Trouble shooting an amp problem guidelines

[tubenit]

I thought it might be useful to create a thread on amp trouble shooting that we can post in the ARCHIVES.  Hope you guys will post what has been useful to you.

Power up an amp for the 1st time from ARCHIVES:   http://el34world.com/Forum/index.php?topic=2376.0

"Servicing Fender Amps"  from Hoffman's Library of Information

https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm

http://www.geofex.com/ampdbug/ampdebug.htm

http://www.trinityamps.com/ForumGallery/trinity/resources/geofex_tubeampdebug.pdf

http://www.egnateramps.com/TechTalk/Volume4.html

How to trace a circuit with highlighter pen    http://el34world.com/Forum/index.php?topic=17701.0

With respect, Tubenit

[tubenit]

Some questions on my personal checklist based on experience are:

1)  Is the speaker plugged in?  Have I tried a different guitar cord?  Have I tried a different guitar?  Have I tried a
     different speaker?  Have I plugged an effects pedal in "backwards" into an FX loop?  Is the speaker wiring attached
     to the speaker?

2)  Have I checked voltages & are they reasonable for the amp design and tubes used?  This includes heater voltages. 
     IF there is no voltage, have I checked the fuse or fuses?   Were the fuses the correct value?

3)  Have I taken a highlighter pen and compared schematic & layout to my actual build?   I have done this 3 times and
     gone beyond double checking. See reference:  http://el34world.com/Forum/index.php?topic=17701.0

     Have I quadruple checked the input jack wiring?   IF there is a passive FX loop,  have I quadruple checked the wiring
     on the FX jacks?

4)  Have I tried different tubes?  If I am suspecting a bad tube then I change that one first.  If that doesn't work,  I
     change all the preamp tubes. If that doesn't work,  I change the power tubes.  (you can sometimes locate a
     microphonic "bad" tube by gently tapping on the suspected bad tube with a wooden non-conductive chopstick)

5)  Have I measured values of resistors and pots to make sure I have the right values?

6)  Have I checked for continuity?   Also check that the inner wire on shielded wiring isn't accidently grounded to the
     shielding wire mesh.

7)  Have I checked using the "chopstick" method? 

8)  If I've gotten really odd or inconsistent voltage readings, have I checked my voltmeter to make sure there is a good
    battery?

9)  Have I looked to see if there are any loose wire strands touching something they shouldn't be touching

10)  Have I begun isolating the area of the problem?  Be safe.  Use insulated alligator clipped wiring carefully.

       -  if the amp has an FX loop,  I might plug into the return jack of the loop.  If that gives a sound that seems OK,
          then I will look for the problem prior to the FX loop in the preamp
 
       -  if the amp has an FX loop,  I might run the preamp into the FX return jack of an amp that is working well.  That
          might inform me the problem is in the phase invertor and/or power amp section

       -  I may carefully use insulated alligator clipped wiring to jumper past a certain gain stage to try to isolate an area. 
          For example, I might jump from V1a (1st gain stage) directly into the LTPI (phase invertor).  Does that work
          OK?   If so, the problem may be between the V1a gain stage and the phase invertor

       -  I might jumper from the input jack further down the signal chain

       -  Sometimes I have found the problem area when checking voltages carefully.  EX: Using the red probe on my
          voltmeter checking for voltages on phase invertor plates,  I may get a pop or noise thru speaker with amp
          volume on slightly.  Let's assume the phase invertor is an LTPI and is V3 tube.  Then I will check the voltages
          safely on V2-b. Does it make a noise?   IF not, then that may be "the" or one of the problem areas. This has
          not been a foolproof way of diagnosing the problem area but has been helpful in some cases.


11)   Have I checked to see if there are any leaky caps?    (See Hoffman's Library of Information)

12)   If the amp is making noises and/or has low power and it's an old amp, ....... have I checked or replaced the
        electrolytic filter caps and/or cathode bypass caps  ( See Hoffman's Library of Information)  (sometimes
        electrolytic filter caps will have obvious leaking or a bulge/bubble on the side)

13)   If you are having a problem with implementing something like tremolo,  "one tube reverb",  active FX loop,  relay
        switching .........
        have you looked in  ARCHIVES  (which has threads on all of those),  have you done a SEARCH on the forum,  have
        you compared your  "innovative" mod/idea/addition to some proven schematic that is using something similar?

14)  if I have run out of trouble shooting ideas:

      -  do I have voltages to post on the forum
      -  do I have high resolution photos to share of  the chassis build
      -  am I willing to take instruction & then post that I've followed thru with the suggestion or why I haven't tried
         the suggestion (if there is a good reason).   
       - have I also listed what I have already tried?

*   I'll add to this list as I think of other things.  Everything that is mentioned in the list has been part of my experience
     in amp building and trouble shooting some problem where I've made a mistake. 

With respect, Tubenit

[tubenit]

I took the above list of trouble shooting tips ................

And then created a list of amp trouble shooting problems such as:

- won't turn on
- blowing fuses
- won't make noise
- low volume
- intermittent noise or intermittent sound
- odd noises
- excessive hum or buzz
- too much hiss or high end hash
- too much bass or too much treble
- too much gain
- problems with tremolo, reverb, fx loops and relay switching

Each trouble shooting problem has corresponding trouble shooting tips that correspondent to that

With respect, Tubenit

[jjasilli]

Here's Ampeg's Troubleshooting Flowchart from the '60's.

[tubenit]

You need to know how to safely work on an amp to do these.  Don’t attempt any of these if you are not knowledgeable in amp building and trouble shooting.  Instead take you amp to a qualified amp tech.  I am not saying you should trouble shoot in this manner, this is simply how I would approach it.  There are lethal voltages inside a tube amp. 

The numbers listed under each amp “problem” correspond to the trouble shooting technique/procedure that I would use and in the order that I would try things out.  The following "numbers" referenced are in reply #2 of this thread.

Amp won’t turn on

Is the fuse blown?  Is the power cord good?  Is the wall socket providing voltage?  Is the PT getting voltage after the switch is turned on?  Is the PT, on/off switch,  fuse, (& standby wired correctly)
Follow Sluckey’s start up procedure in reply #3 in ARCHIVES thread :
http://el34world.com/Forum/index.php?topic=2376.0

2, & 3

Blowing fuses

Follow Sluckey’s start up procedure in reply #3 in ARCHIVES thread :
http://el34world.com/Forum/index.php?topic=2376.0
Are the tubes known to be good and worked in another amp?

2, 3 & 4

No volume but tubes light up

1, 2, 3, 4, 6, 7, 9, & 10


Low volume ………… OR sound cuts out and low volume with distortion and/or oscillation.

Is an FX pedal plugged in backwards?   Have you tried a different speaker, guitar and guitar cord?  Check and see if the power tubes are redplating. 

2, 3, 4, 5, 6, 7, 9, 10, & 12

Intermittent noise or intermittent sound

If a potentiometer is scratchy/noisey, use an appropriate spray into the pot and/or check for a leaky capacitor going into the pot.
Popping or frying bacon noise can possibly be connected to poor lead dress, bad tubes or bad tube sockets that need to be re-tensioned or replaced.

You may need to figure out whether the noise is "mechanical" from vibration or electrical. One way of doing this is to plug an amp speaker from a separate cabinet into the combo chassis head speaker jack if it's a combo amp.  If it's an amp head then you can pad below the head (sitting on top of a cab to minimize vibration from the cab back into the head.)  IF your chassis/head is plugged into a separate speaker cab and still makes odd or intermittent noises, then the problem is more likely to be electrical such as a bad tube or bad solder joint.

2, 4, 6, 7, 9, 10 & 11

Odd noises

If you get a squeal at start up follow Sluckey’s start up procedure advice and reverse the OT wires.  You can also disconnect the negative feedback temporarily to see if the squeal goes away which is an indication that the OT wires need to be changed.   

A squeal can also be a sign of a preamp tube going bad.  You can try tapping a chopstick gently against the tube to see if a tube is particularly microphonic.

“Motorboating” noise can be connected to poor lead dress, poor grounding, bad filtering or inadequate de-coupling on the B+ rail.  Replacing the B+ electrolytic filter caps in B+ rail may resolve this.

“Ghost notes” are possibly related to inadequate filtering on the B+ rail especially filtering to the power tube plates and screens.  Increasing filter may help resolve this issue.   (Be sure to try another guitar to rule out your strings pinging on an unlevel neck fret).

2, 7, 3, 4, 9 & 10

Excessive hum

Am I using a known proven layout?   Are the PT and OT at 90 degrees to one another?  Can I lessen the hum by moving the heater wires with a non-conductive tool like a wooden chopstick?  Are the signal wires close to B+ wiring?  Have I checked the grounding to make sure things are grounded well in the correct spots and there are no ground loops.  Hoffman offers an excellent way of grounding an amp in his Library of Information.   http://el34world.com/charts/grounds.htm

4, 3, 7, 10 & 12

Too much hiss or high end hash:

Is this a high gain amp?  Are you using a known proven layout?   Have you tried smoothing caps like on a Dumblish style amp?   Have you tried the “enhance cap”  across the entrance plate resistor in a LTPI?  (do forum search)   Have you tried a snubbing cap like the 47p post LTPI on the Fender Twin 5F8A?   What type of plate resistors are you using (carbon comp, metal film,  Vishay-Dale RN65)?   Have you tried moving the heater wires with a non-conductive stick like a wooden chopstick?   Are you using shielded wiring in appropriate places? 

2, 4 (including trying a lower gain preamp tube like a 5751 or 12AY7)

[tubenit]

Too much bass or too much treble tone:

Decrease the values of coupling caps and/or cathode caps to lessen bass.  For example:  Use 5uf or 2.2uf cathode cap instead of 22uf.  Use a .047 coupling cap instead of .1        Increase values to increase bass.

Problems with tremolo,  relay switching, reverb or active FX loop

Tremolo in ARCHIVES:    http://el34world.com/Forum/index.php?topic=604.0
Relay switching in ARCHIVES:    http://el34world.com/Forum/index.php?topic=14348.0
Reverb in ARCHIVES:  http://el34world.com/Forum/index.php?topic=7957.0
FX loop in ARCHIVES:    http://el34world.com/Forum/index.php?topic=10208.0

Not enough headroom and too much gain

Have you tried a lower gain tube such as a 5751 or 12AY7 in preamp or 12AT7 in the phase invertor ? 
 
Consider adding a voltage divider  (do a search  Look at Hoffman’s “hot switch for Plexi” as an example of a voltage divider or the Marshall JCM 800 http://el34world.com/charts/Schematics/files/marshall/Marshall_JCM800_2204_Lead_Series.pdf)   

Are you using a audio or linear gain pot?  A linear pot will increase the gain more quickly then an audio pot.   
Sometimes too large of a cathode cap or coupling cap can add too much grit and it sort of sounds like too much gain.

[tubenit]

https://www.youtube.com/watch?v=GrVtX0QGNls

How to determine between 60 and 120 cycle hum.