Mark V problems

[Chris19]

Hello everyone. It sucks that my first post is a result of problems I've had since day one. I won an older used Mark V head off of eBay l (serial number is mid 400's) so obviously out of warranty. I was pleasantly suprised at how good it looked for an earlier model when I received it. The original owner took extremely good care of it; it looks like a new amp. Channel 1 is great. Channel 2 has been awesome up until lately, and channel 3 has been mostly terrible. Channel 3, and only recently channel 2, have significant fizziness in the high end and muddy flub in the low end. No amount of fiddling can dial any of it out, and I'be become reasonably competent with dialing this thing in. There was one or two times where it sounded phenomenal. Also, I was playing last night with the effects loop toggle engaged just for using the output to control volume better and had a sudden, massive loss of volume. Switched channels and it was still there. Bypassed the loop and volume returned. I swapped v6 and the problem was remedied. Replaced the original just to verify it was the tube and not the amp, and the problem didn't follow. Do tubes cause intermittent problems like these? I thought that once one went bad, it was done for. The fizziness, flubby, farty low end, and general lifelessness are across channels 1 and 2 in all power settings. I swapped the inner and outer pairs, attempting to isolate a possible bad tube in the inner pair in 45 watt mode, but nothing helps. I swapped every preamp tube 1 for 1, but if multiple preamp tubes are bad then I guess that won't help. I have v4-v6 swapped with spares I have around and still have the bad tone problems. Volume problem is fine for now. I'm looking at ordering a new set of output tubes (been wanting to try el-34's anyway) but I have no clue if that'll fix anything. I don't have any 6L6's around. I have a pair of kt-66's but I don't know if they'll run safely with the 6L6's. Any other ideas guys? Any help would be extremely appreciated. Gonna give Mesa a call as well.

 

[Monstercastle]

New tubes are always the first place to start , at least in my experiences. I always keep a full set as spares for all my amps. The set of EL84's in my V:25 went bad in a couple weeks, and Mesa sent me a new set. I've only had my Mark V head for about 3 months (new) and so far so good.

 

[Chris19]

I'm thinking the same, especially since it was in shipping for 5 days, from the east coast to San Diego. Probably got roughed up a bit. Now, to go tube shopping 😎

 

[bandit2013]

Tubes are most likely the cause of the issue. Usually when there is a sudden drop in volume that is a sure sign. The Mark V drives the center pair of tubes in an extended class A mode. There is only one tube that does all the work and the other pulls or pushes the current though the output transformer, in essence it acts like a load. When you are operating the amp through the FX loop, V9 will be the primary class A tube and V8 acts like the load. When you turn off the FX loop (hard bypass), there is a phase shift that occurs (typically with the FX loop active, the amp is out of phase) and the role of V8 and V9 will change. This will hold true for both 45W and 90W mode. What I suggest, move the amp such that you can view the power tubes in operation, If a tube is running too hot it will begin to glow at the seams of the plates (outermost metal structure inside the power tube). In order to see it, you need to be in the dark. The bright yellow glow is the heater element no need to worry about that unless one is very dim (would indicate damaged tube, loss of vacuum). Tubes can easily be damaged from sudden shock or drastic temperature changes or may just be a bad tube. A 6L6 tube will have an odd blue hue dancing around the inner edge of the glass. Loose connections on the pins of the socket or other may cause the tube to shut down suddenly the troublesome tube will not be glowing blue but looks okay otherwise. Note that not all tubes have a blue glow (EL34 or KT77).

As for the KT66, I would not use them unless you can confirm the bias current and applied voltage are suitable. Also the heater supply may not be adequate. The Mark V has a hot bias on the inner pair and a colder bias on the outer pair. I am clueless on what the spec is, so I cannot help you with that. A new set of tubes will probably fix your issue. Do not forget to replace the Rectifier tube as well. If you are going all Mesa brand tubes, I will recommend an SPAX7 for use on V1.

 

[Chris19]

Well the effects loop volume problem is gone. V6 was bad. I went out and picked up some Mesa brand EL-34's and slapped em in, no real difference. While I was there, I fired up a Mark V combo they had on the floor and it was phenomenal, even at lower volume I use at home with a lesser guitar. Doesn't seem right. Anyways, I put back the 6l6's and checked for red plates and loss of blue hue. Nothing obvious was wrong. On a whim I was like "F the neighbors" and opened it up for a few minutes and it just transformed. Channel 3 sounded monstrous. Even with the gain maxed AND a clean boost up front it was super tight and aggressive. Total Mark series tone. Not very surprising giving the nature of tube amps, but should channel 3 be completely unuseable at bedroom volume? This was definitely not the case with the combo version I played at the shop.

 

[bandit2013]

No, All channels will sound good at any level. Usually the amp may sound a bit fizzy if you set the master (using the FXloop) at a low setting especially in the 90W mode since you get more headroom from the outer tubes. There is a difference between the head and the combo in terms of the speaker enclosure. Combo being a 3/4 back cabinet vs a 212 or 412 which are typically sealed enclosures. Not sure what cabinet you are using the head with. The perception of the open cabinet of the combo will seem louder at lower volumes when compared to a sealed cabinet since there is no compression of air by the speaker inside the enclosure, also the sensitivity of the speaker also plays an important role in how loud the amp sounds relative to the volume control. There is a reason why I converted my Mark V from head to combo was to take advantage of open back sound for the clean channel. Also the CH2 and CH3 do not suffer either. I have 2 OS recto 412 cabs to use as well so I can make the Mark V more flexible, either run the combo speaker by itself (8ohm or 4ohm mismatch to drop the top end a bit), couple it with an extension cab (which I do often) or channel it though one or both 412 cabinets (nothing beats the full stack sound!). If you had a combo and head to compare side by side, the head though a 412 will definitely be louder than the single speaker combo. The Mark V does have a dark side to it along with a prominent top end. It will take some getting used too in order to dial in the character of tone you are seeking.

 

[Chris19]

I have a 4x12 Peavey with Peavey "Supreme XL" speakers. I rewired it in parallel for 2x12 operation with an 8 ohm load because the 4 12's in series/parallel (16 ohm) had a bit of a flat, lifelessness to it. Strange, as the cab actually sounds great with my other amps, in either wiring configuration. It does seem more alive and responds to pick hand dynamics better with matched impedance. I'm going to upgrade soon but I can't make a decision on what to get. I'm thinking either a Theile or a Port City 2x12. Anyway, I now strongly suspect that the fault is isolated to channel 3. It's very intermittent though. Earlier I was playing on channel 3 in 90 watt mode, and had another incident of sudden, extreme volume loss. Switched to channel 2, then 1, and the problem was not present. Went back to 3 and the volume has returned, but now there's a low frequency hum that's modulating probably 2-3 Hz. It's only present in 90 and 45 watt mode. It disappears on the 10 watt setting. I turned it off for a little while, then went back, now it's fine again. I suspect an internal component failure, maybe a cap or resistor bit the dust somewhere, or a weak solder joint that got knocked loose during shipping?

 

[bandit2013]

That almost sound like on of your power tubes is shutting down (not red plating or over loading). Is the switch on the back in pentode or triode, Try the other position and see if the problem is persistent. I would definitely watch the tubes in the dark and observe for loss of blue hue. CH1 or CH2 do not use the pentode function and the connections are switched to triode when you channel change. Also in 10 W mode, the two tubes closest to the Rectifier operate with class A using a Cathode bias vs control grid bias. I found that quite interesting while studying the partial schematics. The control grid is disconnected from the bias supply and pulled to ground though a 220k ohm resistor.

If not power tubes, V4, V5 or V6 may be having issues maintaining operation. Also check to see if the LED for the EQ is turning on and off as that may be caused by the cable if you are using the footswitch.

 

[Chris19]

Well I have a bit of an update. It was red plating with el-34's (just v-8). It got worse when switching to 45 watt mode. One thing to note is that I was running a "green" pair in the inner two and a "blue" pair in the outer pair. I swapped the pairs and it stopped. Strange? Also there is a loud low frequency hum only present on channel 3 in 90 and 45 watt modes. The hum disappears in 10 watt OR when switching to pentode. I'm gonna have a tech take a look as swapping all the tubes has done no good. It's not red plating with 6L6's but it still has intermittent, yet significant performance issues in channel 3.

 

[Wolfgar926]

I HOPE YOU REMEMBER! IF USING EL34 TUBES. YOU CAN ONLY RUN THE AMP ON VARIAC MODE ! OR IT GETS TOO HOT !
are you running it at normal setting for 6l6?

 

[Wolfgar926]

Well I have a bit of an update. It was red plating with el-34's (just v-8). It got worse when switching to 45 watt mode. One thing to note is that I was running a "green" pair in the inner two and a "blue" pair in the outer pair. I swapped the pairs and it stopped. Strange? Also there is a loud low frequency hum only present on channel 3 in 90 and 45 watt modes. The hum disappears in 10 watt OR when switching to pentode. I'm gonna have a tech take a look as swapping all the tubes has done no good. It's not red plating with 6L6's but it still has intermittent, yet significant performance issues in channel 3.

were you running your el34 tubes in variac mode? thats the only position your are suppose to run them in ....can run them in normal mode...