Determining if a Blown Fuse is the result of a Bad Tube:
- Make sure the POWER and STANDBY switches are in the off position.
- Remove all the POWER and RECTIFIER tubes ** (See Note 2 below).
- Inspect the tube sockets - make sure there are NO burn marks (any burn marks must be removed before the amp will work).
- If the amp has a Rectifier Select switch, put it in the SILICON DIODES position.
- Install a new fuse of the proper rating. (It is OK to use a fuse of slightly LOWER rating. It is NOT OK to use one of HIGHER rating!).
- Amps featuring Multi-Watt selector switches should be set to full power to make sure that all the tube sockets are "live".
- Push the POWER switch up (on). Does the pilot light come on and stay on?
- If YES, then the amp is likely okay, and you simply have a bad tube! (If NO, and the fuse has blown again. The amp probably has a bad SILICON DIODE. This problem, although minor, must be fixed by a tech).
If you went through this test and it suggests a bad tube, continue on to locate the bad one:
Testing Rectifier Tubes (5U4, 5AR4, 5Y3):
- Start with the POWER and STANDBY switches down. Put the Rectifier Select switch into Vacuum Tube Rectifier position.
- Load either one of the Rectifier tubes into either one of the Rectifier tube sockets. Be careful not to put a Rectifier tube in a power tube socket! **(See Note 2 below).
- While observing the Rectifier Tube, give the amp a quick (1-sec) burst of power with the POWER switch. Make sure there are no “lightning” flashes. If not, place and leave the POWER switch up (On). If the amp remains lit, that Rectifier Tube is OK. Repeat this test with the other Rectifier Tube(s). “Lightning” (arcing) or amp shutdown (blown fuse) at this stage indicates a bad Rectifier Tube.
If the Rectifier Tubes are OK, move on to testing the Power Tubes.
Testing Power Tubes (6L6, EL34, 6BQ5, EL84, 6V6, KT88):
- Start with the POWER and STANDBY switches down (Off). Load in one Power Tube (any) ** (See Note 2 below).
- While observing the Power Tubes, push the POWER switch UP (On), let the tubes warm up for about 30 seconds (3 to 5 Minutes for KT88s), then give the amp a quick burst from the STANDBY switch. Make sure there are no “lightning” flashes from the tube. If you saw no “lightning”, then push the STANDBY switch up (On) and leave it there. If the amp remains stable and powered up, that tube is OK. Repeat this test as you add in Power Tubes, one at a time. If / when you get to a bad tube, it will visibly “flash” and/or blow the fuse again. * (See Note 1 below).
* NOTE 1: on almost all Mesa amps that have four Power Tubes, sockets 1 & 4 should contain a matched pair, and sockets 2 & 3 should contain another matched pair. Use the “color word” (GRN, YEL, RED, etc…) on the base of the tube as the matching indicator. (One exception is the ROAD KING amp, in which sockets 1 & 2, 3 & 4, 5 & 6 paired). Refer to Tube Replacement Chart on left/inside of cabinet if necessary.
** NOTE 2: Power & Rectifier Tube bases have 8 metal pins, and a plastic “guide pin”, which assure proper alignment of the tube in its socket. Do NOT twist or rotate tubes when removing. When re-installing, do NOT force a tube into its socket - when aligned correctly, it will easily slide into the socket. Power & Rectifier Tube socket positions have thin, flexible metal “clips” designed to grasp the plastic base of the tubes. Spread these out of the way before removing a tube. Before re-inserting a tube, squeeze the clips so they will apply friction on the tube base as it goes into its socket.
We’ve got a troubleshooting video that is worth a view.
If you’re not comfortable replacing tubes yourself or should the steps above not resolve your issue, you could certainly take your amplifier to an authorized repair center. Click here to locate the closest.
If taking an amplifier still under warranty to an authorized service center, please provide them a copy of your sales receipt, so that any warranty service provided can be claimed.
You're also welcome to make arrangements to send your amplifier to our shop for service. Please contact us for an Estimate, Return Authorization Number, and Shipping Instructions. We will need to know what issue your amp is exhibiting or what service you wish to have done, what model you have, its serial number, and your contact information.