simple tube amp questions

dbw

Senior member
Messages
4,531
Okay, everyone can probably answer the first one and probably only -CB- can answer the last one.  Here are my latest batch of tube amp questions...

1. Is it safe to have a tube amp on, in standby, with no load?
2. Is it safe to run an amp expecting a 4-ohm load through a 8- or 16-ohm cabinet?
3. If I "strip" the cathodes in my tubes, as -CB- explained way back on my Tubes Tubes Tubes thread, what happens in terms of the amp?  Does it get quieter?  Does it run hotter? colder?
4. Are both power and preamp tubes prone to cathode stripping?

Thanks guys!
 

Volitions Advocate

Senior member
Messages
1,239
1. Yes, perfectly safe. I do this with my head while i record sometimes. or at least its safe with my amp.
2. CB just told me that you can hook cabs up with a higher impedance okay.. but that was for a solid state amp. so i'm not sure about tubes.  I've heard people say that so long as you're only one increment away than it should be okay.. like if you're head wants 4 ohms its okay to use an 8 ohm cab, or if it wants 8 ohms its okay to run a 16 ohm cab.  but i've found no evidence that says its okay and I havent tested it out so dont take my word for it. wait for CB :p

i dont know about cathode stripping
 

-CB-

Senior member
Messages
5,427
dbw said:
1. Is it safe to have a tube amp on, in standby, with no load?
2. Is it safe to run an amp expecting a 4-ohm load through a 8- or 16-ohm cabinet?
3. If I "strip" the cathodes in my tubes, as -CB- explained way back on my Tubes Tubes Tubes thread, what happens in terms of the amp?  Does it get quieter?  Does it run hotter? colder?
4. Are both power and preamp tubes prone to cathode stripping?

1.  Yes, and no, and maybe.  Its never "safe" to have no load on a tube amps output.  It will self destruct.  Marshall is famous for this - because even a high impedance mismatch gets their output all bothered.  The trade off is greater saturation in their transformers... and the tone.  Fender actually "dead shorts" the output when the plug comes out of the jack.  This is a safety thing, as its better to dead short the output then run it "open".  While the amp is on standby, the B+ is disconnected, so technically it would be safe to have no output when in standby... but you can see the handwriting on the wall for disaster in that scenario. 
2. Most Fender amps can handle double the rated impedance, or half the impedance.  Marshall iron is not as over spec'd as Fender and can cause problems - particularly if you run a 16ohm cabinet on a head seat for 8 or even worse 4 ohms.
3.  Cathode stripping will result in some noise (maybe) and general loss of power.  The tubes will not be able to conduct as much signal, and so you'll have a softer sounding amp.  This will not result in loss of overall volume, but will result in the "crisp" and the "thump" not being what they should be.
4.  Yes and now.  Power tubes, defiantly.  Preamp tubes are generally run at comparatively low voltages on the plates - in the range of 150-225 volts, depending on the design.  Most older amps ran lower voltages, newer amps tend to run them up a little past 200v or so.  The higher the voltage, the worse the stripping will be.  But, preamp tubes also get up to heat faster then those big ol' power tubes, so generally in a comparison on the same amp, the problem will be evident on the power tubes first.  And.... preamp tubes are lower current handling, they're acting as true "amplifiers" rather than just control output current.  Very little current flows in a preamp so the stripping effect is way lower.  In an output tube, the tube is there to not multiply voltage across a resistor, but to control the current flow through the output iron.

If you run a solid state rectifier - stripping is worse, since the B+ is RIGHT THERE when you power on (unless you use standby).
The 5AR4 rectifier tube is one that practically eliminates stripping, as it takes a long time to get up to temperature.  5Y3's and 5U4GB's are more prone, as they get hot quicker, giving B+ quicker - and before the 6v heaters get up to temperature.
 
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