The V sounded awesome then poof.

I was jamming along enjoying the tone of the Mark V while it was in CH3 Mark IV voice. Could this amp get any better than this? Then it suddenly cut out and back up to volume level. I knew right away that is not good so I shut it down after looking at the power tubes, all good there, nothing red plating, but the odor that shortly filled the room was not a good sign. I had two guesses what the smell is, first thought it smelled like burning FR4 delaminating due to overheat. Considering it cut out and back on what else could it be but a carbon comp resistor with a resin based body. Pulled the amp chassis but could not see any visual indication of a problem. Well it appears the 1K on V8 decided to swell in the center and split completely from end to end out of sight. It happens. Now I suspect the tubes may have been cause for the resistor, but perhaps not. I have blown many tubes in this amp, seemed like a normalcy

Why is it when I have Mesa Tubes in this amp it dies? Go figure. Resistor may have been weakened from blowing up Mesa tubes. Surprised it lasted this long without issue with SED =C=. Time to order some Resistors. I think I will go with 5W metal film, flame proof type suitable for high voltage. :| This one amp has been problematic since I got it. Never had an issue with the MKIII, MKIV or the RA100. Does not look to difficult to replace so I will do both resistors.

bandit2013 said:

I was jamming along enjoying the tone of the Mark V while it was in CH3 Mark IV voice. Could this amp get any better than this? Then it suddenly cut out and back up to volume level. I knew right away that is not good so I shut it down after looking at the power tubes, all good there, nothing red plating, but the odor that shortly filled the room was not a good sign. I had two guesses what the smell is, first thought it smelled like burning FR4 delaminating due to overheat. Considering it cut out and back on what else could it be but a carbon comp resistor with a resin based body. Pulled the amp chassis but could not see any visual indication of a problem. Well it appears the 1K on V8 decided to swell in the center and split completely from end to end out of sight. It happens. Now I suspect the tubes may have been cause for the resistor, but perhaps not. I have blown many tubes in this amp, seemed like a normalcy

Why is it when I have Mesa Tubes in this amp it dies? Go figure. Resistor may have been weakened from blowing up Mesa tubes. Surprised it lasted this long without issue with SED =C=. Time to order some Resistors. I think I will go with 5W metal film, flame proof type suitable for high voltage. :| This one amp has been problematic since I got it. Never had an issue with the MKIII, MKIV or the RA100. Does not look to difficult to replace so I will do both resistors.

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Have you ever checked the plate current (bias) in your MKV? In mine the Class A sockets run hot, IIRC around 70% max plate dissipation with Mesa GRN STR-440's. I have to be very careful with the tubes I run in those spots.

FWIW- I've had a power tube failure take out a screen grid resistor in my Roadster once. Same scenario as you with the MKV. Replaced it with the original value and it's been fine since (crosses fingers......).

Dom

I have been lazy, I should get a tube meter to check the bias on the inner tubes. This is the first time I had a tube related issue in 90W mode. Typical red plate problems were associated with 45W mode in CH3 in reduced voltage (variac mode). Perhaps after cooking 4 sets of tubes, followed by loss of JJ 6CA7 (probably should not have used those) the lead tube suffered the most. Carbon composition are prone to moisture absorption when the cool down. Not sure how many heat cycles they can endure before the element gets fractures. While I am replacing the resistors I will check the others before I install the tubes again. I guess it was just a matter of time before something was to open up. I did find some 1K, 5%, 3W resistors, metal oxide film, flame proof and are surge rated. Max voltage is 700Vdc so they should be fine. I have used these in many other products as ballast resistors. You can literally cook them until the solder reflows before they fail. Will see how they hold up, hopefully there will be no additional noise by the material change. I did look for military grade resistors but could not find any in the wattage I was looking for. I got plenty of them just in case I get a repeat as well as the 470 ohm resistors for the outer tubes. There was not circuit board damage or any other noticeable problems. I will have to do the repair at work since my solder iron at home is not as good.

May as well check the impedance of the OT primary. I do recall there were two instances I had changed tubes and forgot to reconnect the speaker load, so that may have also contributed. Not sure what amp I would prefer to work on, ideal would be none. If it was the Mark IV it would be easier since the power tubes are basically hand wired compared to the PCB on the Mark V. Hopefully I can repair it when the parts come in and be back in action.

All Fixed and back up and sounding Awesome once again.

The resistors I ordered from Digikey arrived Friday afternoon. Repair went without a hitch. I was concerned that the leads on the resistors would have been bent over on the bottom side of the PCB which would have complicated the repair by requiring removal of the PCB. :| In order to remove the PCB, a few of the wires would have to be removed, I was spared the extra task since the leads were straight and not secured with a bent lead. :mrgreen: Easy repair. Before proceeding although not necessary, I checked for stored energy in the large power supply capacitors. Each had less than a volt. I did not leave the stand by switch off after removing the amp chassis. If you do that, there are bleed resistors that will eventually drain the stored energy of the large supply capacitors. If left in stand by, only two of the capacitors will be discharged and the rest may or may not be drained. :!: Best way to reduced stored energy of the high voltage capacitors is to leave the amp out of stand-by after disconnecting from the AC power. Note: the grid resistors are connected to the high voltage supply circuit so there could be stored energy present. Before attempting repair, :!: check for stored energy on the DC supply. If more than a volt, leave the stand by switch off for at least and hour and recheck. You definitely do not want to be the discharge path for the capacitors. :twisted: You may survive the shock but tissue damage would definitely occur, or it could kill you depending on the path the current takes. Also helps if you have a schematic of the amp. If you do not, one end of the resistor connects to the tube socket on pin 4 (trace is visible on the side of the PCB you will be working on.) The other end connects to the 445VDC rail. Use am multi-meter to check for DC voltage on the resistors. Note, Not all of the capacitors that store energy are axial ( leads accessible at both ends). Be safe and leave the stand by switch off a few hours before attempting the repair. :!: This is not for a Novice, even though the repair itself is easy, if you do not have good soldering skills, do not attempt! I have been designing, building, conducting forensic studies, Failure Mode analysis, Forced failure evaluation, etc... for 24 years as a professional Electrical Design Engineer. I also have disposal of the proper equipment that most home owners would not have, plus the experience of working on live circuits. Safely first. Your amp will not sound good to you if you are killed by it. Take it from SPOCK, live long and prosper, have someone else with the proper skills to do the repair for you. Done over expressing the potential danger of poking around a high voltage appliance. 8)

Below are pictures of the repair and of the resistors that were removed. Note: there is a bend on the leads that forms a means as a stand off for the resistors. You do not want them resting against the PCB. They will dissipate heat so try to bend the leads of the new parts to obtain the same stand off height. Below the bend, trim the lead such that 1/8 inch remains. Removal of the multi-wire ribbon cable will need to be removed either entirely or on one end and secured to improve work area.

**** right that this is not for a novice like me - thanks for the insight :shock: :shock: :shock:

Any noticable changes in sound or power tube behaviour? Interesting you have had issues with tube reliability that I have not had at all with my MKV after +4 years .. could this cure that?

The amp sounds the same. The resistor issue was just prone to happen eventually. I would suspect may have been weakened early on due to blowing up the Mesa STR440, followed by a complete quad of Mesa STR420, and then the Rectifier tube. Typical was the other tubes getting close to red plating. Would not have noticed unless I was tube rolling with the amp chassis removed and inverted on top of the speaker cabinet (you have full view of the power tubes). Only tubes I have ever used that did not exhibit this issue are SED 6L6GC, TAD 6L6GC-STR or the Gold Lion KT77. Everything else will heat up the plate seams (new tubes or old). Basic issue is the hot bias of the class A circuit. When a grid resistor fails typical symptom would be considerable change in tone and not being consistent. Usual outcome may be damage to OT, Power supply, etc.... Still should get a bias meter just for kicks. Most of all, the SED, TAD and Gold Lion tubes were specifically ordered for a hot bias. I have yet to cause failure of the three tubes listed in the Mark V. Everything else seems to suffer a short life span. Go figure. All that matters is the amp is back in service.

Aside from my warning on the lethal potential that may be present, regardless if the standby switch is left off while the amp is not powered, check for voltages present before attempting. Other than that, on the Mark V power tube board, the copper traces are not very large in width (uncertain of the mass of copper that is out of view). Proper soldering skills for repair and re-soldering should be essential to prevent damage to the PCB. Trick is to reflow the solder on the lead and pad of the resistor (pad is the surface of copper surrounding the hole the lead fits into). Start at one end of the resistor and rewet the solder joint, keep the heat on and carefully lift the end of the resistor out. Too much head will lift the copper from the PCB or possibly cause the copper barrel to be removed with the resistor lead (not good). It should not take any force to remove. If you think your skills are not up to par, cut the resistor body out by clipping the leads at the edge of the resistor body. Removal of the leads will be easier. Do not worry about the solder remaining in the hole until you are ready to install new components. Solder wick (flat woven copper fabric with flux ) should be used to remove the solder from the holes. Place an end of the solder wick onto the pad and put the solder iron tip on top of it. Watch for solder to flow into the wick. You can also excite the wick process with solder on the end of the iron while heating the pad and wick. Do not push down on the wick with force as this will cause lifting of the copper pad surrounding the hole. There will be black residue left behind from the solder removal process. Use a Q-tip saturated with acetone to remove the flux residue. Repeat until both holes are cleared of solder and cleaned. Bend the leads of the new part, you should place a standoff bend in the lead so the body of the resistor does not touch the PCB.
insert the resistor into the hole and solder it in place. Residual flux should be cleaned with the acetone on the cotton swab.

If you accidentally remove the copper barrel from the PCB, you will need to solder the resistor pads on both sides of the PCB. In order to do that, you will need to remove the screws that hold it in place, you may also need to unsolder some of the wires on the PCB to gain access to the other side of the PCB. Mark them with tape and remember where they go. Take a picture if necessary so you can return the wires to their original place. Note there are some wires with the same color, Mark each one differently and also mark where they go on the PCB.

Additional notice: Be careful not to burn off the insulation of the wires while working on the repair. For a job like this with limited space, I will not use the typical solder irons you can find at Radio shack or Lowes, Home depot or craft/hobby shop. I have access to a professional solder / re-work station commonly used in the PCB assembly industry typically used for hand assembly of surface mount and though hole PCB. I am sure it could be done with a good solder iron, proper tip selection is important, do not use a sharp point tip, solder cup tip will be the best for this type of repair or a rounded blunt tip would work too. Sharp point tips may cause damage to the copper pads and do not spread the heat where it is needed.