#1
so i originally posted in GG&A but this is probably a better place for this.

i have a VJ combo that i did the impedence mod to. i did also do another mod but changed that mod back to stock the same day. i also changed the output phmage to 8 instead of 4. i also changed the power tube to an old Ei.

anyways, lately the tubes have been burning a lot hotter and not long ago there started to be a farty crackling sound that gets worse louder than 9 o'clock. first thing i thought of was the tube so i swapped it back to the stock sovtek, but no difference.
i also opened it up and inspected the circuitry and found no scorch marks or rattles but i fould a bad solder joint that i fixed, which improved the fartyness but there is still this sizzling/crackling. then i checked the cable and it isnt it.

now i come to the techheads of UG for wisdom

tl;dr: it isnt the solder joints, cable, tube, or guitar but it is still there.

thanks


edit-update: the crackling isnt very much of a problem anymore. i think it was actually a part of the heating vent not too far from the speaker cab that rattled, as it started to 'crackle' running my valvetronix as well but stopped when i moved my gear to the basement. now the problem is with the volume almost always cutting out.

help/suggestions most welcome!
Last edited by boardsofcanada at Apr 15, 2008,
#2
I dated a girl who's VJ used to fart and crackle all the time. Just keep the volume up on the tv a bit all the time if shes hot.


Edit: Just read post, sorry I dont know anything about valve juniors
Jackson Dxmg w/dimarzios up n down
Jackson DK2t
Carvin DC127
ValveKing 100 head
5150 head
Recto-Verb 112
1960a Marshal 4x12
FFFDFEFRFKFFF,jeremylp,atreyurock9, noahfor, Vangkm, Used666, and sgtshak- great sellers/traders!!
Last edited by fuzzyDXMG at Apr 11, 2008,
#3
Try the Valve Junior Resource thread or the sewatt forum.
Gear:
- Bugera 333
- VJ & VJ cab
- Jackson JS30
- TS9

Bugera Users Militia. We are horrible people. With a sprinkler fetish.
~ BUM: For all things extinguishing

Rackmount Tube Amp Project <<< Updates!
#4
Hi boards!

I thought it would be a good idea to put this conversation on your thread. That way it won't get confused with the general conversations happening on the resource thread. You might want to update your OP by editing it. That will make it easier for others to contribute ideas.

Quote by boardsofcanada
sorry to temporarily hijack, but im having a fairly common problem- volume cutting out (and only sometimes back in).
i've been googling for a while now and everything i've seen points at the tubes (which i've changed out pre and power tubes with ones i know are good to no avail) or a switching lineout jack which the vj obviously doesnt have.
i've posted in GB&C here- https://www.ultimate-guitar.com/forum/showthread.php?t=834993&highlight=valve+junior but the problem is more about the volume than a crackle now.

can anyone guide me in how to approach diagnosing and fixing this? i dont want to 'just bring it in' and i'm going into EET soon so i want some learning material.

thanks!
boards


edit: one aspect i've noticed is that it cuts out only when the amp powers on to max- it fades in and when it hits operating volume it goes out every time.



Quote by boardsofcanada
for #1 R8 and R9 (cathode resistors) both measured ~1.8v, .1v from the target, so that is ok.

#2 gets interesting. R3 and R4 (plate resistors) both measured 78v, Far below the 207v mark, meaning there is a lot more current going through the tubes than I want.

the only thing is now i dont know where to go from here. what do i check knowing the plate voltage is way low? i'm still just bumbling along tring to diy fix my baby.

I think your preamp section seems to be working normally at this point. 78 volts sounds VERY familiar, doesn't it? We have 0.78 mA in both of the triodes in the preamp section. With 0.78 mA flowing through a 100K resistor, you'll have 78 volts dropped across the resistor. That's how you measured it, isn't it? The 207 volts would be from the plate to ground. So unless I read you wrong, so far, so good. You have the correct idle current in both triodes.

I want to ask more about this statement:

"one aspect i've noticed is that it cuts out only when the amp powers on to max- it fades in and when it hits operating volume it goes out every time"

Does this mean when the amp warms up, even without putting a signal through it,

OR

do you have to play loudly through it, before it cuts out?
Meadows
Quote by Jackal58
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Quote by SK8RDUDE411
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#5
Quote by SomeoneYouKnew


I think your preamp section seems to be working normally at this point. 78 volts sounds VERY familiar, doesn't it? We have 0.78 mA in both of the triodes in the preamp section. With 0.78 mA flowing through a 100K resistor, you'll have 78 volts dropped across the resistor. That's how you measured it, isn't it? The 207 volts would be from the plate to ground. So unless I read you wrong, so far, so good. You have the correct idle current in both triodes.

I want to ask more about this statement:

"one aspect i've noticed is that it cuts out only when the amp powers on to max- it fades in and when it hits operating volume it goes out every time"

Does this mean when the amp warms up, even without putting a signal through it,

OR

do you have to play loudly through it, before it cuts out?


if i have the guiter plugged in and just kind of strum until it warms up at about .5 volume on the amp it almost 100% cuts out as soon as it's warmed up, and if i let it warm up without a signal it is either the first or second strum that kills it, again at even a low volume.

and yeah i measured it across the resistor not plate to ground. i'll check it out again when i get home.

thanks again for the helping

edit: what are you saying with the 1 2 and 3 numbers in red exactly, where i should touch the leads to measure things?

edit-edit: and just as a disclaimer i'm not quite brushed up on my electronics math yet. i have a couple army textbooks for radio electronics and electronics math that will come in handy im sure, everyone seems to say an amp is close to a radio.
Last edited by boardsofcanada at Apr 15, 2008,
#6
Quote by boardsofcanada
edit-edit: and just as a disclaimer i'm not quite brushed up on my electronics math yet. i have a couple army textbooks for radio electronics and electronics math that will come in handy im sure, everyone seems to say an amp is close to a radio.
A radio has a bunch of other stuff, too. But it does have an amplifier in it.

Quote by boardsofcanada
edit: what are you saying with the 1 2 and 3 numbers in red exactly, where i should touch the leads to measure things?
Yeah, The arrows marked 1 are the cathode voltages. 2 indicates the plate voltages and 3 indicates the part of the power supply after the series resistor and filter cap for the preamp section. Usually the first thing I look at are each of the cathode voltages. The voltages on that print are measured to ground, so one lead needs to be connected to ground, the other on the place where you're measuring.

Quote by boardsofcanada
and yeah i measured it across the resistor not plate to ground. i'll check it out again when i get home.
Well the voltages add up to the right value, so I don't think you really need to worry.

Quote by boardsofcanada
if i have the guiter plugged in and just kind of strum until it warms up at about .5 volume on the amp it almost 100% cuts out as soon as it's warmed up, and if i let it warm up without a signal it is either the first or second strum that kills it, again at even a low volume.
That's gonna make it more difficult to troubleshoot, isn't it? Think about it for a minute. All the conditions are right, even after it warms up, until you strum a couple of times and it dies. So all the voltages that you measure will be right until you put a signal through it and cause it to die.

You'll need to take it apart and connect a speaker and send a signal through it until it cuts out. Then take the voltage measurements after it cuts out.

I guess the really good part about it is that you can turn it off for a while then when you turn it back on, you can measure what the voltages are supposed to be.
Meadows
Quote by Jackal58
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Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#7
Quote by SomeoneYouKnew
A radio has a bunch of other stuff, too. But it does have an amplifier in it.

Yeah, The arrows marked 1 are the cathode voltages. 2 indicates the plate voltages and 3 indicates the part of the power supply after the series resistor and filter cap for the preamp section. Usually the first thing I look at are each of the cathode voltages. The voltages on that print are measured to ground, so one lead needs to be connected to ground, the other on the place where you're measuring.

Well the voltages add up to the right value, so I don't think you really need to worry.

That's gonna make it more difficult to troubleshoot, isn't it? Think about it for a minute. All the conditions are right, even after it warms up, until you strum a couple of times and it dies. So all the voltages that you measure will be right until you put a signal through it and cause it to die.

You'll need to take it apart and connect a speaker and send a signal through it until it cuts out. Then take the voltage measurements after it cuts out.

I guess the really good part about it is that you can turn it off for a while then when you turn it back on, you can measure what the voltages are supposed to be.



edit: cathode voltages are: 1.8v for R9 and R8 and 200v for R14
plate voltages are: 200v for v1a and v1b and 349 for v2
and for -R3 to gnd is 200v.

these are the 'non-functioning' values, it cuts out wth no strums as well so i wasnt able to get the Functioning values but i'll try again tomorrow when it isnt midnight and the amp is cool.

tell me what you think of the values
Last edited by boardsofcanada at Apr 16, 2008,
#8
Quote by boardsofcanada
edit: cathode voltages are: 1.8v for R9 and R8 and 200v for R14
plate voltages are: 200v for v1a and v1b and 349 for v2
and for -R3 to gnd is 200v.

these are the 'non-functioning' values, it cuts out wth no strums as well so i wasnt able to get the Functioning values but i'll try again tomorrow when it isnt midnight and the amp is cool.

tell me what you think of the values
200V @ the cathode of V2 tells you there's a huge problem there. If there is really 200V across a 220 ohm resistor, that would be dissipating 180 watts. Your amp would be on fire right now. Either the resistor is defective, or there is a poorly soldered joint, or a crack in one of the foil traces leading to that resistor. If you have 200v across C5, it might be permanently damaged now, too.

Look carefully at all the connections and traces that lead from pin 3 of V2 through R14 then to ground. If you can't spot any problems, replace R14 and remove C5 until you're sure you have the problem solved. Then put in a new cap for C5. I wouldn't trust that one now.

The amp will work without C5. It just won't be nearly as loud.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#9
Quote by SomeoneYouKnew
200V @ the cathode of V2 tells you there's a huge problem there. If there is really 200V across a 220 ohm resistor, that would be dissipating 180 watts. Your amp would be on fire right now. Either the resistor is defective, or there is a poorly soldered joint, or a crack in one of the foil traces leading to that resistor. If you have 200v across C5, it might be permanently damaged now, too.

Look carefully at all the connections and traces that lead from pin 3 of V2 through R14 then to ground. If you can't spot any problems, replace R14 and remove C5 until you're sure you have the problem solved. Then put in a new cap for C5. I wouldn't trust that one now.

The amp will work without C5. It just won't be nearly as loud.



im sorry, its 10.7v for R14, i accidently put the value for R3.

edit: im thinking it's a problem somewhere in the power section, as the power supply, cathode, and plate voltages are pretty close to target, but the power plate voltage is 40v over, and i dont know what the voltage for R14 is supposed to be.
the only components AFAIK that are in the 'power section' is R14 (which is 218 ohms, right on the money) and C5.

when you said that the amp just wont be nearly as loud without C5, how much quieter are you saying? when the volume cuts out it doesnt cut Entirely out (and actually sounds pretty good as i can crank it to hell without it being too loud). it boils down to:
could C5 be faulty anyway?
is C5's capacitance what you hear when the amp fades in?
Last edited by boardsofcanada at Apr 17, 2008,
#10
Quote by boardsofcanada
im sorry, its 10.7v for R14, i accidently put the value for R3.
No worries.


Quote by boardsofcanada
edit: im thinking it's a problem somewhere in the power section, as the power supply, cathode, and plate voltages are pretty close to target, but the power plate voltage is 40v over, and i dont know what the voltage for R14 is supposed to be.
The voltage across R14 is supposed to be around 9 Volts.

Quote by boardsofcanada
the only components AFAIK that are in the 'power section' is R14 (which is 218 ohms, right on the money) and C5.
There is also the tube itself and the output transformer.

Quote by boardsofcanada
when you said that the amp just wont be nearly as loud without C5, how much quieter are you saying? when the volume cuts out it doesnt cut Entirely out (and actually sounds pretty good as i can crank it to hell without it being too loud). it boils down to:
could C5 be faulty anyway?
is C5's capacitance what you hear when the amp fades in?
If you think C5 might be the problem, listen to the amp when it fades out and make a mental note of how loud it is. Then without changing any settings, remove C5 from the circuit and try it again.
Is it the same volume? Then C5 would seem to be the problem.
Is it even quieter? Then C5 wasn't the problem.


Having a somewhat higher than normal voltage on the cathode resistor means the current in the tube is higher than normal. In that condition, all other things being equal, the plate voltage should be lower than normal, not higher. What could cause that? If there is less resistance between the plate and the power supply than normal, the same amount of current will have less voltage drop across that resistance. The resistance we are talking about in this case is the primary winding of the output transformer. If some of the windings are shorted, then the resistance will be lower. Also, you will couple less of the signal through the transformer to the speaker.

I'm not certain your transformer is cooked, but it definitely seems a possibility.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.
#11
Quote by SomeoneYouKnew
No worries.


The voltage across R14 is supposed to be around 9 Volts.

There is also the tube itself and the output transformer.

If you think C5 might be the problem, listen to the amp when it fades out and make a mental note of how loud it is. Then without changing any settings, remove C5 from the circuit and try it again.
Is it the same volume? Then C5 would seem to be the problem.
Is it even quieter? Then C5 wasn't the problem.


Having a somewhat higher than normal voltage on the cathode resistor means the current in the tube is higher than normal. In that condition, all other things being equal, the plate voltage should be lower than normal, not higher. What could cause that? If there is less resistance between the plate and the power supply than normal, the same amount of current will have less voltage drop across that resistance. The resistance we are talking about in this case is the primary winding of the output transformer. If some of the windings are shorted, then the resistance will be lower. Also, you will couple less of the signal through the transformer to the speaker.

I'm not certain your transformer is cooked, but it definitely seems a possibility.



it is the same volume with or without C5 in the circuit.

but the curious thing is it still does that 'powering up-click,' it isnt all-around quieter, the problem still persists. is this to be expected or imply something else?
in the meantime, while my iron is hot, i think i have a spare 22uF cap floating around.

funny enough i was planning on replacing the OT with a Hammond 125DSE some time or another, i was just hoping it wouldnt be a necessity when money's tight.


edit: nope ill have to buy the replacement cap.

edit-edit: i bought the cap at radioshack ( lol) and put it in and now it's just much quieter than normal. there is no fadein-pop thing which is promising, but it is still not normal volume. it may just be my head but it sounds ever so slightly louder than with the old cap/no cap as well. not sure what this implies.
Last edited by boardsofcanada at Apr 17, 2008,
#13
Hi boards,

I'm still wondering about the output transformer. I think you should measure the DC resistance of both the primary and secondary. For the primary, make sure the amp is unplugged, and the filter caps are discharged. Then measure the resistance from pin 7 of the EL84 to ground. For the secondary, unplug the speaker and plug a cable into the jack. Then measure the resistance from tip to sleeve of the plug on the other end of the cable.


Go to the resource thread and determine which version your VJ is, from the information in post #2.

https://www.ultimate-guitar.com/forum/showthread.php?t=800422

Then maybe Kurt lives or Mr Carrot can make measurements of the resistance of their OTs, assuming one of them has the same version that you do.
Meadows
Quote by Jackal58
I release my inner liberal every morning when I take a shit.
Quote by SK8RDUDE411
I wont be like those jerks who dedicate their beliefs to logic and reaosn.