How long it will last?

[Mas Penk]

Hi all,
I built EL84 direct coupled michael "Tube Master". I like the sound very much (thank you tube master...)
But due to the driver is pushed harder (8mA) my friend said that this will shorter the tube life. And when driver die it will kill the output too since grid of output tube will recieve high voltage. my friend said if i played in normal listening for 3 hrs/day then the tube will last about a year....

is this true? is there anything i can do to prevent this.... at least is there anything i can do to make the output tubes don't die together with the driver...


rgds

Mas Penk

[Joel]

8mA is not pushing a 12AU7 hard.

Joel

[alexistheo]

if the direct coupling really scares you, you can always put a fuse in the B+. however, joel is correct: the 12au7 can take 8ma easily

[tubemaster]

Hello Mas,

congratulations on finishing the project!

Originally posted by Mas Penk
But due to the driver is pushed harder (8mA) my friend said that this will shorter the tube life. And when driver die it will kill the output too since grid of output tube will recieve high voltage.

Don't worry about that at all. The typical rating of the ECC82 according to the Philips Datasheet is 10-12 mA. The maximum allowed cathode current is 20 mA so you are well within the limits. Depending on the model expect the tube to last for many years indeed! I have seen ECC82s which have been in service for more than 30 years.

Apart from that the tube normally does not fail instantly but will slowly go weak with less gain and a change in sound so you have plenty of time to notice. It is very rare for a preamp tube to fail completly, I have never seen this happen myself.

So don't worry and enjoy the music!

Best regards
Michael

[Mas Penk]

Thank You Guys...
All your info make me feel good to keep this circuit on my system...

Thank you....

rgds

Mas Penk

[Johan Potgieter]

Just to further reassure (I have used them for decades).....

The RCA Tube Manual gives the maximum allowable continuous anode current as 22 mA and the plate dissipation (if both triodes are used) as 2.75W. You are drawing 8 mA and dissipating 800 mW. So sleep peacefully.

[SY]

Connect a reverse-biased diode between the grid and cathode of the output tube to protect it in case the driver goes out. In normal operation, the diode is an open circuit, but if there's a fault which tries to drive the grid positive, it will act as a clamp.

Hmmm, second time today I've given that advice.

[cogsncogs]

Guys I believe Mas Penk was talking about his output tubes; EL84

is this true? is there anything i can do to prevent this.... at least is there anything i can do to make the output tubes don't die together with the driver...

Sy is correct. I would use a 1N4007 from grid to cathode, the anode side of the diode connected to the grid and cathode to cathode. Say the filement burned out, went out, or some other fault condition like a bad tube socket, that diode will "protect" your output tube. It will shunt current from the grid to cathode and then to ground through the cathode resistor.
Also Me thinks Mas Penk may be worried that the 12AU7 may pull too much grid current from the EL84 during maxed out operation. Sy's suggestion will prevent that from happening. We don't want your amp to go into class "C" operation! joke! Well we would if we wanted a RF transmitter!
You may hear some very nasty asymentrical clipping if driven way past the operating point due to positive grid current. You would also hear it with the diode in place due to it clipping the positive going output of the driver, starting at about 550mV. Which means you've overdiven the EL84 as a result of the cathode not keeping up with the grid; grid going positive with respect to the cathode.
Whew! So as long as you don't push it that hard and use a protection diode, you're fine! Always use protection, son...yeah!

Wayne

[Mas Penk]

Do you mean like this....? for me it looks interesting
Will this diode have any effect on sound?

thanks

[ErikdeBest]

Don't know if it is of much use, but thought I would post it anyway.

http://www.glass-ware.com/tubecircui...Amplifier.html
http://www.glass-ware.com/tubecircui...plifier_3.html

Erik