Dead preamp tube or something worse ?

[IEHAGY]

It started with louder than normal noise floor / hiss that I was able to control by adding a passive preamp into my chain ... then it started taking longer than normal for it to heat up .... and now it wont heat up at all.

I'm wondering if its time to take my beloved Juicy Music Extreme to a tech or if perchance a better first step would be to try some tube replacements on my own.

This particular preamp has 4 tubes I can see .. 2 12AT7's, 1 12AX7 and a what I think is a 6h30 supertube.

After taking off the clothes, and let it power on for about 30 minutes ... the 6h30 gets HOT, one AT7 is HOT and the other is WARM. The AX7 however, remains completely cool to the touch.

I am a pretty big novice at this but would it be worth replacing the AX7 or should i be safe and just go straight to the shop ( speaking of which, any good ones in LA ) ?

Appreciate the advice, thanks.

 

[gadget73]

can you see any glow from the cold 12ax7 with the lights off? If no glow at all, my vote is on a dead tube and you don't have much to lose by trying another one.

 

[IEHAGY]

So I just swapped in another AX7 I had lying about and the problem remained, so I am guessing I have a bigger issue

 

[gadget73]

maybe try the 6h30 tube if you've got a spare. Don't know anything about the way this one is made, but if its both channels it makes me think something common to both. Further guessing, I'd expect that since there are two 12AT7, its one per channel. The other tubes are probably used for both channels, so if it was going to be a tube I'd expect it to be one of the singles.

beyond that if you're not of the poking about internally type, you'll need to find someone to take it to.

 

[egellings]

6H30 is a big duo triode.

 

[GordonW]

I've had issues with heater power supplies on these Juicy Music preamps. There's a regulator in the supply of some models, that if it gets weak (overheats, etc), it will start to limit current- which makes the heater supply get weaker and weaker (the voltage goes down as the regulator tries to "protect" itself). Eventually, the tubes don't get enough heater voltage to function properly- which could explain things like a tube feeling "cold".

I finally gave up trying to fix the one I had, and sold it to someone who had more time to test each individual part, and to find a replacement to what was looking to be a hard-to-find transistor, at least in the one I had. Hopefully, yours isn't the same issue...

Regards,
Gordon.

 

[Lavane]

I've had issues with heater power supplies on these Juicy Music preamps. There's a regulator in the supply of some models, that if it gets weak (overheats, etc), it will start to limit current- which makes the heater supply get weaker and weaker (the voltage goes down as the regulator tries to "protect" itself). Eventually, the tubes don't get enough heater voltage to function properly- which could explain things like a tube feeling "cold".

I finally gave up trying to fix the one I had, and sold it to someone who had more time to test each individual part, and to find a replacement to what was looking to be a hard-to-find transistor, at least in the one I had. Hopefully, yours isn't the same issue...

Regards,
Gordon.

Would adding a heat sink, or a larger heat sink on the regulator help? I've done this with several pieces of SS gear that lowered the temp of the regulator. I did this also with a dual 5v regulator for a heater supply. The heater regulator runs much cooler now and hasn't failed.

 

[GordonW]

The one I had, already had a heat sink on the regulator. I don't know if it needed to be bigger, or if it just needed to be better attached.

Regards,
gordon.

 

[JohnT123]

The high background noise could be pointing to a bad plate resistor. If it started to break down and then opened it would explain your symptoms. I would check for plate voltage. If there's none, there's no output.

 

[analog]

If you are in need of a shop in the Los Angeles area, I have used the Audio Specialist in Studio City with good results. He has repaired A tube integrated amp with a bad phono stage. Google him for further info..