Holden Wasp 4xKT88 power tube arcing.

[Colonel_Sanders]

Hi guys,

I'm repairing a Holden Wasp at the moment that keeps blowing fuses. It seems that pin 3 (anode) is arcing to pin 2 (heater) on one of the KT88's. The amp runs fine without the power tubes in place, and won't blow fuses with 3 KT88's minus the 4th arcing tube in place.

I've read before that arcing causes a carbon track to form. Do you guys think that this is the problem with the amp? And that when I put the arcing tube into place, it blows the fuse? If so, can I scratch the track off? Kt88's are expensive .

Cheers,
C_S

[Rick Erickson]

Kt88's are expensive

What's your time worth? You've got it down to the tube that causes the problem. Do you see any sign of external arcing? If not, replace the tube. But wait - have you put one of the good tubes in the socket the bad one came out of to be sure it isn't a socket related problem? If not - I would inspect the sockets carefully first for arcing signs, or you may be ordering a pair of KT88's.

RE

[Colonel_Sanders]

Hey Rick, cheers for the reply!

Tried all tube-socket combinations with the good KT88's, so its definitely the tube. We're all poor! But I shall chat with the client...

[Rick Erickson]

I usually don't recommend replacing a single power tube - especially if you are mixing a brand new one with some worn ones, but recently did just that with a Fender Rumble-Bass amp. Someone had loaded it with EH KT88's and one was dead. That was the least of that amp's problems. Imagine an SVT type amp built with 1/4" crimp connectors on virtually every wire connecting to the PC boards of the pre-amp and power-amp sections and an oversize circular AMP connector with bad crimps on all the shielded audio connections. Add to that a very bad 120V fan install with wires running between the PT and chassis. I don't think the fan was the original factory one, but I don't know how they would have run the wires to it. Maybe it was a prototype or a one-piece-at-a-time employee build.

RE

[unparalleled]

Kt88's are expensive

What's your time worth? You've got it down to the tube that causes the problem. Do you see any sign of external arcing? If not, replace the tube. But wait - have you put one of the good tubes in the socket the bad one came out of to be sure it isn't a socket related problem? If not - I would inspect the sockets carefully first for arcing signs, or you may be ordering a pair of KT88's.

RE

Every standard Wasp 200 I have worked on has had series connected filaments on the 4 output bottles, however the holden chassis for the 100Watt version had 2 more socket positions unused although punched so it was very common to find two extra tubes and sockets added as a mod. The transformer sizes are a dead givaway.
They can, and did happily run 6550 tubes.
For circuits & info google: " anz amps"
There is a Wasp 200 on the site which I restored.
Alan.

[Colonel_Sanders]

Thanks for the info guys! I'll definitely check out the amp site.

Ended up measuring the resistance of the carbon track and taking a flat head screw driver to it. When it measured infinite I popped it back in, soldered a few dodgy connections and now all is merry! Owner'll be stoked.

Cheers,
C_S