January 2009 \ Tech Tips \ Ask Amp Man \ Diagnosing a Distressed Tube Amp

Diagnosing a Distressed Tube Amp

Jeff Bober

A Fender Hot Rod Deluxe is on the fritz, and Jeff has five possible causes to check out
Premier Guitar January 2009

Hi there,
I have a Fender Hot Rod Deluxe that is just six months old and well looked after. It has recently developed a popping, crackling sound when warmed up—even when there are no guitars or cables plugged in. What is wrong? Could the valves be on the way out? Is it something more serious?
Please help!
Ken Morton


Hi Ken,
Thanks for your question. The Fender Hot Rod series of amps is a very popular one, possibly their most popular. More than a few have crossed my bench over the years, some coming in for a “crackling” issue. There are many types of failures that cause this symptom. I’ll go through most of the typical ones here and tell you which ones I suspect might be the cause of the failure in your amp.

1. Input, footswitch and effects loop jack connections.
Jacks are probably the most physically used and abused parts in the amp. They are soldered directly to the circuit board, but are also attached to the front panel via the jack nut. This is usually not an issue unless the jack nut loosens due to vibration. Once the nut loosens slightly, the weight and motion of the guitar cable is supported solely by the circuit board solder connections. Over time, these connections can be compromised and cause crackling and an intermittent signal.

2. Broken solder joints on large components.
Another vibration-caused failure, the solder connections of larger-sized components, such as large 5-watt resistors, can eventually be compromised from the constant vibration of playing, or simply a rough ride in your trunk or the band truck.

3. Simple tube failure.
Although a tube may look good from the outside, there’s no way to tell for certain if it’s good or bad just by looking at it. Tubes can fail in many different ways. Output tubes will generally short internally and cause a fuse to blow, or they can become mechanically noisy and exhibit a “rumble” sound through the speaker when vibrated. Preamp tubes, on the other hand, will generally either become microphonic and make high pitched ringing or squealing noises, or will emit noises such as crackling and popping during idling. If the level of the noise is controllable in any way by the setting of the volume or tone controls, then there’s a good chance that the symptom is caused by a preamp tube and not an output tube.

4. Stressed component failure.
Some components become stressed over time when the typical operating parameters of the circuit are exceeded. This can easily happen with the failure of an associated component. Take for example an output tube. While a typical 2-watt resistor (used by many companies as an output tube screen grid resistor) will function fine under most normal operating conditions, that resistor may be pushed past its capabilities by constantly playing the amp at very high volumes or by an output tube shorting. This may cause excessive screen grid current to flow through the resistor, stressing it or causing complete failure. A stressed screen grid resistor can definitely cause crackling noises to occur. (As a side note, this is one reason to overbuild amps: to ensure the least possible chance of failures—using a 1-watt resistor instead of a half-watt resistor throughout the amp, for example, or a 5-watt in place of a 2-watt, etc.)

5. General component failure.
Any electrical component can fail at any time, causing anything from compromised operation to a complete shut down. I did a bit of research on this question to see if there are any known failures of this particular amp that I had not personally encountered, and it turns out that there may well be. According to the research, there may have been a bad batch of resistors used as plate resistors in the phase inverter circuit of the amplifier that were responsible for crackling noises in some amplifiers— a very strong possibility for the cause of your symptoms.

Having described the most likely candidates for the cause of your amp problem, I would say that based on your description of the amp needing to warm up first, and no guitar needing be connected, the most probable in your case is either 3 or 5. The easiest way for you to do some basic troubleshooting would be to first purchase one replacement 12AX7 tube. Then, one by one, replace each preamp tube and see if that alleviates the problem. If you still have the symptom, you should next replace the output tubes.

For the purposes of troubleshooting, you can install a new set without re-biasing the amp, but if they are indeed the cause of the noise then you should definitely take the amp in for biasing. If neither of the replacement tubes cure the problem, the amp will need to be taken in to your local experienced tech for further troubleshooting— but at least you’ll have yourself a couple of spare tubes for the amp, which no self-respecting tube-amp-playing guitarist should be without! ‘Til next time…

Jeff Bober
Co-Founder and Senior Design Engineer – Budda Amplification
jeffb@budda.com
budda.com
©2008 Jeff Bober

     

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Comments

 (10 comments) display by
UsernameComment
Mick Maughan
on 01/20/2013 		I have a 15watt Fender Pro Junior amp. The sound has become more distorted and there like a harmonizing effect occurring when I play single overdriven notes. I replaced the pre amp valve with new ones but the problem is still there any ideas, is it worth replacing the power valves. They were replaced 18mths ago but I do play a lot of gigs approx 300 a year ?
si
on 07/14/2012 		Hi, i have a fender Hot Rod delux that is making a very bad vibrating type noise on the drive channel, no instrument pluged in, clean channel works ok, although slight hum, the volume control affects the level of the noise ? any help please.
Ashe
on 08/15/2011 		Havethe same problem with my Rivera TBR-1, very helpful thank you
Jim
on 01/12/2010 		Have a Vibrolux that crackles and pops very reverby when I firs wrm it up then goes away afer about 3 seconds and amp performs fine all night. ??? Help?
a
on 11/09/2009 		The manual says the send provides a prereverb signal, but doesn't seem to say if it is pre gain setting too. Okay, I tried it an can confirm that both the send out and the line out in the back are “post gain setting” in other words, it doesn't seem possible to get a clean out from the amp while micing a distorted out at the same time? Any insight? Even if one of those outputs did work for my purposes, do you know if a speaker cable, patch cable, or guitar/insturment cable should be used to go out to the audio interface from either the line out or the send jack? Some reviews of this amp: http://reviews.harmony-central.com/revi ews/Guitar+Amp/product/Fender/Roc+Pro+10 00+Head/10/1
z
on 11/09/2009 		“4. LINE OUT - This jack provides a signal output suitable for connection to recording and sound reinforcement equipment. Using a 3-conductor stereo cord will provide a quasi-balanced signal and a 2-conductor mono cord will provide an unbalanced signal. When hooking up this line out to my audio interface, the signal will be distorted (ie on a gain setting) if the amp is on a gain setting. This presents a problem. I want to record a clean tone either direct to my interface or routed through the amp to the interface while at the same time recording with a mic to the cabinent a distorted tone. Recording a clean signal allows for flexibility later with amp modelers/simulators, etc... However, the line out on this head will not seem to output a clean tone if the amp is on a distorted channel? Is there some way to get both at the same time with only one head and one cabinet? One workaround I have used is to buy a Y-cable female mono 1/4” to two male mono 1/4” splitter, YPP-111 by livewire (that brings up another question, if I plug a 1/4” guitar cord from my guitar into the female end on this splitter, then go out to the interface with one of the males and to the amp with the other, does this splitter need to be guitar cable, patch cable, speaker cable, etc? It would seem like it could cause a bottleneck type problem if its the wrong type of cable used in the splitter?). The manual also shows an inpute for “5. SEND - Connect cord to external equipment at this output jack. This unbalanced jack provides a prereverb signal output from the preamp and may be used to drive effects units in an effects loop, to drive additional amplifiers in a multi-amp set up or as a preamp send to a mixing console.” So maybe I should go from the send in the effects loop straight to my audio interface? Would that be a fix for the problem of recording both a clean track and a distorted track at once? The manual says the send provides a prereverb signal, but doesn't seem to say if i
zach
on 11/09/2009 		have one electric guitar, a Fender Strat '94, american made....Could it be my guitar causing the crackling? The crackling seems like it might be related to right when I hit a chord on the guitar...but it occurs with single notes too...It seems like maybe the crackling occurs right after the strings are struck, but I am not sure it wouldn't occur during in the middle of a sustained note... However, when I unplug my guitar from the amp and tap on the amp it seems like the crackle is still there, so that would seem to indicate it is not the guitar. I also wonder if the input jack on the amp could be the problem? I believe I have had some issues with it in the past and the lip of it seems like the bottom might have chipped a little, but it still holds a ¼ jack pretty snug. The jack makes a cracking sound when I take it out while the amp is on, but I am not sure that is anything abnormal... The manual says the head, in the back, has a “4. LINE OUT - This jack provides a signal output suitable for connection to recording and sound reinforcement equipment. Using a 3-conductor stereo cord will provide a quasi-balanced signal and a 2-conductor mono cord will provide an unbalanced signal. When hooking up this line out to my audio interface, the signal will be distorted (ie on a gain setting) if the amp is on a gain setting. This presents a problem. I want to record a clean tone either direct to my interface or routed through the amp to the interface while at the same time recording with a mic to the cabinent a distorted tone. Recording a clean signal allows for flexibility later with amp modelers/simulators, etc... However, the line out on this head will not seem to output a clean tone if the amp is on a distorted channel? Is there some way to get both at the same time with only one head and one cabinet? One workaround I have used is to buy a Y-cable female mono 1/4” to two male mono 1/4” splitter, YPP-111 by livewire (that brings up another question, if I
zach
on 11/09/2009 		The manual says the head, in the back, has a “4. LINE OUT - This jack provides a signal output suitable for connection to recording and sound reinforcement equipment. Using a 3-conductor stereo cord will provide a quasi-balanced signal and a 2-conductor mono cord will provide an unbalanced signal. When hooking up this line out to my audio interface, the signal will be distorted (ie on a gain setting) if the amp is on a gain setting. This presents a problem. I want to record a clean tone either direct to my interface or routed through the amp to the interface while at the same time recording with a mic to the cabinent a distorted tone. Recording a clean signal allows for flexibility later with amp modelers/simulators, etc... However, the line out on this head will not seem to output a clean tone if the amp is on a distorted channel? Is there some way to get both at the same time with only one head and one cabinet? One workaround I have used is to buy a Y-cable female mono 1/4” to two male mono 1/4” splitter, YPP-111 by livewire (that brings up another question, if I plug a 1/4” guitar cord from my guitar into the female end on this splitter, then go out to the interface with one of the males and to the amp with the other, does this splitter need to be guitar cable, patch cable, speaker cable, etc? It would seem like it could cause a bottleneck type problem if its the wrong type of cable used in the splitter?). The manual also shows an inpute for “5. SEND - Connect cord to external equipment at this output jack. This unbalanced jack provides a prereverb signal output from the preamp and may be used to drive effects units in an effects loop, to drive additional amplifiers in a multi-amp set up or as a preamp send to a mixing console.” So maybe I should go from the send in the effects loop straight to my audio interface? Would that be a fix for the problem of recording both a clean track and a distorted track at once? The manual says the send provides a
zach
on 11/09/2009 		That manual says the head comes with: A choice of two pre-amplifiers in the drive channel, using a 12AX7 / ECC83 tube for its well known warmth of tone. • 100 watts of power. I took out the original 12AX7 tube and replaced it with a new and compatible Sovtek 12ax7 tube. When I removed the original tube and then tried to amp with no tube in the socket, the crackling sound was still present on the clean setting, as well as on both distortion settings...(i am not sure how the yellow gain setting could work without a tube in the preamp's socket?). I used dexoit's DN5 and GN5 on most all of the board inside the amp. The pots seem to have a little hole on top of them that would allow for the Deoxit to get inside. I sprayed into that. I also tried to spray up into the pots from where the 3 tracers connect to the board, but am not sure I hit the right angle. I guess I could try to get some of the Deoxit to pile up in a bowl and then drip it into the pots when they are upside down....). The dexoit didn't seem to fix anything. This amp has an Fx loop. I tried pluggin a cord into the in and out of the Fx loop to see if that fixed the problems that can occur with a bad connection in the bypass part of the effect loops. No improvement. I inspected the top of the board. I have yet to try to inspect the bottom of the board (where I believe the solder joints reside) because I am afraid of electricity type safety hazards, even when the amp is unplugged because I read that the amp can retain a powerful charge even when unplugged for quite some time. To look under the board at the solder points (which I believe is how one fixes “cold solder” joint problems, which can be associated with crackling) I think I would need to unscrew about 6 screws on the board. I don't know if I should disconnect the transformer or what before doing that and whether doing so would be insufficient to ensure proper safety. I have one electric guitar, a Fender Strat '94, american made...
zach
on 11/09/2009 		Hi, I have a Performer 1000 guitar amp by Fender (it is my understanding that the Roc Pro 1000 by Fender is just a rebranded version of the Performer 1000, there is a 650 version too, the following schematic is dated as early 1990's and they didn't even sell the Roc Pro till late 1990's, I believe): http://www.fender.com/support /amp_schematics/pdfs/Roc_Pro_1000_Schema tic_B12.pdf This amp head is a hybrid. I have ve a 412 fender cabinet that says it is “stereo ready” but I have eliminated the cabinet as a cause because I get the crackling when I go from the line out in the back of the head to my audio interface, when no speaker is plugged in. I am not sure exactly what being a hybrid means (for instance, when on the clean setting, does the tube preamp come into play at all, or is that only a consideration when on the yellow gain setting- I believe the red gain is solid state)? http://www.manualnguide.com/dl/ 3637/ also, here is the longer Reference and Owner's Manual: http://reviews.harmony-central.c om/reviews/Guitar+Amp/product/Fender/Roc +Pro+1000+Head/10/1 Looking at the board I can see that CR56 and CR 57 and r119 and r118 on the board seem to have some issues with regard to having discoloration around them, like reddish brown or something, nothing too severe looking but noticeably different from the other items on the board. Also, the transformer looks like it has some dried maple syrup looking stuff spilling out just a little past the copper band around it.



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