Roaster blows fuse when turning on in bold mode

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dlabrecque
Mark I
Posts: 44
Joined: Sat Oct 24, 2009 12:54 am
Location: Tucson, Arizona, USA

Roaster blows fuse when turning on in bold mode

Post by dlabrecque » Wed Mar 28, 2018 9:33 pm

Turning it on in Spongy mode works fine. Yes, it's set to Standby when I fire it up. But when flipping it to bold to turn it on (the correct switch-on procedure according to the manual), the fuse blows after a few seconds. Ideas?
Dave Labrecque
Becket, Massachusetts

Dual Rectifier Roadster
4x12 Rectifier Slant Front
1x12 Compact Open Back

Boss MS-3 and various pedals
2002 Gibson SG
2006 Fender MIM Stratocaster
2006 Fender MIM Telecaster
2004 Taylor 510CE

bandit2013
Triple Recto
Posts: 3097
Joined: Fri Feb 22, 2013 7:31 am
Location: North Carolina

Re: Roaster blows fuse when turning on in bold mode

Post by bandit2013 » Mon Apr 02, 2018 6:24 am

You may have a faulty rectifier tube. Set all channels to use silicon diode and then remove the two Rectifier tubes and see if that blows a fuse. I am assuming that there is a fault in one of the Rectifier tubes. Even if you are not using them, they are still active and parallel to the silicon diodes. Just an idea but may be something else.

bandit2013
Triple Recto
Posts: 3097
Joined: Fri Feb 22, 2013 7:31 am
Location: North Carolina

Re: Roaster blows fuse when turning on in bold mode

Post by bandit2013 » Mon Apr 02, 2018 6:23 pm

Here is another trick, Remove all of the power tubes (including the rectifiers) and power up in bold. If the fuse goes it may be possible a preamp tube is drawing some current more than expected but that generally should not happen. Give Mesa Tech service a call, even if you have to leave a message, they will call you back.

dlabrecque
Mark I
Posts: 44
Joined: Sat Oct 24, 2009 12:54 am
Location: Tucson, Arizona, USA

Re: Roaster blows fuse when turning on in bold mode

Post by dlabrecque » Tue Apr 03, 2018 11:12 am

Thanks for the tips. I found this in the manual, which talks about power tube issues:

FUSE: This is the A.C.’s (Alternating Current) main fuse and provides protection from outside A .C. fluctuations as well as power tube failure damage. Should the FUSE blow, replace it with the same rating in a Slo-Blo type package. The domestic U.S. version requires a 4 amp Slo-Blo fuse. A power tube short or failure is often the cause of a blown fuse...Follow the cold start procedure mentioned in the STANDBY switch section and watch the power tubes as you flip the STANDBY to the ON position. If a power tube is going bad or is arcing you will see it! Flip the STANDBY switch down immediately and replace the faulty power tube and the FUSE if necessary.

If you see nothing abnormal as you lift the STANDBY switch, it is possible that a power tube shorted temporarily and blew the FUSE. If this is the case it may work again normally. To be safe, you might want to replace just the adjacent tube or all power tubes in the “shotgun” troubleshooting tradition and save the replaced set as spares. Spare fuses are a must for the fabled cord bag along with your spare tubes. Always carry both for they could be worth their weight in gold someday.
Dave Labrecque
Becket, Massachusetts

Dual Rectifier Roadster
4x12 Rectifier Slant Front
1x12 Compact Open Back

Boss MS-3 and various pedals
2002 Gibson SG
2006 Fender MIM Stratocaster
2006 Fender MIM Telecaster
2004 Taylor 510CE

bandit2013
Triple Recto
Posts: 3097
Joined: Fri Feb 22, 2013 7:31 am
Location: North Carolina

Re: Roaster blows fuse when turning on in bold mode

Post by bandit2013 » Tue Apr 03, 2018 6:49 pm

The advanced method if you cannot determine if it is tube related or something else (most likely it is tube related) is to pull power tubes. You can do one at a time if that helps. I generally mark my tubes with an I and an O for 100W amps. I am sure you could use a similar scheme with a 120W amp but with I, M, O. for a quad array, I mark the ones on the outside with an "o" or a dot on the Mesa label or on the plastic base (depends if I am using a black or silver sharpie pen). Inner pair get a line or an "I" just so you can keep the tubes straight if and when you have to pull them or if you want to swap the inner and outer pair if you use the 50W mode more often.

With one power tube removed, no blown fuse, you found the bad tube, the one you removed. Do not leave the amp on for too long or play it with one tube missing as this may cause the other tube of the pair to heat. It is okay to do this for diagnostic purpose. Just a short term to confirm the fuse did not blow. Removing all of the tubes is just a quick way to determine if the cause is something else.

Why it works in spongy mode and not bold, at spongy mode or reduced power mode, the issue may not be apparent until the voltage is increased at full power. There is not much of a voltage drop in spongy mode but enough to retard the current though the tubes as the plate voltage is reduced. You may have a bad tube (hopefully that is all it is.)

dlabrecque
Mark I
Posts: 44
Joined: Sat Oct 24, 2009 12:54 am
Location: Tucson, Arizona, USA

Re: Roaster blows fuse when turning on in bold mode

Post by dlabrecque » Tue Apr 03, 2018 10:49 pm

bandit2013 wrote:The advanced method if you cannot determine if it is tube related or something else (most likely it is tube related) is to pull power tubes. You can do one at a time if that helps. I generally mark my tubes with an I and an O for 100W amps. I am sure you could use a similar scheme with a 120W amp but with I, M, O. for a quad array, I mark the ones on the outside with an "o" or a dot on the Mesa label or on the plastic base (depends if I am using a black or silver sharpie pen). Inner pair get a line or an "I" just so you can keep the tubes straight if and when you have to pull them or if you want to swap the inner and outer pair if you use the 50W mode more often.

With one power tube removed, no blown fuse, you found the bad tube, the one you removed. Do not leave the amp on for too long or play it with one tube missing as this may cause the other tube of the pair to heat. It is okay to do this for diagnostic purpose. Just a short term to confirm the fuse did not blow. Removing all of the tubes is just a quick way to determine if the cause is something else.

Why it works in spongy mode and not bold, at spongy mode or reduced power mode, the issue may not be apparent until the voltage is increased at full power. There is not much of a voltage drop in spongy mode but enough to retard the current though the tubes as the plate voltage is reduced. You may have a bad tube (hopefully that is all it is.)
Cool, thanks for all the info.
Dave Labrecque
Becket, Massachusetts

Dual Rectifier Roadster
4x12 Rectifier Slant Front
1x12 Compact Open Back

Boss MS-3 and various pedals
2002 Gibson SG
2006 Fender MIM Stratocaster
2006 Fender MIM Telecaster
2004 Taylor 510CE

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