Tube amp distorting when not supposed

[Le québécois]

Hi all,

I have a Laney Cub10 little tube amp for practicing at home. This thing can be hooked to a 4 x 12'' Cab if you want so, what the heck... let's try. So I tried with my band and I did make shure to have impedance matching. According to the Amp spec: it can handle impedance of 8 - 16 ohms. The 4 X 12'' Cab I use was 8 ohm. The result was good. In fact the sound was cool but not loud enough for the jam and we had to mic the amp.
Long story short:
When plugging my amp today back at home. I realised that I have lost some clarity (headroom) .... It is not horrible but it is definitely there .. little tiny overdrive or gritness on the lower E string and low chords. I never use that amp at more than 3-4 o'clock volume with very low gain about 1 to 2.  During the Jam, I had to push it more despite the mic (volume of about 6 to 7) but the gain remained low to avoid distortion (I have stompbox for that ....).
Is it possible that I have damage my tubes or something. Can impedance be a problem even if the number are matched (it may be to much for a 5W amp?).

The amp is not old, the tube are 2 years old and the amp is never push hard except for that trial.....

Advises would be fun! I really like that little amp.
Charles
 

 

[iccaros]

This is a 10watt class ab amp, so I doubt that you hurt it, what you may be hearing is the difference between the 4x12 and its internal, but now that you heard the 4x12 the internal does not sound as good. 
if you had the Volume all the way, and they set those tubes to max, which 10 watts is not max for them, then being run that hard could have made a week tube get worse.

Try new power tubes.. the 12ax7 should be fine..

but if you want  a cleaner amp 12au7's would make the amp run cleaner..

[Tacoboy]

Be careful swapping 12AX7 for a 12AU7: 12AU7 draws significantly more current than a 12AX7 and might possibly burn up a plate resistor...

[iccaros]

valid point,
12au7 does draw more current a max of 10 ma and the 12ax7 draws much less 1.2ma. the plate resistance drops most of the voltage, as its a voltage divider between the plate resistor and the plate, but the 12au7 has a lower plate resistance so the resistor will drop more voltage. . The load resistor drops maybe 20v - 30v with 10ma.. which is .3 watts. So if you have a amp that is using 1/4 watt resistors on the plate this would be an issue, but 1/2w is standard. your not in X2 safety margin but your not maxing the resistor.  Its worth looking at and should not be ignored, if there is a doubt, open the amp and look at the plate resistors, easy to replace. Most likely they are 100K 1/2 watt.

[familyortiz]

Any chance it's not the amp causing your distortion? What are you plugging into it? If you have an active guitar, I've heard distortion increase as the battery dies. Just a thought...

[sault]

If you have an active guitar, I've heard distortion increase as the battery dies. Just a thought...

+1, for any onboard electronics. However, its not necessarily "distortion" as it has been loss of bass, clarity, and increased noise. The big thing was definitely noticing that all of a sudden the guitar didn't seem to have as much chunk/low end as it used to. That's been the case for both the guitar with passive pickups/onboard buffer and the other with active pickups.

Loss of bass = first indication that the battery is going

[DavenPaget]

If you have an active guitar, I've heard distortion increase as the battery dies. Just a thought...

+1, for any onboard electronics. However, its not necessarily "distortion" as it has been loss of bass, clarity, and increased noise. The big thing was definitely noticing that all of a sudden the guitar didn't seem to have as much chunk/low end as it used to. That's been the case for both the guitar with passive pickups/onboard buffer and the other with active pickups.

Loss of bass = first indication that the battery is going

ALL pickups are passive , it's only whether they come with buffers/preamps .
He might have probably got used to the 4x12 O_O

[Le québécois]

Thanks for all the answer!!
OK, first I have on board preamp in my guit (tilman preamp) but I manage to feed it with a stereo jack from my wah so low battery is not the cause since it's connected to a PSU. Plus I have a switch to bypass the preamp and go back to the passive pickup and the problem persist. For all the rest (my stompbox and all) they remained the same (cables, pedal order, PSU......etc), I wouldn't suspect that since the problem is present with clean sound (no stomp at all).

In regard to being used to the sound of the 4 X 12. I doubt since I use it for 3 hours only compare to 2 years at home with the small 10 inch celestion. The exception is not the celestion sound but the 4 X 12.

Maybe as some have suggest, the power tube need replacement? Other thing that float around in the realm of tube is to push the tube of a brand new amp to ''open'' the sound?? or something like that? right? (I don't no the word in English maybe "conditioning" of tubes) I never really did it. Maybe the 3 hours with the cab at higher level did the same? If so, don't push a Laney CUB10 since you'll lost the bass... 

[DavenPaget]

Thanks for all the answer!!
OK, first I have on board preamp in my guit (tilman preamp) but I manage to feed it with a stereo jack from my wah so low battery is not the cause since it's connected to a PSU. Plus I have a switch to bypass the preamp and go back to the passive pickup and the problem persist. For all the rest (my stompbox and all) they remained the same (cables, pedal order, PSU......etc), I wouldn't suspect that since the problem is present with clean sound (no stomp at all).

In regard to being used to the sound of the 4 X 12. I doubt since I use it for 3 hours only compare to 2 years at home with the small 10 inch celestion. The exception is not the celestion sound but the 4 X 12.

Maybe as some have suggest, the power tube need replacement? Other thing that float around in the realm of tube is to push the tube of a brand new amp to ''open'' the sound?? or something like that? right? (I don't no the word in English maybe "conditioning" of tubes) I never really did it. Maybe the 3 hours with the cab at higher level did the same? If so, don't push a Laney CUB10 since you'll lost the bass... 

What kind of wiring did you use on the bypass switch ? I used a DPDT for bypass and power and it ate tone  so it's not a true bypass

[iccaros]

Its simple to replace a tube and see, its may also be a placebo effect, but as long as your happy.

I brought up that you may have gotten used to the sound of the 4x12, because sound is subjective, and once people hear an amp one way other ways sound bad, This is how our ears work.  also having the amp in a different position changes how it sounds, lean it back, elevate it off the floor, all change how the amp sounds. how long it is on, I turn mine on 10min before I play, let everything stabilize and heat up...

[sault]

That's the purpose of the standby switch... give the tubes a chance to warm up before you go full-bore, then give the tubes a chance to cool down a little before you kick the power off. Good way to help keep your tubes alive longer...

[iccaros]

if you have a stand-by  ,
I know what you are saying, and I am not trying to pick, just pass on some things..
  most modern amps do not need them as they do not pick  parts that  so close to max running voltage (like 25v cathode caps on a 6v6) . A standby switch does not cool your tubes at all, as the heaters are on.. and they pull 50 - 100X the current the plates draw

to Quote Merlin  http://www.freewebs.com/valvewizard1/standby.html
The standby switch usually allows the HT to be turned off while the heater and bias supplies are always on. Contrary to popular belief, the standby switch is not there to prolong valve life-span. The theory is that if the HT is applied while the cathodes are cold they will be 'stripped' by ions crashing into the unprotected cathode. However, this simply does not happen. It is an urban myth borrowed from transmitter and cathode-ray tube technology NOT ordinary 'receiving' valves. The only valve which might ever be at risk of failure is a rectifier valve, because it has to supply inrush current to the reservoir capacitor, and some guitar amps fail to use enough limiting resistance to protect against this (see more on rectifies).
On the other hand, leaving a cathode hot without any anode current flowing does lead to the very real effect of cathode poisoning, which reduces the gain (transconductance) of valves. Fortunately this phenomenon really only becomes significant if the valves are left on standby for hours on end.
If you're wondering why all those old amps use a standby switch, its because Fender was designing complicated amps on the cheap. In the bigger versions of the Bassman, money was saved by using power supply caps that were rated only for the working voltage, not the peak voltage which occurred before that valves start drawing current. As everyone knows, Marshall simply copied the Bassman without a second thought, complete with standby switch, so now we have the two biggest names in the industry using standby switches, and the rest is history. The other big players, Vox and Gibson, never used standby switches since they didn't need them. Only very recently have they started adding them, purely because too many guitarists want their amp to look just like a Fender/Marshall, even though nowadays no designer (who values his reputation) uses underrated capacitors. But the average amp tech doesn't know this.
Note that even the RCA Transmitting Tubes Technical Manual No. 4, p65, states: "Voltage should not be applied to the plates or anodes of vacuum, mercury-vapor, or inert-gas rectifier tubes (except receiving types) until the filaments or cathodes have reached normal operating temperature." [My italics]. In a properly designed amp, a standby switch is nothing more than an expensive, oversized mute switch.

[Le québécois]

What kind of wiring did you use on the bypass switch ? I used a DPDT for bypass and power and it ate tone   so it's not a true bypass

I use DPDT (push/pull pot) and I don't have tone sucking. I think that with this setup it is true bypass because I don't use a LED indicator (which would require millenium or 3PDT). Both the In and the out of my onboard preamp are removed from the sound path with the DPDT. If it suck tone in your setup, you may have wired it differently and have interaction with volume knob or tone?

sorry, I just reread your post (my English is working but .... slowly): In fact, I don't use the DPDT to turn the power of my preamp on or off as I understand you seem to do. My onboard is a non-environmentaly friendly unit since it's sucking power all the time even when idle (like a car trying to heat is passenger in winter....).
How come you still have sound coming out when no DC reach your preamp?! Maybe this is the source of bypass tone sucking right there?

Its simple to replace a tube and see, its may also be a placebo effect, but as long as your happy.

I'm not there yet (I'm afraid of biasing!). I will assume it's placebo effect and convince me that I'm still happy with the sound. Base on all answer, nobody seem to say that I did a mistake or have done something that was clearly bad for my amp. I'll wait a bit for changing the tube or building my own amp!!

Charles  

[iccaros]

if you replace with the same tube type, no need to bias.. not really, start with preamp tubes, as most of what you get tone wise is there.

Its once you change the power tube type, others will say you have to re bias, but if the tube is good and the Amp is designed well, then different tubes will put out different emissions (current) but it will be with in the acceptable range.  This is how a tube tester works, the makers know the acceptable range of that tube type and set a bias to where the needle falls in the good zone on a ampmeter. 

[petemoore]

 Dunno, my preamps are clean like M and F types, perhaps a bit in the PI...OTube Disto is what I build/use little and other tube amps to do.
  Preamp tubes are swap quick, replace with the  same type but try 12au7 or 12a_7 in the first stage, PI...wherever, lower gain can be a nice sound.
  Otubes generally 'just swap' anyway, regardless...if running some 'super tube' super hot and replacing them [especially with a less 'durable' type] a re-bias might save the lesser tubes from over current/heat damaging conditions.
  ...and depending but usually in modern amps it's just a matter of accessing good replacements. If they're redplating that's another matter entirely of course.
   Considering the danger and hassle of bias checking...and the increased reliability or tonal preference involved with hot or cold bias...knowing where it 'sits' at some point is a good idea...somewhere's in the middle is fine for most tubes, and sounds as good as super hot settting, way better when redplating sets in.

[DavenPaget]

> sorry, I just reread your post (my English is working but .... slowly): In fact, I don't use the DPDT to turn the power of my preamp on or off as I understand you seem to do. My onboard is a non-environmentaly friendly unit since it's sucking power all the time even when idle (like a car trying to heat is passenger in winter....).
How come you still have sound coming out when no DC reach your preamp?! Maybe this is the source of bypass tone sucking right there?
It doesn't exactly isolate the signal at all , it's just like the boss pedals .
I know about that since i work on boss pedals sometimes .

[Astronaurt]

Maybe as some have suggest, the power tube need replacement? Other thing that float around in the realm of tube is to push the tube of a brand new amp to ''open'' the sound?? or something like that? right? (I don't no the word in English maybe "conditioning" of tubes) I never really did it. Maybe the 3 hours with the cab at higher level did the same? If so, don't push a Laney CUB10 since you'll lost the bass... 

At least from what you've described, it sounds to me like a Power tube replacement would do you well. If you've had the amp for a couple years without replacing the Tubes, it's likely that pushing it hard into the 4x12 would've done exactly what you're describing; Common symptoms of Power tube fatigue are reduced volume and headroom, as well as having a 'mushy,' unclear, or unfocused bass response.

A couple weeks ago I actually just had my Bogner Alchemist re-tubed for the first time since I got it last year, and I swear it's like putting on glasses and seeing for the first time. You don't notice the gradual degradation in sound quality because it happens over months or years. It's not unheard of that a big jolt of 4x12 being hit by the already weak tubes knocked them further out of balance with each other. Luckily for you, Amps in the 1 to 15 watt realm are often Cathode-biased, which means that new tubes don't need to be manually adjusted. I'd look up the specs for your Laney and figure out if that's the case or not, if it is in fact fixed adjustable bias then I'd go ahead and take it to a guitar shop where they know what to do.

[gmoon]

Good advice about the tubes...

Don't overlook other possibilities--if the amp was used only at home, the connectors they normally use in a modern PCB tube can become loose, especially if it's been tossed in the trunk and taken for a bumpy ride. Those consumer grade spade-style connectors are prime suspects.

Clean and re-tension the sockets, too, if it's been a while. A little amp like that's probably cathode-biased, and sometimes the PCBs and other components get hotter than is wise (which can build up some oxidation on the sockets, if one is too close to the bias Res).

Move on to more serious debugging after the easy, obvious stuff.