Super Twin Reverb blowing fuse

[Justin Thomas]

Someone probably had a blowout sale...
Though I have heard a technical explanation that they wanted full signal passing all the way down to DC. They can always limit it later on, I reckon. But I still think blowout sale.

Justin

[yann]

One side of the reverb tank is reading open. I need to have a look inside to see if this can be fixed. Another thing to add on the bill.

[yann]

Pretty much done with this amp. All electrolytics caps has been replaced, reverb has been fixed (broken wire inside). One of the two speaker has been fixed (thanks to people who helped me in another thread ).
Now I have checked bias and adjust the balance pot to have each side of the push pull stage drawing approx. same current.
I have now the following measures
Vplate = 508 Ia = 25mA which give a static power dissipation of 12.7W per tube. It seems a bit cold for this 180W amp but I heard that it has been design this way.

If some of you do have some experience with this particular amp, I would be happy to hear from you :-)

[g1]

I think that bias should be ok.
The balance pot is usually better to adjust by ear for minimum hum.
Hum balance pot is adjusted for minimum hum with all controls turned up, balance (matching) pot is adjusted for minimum hum with master turned down to zero.

[yann]

I have tested the chassis with a resistive load and could get the full power with a proper sinewave (no crossover distortion).
So I have mounted back the chassis into the cab, plug the thing and did not come up. Checked and find main blown fuse...
Replaced the fuse and the amp worked fine without touching or moving anything. Tested a few power cycles and it seems ok but I don't want to give back the amp that blowed a fuse after remounting.
I am afraid there could be intermittent shortcut somewhere (despite I have checked the thing a few time but I may have missed it)
This one is gonna be difficult to find...

[yann]

Another thing I have noticed is when switch from standby to play I hear a sound (like a "vvvvvmph vvvvvmph"). But it did not blow fuse at this time. Could this be drawing too much current at that time and push the fuse to its limit so it ends blowing after few power cycles ?

[yann]

This thing has again blown the main fuse after 2 good power cycles today.
At the 3rd attempt, when I flip the power switched, just nothing happend. The amp has not been moved at all during these 3 tests.
I will pull the chassis and try some chopstick tests and see what happen on the limiter.

[Pedro Vecino]

Power and standby switches in that amplifier are very exposed. It is common to find them mistreated.
Make sure the fuse is 3A slow (T3A).

[yann]

The standby and power switch looks normal. The fuse I use is supposed to be a slow blow one.
It is marked as T3.15A 250V and looks like this :

[DrGonz78]

Why not spray a bit of contact cleaner into the standby switch? Rock it back and fourth a bunch to make sure it's making good contact inside. I had an amp recently that had a dirty power switch and it made noises as it took the amp at least 5 seconds to act normal. Just a thought out here in left field.

[yann]

I have done several test with chassis out of the cab. These tests has been done with the amp in stanby mode, so no HV applied to the tubes.

Without power tubes inserted, the light bulb limiter is acting as expected. The light dim quickly right after power on. Done several times.

So I run the same test with the 6 power tubes inserted.
When I switched the amp on, the bulb limiter was nearly full brightness for approx. 3sec and then start to dim slowly.
It seems to be a bit long, could this explain why the fuse blow after a couple of power cyles ?

[yann]

I am not sure this amp is really having an issue and I start to suspect that the type of fuses I have been provided is not the right slow blow ones.

These are ESKA fuses (made in germany) and they have several type of fuses according to the datasheet :
- Medium time-lag
- time-lag
- very time-lag
- quick acting
- super quick acting

My supplier was selling these fuses simply as slow blow, but I don't really know if these are medium time-lag, time-lag or even very time-lag (I have received them in plastic bag so no ESKA references available to double check).

I will source the 3 types of slow-blow ones from another supplier where you can order the exact ESKA reference and fit each type of fuse in the amp (starting from the medium time-lag one) and leave in the amp the one that will survive at least 10-15 cold power cycles (for now I never went over 5).

[yann]

Ok, I have done some more search. The originial fuse was a littelfuse 313 3AG (after searching in the trash I was lucky to find it, the only one 3A fuse among the dead fuses ^^). This is a slo blo one with following specs found in datasheet :



So I went through the Eska datasheet and find that the one I need that meets the spec is the time-lag UL 632.300 series :



Which is not exactly the same as the 632.300 series (without UL). Fortunately I have ordered both version. I will test directly the right one and hopefully this amp will stop blowing fuses !

[Jazz P Bass]

If you think the amp is o/k, remove the limiter.

[yann]

With the limiter removed, it will start ok a few times, then the fuse will blow. On the limiter I cannot see weird behavior of the bulb even after 20 starts. Except some times when it takes a bit more longer to dim (less than 3sec.).