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Thread: Bugera 333 212 combo, very low output

  1. #36
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    Quote Originally Posted by bwheat View Post
    Is there a good way to actually test that the tube is doing what it is supposed to when it is in the amp. I have thought about buying some cheap Russian reflector brand tubes just to test this thing out and keep them as spares.
    DMM Checking the voltages on the pins will tell you how the tube is performing.
    Do you have one? Even the cheap Harbor freight ones will work. My problems has always been when the 9v battery gets low readings go wonky.

    nosaj
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    Lifetime Member Enzo's Avatar
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    Sure, by the performance. A tube is noisy or not, it hums or not, it amplifies or not, etc.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by nosaj View Post
    DMM Checking the voltages on the pins will tell you how the tube is performing.
    Do you have one? Even the cheap Harbor freight ones will work. My problems has always been when the 9v battery gets low readings go wonky.

    nosaj
    nosaj
    I have a nice one, I will get the tubes back in and take some measurements. I have a few of the harbor freight ones as well, they work fine, but the leads always break on them. I am usually upside down under the dash of a car with them so I am kind of hard on them.

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    Finally got a little extra time and cash to work on this thing. I replaced the preamp tubes, and it is a tad bit louder, but still about the same as before. Still has a little bit of a hum also. I will make a video here in a bit as to what it sounds like.

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    Would is work to test the pre amp and power amp to run a cable from the line out or head phone jack on a separate amp to the effects return to test the power and then run the cable from the effects loop out unto the front of the other amp to test the pre amp? Also when the effects loop is kicked on should there still be signal going thru to the speaker without anything hooked to the effects loop. I get a very low sound that still goes thru. Also a loud popping noise thru the speaker and then more hum when it is switched on. I recorded a clip of this, but my iPad video wouldn't load on here for some reason.

  6. #41
    Lifetime Member Enzo's Avatar
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    Yes, you can apply a signal to the FX return from elsewhere. Hell you can even just plug the guitar in there. And yes, you can send the preamp out from the FX send to some other amp.
    Pdavis68 likes this.
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    Quote Originally Posted by Enzo View Post
    Yes, you can apply a signal to the FX return from elsewhere. Hell you can even just plug the guitar in there. And yes, you can send the preamp out from the FX send to some other amp.
    I have tried the guitar plugged into it, and I just get some light popping noise when you strum it. Trying to find my spare cable to hook up to another amp. Is it normal to get sound when the effects loop is on and the guitar plugged in the front? Seems as though nothing should make it thru as the preamp is not hooked to the power amp when it is switched on, is this correct?

  8. #43
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    It depends if it is a series loop, or parallel.
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    I finally located my extra guitar cord today and tried hooking the other amp up. When I do effects loop out to the input on the other amp I get the same thing, a humming sound and very low volume on both amps at the same time. When I hooked the line out from the other amp into the effects return I get full power, very loud volume, but it is very distorted even on the clean channel. It seems that the power side and output transformer are working good. I am going to pull it apart and do some investigating in the effects loop, preamp side of it here in a bit. The loud noise and hum when it is activated leads me to believe that there is something going on in the area where the pre amp and power amp are connected.

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    Didn't find anything obvious, I measured the resistors in the board that the effects loop jacks, are on, and also in the section on the front board where the harness goes to. Everything I tested seemed to be in spec. Any ideas on where to look next?

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by bwheat View Post
    ......When I hooked the line out from the other amp into the effects return I get full power, very loud volume, but it is very distorted even on the clean channel.....
    Can you explain what you mean by that? If you are feeding the effects return directly, the preamp shouldn't have anything to do with the test, yet you mention "the clean channel".

    At any rate, if I correctly understand your post, we have determined the power amp works and the preamp has problems? If that's the case, I would next do a quick check of preamp plate voltages. You may have an open or out of tolerance plate resistor.
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    Quote Originally Posted by The Dude View Post
    Can you explain what you mean by that? If you are feeding the effects return directly, the preamp shouldn't have anything to do with the test, yet you mention "the clean channel".

    At any rate, if I correctly understand your post, we have determined the power amp works and the preamp has problems? If that's the case, I would next do a quick check of preamp plate voltages. You may have an open or out of tolerance plate resistor.
    I was referring to the amp that I had hooked up to this amp being on its clean channel, not the actual amp that I am working on. I have done all the voltage measurements with tubes in and out as well as plate resistors. The first page of this has all the results from those tests.

    Actually just looked again, those measurements were from the power tubes and phase inverter. I will get some numbers from the preamp tubes tomorrow. The plate resistors I tested were for the preamp and all were in spec according to the schematic I found, even though it is a little different as it is for a 333xl and I have just the 333.

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    I got a little free time today and measured all the pins for the 3 preamp tubes.

    Valve6
    P1 207v, P2 0v, P3 1.5v, P4 16v, P5 5.2v, P6 211v, P7 0v, P8 1.5v, P9 10.8v

    Valve7
    P1 213v, P2 0v, P3 1.5v, P4 -5.6v, P5 -16.8v, P6 156v, P7 0v, P8 1v, P9 -11v

    Valve8
    P1 174v, P2 0v, P3 1.3v, P4 -5.4v, P5 5.2v, P6 150v, P7 0v, P8 1v, P9 0v

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    Could anyone tell me if these numbers seem fine.

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    Quote Originally Posted by bwheat View Post
    Could anyone tell me if these numbers seem fine.
    Someone correct me if I'm wrong but pins 4,5,9 The numbers don't seem right or even correct polarity.
    To me your heater (filament) voltages are not correct for a 12ax7.
    nosaj
    Bugera_333XL_Schematics.pdf

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    Quote Originally Posted by nosaj View Post
    Someone correct me if I'm wrong but pins 4,5,9 The numbers don't seem right or even correct polarity.
    To me your heater (filament) voltages are not correct for a 12ax7.
    nosaj
    Bugera_333XL_Schematics.pdf
    I will measure those again, just to verify, I thought that the heater voltages seemed all funky.

  17. #52
    Lifetime Member Enzo's Avatar
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    The heaters are fine, I bet if you look at them they are glowing. Your voltages on the triode pins all look reasonable, which means the tubes are conducting - which they do with proper heater.

    Look at the schematic, right in the middle, there are your 12AX7 heaters. They are wired in series across +18 and -18vDC.
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  18. #53
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    Quote Originally Posted by Enzo View Post
    The heaters are fine, I bet if you look at them they are glowing. Your voltages on the triode pins all look reasonable, which means the tubes are conducting - which they do with proper heater.

    Look at the schematic, right in the middle, there are your 12AX7 heaters. They are wired in series across +18 and -18vDC.
    And the reason V8 pin9 is zero is because it is directly at Earth ground?

    Thanks,
    nosaj

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    Quote Originally Posted by Enzo View Post
    The heaters are fine, I bet if you look at them they are glowing. Your voltages on the triode pins all look reasonable, which means the tubes are conducting - which they do with proper heater.

    Look at the schematic, right in the middle, there are your 12AX7 heaters. They are wired in series across +18 and -18vDC.
    Should valve 8 have more than 5 volts present on either pin 4 or 5 then like valves 7 and 6. I believe that there

  20. #55
    Lifetime Member Enzo's Avatar
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    DO it this way: measure voltage from pin 4 to pin 5 of each tube. I bet you find close to 12v on each tube. VOltage to ground doesn't matter, what matters is the voltage across the heater - its working voltage./

    They are all three in series from +18 to-18v. That means one tube will have +18 and +6 on pins 4 and 5 (or 5 and 4) The next tube will have +6 and -6v on pins 4 and 5, and the remaining tube will have -6 and -18v on pins 4 and 5. The tube with 6v on each end will have pin 9 grounded.

    If the three tubes have glowing heaters, and the amplifier amplifies, then move on from the heaters, they are fine.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    DO it this way: measure voltage from pin 4 to pin 5 of each tube. I bet you find close to 12v on each tube. VOltage to ground doesn't matter, what matters is the voltage across the heater - its working voltage./

    They are all three in series from +18 to-18v. That means one tube will have +18 and +6 on pins 4 and 5 (or 5 and 4) The next tube will have +6 and -6v on pins 4 and 5, and the remaining tube will have -6 and -18v on pins 4 and 5. The tube with 6v on each end will have pin 9 grounded.

    If the three tubes have glowing heaters, and the amplifier amplifies, then move on from the heaters, they are fine.
    I will give that check. I appreciate your advise. This tube audio stuff is a bit different than a lot of the electronic stuff I have expierence with. I have a real good understanding if I am looking at as far as the circuit looking at say a handwired jtm, or a twin reverb, or even some of the modern stuff with pcb boards that are arranged similar. The way these are built with the switching circuitry, the effects loop, and partially correct schematic, has me a bit confused because I can't see the actual traces on the preamp board as they are on the bottom.

    Where I am at is I guess going thru and testing the components in the signal path, grid stopper, coupling caps, and the grid leak resistors. I assume having a scope and signal generator to trace the ac signal would be very helpful for this.
    Last edited by bwheat; 11-04-2017 at 04:42 AM.

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    Alright, I decided to make a signal probe to track the signal thru the circuit, and am getting something which doesn't seem right. I am getting signal to the first tube pin 7, and an amplified signal out of 6, so the tube is working to amplify the signal. What doesn't seem right is that the signal into the second stage of the first preamp tube is dropped to what it is coming into the amp. Shouldn't the signal going into the second stage (pin 2) be amplified from the first stage? Than from there further amplified by the second stage and onto pin 7 of the second preamp tube? The signal going into the second preamp tube is also equal to the input of the first stage, no amplification. There should be more than one stage of amplification in the preamp correct?

  23. #58
    Lifetime Member Enzo's Avatar
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    The tone stack has huge losses. So I don't think this sounds wrong at all.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    The tone stack has huge losses. So I don't think this sounds wrong at all.
    So should the final amplified signal coming from the preamp to the power amp be about the same as coming out of the 2nd stage of the preamp tube. I am going to check at the point where the signal enters the power section to see if it is weak there vs the signal coming out of the preamp tube.

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    On what is x17 on the schematic, which comes from the front board to the power board I have just a very faint signal that is lower than what the signal coming into the front of the amp from my laptop signal generator. Very strong signal on the few 2 pin harness's around that one on the front board, but little to nothing coming out to the power board.

  26. #61
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    You can make a little probe with a ground and a hot on alligator clips that plugs into the headphone output of a small fm radio, ghetto fabulous sig gen. Then you are not trying to play guitar at the same time with your hand on the strings and in the power section

    *hijack on*I find that my Hickok 533 is excellent at testing small tubes for function and transconductance, and for wear with the "life" test. It really helps eliminate the situation with one or more bad preamp tubes being swapped back and forth (regardless if they are nos or used or new production shiite ) in a guitar amp with 4+ 12a*7's. There's a high likelihood that those tube failures were caused by insertion and removal at some point while troubleshooting.

    A functioning transconductance tester like my 533 is useful in testing NOS and and used and new production power tubes for transconductance and life. You can easily tell when its' time to replace power tubes. I run large tube power amplifiers 24x7 and test tubes only when necessary as the voltages they put on the elements can be hard on tubes. *hijack off*

    Excellent thread so far! hope you get 'er straight.

    Pete

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    Quote Originally Posted by bwheat View Post
    On what is x17 on the schematic, which comes from the front board to the power board I have just a very faint signal that is lower than what the signal coming into the front of the amp from my laptop signal generator. Very strong signal on the few 2 pin harness's around that one on the front board, but little to nothing coming out to the power board.
    Do you by chance have a signal injector? You can start at the speaker then the grids of the power tubes and work backwards.
    I believe this was also in Jack Darr's book.
    Nosaj
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    I don't have a way to actually put a signal into a specific part of the circuit, I am just running a line out from my laptop to the front input jack of the amp with a signal generator program. I built a signal probe using a set of computer speakers and a multimeteter lead with a capacitor to block Direct current into a 1/8 audio cable, into the speakers. It works very well to trace the signal thru the circuit. I feel like I was wasting my time without it before. I have verified that the power amp section actually works a while back, something between the preamp tubes and power amp. Going to go probe around a little more on that front board and find where it stops I guess.
    Last edited by bwheat; 12-02-2017 at 02:10 AM.

  29. #64
    Lifetime Member Enzo's Avatar
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    I just plug a CD player or tape deck into the front of the amp. Test tones are boring.

    Isolate your problem. So you have a weak signal at the power amp input.

    You want a simple signal injector? Use a screwdriver. Seriously. X17 feeds the phase inverter via C1 and R15. If I touch a small screwdriver to pin 7 of V5, I should hear some hum. Move back to R15, touch that. No screwdriver? Use your meter probe, but leave the black one not connected. Now the meter probes act like antennas for hum.

    If those work, back to preamp board. MAx the master volume, and touch the hot end of the control, get hum? Pin 2 of V5 is the master driver, so touch pin 2, get hum? Want a different way to test that point? Turn up the master and the reverb, now bop the reverb pan to crash the springs. Does that noise come out loud or real weak? Reverb mixes in at pin 2 there. Or I forget, is this one a digital reverb?

    Unplug the FS so the FX loop stays on. Does a signal into the FX return make sound or not?

    With input signal present, does it matter which channel is on? Did we already find if the FX send is strong or weak?

    At the output of the preamp is relay RL5C, where you select which channel. From there through the connector and R57, then a mute transistor. Is that stuck on? See D8, D9? If one shorts, you get 15v on R57/R62. Is there? Then with the amp running measure resistance to ground from the junction of R57, R62, and the diodes. If it is low, then the transistor is bad or it is not being turned off at its gate.
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    Quote Originally Posted by Enzo View Post
    I just plug a CD player or tape deck into the front of the amp. Test tones are boring.

    Isolate your problem. So you have a weak signal at the power amp input.

    You want a simple signal injector? Use a screwdriver. Seriously. X17 feeds the phase inverter via C1 and R15. If I touch a small screwdriver to pin 7 of V5, I should hear some hum. Move back to R15, touch that. No screwdriver? Use your meter probe, but leave the black one not connected. Now the meter probes act like antennas for hum.

    If those work, back to preamp board. MAx the master volume, and touch the hot end of the control, get hum? Pin 2 of V5 is the master driver, so touch pin 2, get hum? Want a different way to test that point? Turn up the master and the reverb, now bop the reverb pan to crash the springs. Does that noise come out loud or real weak? Reverb mixes in at pin 2 there. Or I forget, is this one a digital reverb?

    Unplug the FS so the FX loop stays on. Does a signal into the FX return make sound or not?

    With input signal present, does it matter which channel is on? Did we already find if the FX send is strong or weak?

    At the output of the preamp is relay RL5C, where you select which channel. From there through the connector and R57, then a mute transistor. Is that stuck on? See D8, D9? If one shorts, you get 15v on R57/R62. Is there? Then with the amp running measure resistance to ground from the junction of R57, R62, and the diodes. If it is low, then the transistor is bad or it is not being turned off at its gate.
    Running a signal into the effects return gives me full volume, I used another amp and did a line out from that and into the return. The power amp amplified it and it was as loud as the amp should be. I also ran the effects send to the input jack on the other amp and got a very weak signal out of that amp. Incredibly low volume. So yes, signal into the effects return makes the amp work, and the signal from the send is very weak. Power amp seems to work good. Wouldnt this also mean the phase inverter is working properly since it is before the power tubes? Also this one has digital reverb. I will have to locate the transistor and diodes, i can't really go off numbers listed in the schematic, they are mostly not right for this model. I will check that out though.

  31. #66
    Lifetime Member Enzo's Avatar
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    I have 333 and 333XL. The circuits are real similar if not identical, but the numbers on things are different.

    For example, your X17 as input is on 333XL. On 333 that same connector is X15. My R57/62 are on the 333XL, the same parts on 333 are R62/63. The mute transistor is T3 on 333, and T4 on 333XL.


    The phase inverter is part of the power amp, which is working. We are exploring the preamp.

    Please tell me whether you have 333 or 333XL.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    I have 333 and 333XL. The circuits are real similar if not identical, but the numbers on things are different.

    For example, your X17 as input is on 333XL. On 333 that same connector is X15. My R57/62 are on the 333XL, the same parts on 333 are R62/63. The mute transistor is T3 on 333, and T4 on 333XL.


    The phase inverter is part of the power amp, which is working. We are exploring the preamp.

    Please tell me whether you have 333 or 333XL.
    Mine is the 333.

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    Lifetime Member Enzo's Avatar
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    But you are using the 333XL schematic.

    The 333XL drawing is #P0623.

    The 333 drawing is #P0406

    Just looking them over, I think the circuits are identical, but the numbers differ. SO one could look at both and translate, , but that is a ton of work. See if you can find the plain 333 drawing.


    I am guessing, but I suspect the reason for this is they made up a new board set with the layout changed and the parts are sitting in different order, so the numbers changed.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    But you are using the 333XL schematic.

    The 333XL drawing is #P0623.

    The 333 drawing is #P0406

    Just looking them over, I think the circuits are identical, but the numbers differ. SO one could look at both and translate, , but that is a ton of work. See if you can find the plain 333 drawing.


    I am guessing, but I suspect the reason for this is they made up a new board set with the layout changed and the parts are sitting in different order, so the numbers changed.
    It is a pain in the butt for sure trying to find things with different numbers than the schematic. For the most part I can do some continuity tests to find where things are, and what is connected to what. This black pcb is very hard to see the traces also. Getting ready to drag it out of the closet and check the function of that transistor.

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    R62 and r63 are both right at .07 volts.

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