Carvin X100b Help

[fxdrider2004]

What I thought would be simple since this late 80's amp has fixed bias was a simply tube change, Wrong. I put a matched set of 6l6gc's and 12ax7's in and one of the power tubes only lasted about 2 minutes and quit. I just thought it was a bad tube so I ordered a new set of matched tubes from Eurotubes and cleaned the tube sockets and installed them, they lasted about 10 minutes and the sound just quit, power tubes were lit up but flashing blue in the bottom of the tubes. I turned it off and put the original tubes back in along with a bias probe and the bias was fluctuating between 17 and 32 and the old tubes were flashing then. The amp was working fine, I bought it used and figured a new set of tubes could only help, no so. Anyone have any ideas on what could be wrong with it?

[Twist]

When you say the output tubes "flashed blue" did you mean a constant blue or were you strumming your guitar and the blue glow was fluctuating with no sound? Did you have your speaker plugged in? Pull out your output tubes and monitor the negative voltage on pin 5 of each socket (with no signal into the amp). What method are you using to determine the idle current? I hate to ask what may seem to be dumb questions but it helps to know the whole picture.

[fxdrider2004]

They were flashing blue with no guitar input signal, guitar was plugged in but was sitting on a stand. Speaker cab was plugged in and my son was playing when the sound just shut off, that's when I looked at the tubes and saw the blue flashing. When I hooked up the bias probe the flashing seemed to be about the same as the fluctuation in the bias. I didn't measure voltage yet. Was unsure if bias resistor went bad or really where to start. Thought if it was something simple I would do it myself.I'm using a tube socket bias probe.

[Twist]

As you play a guitar through an amp, the current your monitoring will fluctuate and the blue glow will change in intensity. Actually your probe is probably reading a small change in DC across an internal 1 ohm resistor which indicates a change in current. Anyway I would first check my speaker cord and speaker cabinet. I assume we're talking about a separate amp and cab.
If the tubes are fluctuating with the guitar being played it means everything for the most part is working in the power amp section. It is not advisable to keep playing the amp until we rule out the status of the speaker cord and cabinet.

[fxdrider2004]

The tubes had the blue fluctuating without the guitar being played, the looked normal on standby, but when it took it off standby they started flashing. Yes it is a seperate 4x12 speaker cab and I didn't replace tha cable yet. Everything I read and even the videos on bias setting showed the voltage steady when it was set. Thanks for your help and patience with an old novice.

[Twist]

Then it sounds like there is an issue with the bias fluctuating or the screen grid supply. The next step would be pulling out the output tubes and monitoring the negative voltage on pin 5 of each tube socket. I'm a little confused right now because even with a fluctuating bias you still should hear some sound when you play. It might sound crappy but there should be some kind of noise.

[fxdrider2004]

Thanks, I'll pull the tubes and check the voltage, but first I'm going to do as you suggested and check the speaker cable and fuse to the speaker cab itself. In the event the grid voltage is fluctuating, what next?

[Twist]

If you find no fault with the tubes and the power supply is fluctuating then your looking at opening the amp up and checking supply voltages. There are very lethal voltages inside and if you're not comfortable with that I would suggest taking it to your local amp tech.
It's past midnight here, guess I'll go to bed now.
If someone else doesn't help you, I'll check back later.
Good Luck

[fxdrider2004]

Yes, it is getting late and I think I'll work on it tomorrow when I'm not so tired. I'll let you know what I find. Thanks again.

[fxdrider2004]

Problem Solved

Apparently the new tubes and the Ibanez tube sceamer were enough to shake some wires loose in the speaker cab. I found one wire completely off the speaker and the ground on another was loose. I cleaned and tightened all the connections and cleaned the cable plug in connector and it works great now. There was a hum in the amp when I got it and I blamed it on weak tubes. Could have been the tubes and the bad connections in the speaker cab. The amp is totally quiet with the L.P. and the S.G now, no hum or squeal, but when you play, hold on, it's very loud at 3 on the master setting. Thanks, Twist, I was stuck on the bias thing and you got me on the right track when you said it should still have sound out of the speakers even if it was crappy. Thanks again for the help.

[Twist]

Great !!! I hope the amp keeps working ok.