Tube Amp Problem [SOLVED]

[zayance]

Hi Guys,

Well a friend of me called me today to tell me that he had some kind of weird issue with his Guitar amp, so i went to just have a visual look and know about what he did before that, and from visual everything seem to be fine inside the amp no burned components, all tube glowing etc...
I'm not a pro in Tube amp repairs and all, and a little knowledge from you guys would be great, here are the symptoms and what he did before they appeared:

First he builded himself a cabinet to fit two 12" Eminence speakers, for now nothing fancy.
Second he hooked up + beeing the tip of the jack, and - beeing the ring of it, everything ok.
Third after a quick test everything is ok, sounds nice, but he had some ECC83 / 12AX7 tubes from Sovtek and Tungsol to try out.
So he openend the amp and tried some different combinations of tubes in the V1/V2/V3 Section of it, wich is i believe Clean/Distortion/Phase or something like that? Sorry for my noobiness  .
But after some try outs, well no more sound, very weird, he told me that he swapped them while amp was off of course, and waited a minute before replacing, and a minute before lighting up again, and that for about 5 or 6 different combinations for some minutes before nothing, when doing the last combination and lighting up the amp again, and then nothing, all the amp seem to be feeded, LED lighting up with footswitch, all Tubes glowing etc....
Anybody Knows what could be the issue  ???, i mean normally changing tubes with same ref tubes is no prob right?, or where can i look at and check with DMM, to spot the prob.
I don't have huge experience in troubleshooting Tube Amp, i know that i have to be carefull while working on them that i know, safety first .
Anyway Thanks for your help.

 

[pucho812]

check solder connections. Check  voltages with out the tubes in it for the plates and heaters... work your way up from the PSU. a scope is really handy in these kind of situations... does he have a schematic?  I would suggest going back to the original tubes that were in it and see if it still works... then move around one tube at a time. so for example start with the original tube set. the replace v1 until he gets the fault or the right v1 sound he is looking for... then move on from there. It's possible there is a faulty tube but unlikely....

 

[PRR]

> then nothing

Not "nothing". It lights up. Put your ear in the speaker. Is there hiss? Hum? Does that sound change with any controls?

Is the speaker really plugged-in? Are his speaker solder-joints working?

It is difficult, but not impossible, to jam a 9-pin 12AX7 into the socket "wrong". In some tight amplifiers it is much too easy.

Don't be in a hurry to open it up. Check the tubes. Check ALL the essential connections.

 

[MagnetoSound]

Sorry if this is too obvious, but if the filaments are lit, then no sound at all suggests that the HT is absent.

Check the HT fuse - there is one, right? If your filter caps are on the large side, a quick-blow fuse, especially one that is conservatively rated, will easily open up after repeated powering up and down.

 

[isophase]

yes,
i would check the tubes first, this happened to me once, i changed a phase splitter tube with a brand new one that was faulty = no sound at all.
check the pins on the tube sockets visually, and make sure the speaker is well connected to the amp.
if the amp is a bit old, i sometimes try to gently bend the pins of the tube socket inside the amp for a more solid contact.
make sure the amp is turned off ! keep one hand in your pocket for safety and try to use a chinese stick or something non conductive to do this, especially if you don't have the right tool to discharge the filter caps.
bear in mind that even if the big PSU filter caps are discharged properly there is still one cap that can still retain some charge and this is in the negative bias circuit (if there is one) so be careful when changing the power tubes.
but of course we all know that.

 

[zayance]

Thanks guys for all the answers.

I would suggest going back to the original tubes that were in it and see if it still works... then move around one tube at a time. so for example start with the original tube set. the replace v1 until he gets the fault or the right v1 sound he is looking for... then move on from there. It's possible there is a faulty tube but unlikely....

I've tried that yesterday IIRC, i'll do again to be sure, and check voltages also. Thanks

> then nothing

Not "nothing". It lights up. Put your ear in the speaker. Is there hiss? Hum? Does that sound change with any controls?

Is the speaker really plugged-in? Are his speaker solder-joints working?

It is difficult, but not impossible, to jam a 9-pin 12AX7 into the socket "wrong". In some tight amplifiers it is much too easy.

Don't be in a hurry to open it up. Check the tubes. Check ALL the essential connections.

Essential connections are correct will check back, no hiss or hum from the speakers IIRC and he seemed to have inserted the tubes correctly.

Sorry if this is too obvious, but if the filaments are lit, then no sound at all suggests that the HT is absent.

Check the HT fuse - there is one, right? If your filter caps are on the large side, a quick-blow fuse, especially one that is conservatively rated, will easily open up after repeated powering up and down.

That i will check first, because actually i didn't had no spare fuses with me to try out, could be one of the probs, he did turn on and off pretty repeatedly.

PS: He said to me with the combinations of Sovtek in V1, TungSol in V2, and the unchanged JJ Tesla in V3, the devil was in his amp , he had a great moment with his amp for a couple of minutes, i would love to help him have this feeling back, he's one great guitarist and you play even better when the sound is right, right  

Thanks again for your help guys, will report back.

 

[zayance]

Ok essential connections are all ok, there is some clicks coming out of speakers when switching on footpedal and i can hear the relays inside amp also switching and some hum and little buzz when overdrive engaged tough, the circuitry inside looks healthy, what about the audio transformer it's a noratel 30W, maybe a failure because of different tubes ??? Just a tought, and how can i check it, i presume i have to take it out of the circuit to do so, but don't want to go that far still.
Fuses inside amp are replaced and ok.
I've Checked voltages of the socket  for the 12AX7 without the tubes and here is what i get:
pin 4 and 9 on V1/V2/V3 are all at about 310V
all the rest of the pins were at OV, but there was crackling noise when testing Pin 4 and Pin 9 of the V3 socket, normal, not normal???

Any toughts?

Thanks a lot for your help.

 

[MagnetoSound]

I've Checked voltages of the socket  for the 12AX7 without the tubes and here is what i get:
pin 4 and 9 on V1/V2/V3 are all at about 310V

... Any toughts?

Not really relevant to your problem but you're counting backwards! Pins 1 and 6 are the plates. 

 

[MagnetoSound]

Ok essential connections are all ok, there is some clicks coming out of speakers when switching on footpedal and i can hear the relays inside amp also switching and some hum and little buzz when overdrive engaged ...

there was crackling noise when testing Pin 4 and Pin 9 of the V3 socket, normal, not normal???

The output stage is working. Insert the driver tube and then the others in reverse order, taking your voltage measurements as you go. Make sure that volume and tone controls are turned up at least half way.

When you no longer hear a pop as you touch the plate pins with your probe, you have found the faulty stage.

 

[pucho812]

just because a solder joint looks healthy does not mean it is healthy.... IMO i would scope the amp out... run a 1k sine wave into it and use an o-scope to see where the connection is not happening... oh and if possible run the tubes on a tube tester....

crackle when testing a pin or part usually points to a cold solder joint...

 

[zayance]

Not really relevant to your problem but you're counting backwards! Pins 1 and 6 are the plates.  

Oops yes  

BTW checked the Transformer output, all voltages are ok, i have 6.5V, 12V and 250V, going in circuit, seem to be good, i just wanted to start from PSU, after all seem to be ok, on rectifiers...

The output stage is working. Insert the driver tube and then the others in reverse order, taking your voltage measurements as you go. Make sure that volume and tone controls are turned up at least half way.

When you no longer hear a pop as you touch the plate pins with your probe, you have found the faulty stage.

You mean insert V2 for instance, check voltages in V1 and V3, and after that insert V1 check voltages at V3 etc... thanks for claryfing my noobie brain  

Insert Quote
just because a solder joint looks healthy does not mean it is healthy.... IMO i would scope the amp out... run a 1k sine wave into it and use an o-scope to see where the connection is not happening... oh and if possible run the tubes on a tube tester....

crackle when testing a pin or part usually points to a cold solder joint...

You think that taking IN and OUT the tubes from the sockets may have damaged pins soldering? Could be, he told me that the Tubes were pretty hard sometime to take out who knows, i don't want to open up all the circuit board to have access at the bottom, it's not possible to take out the bottom of the case, one solution is that i heat up the pins from top and add a little solder just to make it new, the PCB is doublesided so???
And if this is too hard then take the guts out after taking all voltages out of it...
I really need to learn a little more on Tube circuits, really, this and my G7 project actually got me motivated...
BTW, i don't have Tube tester  , i can still check with DMM right, i remember something about the filament resistance .....

PS: BTW it's a pretty new amp, not a vintage one, so circuit is clean as..... (don't have any comparison in mind)  

Thanks again, i really appreciate

 

[pucho812]

tubes are sometimes hard to pull in an out even if going in at the correct position.... nature of the beast. testing tubes with a dmm? I do not know... I happen to have 2 tube testers so I don't even think about it....

 

[MagnetoSound]

The output stage is working. Insert the driver tube and then the others in reverse order, taking your voltage measurements as you go. Make sure that volume and tone controls are turned up at least half way.

When you no longer hear a pop as you touch the plate pins with your probe, you have found the faulty stage.

You mean insert V2 for instance, check voltages in V1 and V3, and after that insert V1 check voltages at V3 etc... thanks for claryfing my noobie brain 

No, I am referring to the fact that each triode amplifies the signal from the preceding one. This is simple signal tracing. (Although you are correct in that the voltages will drop slightly as you add each tube.)

With V3 in place, you can insert V2 and listen for pops when you probe the pins. If you hear the pops at V2, insert V1 and repeat the test. (If you can hear pops all the way back to V1a, you know the problem is between the input jack and V1a.)

Best to probe the grids to listen for the amplification at each stage.

Of course, an alternative method if you have a 'scope is to inject a sine wave into the input and work forwards through each stage and see where the signal 'gets lost'. Again, probe both grids and plates for signal at each triode.

(Note: In a dual - or multi-channel amp, not all triodes will be in the signal path all the time, so make sure you have a fair idea of which triodes are in use in each channel!  )

It is most likely that you will find a dead stage where either there is no plate voltage (perhaps from a failed plate resistor), a bad cathode connection, or a break in the signal path due, as pucho says, to a dry joint or other problem.


(EDIT: Yes, you can check filament continuity with a DMM, but if they are lighting up you know they are good anyway. You can't test any other tube characteristics with a DMM.)

 

[johnR]

Another thing to check is that the contacts in the valve bases are gripping the pins on the valves properly. Sometimes they open up and go loose. They can often be bent back to the correct shape with the end of a small, straight bladed jeweller's screwdriver if you're careful, but make sure the power is off and all power supply capacitors are discharged first.

 

[PRR]

> crackling noise when testing Pin 4 and Pin 9 of the V3 socket

Yes, when you poke a live tube amp with a volt meter the change of load makes a click/crackle.

Therefore we know the power supply, output transformer, output tubes, and some of the wiring around the driver tube "works".

The other sockets did not give BIG crackle because you had V3 out. Put it back and poke V2's holes.

In fact: put all the tubes in and wiggle each one. All sockets crackle when you slide the pins in the sockets. If V2 crackles and V1 does not, either the problem is in that area _OR_ V1 does something that is not turned-on or switched-in. (If Reverb is turned off, wiggling the reverb tubes may "do nothing"; modern amps have extra stages which may be bypassed for "clean".)

This would be a lot easier if you had easy access to the sockets while the tubes were in. That's why Fender built his amps to come out on 4 screws and everything exposed. Much wisdom has been forgotten since 1957. PCB is bad enough because "wires" are hard to trace (yet so cheap that they multiply like weeds); a chassis-box that won't let you lay a finger (or meter probe) on live traces without major disassembly is just stupid.

Don't try to be a hero. If you have not worked a lot on tube amps, and particularly guitar amps, it is honest and honorable to check the easy stuff (Plugged in? Turned on? Fuses? Loose parts which are maybe missing somewhere?) then suggest a local guitar tube-amp specialist. If you brought it to me, if I could get it open, I could narrow the problem very quickly; but it is hard to put all the accumulated tricks and mistakes in words.

Don't forget it has lethal voltage in there. So does the G7, but this is a little more, and trouble-shooting is more chaotic and dangerous than building.

 

[zayance]

Yes i hear you loud and clear PRR, i want to go on the easy side actually, because i actually don't want to be hero, my friend is just very sad that this kind of thing happened while just changing tubes, it's not like he had messed with the overall circuit, and a Tube Amp specialist is pretty expensive around here, i know there is lethal voltages, and i remember that i could discharge all caps with a thick wire and a big wattage resistor if i remember correctly, and check back if there is still some voltage on those caps afterwards,  but in this amp there are only two Medium size caps, and they are not radial but axial, and so getting a grip to the +leg will be hard, since and as you said, this box is made pretty stupid for servicing, well i'll try my best with the latest informations you guys gave me, check with Oscilloscope etc...., and i'll report back.

Thanks you very much for your help guys  

PS: Will post a picture of the guts when the amp is on my desk, if you're interested to see what it looks like???

 

[zayance]

I just did a quick search on google, and found something, and it actually made me think of something i've noticed, but forgot to mention, my friend plugged a jack in the loop return, Effect Loop activated from footswitch i believe IIRC, and touched the tip, and i could hear a loud buzz coming out of the amp, telling that the Loop was ok and actually this little post tells another thing i could check, it will just give some more hints on the area of the problem:

...You can also try plugging the guitar right into "return" but you'll likely need another cable to plug into the send just to activate the return. If that works, you know your power section is ok and it's most likely a preamp tube...

 

[mad.ax]

The more info you give, the easier it will be to help you...i.e: the loop return test confirms that the driver/PI/output stage/OPT/PSU are all good.

You still haven't told us what amp is it... Is it that much a shame of a brand, like Behringer or something? ;D
Seriously, a name, model, characteristics would help... May even lead to a schematic...

What may have happen, is that while (difficultly) plugging the tubes in, a pcb trace may have broke under one of the tube socket... If it's the case, I'm afraid you will have to disassemble the whole thing.

Another simple thing to check, the 'send' loop jack... Connect it to another working amp, plug a guitar in the faulty one and see if you hear something... Sometimes, the internal connections of such jacks are faulty.

Even if you can't access the PCB traces, you can still discharge the caps... Solder some probe on your power resistor, plug off one of the power tube, and insert one of the probe in the plate pin while the other one is at ground. It's useful to have a auto-ranging voltmeter permanently wired to your bleeding resistor. Since I haven't seen the schemo yet, I would advise to repeat this procedure for preamp plates (Who knows, may this is a dual HV supply thing?)

Axel

 

[mad.ax]

Your tools should do the job indeed. Keep one crocodile clamp for the ground, and use a probe...

Ok, we won't see a schematic unless you trace it out! Looking at the specs of this amp, I have a feeling it may be an hybrid, cause it have a lot of possibilities for 3 X 12Ax7s...
A few pics would help... If you see 'unusual' components, like opamps, transistors, optos and the like, tell us what they are...

In order to confirm that V3 is indeed the driver, you can plug it out and see if the 'return loop jack trick' still sounds well... If it doesn't and plugging it back make the sound appears, then V3 is Ok and we can concentrate on V1 and V2 (and whatever else there may be between them and the send loop jack...)

Did you try  to check what comes out of the send jack?

If you can probe with the scope with the tube installed, then trace the signal from the input. Keep in mind that one or more triode may be out of the circuit depending on channel switching. As you go try to figure out the configuration... i.e: V1a = first stage, V1b=boost (only active when switched), V2a= second stage... You get the idea...

If you find no signal on a triode in any configuration, but there is signal on its grid, then check plate and cathode voltage with a DCV meter.

Axel

 

[zayance]

Thanks a lot.

I'll have the amp in my Little Lab tonight or tomorrow, so every checking and testing will be easier for me.
Will post pics also so for you to see, but i don't remember seeing some unusual components, but not sure.
Anyway will report back and post pics.