Amp cuts out when using 8 ohm tap and 8 ohm load, ok@16-16

[joitheboat]

I've been using my head with the 16 ohm tap as long as I've had it, with no issues.

I just bought a 4x10 cab that has a 16 ohm input and an 8 ohm input. The amp works fine with the 16 ohm, but cuts out very soon after starting to strum my guitar when I connect the 8 ohm output tap to the 8 ohm input on my new cab.

I popped open my cabinet to ensure there wasn't a wiring issue. The 16 ohm jack is fires up one pair of the 8 ohm speakers, wired in series (so a 16 ohm load).

The 8 ohm jack is wired so that the left speakers are wired in series (16 ohm) and then paralleled to the other series pair (16 ohm)... so and 8 ohm load.

Obviously, I'd highly prefer to use all 4 of my webers on 8 ohm, rather than just 2 of them at 16 ohm.

Any ideas what could be causing the amp to cut out when using the 8 ohm tap?

The previous owner always used the 4 speaker (8 ohm) input and had no issues. Since the 8 ohm uses two more speakers than the 16 ohm, I figured I'd do what I could to 'test' them for a short or whatever. I disconnected the speakers and tested each with an ohm meter across the terminals- all 4 tested fine (meaning the two alnico's both showed the same resistance and the 2 ferromax's showed the same resistance, so there doesn't seem to be any obvious problem with the speakers that could be causing too high of a load.

My speaker cable is good (obviously, it works on the 16 ohm) and is definitely not introducing any extra impedance. I built it myself out of doubled up heavy gauge speaker wire and quality 1/4" plugs.

I've checked to make sure all the speaker terminal wires and jacks on the cab are making a good connection and are clean. One of the terminals on one of the speakers was painted and the connection was on that spade- I moved it to the bare metal one. I haven't tested after that yet, but again, the previous owner had no issues, so I'm not holding much hope that that was the problem.


More info:

The head is a 20 watt (egnater rebel)
The 4 speakers are 8 ohm, 2 rated at 30 watt/each, 2 rated at 25 watt/each.

[xtian]

Do I understand, that there are separate output jacks for 8 and 16 ohms? Check to see if the 16-ohm jack, the one that you said works properly, is a switching jack. The switching terminals may be dirty, and not making a good connection when there is no plug inserted. You'll have to open up the amp to do this.

[Structo]

Could have been that speaker wiring being intermittent.

[joitheboat]

Thats correct. The amp actually has 3 outputs: 16, 8 or 4.

Looks like they are switching jacks. Detail of the output jacks from the schematic attached.

[Structo]

Are the jacks soldered to a pcb?

Over time jacks that get more use (say speaker jack) the mounting tabs and where the contacts connect to the board, can fracture the solder joint.
So check out input jacks, FX loop jacks and Speaker Out jacks.

First thing I would do is get that bias voltage straightened out.

Then go back from the power tubes sockets and look for bad joints, bad jack connectors.

[joitheboat]

Looks like they are soldered to a pcb based on this picture I found of this amp with the chassis open, haven't cracked it open yet. I'll hit solder joints.

Get the bias voltage straightened out? Original tubes, no adjustments have been made.

[Firestorm]

Could it be the switching jack in the cab?

[joitheboat]

Im not seeing anything awkward with the jacks. The ground or ring side is operating normally. Im going to check the tip side in a minute... was a little harder to get to.

I did go ahead and adjust the bias on both sets of output tubes... they had drifted a bit... like 5 to 7 mv. I have them both within half a mv of spec now. I have no idea if this says anything, but the bias reading were consistent when tested with the 8 ohm out to 8 ohm load and the 16 ohm out to 16 ohm load.

[joitheboat]

Found nothing that seemed strange on the jacks after looking more deeply. Maybe it was a fluke.

We'll see how it goes at practice tonight. I only had it happen once and immediately shut down and switched to 16 ohm out to 16 ohm load because the cabinet was new to me and I wasn't sure it was wired correctly (couldn't wrap my head around how this one 4 speaker cab was pulling off 8, 16 and 32 ohms, coupled with the fact that the guy I bought it from kept calling the 32 ohm jack a 3.2 ohm jack, when it was clearly labelled 32. Since then, I've taken the cabinet apart, traced all the wires, checked everything with my multimeter and found that everything was labelled and wired the same- and that the way they pulled off 16 ohms from 4 8 ohm speakers was by keeping 2 of them unused when using the 16 ohm jack.

[joitheboat]

Also, one more possibility: My speaker wire has shrink tubing over the plug. It may have extended a little too far and caused the plug to not make a solid connection if the stars aligned just right. I trimmed it back.

I won't be able to plug it in until tonight- so I guess I'll find out if I need to keep digging then.

[M Fowler]

How about posting that schematic. NEVER MIND I FOUND IT on Google.

[joitheboat]

How about posting that schematic.

Sure thing.

[M Fowler]

Some day I'll have compare the schematic Colossal did to this schematic but so much inside these amps it get's nerve racking.

I had to fix the bottom board which meant tearing the whole amp down what a PITA.

Mark

[Guus]

Hi Joitheboat,

Indeed a bad connection.
I suggest you first locate the problem, ie the amp or the speakercab.

When you have no sound with 8 Ohms, switch the amp off, pull the speaker jack, take an ohmmeter and messure the resistance from the cable end jack (tip/sleeve) to the speakercab. (so messure the cable plus speakercab)

When you get about 6 ohms the speakercab is ok and you have to check the amp output. If you get nothing, the problem has to be find in the speakercab or cable.

If you don't have an Ohmmeter you can take a 9 volt battery and make contact between tip and sleeve of the speaker cable jack. If you hear a plop the speaker and cable are ok.


Success

Guss

[tubeswell]

Im not seeing anything awkward with the jacks. The ground or ring side is operating normally. Im going to check the tip side in a minute... was a little harder to get to.

You probably won't be able to tell just from looking at them

Have you checked for DC continuity on all the socket switch poles with (and without) the plug in?