Help with troubleshooting slo 50 robrobinette.com build

[Canezila]

This is a hard one to write.  I had an amazing, perfect 50watt slo nakid build.  I have specs on an older post and can find them if it helps... But I am afraid it won't get that far as to be needed....

Today, I took my working amp, that has the Metropoulos fx loop.  I wanted to see how it would sound if I plugged into a bass preamp and sent the output into the return.   But I am not using my guitar speakers, instead, I am using a bass cab from a former fender rumble 75 combo. The electronics were fried and pulled from the amp.
I plugged in a speaker cable from my guitar amp and hooked up the speaker cab. When I started playing, it wasn't loud.  I wasn't sure if I had something misconfigured within my preamp.  I tried a few things with the preamp.... But then.... I started smelling something.  Then the amp shut off. My fuse blew on my amp. The power amp and OT were very hot. The smell was coming from that and one of the tubes.  It would hum or lower buzz for 2 seconds and turn off.  Tried a different set of tubes (also different type because I don't have a replacement. So I pulled the 6L6GCM-STR and put in el34). The hum and buzz went away but it still shut off after 2 seconds.

I pulled the power tubes and I get voltage at the HT before the bridge diodes but it still shuts off after 2 seconds.

Could a faulty speaker have blown the fender rumble 75 and then did something bad to my pt or ot?  Can a speaker actually do that?

Did I ruin everything by having a total mismatch of ohms out of my ot?

Could my bridge diodes have gone bad?

I will be going through the troubleshooting guide on robrobinette.com website later this evening.  I was wondering if there is no hope and my pt and ot could be fried?

[mresistor]

To clarify.  You inserted a bass preamp into the effects loop return (used your power amp section of the naked slo)  then plugged your amps speaker output into a repurposed Rumble 75 cabinet's 12" speaker and played guitar into the bass preamp.

If so what is the identity of the bass preamp?  What is the impedance of the bass speaker? The amp is 8 ohm output and BF OTs can usually tolerate 1 step up or down in impedance as long as you don't run the amp wide open, so to speak. 


So your amp is this https://robrobinette.com/How_the_Saldano_SLO-100_Works.htm#SLO-Nakid_6L6  with this installed during the build https://store.metropoulos.net/products/zero-loss-fx-loop-kit

[mresistor]

Did you test the Rumble speaker to make sure it was good before you did this?

[Canezila]

Did you test the Rumble speaker to make sure it was good before you did this?

No, I didn't test the bass speaker.  Thinking back, I wish I had.  I will search how to check a speaker. I will report back

[mresistor]

Probably need to check the impedance of the output transformer. Pull power tubes measure the ohms of each plate lead to center lead and measure the output lead to ground resistance.


It kind of sounds like there was a short on the output and could have damaged the OT.


Guess a tube could have shorted out too.

[mresistor]

How did you connect to the speaker?  I ask because on that amp if I'm not mistaken there is no speaker jack it seems to be wired internally.

[Canezila]

Yes you are correct.  I crimped a blade onto each speaker wire and then inserted the blade from the speaker cable /amp into the slide connection that runs into the speaker cabinet.  Black to red. White to white.

[Canezila]

Fender rumble 75 speaker measures 7.20 ohms.

With power tubes out and no power, I measured a cable connected to the output of the amp.  0.81 ohms.

Next, I will follow your directions as you listed.  Maybe it's the OT by this super low ohms reading?

[mresistor]

the outputs are all super low ohms normally the input is higher  like for example 120 ohms per side from center or maybe higher.  They was almost always not be prefectly balanced.

[Canezila]

Probably need to check the impedance of the output transformer. Pull power tubes measure the ohms of each plate lead to center lead and measure the output lead to ground resistance.


It kind of sounds like there was a short on the output and could have damaged the OT.


Guess a tube could have shorted out too.

Blue /plate and OT CT = 103.50 ohms
Brown /plate and OT CT = 84.80 ohms
OT output and chassis ground = 0.66ohms

[mresistor]

It doesn't appear to me that your OT is bad, although it is a tad unbalanced. But I don't know what your OT is and if that is low readings for it or not.
  Did you check the rectifier diodes? 
So it's blowing the fuse after 2 seconds being turned on with no power tubes installed.
Do you have any pictures you can post up of the innards?


Anyone else want to chime in ?

[mresistor]

I'd check the filter caps. Something is shorting but not a dead short or the fuse would blow immediately. 

[Canezila]

I removed my diodes within the bridge.  HV is removed from the board and at least that should allow the power transformer to power up.  Still failed.

I unsoldered the b+ going to my metropolis fx loop and then pulled the preamp tubes. I was hoping something was shorting out there. Still no luck.

I am thinking my PT is shorting out.  There is no load now and it still fails.

Unless I am way off, I will replace the PT and look into putting fuses at the HV lines and possibly other places, too.  I remember reading a layout on a Doug Hoffman's layout that had a number of fail safes.  When I put my new pt in, I will be smarter and try to protect things with additional fuses.

[Willabe]

You have to test the PT by itself.

You have to be extremely careful doing this!!!!  You need to secure the secondary wires so they don't move when you put the ACV on the PT's primary!

Disconnect ALL the PT AC secondaries.

When you have the secondary wires disconnected and they are safely secured, use alligator clips on your meters leads to measure each secondary wind pair. Put the gator clips on the wires BEFORE you turn on/put the AC on the PT's primary. 

Right these all down; Measure the B+; 1 leg to CT, then the other leg to CT, then out side leg to out side leg. Do the same with the heater wind and then with any other winds the PT has. 

[GlideOn]

Sucks to hear you're having trouble with your build

Recently went through virtually same thing with a build; mine was from inadvertently grounding the Screen/Ultralinear tap incorrectly, thinking it was a transformer shield wire instead of leaving it unconnected and it did indeed damage the Output Transformer enough to produce no signal. Annoyingly, the effect was intermittent and I chased my tail and pulled my hair out a few times.

Fortunately I had a tech I've known for over 15 years confirm this after about 20 minutes at his bench in a well-lit room with tons of tools, beefy multimeter and oscilloscope. He figured out in a few minutes what took me days with no avail.

I guess what I'm saying is: don't be afraid to get professional help. Take to a reputable tech. I promise you they love challenges like these to break up the monotony from repairing Behringers and do fair work, charge a fair rate. Bite the bullet, save some sanity and learn a lesson that sometimes we try to do things that for whatever reason are a little beyond us in that moment.