Pops 300B tubes on turn on in my Vac

I am a complete amatuer but it appears to be arcing which is very serious. Does this happen while playing ? Does any other tube you use do the same. If not I would pull these immediately.

Did you set the bias "low" before installing the new tubes?

When circuits are not self-biasing I always adjust the bias to a much lower setting and then increase as needed when changing tubes.

Good luck.

no, not yet anyway the spark has occured in the back row on the left channel and the right channel also.I have checked the seating of the tubes and they seem tight.I have powererd the amp down and will try again later to power up.Can arching blow my amp or damage it this is a push pull design.

thanks

Hello Bill,
Yes i set the bias at the lowest setting.I am going to send the 2 back as this amp uses 8 of these and it's a chance i don't want to take to blow this baby.

Are they solid plate or mesh plate? You should not run mesh plate 300B's with VAC Renaissance amps, as they require a straight WeCo spec 300B - meshplates can't handle the voltage in the circuit.

Also, VAC's Renaissance amps all autobias the output tubes, so how could you have set the bias "low"? Makes no sense.

I've owned VAC Renaissance amps for eight years. They require well vetted output tubes. In fact, Sophia used to market a special set of the Sophia tubes for VAC Renaissance amps, which were the basic Sophia tube, but particularly carefully tested at plate voltages that resemble the voltage that 300B's see in the Renaissance circuit. Generally, as for transconductance, output tubes for the Renaissance amps should test in the 3,000-5,000 range. In addition, the plate-to-cathode voltage is approximately 430 volts dc, with idle current approximately 85 to 90 milliamperes in a self-bias (cathode bias) circuit. Again, this is approximately 5% below rating for the WeCo spec 300B. The milliamp and transconductance testing for purposes of matching must be done at those voltage levels.

I've always gotten 8,000-10,000 hours out of my output tubes, but tubes that are not good tubes will not hold up in the circuit.

In any event, call Kevin Hayes of VAC and the fellow at Sophia (Henry?) - they will confirm what I have written.

I forgot to respond to your question about whether defective output tubes can damage the amp -- the answer is no, because VAC Renaissance amps have a "sentry" output tube shutdown circuit that shuts down power to any output tube pair that goes out of spec (i.e., that contains a dying or defective tube). A red light will illuminate for the affected tube pair, meaning voltage was killed to that pair. There is no worry that a defective output tube can harm the amp. Given that the Renaissance amps autobias, use super long-life 300B output tubes, and have the tube shutdown feature, they are as easy to own as a solid-state amp (easier, because if you blow an output transistor, your stuck with a repair, whereas with a tube amp, once you have retubed, you basically have a new amp).

Have you arranged the tube pairs properly? Tube pairs must be installed "front-back", up and down the "V" layout of the tube sockets, not "left-right". As for other operational issues: (1) regarding the adjustable feedback, DO NOT run the amp with any feedback, as even 2 decibels of feedback kills the magic of the amp. If you don't have sufficient woofer control using zero feedback, sell the amp or replace the speakers (the feedback feature was a nod to marketing); (2) run the amps off the 8 Ohm tap.

You may not know this, but the Renaissance amps are the best amps VAC has made -- they are entirely point-to-point wired (in comparison, all PHi components use circuit boards), have outrageous parts quality, and superb output transformers (that's why your 70/70 retailed for $14,000 ten years ago). This explains why, at 65 watts/channel, your 70/70 sounds like a good 150 watt/channel amp (it helps that it is totally dual-mono, all the way down to two power cords and two on/off switches).