Luxman Amp Bias

[john]

Hi everyone, I bought recently  a Luxman integrated amp ( SQ-38 Signature ) whilst checking it I noticed one output tube running a lot cooler than the rest. I checked the bias setting on that particular tube, there isn't any !! absolutely nothing. The other three bias fine
 ( 40mv )  there is continuity from pin three to the circuit board, swapping tubes around makes no difference, any ideas? tube compliment is four EL34, two BH7A and four 12AU7. Unfortunately I don't have a schematic but found this on the web.

 http://audio-database.com/LUXMANALPINE-LUXMAN/amp/sq-38signature-e.html

[MylesAstor]

Hi everyone, I bought recently  a Luxman integrated amp ( SQ-38 Signature ) whilst checking it I noticed one output tube running a lot cooler than the rest. I checked the bias setting on that particular tube, there isn't any !! absolutely nothing. The other three bias fine
 ( 40mv )  there is continuity from pin three to the circuit board, swapping tubes around makes no difference, any ideas? tube compliment is four EL34, two BH7A and four 12AU7. Unfortunately I don't have a schematic but found this on the web.

 http://audio-database.com/LUXMANALPINE-LUXMAN/amp/sq-38signature-e.html

Could each tube be individually fused?

[ironbut]

Hey John,
When you say that switching tubes made no difference, I'm guessing that not matter which tube is in that position, it still runs cool?
If the temp. difference is significant, it's probably not something that you could service yourself unless you have a good deal of experience in these matters.

I believe that's one of the deParavicini designs. Pretty nice amp from what I've read (I think that's one that I was looking for a while back). It may need some work on the power supply but even so, I think that amps probably worth the expense.

[ironbut]

I should have suggested this in my first post but you should try getting a new set of tubes and see how they work. Just get 4 plain jane off the shelf tubes that have been tested by the seller. I have a set of the new production Mullards for testing purposes. Not too expensive and if you get them from a seller like Jim McShane, you can be pretty sure that they're well tested.

[steveidosound]

It's kind of hard to see from the small schematic, but I did not see any bias adjustments. Have you checked the cathode resistor to ground for proper value and connection on the suspect tube position?

[john]

Thanks Steve, yes I did change the output tubes around. When I bought the amp it still had the original Luxman branded Golden Dragon EL34 and 12AU7 tubes, the 12BH7A's had been changed for Philips. Not really knowing the history of the amp I decided to invest in new tubes and bought some Shuguang Treasure EL34, NOS Mullard 12AU7WA and RCA 12BH7A.Thanks Myles also, I checked the fuse's but there isn't one for each tube, what fuse's there are were ok. It was a resistor Steve thanks, not on the cathode but the screen grid.
        Hopefully that's all it is, pin four on all four tubes are linked together via 100 ohm resistors, one for each tube. Out of the four the resistor for the tube with no bias was open circuit, two others were ok and one was out of spec at 120 ohm. I've ordered better quality   1% Tantalum  resistors from Partsconnexion.com to replace all four. It will probably be a couple of weeks before I get them, thanks again for all the help and suggestions.

                                                                             John.