Bogner 20th Anniv Shiva KT88 output tubes won't light

[wiredsince94]

Turned on my 2019 Shiva 20th Anniversary this weekend & got no output. Plugged straight in, disconnected pedals from effects loop, still nothing. I checked both fuses for continuity & both tested ok. All preamp tubes are lit, but the KT88's won't light up & remain cold. Speaker cab & cable are both fine & it's always been connected to a cab. I'd appreciate any thoughts or guesses on what the problem might be before I take it to a tech. This is my favorite amp & it's never been used outside my home since I bought it new. Thanks!

[Jazz P Bass]

I do not have a schematic for that one that actually shows the heater circuit.
If the KT88 heaters are in series, then one dead one will turn both off.

[Enzo]

And did you remove the fuses individually to test them with an ohm meter? If you try to test a heater fuse while it is in the clip, the false path will fool your meter.

[wiredsince94]

And did you remove the fuses individually to test them with an ohm meter? If you try to test a heater fuse while it is in the clip, the false path will fool your meter.

Yes, took each out individually & tested for continuity with a multimeter.

[g1]

Remove the KT88's and measure the heater resistance (between pins 2 & 7) on each. Should be a low resistance.

[wiredsince94]

Remove the KT88's and measure the heater resistance (between pins 2 & 7) on each. Should be a low resistance.

You’ve just exceeded my capabilities Tried calling Bogner several times today & no one answered. Did end up speaking with the store where I bought it & going to drop it off to have their tech check it out. Whatever the issue is, hopefully it’s covered under warranty.

[nevetslab]

Remove the KT88's and measure the heater resistance (between pins 2 & 7) on each. Should be a low resistance.

Heater resistance is typically 0.62 ohms, as found on the last Ampeg SVT-CL I had to deal with, where one tube's heater pins were open, but it was a solder joint issue within the tube's Pins 2 & 7. I'd doubt you'd ever finding all four power tubes having that problem on this amp. There may be internal fuse that's NOT on a panel...only accessible inside the chassis. Marshall and Vox have placed heater fuses in their chassis' on some products. More likely an open heater fuse inside the amp.

Does your amp have the 'half-power' switch? This removes two of the four power tubes in the output stage. Might be an issue with that switch/circuitry.

[wiredsince94]

Heater resistance is typically 0.62 ohms, as found on the last Ampeg SVT-CL I had to deal with, where one tube's heater pins were open, but it was a solder joint issue within the tube's Pins 2 & 7. I'd doubt you'd ever finding all four power tubes having that problem on this amp. There may be internal fuse that's NOT on a panel...only accessible inside the chassis. Marshall and Vox have placed heater fuses in their chassis' on some products. More likely an open heater fuse inside the amp.

Does your amp have the 'half-power' switch? This removes two of the four power tubes in the output stage. Might be an issue with that switch/circuitry.

No half power switch, and the output section is one pair of KT88s

[Enzo]

The heaters on the power tubes are dark. Unlikely both tubes have bad heaters. Your power transformer will have two wires - typically heavier wires than the rest - that carry the 6vAC for those heaters. Is there 6vAC between the two? They then likely go to a circuit board, perhaps with a connector. Is that so? Any burnt looking pins there? Are the power tube sockets on a board or hardwired? If on a board, is it separate from the board the transformer is wired to? If so, are the connections between that board and the power tube board or sockets intact? Follow the 6VAC. check any fuses you encounter also. The 6v gets to each step or it doesn't.

[g1]

I'd doubt you'd ever finding all four power tubes having that problem on this amp.

There was speculation earlier that the power tube heaters could be in series. Bogner Shiva has a small 12V smps board just for heaters. Not sure if this model has same, but if so it could power 2 power tubes heaters in series.
If that were the case, just one bad power tube heater would make them both go dark, and preamp tubes would still be lit.

[wiredsince94]

Wanted to thank everyone for your feedback & let you know what the solution was. As a couple of people mentioned on here, there is a separate circuit for the power tube heater & I guess the fuse in that circuit was defective. The tech said it didn't appear to be blown, but once he changed it, the amp was good to go. Thanks again

[Helmholtz]

The tech said it didn't appear to be blown, but once he changed it, the amp was good to go.

What a strange and disappointing answer from a "tech".
What if he missed intermittent fuse holder contacts or solder joints?

[Enzo]

Or he left the fuse in the holder to check it with an ohm meter. That usually results in the parallel path around through the transformer and the tubes for a low resistance making the fuse appear good.

[Helmholtz]

What I meant is, that it's essential to verify the fuse was actually bad.
Otherwise a new fuse will likely not be a permanent cure.