Powertubes - need replacing?

[witeter]

So i got myself a new amp (second hand) and tried it at practice yesterday yet there was a problem - i seemed to be having some volume issues. It seemed as if the amp was very underpowered and i would have to turn up the volume incredibly loud to get anywhere near rehearsal volume. So, could it be the powertubes? previous owner has had it 2 years with not a huge amount of use and never changed tubes.  Im attaching a pic of the tubes (sorry about the cr@p focus)-as you can see the 2 inner tubes are bright at the top but the outer ones arent-is this a sign of faulty tubes? thanks guys!


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[bandmaster188]

WHATS THE AMP

[WeAreNotGentlemen]

WHATS THE AMP

Picture shows Peavey JSX.

[Twinfan]

Looks like the amp is set to half power and only using two output valves?  The heaters are on for the other two which means they're firing up OK?

[Twinfan]

If it's a head, I don't think there's a half power switch so the outer pair of valves may not be working.

Swap the inner two with the outer two and see if you get the same problem - if so, the amp's at fault and will need to go to a tech.

If the 'glow' moves with the valves then you have valve issues - time for a new pair or set.

[Dmoney]

The heaters will always be on, a half power switch wouldn't turn those off, not in a JSX anyway. There is no power half power switch on the JSX anyway as it happens.

The filament supply in the JSX has a 10A fuse in it and all the filaments (for the power valves at least) will be in parallel, so there shouldn't be an issue with the filament voltage itself. Since if the fuse clearly hasn't blown and since some valves are being heated.

Some valves I think do glow a bit dimmer than others and I don't think looking at the filament glow is the best way to diagnose a fault (unless the filament clearly isn't glowing). You can get blown valves with perfectly fine bright filaments. I've seen quite a few! I'd suggest moving the valves around but if you do have a valve with a short in it you can potentially cause extra damage to other screen grid resistors, which is something I found out the hard way in a 5150.

Even if you took two valves out and ran it at 50 watts I'd still expect it to be plenty loud!

This might seem like a silly question, but did you have the loop engaged? and if so, where did you have the effects send and return levels set?

[Telerocker]

This might seem like a silly question, but did you have the loop engaged? and if so, where did you have the effects send and return levels set?

That's a good question. I experienced a lot of loops suck volume and tone.

[witeter]

Hey guys thanks for the replies.
The amp was still loud just nowhere as loud as i remembered it when trying the same model previously, i had to really but really crank the volumes to get it at a decent rehearsal volume to compete with my drummer (the single rec i had before wuold easily reach these volumes being set at 3 or 4 - out of 10). I was having to set the peavey head at channel volume close to 7 and master also 7 and even then it wasnt quite getting there!
I did try engaging the loop and setting the send and receive as peavey suggest, i also tried it without the loop. This is when i started observing the tubes and remembered that in previous amps they have always been quite bright at the top. What would you suggest as my next step? move some powertubes around and see if they brighten at the top also? i just dont wanna wreck the amp and btw the tone is 100% fine. Thanks guys!

[Dmoney]

You can swap them around like Twinfan suggests. The issue I've had with doing that in a 5150 is that a short inside the valve (which did have a decent looking filament glow) caused a screen grid resistor to burn out and go open so no voltage would flow through it, effectively shutting down the valve socket. Moving that valve around just fried more resistors until I figured out what was going on. I had to replace more screen grid resistors than I would have needed to originally. So now I check those resistors before I move valves around.

Even if you move the valves around and the filament glow remains the same and follows the valves in question, I still don't think that the glow itself is a good indication of a fault. Your best bet to completely rule out the valves is to test the valves and I'm afraid moving them around the sockets and looking at the filament glow won't give you a definitive answer to how well the valves are functioning.

Even if you decide to get new valves anyway, the JSX has an adjustable bias, so it's worth getting the bias checked when you swap them anyway, although it's probably set at some weird voltage out of the factory due to the 6L6/EL34 switching. Overall I'd say the best bet is taking it to a tech. Probably worth getting it looked over anyway if it is second hand.

[witeter]

Thanks man will probably take it to a tech-just wondered though, is there any reason why those powertubes wuoldnt glow as bright as the ones in the middle? even compared to every other tube ive had? just seems kinda strange i guess

[hunter]

Can be a preamp tube, too. Phase Inverters can be over with after 2 years.

[witeter]

Thanks Hunter-anything in particular i should look for if the PI tube is going? how could i tell its that one? thanks

[Dmoney]

You'd have to swap it with another valve.
One way I can think to do it, is to plug your guitar into the FX return (your master volume and FX return levels should still work to control volume) and then swap V1 or V2 with V4. Half of V3 is between the FX return and the PI. You could also try swapping V3. If you leave the volume controls and you hear a difference in volume when you swap something, that might be an indicator.

[TheyCallMeVolume]

I feel like any time you get a used tube amp, it's best to just change them out. Better safe than sorry, even if they could be relatively new or unused tubes.

[witeter]

Hey guys thanks for the help-i dont have a practice till this monday so wont be able to fully test it out till then-had a play yesterday and it seemed fine?! though i didnt get to fully crank it. Could be that the powertubes are 'on their way out' hence why it sometimes seems to be ok and then suddenly itll go underpowered. I think i will order some more tubes in case. Twinfan- i will do what you suggested beforehand-thanks

On a positive note just want to say that i love this amp, it just destroys the rectifier in so many ways and the way it takes pedals in the loop is just glorious! thanks for your help guys and will keep you posted!