Strange thing happens when I bias my amp

I once accidentally touched the case with a hot probe. Blew a fuse in the amp but also that particular function in the meter. It behaved exactly the same way as the op's afterward.

Angiernc - I was looking at my manual, as well, when I quoted the 75 mA recommendation. Perhaps the F1 model I have is different from the standard model or maybe recommendations changed during the course of production. I definitely got the recommendation of 65-70 mA directly from Steve Witek at Cary, wrote it down in the back of the manual while I was talking with him.

I wasn't trying to make an issue of the bias value, I was just curious since the 100 mA figure was different from both what was in my manual and the verbal advice from Steve at Cary. The real issue here, of course, is the erratic behavior during biasing that Dbrewer12345 is experiencing.

Dbrewer12345 - One thing I forgot to ask in my first post is whether you're using an analog or digital meter. Don't know if that makes any difference but my experience with the SLI-80 is only with digital meters.

I just read through the SLI-80 manual and they state that the bias should be readjusted for 100 ma. AFTER A TEN MINUTE WARMUP. It's not clear that you followed that recommendation.

It's also possible that a poor connection in the 1/4" plug you're using to connect the ammeter is causing the problem you're seeing. You might also try wiggling or twisting the ammeter plug when the cathode current is seen to be very low and observe if the meter reading jumps up.

By the way, solid state amp owners who have read the TNT document mentioned above should take the info to heart. Too high a DC offset at the speaker terminals will have a greater effect than just generating heat in the voice coils. The speaker cones will be physically offset by the DC current, compromising the sound quality and dynamic range of the drivers involved. And, you may damage the voice coils. The info about replacing the very poor quality trim pots typically used to set the DC offset and bias voltages is spot on. In many cases, with the original pots,it's simply impossible to properly set the offset and bias even when the pots are new.

I have an SLI-80 F1 and have never had any issues biasing a variety of tube types. I haven't seen the behavior you're describing.

I would agree with Celtic's recommendation to look first at the multimeter itself and test with a different meter. You can get a Craftsman meter from Sears for around twenty dollars that should work well enough to see if the issue is with the amp or the meter. There's a Fluke available on ebay for $28 that might be better than the Craftsman.

Advice from Cary's service department has always been good but for an issue like what you're describing I'd be willing to bet they'll want you to ship the amp to them. That's an expensive and potentially risky proposition because of the risk of shipping damage. As you know, Cary's packaging is very good but that's a very heavy amp.

What kind of tubes are you biasing at 100 mA? The manual says 75 mA without mentioning the tube type and the advice I got from Steve at Cary suggested 65-70 mA for El34 and 6L6 tubes.

Basic guesses are usually the best guesses and that is:

your multimeter settings are wrong

you are grounding improperly

some connection sequence is incorrect

Regardless, contact Cary for further instructions.

Csontos...the Cary SLI-80 is a TUBE integrated amp not solid state.

For ss amps, TNT Audio has a great article explaining how to set the bias of your amp. Google 'adjusting the bias of your amp'. But it doesn't talk about mosfets, only bi-polars. Mosfets are adjusted by removing a rail fuse and putting an ammeter (setting your multimeter to mA) across the contacts. One channel at a time. The fuse contacts, that is. Not the mosfets. Ideally both channels at the same time with separate multi-meters.