) and disconnected (when not). They all seem correct, but I am continuing to double and triple check.

    Just as a sanity check, I played a 5W Fender Champ in a local store over the week and it is louder on 2 as mine is on 12 (max).

    Again, I appreciate everyone's input on this.
    All the HT voltages seem low to me. Anyone else?
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    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Escherton, yeah, dug around and found an old post from ... you guessed it

    http://music-electronics-forum.com/a...n-voltages.gif

    Depends on the transformer, of course, but looks like the plate should be closer to 350 on the 6V6.

    JDodd: maybe this will help.

    What transformer do you have, make, model, etc? Do you have a data sheet for it? If you yank out all of the tubes and read the voltages unloaded what do you get?

    Any chance you can post a few photos of the chassis guts? The people here are really sharp at finding stuff.

    Another question: its a Weber kit, on his schematic it says you can either use webers solid state WY3GT or the standard 5Y3GT tube,

    https://www.tedweber.com/media/kits/5f2a_schem.jpg

    (Sorry EVERYONE for the dozen edits, Im reading through the Weber web site, some cool stuff there)
    Weber's 5F2A BOM says your transformer is this one:
    https://www.tedweber.com/media/import/w022772sch.JPG

    It says that there are two sets of secondary wires, Red-White/Red-White will give you 600Vac, Red/Red will give you 660Vac, huge difference. Also, this transformer has a center tap on the 6.3V secondary so you don't need 2x 100ohm resistors. You might want to double check that wiring.

    surprised that the Weber schematic doesn't have voltages on it. <frown>
    Last edited by mikepukmel; 11-05-2017 at 01:44 AM.

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    Plate voltage on 6V6 and the 12ax7 are low. Measure the voltages at points A, B, and C. If the transformer is wired for 300-0-300 and you are using a new production 5Y3 you should be getting about 390 VDC at point A. About 340 VDC if using NOS 5Y3.
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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by eschertron View Post
    All the HT voltages seem low to me. Anyone else?
    Yes. They do seem a little low. However, they are still within ~20% of the original Princeton voltages and they are not low enough to be the primary cause of the very low volume problem that JDodd described. Bit...the fact that the voltages are lower than we would normally expect may be a secondary result of the primary issue that is causing the low volume.

    It would be a good idea to test the amp with another speaker.
    Last edited by Tom Phillips; 11-05-2017 at 10:52 PM. Reason: Further clarification
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    Here is the Power TX
    powertx.jpg

    I measured across the pairs with no load and get Red 595 VAC, Green 6.88 VAC, and Yellow 5.8 VAC.

    and the Output TX (in case it matters))
    outputtx.jpg

    I measured the voltages at A, B, and C as follows
    A - 368.7 V
    B - 235.8 V
    C - 201.3 V

    A seems fine, but B and C look significantly low. I guess that will be a good place to start looking.


    And I am using a 5Y3GT tube, (not the WY3GT). I have a mix of parts from Weber and from Hoffman as I purchased things in three different orders (in case my patience wore thin ;-) ), so sorry if this causes any confusion.

    I will address the other suggestions as I can. Thanks again for the interest.

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    Stray Cap DrGonz78's Avatar
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    Please show pics of your build so we can look over the wiring and connections points. Show some close up ones with detail then some that show the whole thing for good reference.
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    You have something strange going on between point A where you are reading 368 VDC and pin 3 of the 6V6 where you are reading 294 VDC. There should be only about 5 volt drop across the OT primary windings. Make sure you have the OT wired correctly. You should be reading about 320 VDC at pin 4 of the 6V6. Do you have another 6V6 tube that you can try? If not pull the 6V6 and take voltage (DC) readings at pins 3,4, and 5.
    Last edited by mac dillard; 11-05-2017 at 11:45 PM.
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    Don't be worried about posting photos. I posted some and my wiring is hideous! (Mrs Johnsons's second grade class gets a soldering lesson!)

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    Quote Originally Posted by mac dillard View Post
    You have something strange going on between point A where you are reading 368 VDC and pin 3 of the 6V6 where you are reading 294 VDC. There should be only about 5 volt drop across the OT primary windings. Make sure you have the OT wired correctly. You should be reading about 320 VDC at pin 4 of the 6V6. Do you have another 6V6 tube that you can try? If not pull the 6V6 and take voltage (DC) readings at pins 3,4, and 5.
    I checked the OT against the schematic and it looks good. I checked the website for the OT and saw this, so I reversed the leads, but it made no difference.
    img_3322.png

    I also rechecked the resistor values (especially the one between point A and point B) and they all check out. The significant drop in voltage between A and B has me baffled. I don't have a spare 6V6 tube, nor do I have a tester at this point, though I am looking for someone local who does.

    Here are shots of the board. It's a bit of a rat's nest at this point, and I've lifted the board so it looks strange.

    img_3317.jpg
    img_3318.jpg
    img_3320.jpg
    img_3321.jpg
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    Pull the 6V6. Power up and measure the voltages at pin 3,4 and 5.
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    Old Timer J M Fahey's Avatar
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    1) please, with amp OFF, disconnected from mains and after you checked +V discharged to zero, measure DC resistance across OT primary winding .

    2) same conditions, measure resistance from 6V6 cathode to ground.

    3) please post the actual schematic you used, so we all talk the same.
    IF a .pdf, please convert schematic to a .gif, .png or .jpg so we can straight see it in the page itself, instead of jumping back and forth to another window, and we can also edit it, add measured voltages, etc.

    4) for the next test, build an earphone to guitar plug cable and download http://www.mediacollege.com/audio/to...6bit_30sec.mp3 and http://www.mediacollege.com/audio/to...6bit_30sec.mp3

    Playing those tones on a phone or MP3 player gives you constant 440Hz or 1kHz tones, about 100 to 200mV, perfect for amp testing.
    Set player to "loop" or "repeat one" so tone plays continuously, original is just 30 seconds.

    Then weŽll be able to follow signal from input jack to speaker terminals and measure actual power.
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    Juan Manuel Fahey

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    Supporting Member Chuck H's Avatar
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    JDodd,

    I'll open by saying that it's not my intention to offend but some of this may come across as overly critical.

    I'm not even going to try and trace the complete circuit from the posted photos. There's simply too much wire going in too many directions. I don't understand why either.?. Did you intend to resolder the whole thing with shorter leads if it worked first with the longer leads? Did you intend to just stuff and mash all that lead wire down into the chassis before sending it home in the cabinet? While there may be a wiring error causing the low volume condition I think another possibility is a lead proximity induced positive phase error causing a parasitic oscillation in ultra sonic frequencies. This would make the amp work hard at amplifying something you can't even hear all the time and seriously limit it's ability amplify audible frequencies. It would explain your low volume problem and also the 1.25V on pin 5 of the power tube. That voltage shouldn't be there when the amp isn't conducting. That tells me your amp is trying to amplify something when it's just sitting there seemingly doing nothing. You really, really need to get all that lead wire under control and follow good lead path, layout and grounding in the build before you can go any further I think. Trying to trouble shoot it as it is could well be a waste of time anyhow since even if you did get it working that could all change when you stuff all that wire down on top of itself in the chassis.

    JM12C
    Attached Thumbnails Attached Thumbnails img_1185.jpg  
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    LOL. In a word, yes, I was planning on cleaning up much of the wiring after assembley was complete, but I have been sidetracked on the volume problem. If this could be a cause then I could change gears and do that first.

    No offense taken.
    :-)
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    Quote Originally Posted by J M Fahey View Post
    1) please, with amp OFF, disconnected from mains and after you checked +V discharged to zero, measure DC resistance across OT primary winding .

    2) same conditions, measure resistance from 6V6 cathode to ground.

    3) please post the actual schematic you used, so we all talk the same.
    IF a .pdf, please convert schematic to a .gif, .png or .jpg so we can straight see it in the page itself, instead of jumping back and forth to another window, and we can also edit it, add measured voltages, etc.

    4) for the next test, build an earphone to guitar plug cable and download http://www.mediacollege.com/audio/to...6bit_30sec.mp3 and http://www.mediacollege.com/audio/to...6bit_30sec.mp3

    Playing those tones on a phone or MP3 player gives you constant 440Hz or 1kHz tones, about 100 to 200mV, perfect for amp testing.
    Set player to "loop" or "repeat one" so tone plays continuously, original is just 30 seconds.

    Then weŽll be able to follow signal from input jack to speaker terminals and measure actual power.
    Will do. thanks.
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  24. #24
    Supporting Member Chuck H's Avatar
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    Here are a couple of things I see wrong in the photos. Notes in yellow.
    Attached Thumbnails Attached Thumbnails acotw12.jpg  
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    Quote Originally Posted by J M Fahey View Post
    1) please, with amp OFF, disconnected from mains and after you checked +V discharged to zero, measure DC resistance across OT primary winding .

    2) same conditions, measure resistance from 6V6 cathode to ground.

    3) please post the actual schematic you used, so we all talk the same.
    IF a .pdf, please convert schematic to a .gif, .png or .jpg so we can straight see it in the page itself, instead of jumping back and forth to another window, and we can also edit it, add measured voltages, etc.

    4) for the next test, build an earphone to guitar plug cable and download http://www.mediacollege.com/audio/to...6bit_30sec.mp3 and http://www.mediacollege.com/audio/to...6bit_30sec.mp3

    Playing those tones on a phone or MP3 player gives you constant 440Hz or 1kHz tones, about 100 to 200mV, perfect for amp testing.
    Set player to "loop" or "repeat one" so tone plays continuously, original is just 30 seconds.

    Then weŽll be able to follow signal from input jack to speaker terminals and measure actual power.
    The easy ones while I have a moment, the other two later.
    1) 284.8 ohms.
    2) 461 ohms.
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    Quote Originally Posted by mac dillard View Post
    Pull the 6V6. Power up and measure the voltages at pin 3,4 and 5.
    3) 406v
    4) 378.8v
    5) 0.7 mv
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  27. #27
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    Check your wiring from point A to the OT to pin 3 of the 6V6. No way you should have a 74 volt drop through the primary of the OT. Should be 5 to 10 volts.
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    I pulled the 6v6 and measured the voltage at 3,4, and A,B, C Plus pin 1 on the 12AX7B, then reinserted it and rechecked as follows:

    6v6 in. 6v6 out
    3) 318v 410v
    4). 255v. 383v

    A). 375v. 410v
    B). 257v. 385v
    C). 219.5v. 327v

    12ax7 1. 152v. 203v
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  29. #29
    Supporting Member Jazz P Bass's Avatar
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    Schematic with voltage readings: http://music-electronics-forum.com/a...n-voltages.gif

    Have you tried to measure the Vac signal that is at the 6V6 grid when driving the preamp hard?
    What is it's amplitude?

    If you have a low signal there, the output Will be low.
    Last edited by Jazz P Bass; 11-13-2017 at 02:14 AM. Reason: spelling
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  30. #30
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    Not being picky, just noticed a few things:


    1) A few spots where your soldering iron melted off some insulation. REALLY inspect all of your wiring carefully for spots of bare wire. If you find any, unsolder one end, and put shrink tubing over the damaged area. I found out the hard way to get a thin piece of cardboard, cut it with scissors and with one hand hold it over adjacent wires to protect them when Im soldering in a crowded area. Important: Don't turn your amp on again until you make absolutely sure there are no melted areas on your wiring.
    melted_insulation.jpg

    2) This looks like a "cold" solder joint, its not shiny and looks crystalline. Use a magnifying light and check all of your solder joints, they should all be shiny and "wet" to the wire or component lead:
    coldsolderjoint.jpg


    3) Input jack tip shorting on case? While you're doing your testing on your new build, put a small piece of electrical tape over the END of the jack tip on your input jack. If that jack tip shorts on the case, you'll lose signal there!!!!!
    jack_tips.jpg

    4) Frayed wire. I have HUGE problems with this on my wiring: when you have a stripped end of wire, sometimes when you're pushing the wire through a jack lug or tube socket pin, one or two strands bend away and don't make it into the lug or pin hole. Unsolder those connections, re-strip the wire, and be really careful to make sure all of the strands go through the connection hole:
    frayedwire.jpg

    5) Your capacitor is too close to the power resistor. If it heats up, as they do when they're dissipating, it could kill your capacitor. Unsolder and move components away from power resistors:
    captooclosetopowerresistor.jpg

    6) When you stripped the wire for this connection, you cut several of the conductor strands. (same image as frayed wire above)
    conductorstrandscut.jpg
    This could be really bad for higher current connections. You should unsolder, re-strip, and re-do any connections where strands were cut off when stripping.
    Attached Thumbnails Attached Thumbnails possible_short.jpg  
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  31. #31
    Old Timer J M Fahey's Avatar
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    6V6GT
    3) 294 V
    4) 241 V
    5) 1.25 V
    8) 14.75 V
    2) 461 ohms.
    Means cathode current is 32mA

    Which across
    1) 284.8 ohms.
    will drop 9V , and that if we assume all current is going through that plate.
    Discounting a few mA screen current, voltage drop across transformes must be significantly less than 9V , and in fact compatible with Marc DillardŽs
    Check your wiring from point A to the OT to pin 3 of the 6V6. No way you should have a 74 volt drop through the primary of the OT. Should be 5 to 10 volts.
    Yet you report 74V , so do what he says.

    This alone hints at a gross wiring error, so signal injection and such must be delayed until this is solved first.

    And please post schematic here inside the thread
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    There are a couple of resistors on top of the chassis, and wiring, looks like heater wiring running to them. what are they for?

  33. #33
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    JD check these out:

    Automatic Wire Stripper WS39 - Cleaveland Aircraft Tool

    I got a pair last year. They strip like a champ and don't nick the wire. I found out the hard way, though, they don't strip the mil spec teflon coated wire, it just rips one side of the insulation off, but will work on your chassis wiring just fine.

    Pretty cheap too.

  34. #34
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    First, let me just say that my Princeton is now working and sounds amazing.

    Second, and more importantly, let me offer a heartfelt thanks to all who helped me troubleshoot my problem and all who made suggestions for improving the quality of my work. I learned quite bit during this build and much of it was from listening to you guys with your suggestions and tips. I plan on cleaning up my work in the next week or so. Thank you all so much.

    Finally, ahem, I'll reveal the source of the problem. Feel free to laugh or roll your eyes; I did both. The secondary on the OT has three wires: black, yellow, and green. I hooked the black and yellow up to the output jack according to the schematic during the main part of the build. Afterwords, I set up a ground wire (from a piece of romex) to solder all of the ground connections to and then attached it to the chassis lug. During this I attached the green wire from the OT to ground, despite the green wire not appearing on the schematic at all. I assume it was a center tap on the secondary and was shorting my signal to ground. I removed it (and capped it) tonight and the Princeton came alive. 😄

    Again, many thanks! I love this group. You guys are nothing but helpful and it is greatly appreciated.

    Jim

  35. #35
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    Quote Originally Posted by JDodd View Post
    The secondary on the OT has three wires: black, yellow, and green...
    Not a "Center Tap," more likely a second tap for a 16R-speaker (assuming you have an 8R in it now). The original Princeton wouldn't have had one, as it was a "student" amp, but most kits & clones will come with at least two taps for different speakers nowadays. Easiest way to find out is get the docs from your OT manufacturer.

    Justin
    J M Fahey and mikepukmel like this.
    "Are you practicing in the lobby of the municipal library? It's still a guitar amp and it SHOULD make some noise (!!!)" - Chuck H. -
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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