Would wrong bias setting hurt my Tremoverb DR in standby?

[thinskin57]

i was trying out some EL34 tubes in my tverb. when i was done i put my 6L6 tubes back in but forgot to set the bias switch back to the 6L6 setting. i turned the amp on and left it in standby for about ten minutes. in this time, i realized what i'd done and ran back to the amp, turned it off, and set the bias back to 6L6. so, if there's a good thing about this it is that the amp was never taken out of standby but i'm still wondering if i might have caused any damage to the amp. anyone?

 

[elvis]

Probably not. Standby usually just turns on the heaters, not the bias.

 

[Gibson07]

Newbie question but out of interest, what sort of damage could/would it do to your amp if you run it in the wrong bias setting? Do you ever need to get the bias adjusted in a rectifier (like if you put non mesa branded power tube in)?

 

[elvis]

Too cold (EL34 with amp set for 6L6, or tubes with weak bias) will not sound very good, but do no damage.

Too hot (6L6 with amp set to EL34, or tubes with too strong bias) will run very hot and overwear the tubes, possibly blow them. The amp should be OK, but a tube detonation can damage the amp. Also, hot enough bias will dissipate a lot of power in the output tranny, maybe cook it.

Bias sets the idle current (DC current) in the output tubes. That current also goes through the output tranny. Bias works by setting the DC input voltage of the tube to get the right amount of idle current. At a given bias voltage, an EL34 produces less current than a 6L6. So the bias voltage is increased when using EL34 tubes.

So 6L6 bias voltage will produce the correct idle current in 6L6, but low idle current in EL34. EL34 bias voltage sets the right current in EL34, and too much in 6L6.

Tubes come with a wide range of current vs. voltage specs. Some work within the intended design characteristics of the amp, some don't. Many drift outside spec after hours of use. The purpose of bias is to make sure that the idle current is correct for the amp design. If the tube is outside of Mesa's design spec, then the idle current will be too low or too high. Adjusting bias can correct that. Without adjusting bias, you would have to replace the tube to correct it.

The guitar signal is AC coupled to the output tubes, and modulates the idle current to produce AC output. Think of the input to the tube as DC bias voltage + AC guitar signal voltage. Output is idle current + AC modulated current.

I know, I ramble. Enjoy!

 

[Gibson07]

Thanks Elvis!! Very informative.

 

[domct203]

Let me add to this subject.......

Not all tubes will draw the same idle current. Just because the amp is set to "6L6" bias does not mean that you can put any 6L6GC tube in the amp. Mesa selects their tubes to operate correctly at their amp's specs. If you want to try non-Mesa branded tubes be sure to have your seller supply tubes that fall into Mesa specs. Same goes for EL34 type tubes.

As a general rule of thumb, do not exceed 70% of the tube's maximum plate dissipation (MPD) at idle (no signal). Plate dissipation is measured in Watts. A 6L6GC can safely dissipate 30 watts (in the form of heat), an EL34 can dissipate 25 watts.

Idle (bias) current is measured in amps, milli-amps (mA) actually. Using Ohm's Law you can calculate plate dissipation if you know idle current & plate voltage (Ohm's Law states that Volts * Amps = Watts). I like to run the 6L6's in my Roadster at approx 55% MPD at idle. The Roadster puts about 460V on the plates, so I set the Idle Current (bias) to approx 37mA. Using Ohm's Law, I multiply Plate Volts X Idle Current to get the idle plate dissipation in Watts (460 * .037A = 17W) 17W is 56.7% of 30W (17/30=0.567 or 56.7%), 30W is the maximum the tube's plates can take before they overheat and "red-plate", the term used for when the plates get so hot that they start to glow red like when you heat metal with a torch. This destroys the tube and risks taking out other components in the amp.

If I was to run EL34's at those setting I would be at 68% of MPD (17/25=0.68 or 68%), which makes them a little too compressed and saturated for my tastes in a Recto.

BTW - when the amp is in Standby, there is no voltage applied to the plates so the tube is drawing zero mA and is essentially turned off.

Hope your head isn't spinning too fast ..........

Dom