Did I blow my OT?

[Ricky Rockhardo]

So, a few days ago I decided to plug in two cabs into my Mojotone 800 clone. One is a 16ohm 4x12 and the other is a 8 ohm 2x12. I set the ohm selector to 4 ohms and played for about 10 minutes at fairly low volume.

Today I plugged into the amp and I get no sound. The preamp and power tubes appear to be glowing and in particular the power tubes seem to respond (glow more) to my strumming. I checked the HT fuses and they seem fine.

Did I blow my OT? Are there any other easy checks I can do?

I have a local amp tech, but I wanted to see if I should go ahead and buy an OT before I take the amp to him.

Thanks!

 

[skoora]

In response to your question….did you take your blindfold off before, or after you realized it wasn’t a banana?

But seriously…
Setting the amp to 4ohms should have been safe with the load you fed the amp to. By searching a calculation it would have been 5.33 Ohms approximately. Maybe some amp gurus here can tell you where to test voltage to check the OT. Open up the amp and check for any blackened or damaged components. If you’ve never done this before, probably just cut to going to the tech. If it’s an OT they can probably order you one at the same price as you anyway and techs don’t usually put a profit margin on parts ordered.

Edit…try another HT fuse all the same. They can be toast but still look good.

 

[311boogieman]

That sucks Ricky.
Do you have a multimeter?
I'm with @skoora though,
Take it to a tech, first step involves getting in there and running some tests.
Edit:

tests

 

[Rocker68]

1.
In response to your question….did you take your blindfold off before, or after you realized it wasn’t a banana?
That is some funny shat ,,,,,,
2.
Test your fuse ( or your banana ) , OUT OF CIRCUIT with KNOWN good batteries in your
Dmm .
3.
If your impedance selector switch has been dormant on the 4 ohm selection , work the switch through all locations slowly with some Deoxit D5 sprayed into the contact areas . Obviously powered down and unplugged.
4.
Try these steps before you call a tech or get out your wallet. Tell us how it works out .
5.
If you are gifted , use BOTH hands .
( with your banana )
6.
YMMV .

 

[Ricky Rockhardo]

So, I replaced the fuses and still no sound.

I do hear a buzz/hum around the power tubes when I take the amp off standby. For lack of a better term (and this hardcore ages me), it sounds like Space Invaders.

I pulled the chasis out and everything looks fine (no obvious sign of burnt components), so it might be tech time. I’ve been wanting to get an FX loop installed, so she was destined for the bench anyway.

 

[Ricky Rockhardo]

1.
In response to your question….did you take your blindfold off before, or after you realized it wasn’t a banana?
That is some funny shat ,,,,,,
2.
Test your fuse ( or your banana ) , OUT OF CIRCUIT with KNOWN good batteries in your
Dmm .
3.
If your impedance selector switch has been dormant on the 4 ohm selection , work the switch through all locations slowly with some Deoxit D5 sprayed into the contact areas . Obviously powered down and unplugged.
4.
Try these steps before you call a tech or get out your wallet. Tell us how it works out .
5.
If you are gifted , use BOTH hands .
( with your banana )
6.
YMMV .

I wish I were that gifted

 

[jasonP]

The ohms miss match really shouldn't have cause a issue of blowing an OT, however I'm not saying that it didn't happen. This amp doesn't have a shorting jack where you have to be plugged into the jack closest to the impedance selector for only one cabinet? You have made sure your speaker cable is good and connections are good to the cabinet?

 

[Racerxrated]

Your OT should be fine. The amp should be seeing 5.33 ohms; so the 4 ohm setting is good. There's a very good chance that one or more of your tubes are bad, even though they are glowing. If you have a tube tester, I'd test each of them. If you don't, then take other known good pre and power tubes and swap one by one. My guess is one or more of your pre tubes are shot, even though they are still lighting up.
Probably V1 pre.

 

[skoora]

I take it you get no hiss from the speakers?

 

[Ricky Rockhardo]

I take it you get no hiss from the speakers?

No hiss at all.

And yeah, I’ll check the preamp tubes. In the past I had a loose socket in the v1 that I readjusted so I guess it could have come loose again.

Thanks guys!!

 

[Ricky Rockhardo]

The ohms miss match really shouldn't have cause a issue of blowing an OT, however I'm not saying that it didn't happen. This amp doesn't have a shorting jack where you have to be plugged into the jack closest to the impedance selector for only one cabinet? You have made sure your speaker cable is good and connections are good to the cabinet?

Ah! So, typically I do the plug into the jack closest to the impedance selector bc there is a notable volume drop if I just use the other jack. When I used both my cabs I used both jacks (I didn’t daisy chain the cabs).

 

[jasonP]

Ah! So, typically I do the plug into the jack closest to the impedance selector bc there is a notable volume drop if I just use the other jack. When I used both my cabs I used both jacks (I didn’t daisy chain the cabs).

There should be very little to no sound out of the amp if the speaker jack is wired in that way and you are plugged into the jack farther away from the impedance selector, typically Marshalls aren't. A gut shot would help determine how they are wired.

 

[jasonP]

https://cdn.shopify.com/s/files/1/0604/9615/0624/files/British_800_WD.pdf?v=1750263792

Judging by the layout provided by mojotone it looks like the output jacks are wired in parallel and there should be no difference in where you plug the cabinet...