Too much signal into a power tube?

[jojokeo]

*Maybe I should rephrase a question: In a SE amp, is it possible to feed too much signal into a power tube (in this case an el84)? If so, what happens? Can it cause the power tube to blow or short out?

[phsyconoodler]

Well you re using a pentode for a preamp that has a gain factor of 150 so it may require further attenuation before it gets to the power tube,but I really don't know for sure.I'm still thinking screens for some reason.

[bobmegantz]

You can apply too much signal, resulting in too much current, resulting in something either melting or opening.  This can be caused by either DC or AC.  However, in your case the failure is caused by a transient, which would make excessive heating unlikely.

I also vote for one of the screens failing.  I would start by increasing the screen grid resistor (no matter what you think of Kevin O'Conner, this will protect the screen grid).  Then, I'd check the coupling cap to the control grid to make sure there's no DC leakage, and that it is operating within its rated voltage at all times.

If you can find a suitable tube tester, perhaps you can find out what failed in the tubes (open or short to something or other)...

[HotBluePlates]

Are you really asking because you're doing a post-mortem on a dead output tube? I ask cause I seem to be missing something.

Otherwise, too much signal -> distortion of the output tube.
too, too much signal -> redplating of the output tube.
too, too, too much signal -> something will fail due to excessive current.

You hope that the design is such that the "something" is cheap and replaceable like a resistor or a fuse, not expensive like a tube, transformer or speaker.

This may be a good time to point out the folly of using "bulletproof parts" like way oversize cathode resistors or maybe screen resistors.

But I don't know for sure what'll happen... I've had 1 output tube ever be bad, and that was from a heater-to-plate short at the tube base, not from abuse.

[jojokeo]

I don't think there was an issue w/ the tube fitting the socket or replating that I've ever seen.

I was never planning on bringing this issue up until this happened again a few weeks ago w/ the pedal board. Then I don't ever recall anyone discussing a tube blowing issue like this so I figured what the heck to bring it up. 

The amp has performed flalessly for so long except these two blips that I feel I've caused them myself, I'm just not sure why it happened exactly and now I'm curious.

As you can see, the schem is pretty basic so I theororized that the only thing would be the sudden impact from the high signal as the root cause?! I'm still pretty stumped. I wonder if Sovtek's are more robust in this regard?

[stratele52]

Too much signal  ? keep the bias colder

[terminalgs]

Not just SE amps, PP amps as well.   Too hot an input signal will eventually find the weak link in an amp,  most likely in the power section.     Whether its too much potential gain engineered into the preamp or too much gain applied to the instrument before the amp (boost pedals, OD pedals,  any pedals that apply +dB ,,, or combinations of all of the above).

If the amp is well engineered,  it'll be the power tubes that give out rather than the OT or screen resistors, etc.   (Reason: tubes are easily serviced).  If you think of the amp as inst. Jack to speaker, which, of course,  we all do,  the speaker might give out first, if undersized (we've all done that).


A touring guitar player gets free amps from an endorsement when in europe. Rather than give him the top of the line amps, they give him mid-tier stuff made in china.  He sets the amp to "4", and then gets the rest of the gain off various combinations off his pedal board.  The manufacturer has to supply him a new head at least once a week because he blows the OTs. 

Of course,,,  at the edge of breaking might be where the amp sounds the best.

[jojokeo]

A touring guitar player gets free amps from an endorsement when in europe. Rather than give him the top of the line amps, they give him mid-tier stuff made in china.  He sets the amp to "4", and then gets the rest of the gain off various combinations off his pedal board.  The manufacturer has to supply him a new head at least once a week because he blows the OTs. 

That's about right as far what I recall my setting was when messing w/ the pedal board combo's.

*I finally had time to mess w/ the amp and various tubes this AM and the tube in question is not actually blown as reported. STRANGE!?! It tests "good" and strong on a B&K 747 tester but it is microphonic now though. I measured the pins w/ a micrometer and they seemed w/in range at appx .036" - .037" similar to a few others I checked. I also have a socket spring clip for the tube since it's hanging upside-down to help hold and keep it in place. Maybe I stressed the living crap out of it and after cooling & sitting a while the electrons/static charges or something bled away? I have no idea?

[PRR]

> I hit the switch for the CF stage to go from being bypassed

Doesn't that POP badly?

The bypass takes signal from a grounded point, volume wiper.

The CF is up at +150V or so.

The tone control reduces the 150V SLAM and the DC level settles to zero, but short-term it is still a HUGE slam to the EL84 grid.

And not necessary if you can afford another buck and 2" of space.

[jojokeo]

Yes, it doesn pop but now I don't switch until lowering the volume first. It's not on a relay where I go back and forth. But I like the idea and learning about it, thank you. I may use this on another build where I incorporate a relay or two.