Hi there!
I bought some NOS 6p14p pentode tubes on internet to replace the EL84 in my Peavey which is supposed to be an equivalent. I can't Bias it propperly. I have tried with its own Rk(150 Ohm), giving 94mA in Plate and changed to a 470Ohm but just drop to 81mA. Should I try with a larger Rk or is there any problem with this tube ( maybe I have burnt it). I also should say that this tube is glowing blue when I switch on the Standby but later it disappears.
I bought some NOS 6p14p pentode tubes on internet to replace the EL84 in my Peavey which is supposed to be an equivalent. I can't Bias it propperly. I have tried with its own Rk(150 Ohm), giving 94mA in Plate and changed to a 470Ohm but just drop to 81mA. Should I try with a larger Rk or is there any problem with this tube ( maybe I have burnt it). I also should say that this tube is glowing blue when I switch on the Standby but later it disappears.
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The most important thing with a pentode is the g2 voltage; have you checked this? What is the target anode current? Remember that if you are measuring at the cathode resistor then you must include the g2 current as well which could be up to 10mA.
The Vade-Mecum states that the 6P14P is a direct equivalent of EL84.
As for the blue glow this is fairly common - especially in 5881s and 6L6GCs. If it disappears then all is probably OK. Persistent blue could imply gas, but this may disappear anyway with use.
Paul
I looked up the schematic elsewhere - it does not have the cathode voltage marked. Your schematic shows 16 volts which is way to high for an EL84. At a guess, 6 to 8 volts is what I'd expect to see.
With the voltages and currents that you have it almost sounds like you have a grid to cathode short in the valve or at the socket, but then you'd get no audio through the amp. Or maybe the grid is floating and not at zero volts... (but if you try to measure it it will be grounded by the meter, so don't let that fool you).
With the voltages and currents that you have it almost sounds like you have a grid to cathode short in the valve or at the socket, but then you'd get no audio through the amp. Or maybe the grid is floating and not at zero volts... (but if you try to measure it it will be grounded by the meter, so don't let that fool you).
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Assuming nothing else is wrong, the tubes are not compatible even though they are supposed to be. They may be defective or fall-outs sold by an unscrupulose source. This is a typical reason why I despise todays tubes from the turd world. You never know what you're going to get. You should always buy tubes from a reputable dealer.
Hi again!
First of all, thank you to all for your unvaluable help. You really do the homeworks.
These are my readings and calculations for my 6pi14pi 0681 labeled pentode before grounding grid and after:
Vg1=3,3V:
Ik=Vk/Rk; Ik=17V/150Ohm; Ik=113.3mA
Ig2=(Vr9-Vg2)/R9; Ig2=(228-215)/980; Ig2=13,26mA
Ip=Ik-Ig2; Ik=100mA (Hell hot plate!!!)
As 12E1 said the meter fooled me with the Vg1 reading.
Grounding g1:
Vg1=0V
Ik=Vk/Rk; Ik=6V/150Ohm; Ik= 40mA
Ig2=(Vr9-Vg2)/R9; Ig2=(257V-250V)/980Ohm; Ig2= 7,14mA
Ip=Ik-Ig2; Ip=32'85mA
So it seems that the VR3 ground pin (It is a 16mm Alpha pot) was the culprit as it doesn't give the 1MOhm expected between its pin 1 and 3 or 1 and 2, I'm afraid I stayed too long with the soldering on it.
I know it's better to buy from a reputable seller to avoid surprises but I understand that in 90% I should check what I buy despite the dealer reputation and for its price on the Ukrainian site (sovtube.com) I think it is wellworth to try with some of this radioctaivity contaminated NOS tubes from Chernobyl
.
Now I'm wondering how this plate tube will last after this stress (Ipmax=65mA).
Billions of thanks to all
First of all, thank you to all for your unvaluable help. You really do the homeworks.
These are my readings and calculations for my 6pi14pi 0681 labeled pentode before grounding grid and after:
Vg1=3,3V:
Ik=Vk/Rk; Ik=17V/150Ohm; Ik=113.3mA
Ig2=(Vr9-Vg2)/R9; Ig2=(228-215)/980; Ig2=13,26mA
Ip=Ik-Ig2; Ik=100mA (Hell hot plate!!!)
As 12E1 said the meter fooled me with the Vg1 reading.
Grounding g1:
Vg1=0V
Ik=Vk/Rk; Ik=6V/150Ohm; Ik= 40mA
Ig2=(Vr9-Vg2)/R9; Ig2=(257V-250V)/980Ohm; Ig2= 7,14mA
Ip=Ik-Ig2; Ip=32'85mA
So it seems that the VR3 ground pin (It is a 16mm Alpha pot) was the culprit as it doesn't give the 1MOhm expected between its pin 1 and 3 or 1 and 2, I'm afraid I stayed too long with the soldering on it.
I know it's better to buy from a reputable seller to avoid surprises but I understand that in 90% I should check what I buy despite the dealer reputation and for its price on the Ukrainian site (sovtube.com) I think it is wellworth to try with some of this radioctaivity contaminated NOS tubes from Chernobyl
Now I'm wondering how this plate tube will last after this stress (Ipmax=65mA).
Billions of thanks to all
Well done! I'm glad you found what the problem was. The valve that got hot will probably be OK so long as you didn't cook it for too long. 7N7's advice to compare it with another is the best option unless there is obvious damage. Valves are pretty tough compared to transistors. As for damaging the pot with soldering - some parts dislike solder temperatures more than others. It's just one of those things that makes life interesting. 
Hi,
In fact, I have seen a very small crack at the end of the carbone crack in the Vr3 pot which cut the circuit to the ground pin. Due to the lack of space inside the amp, the pin was bended against the last coupling cap in the preamp section. It was a Peavey Nanovalve converted to ValveKing8. They are almost the same amp but with tone and master vol. pots and I added the standby and triode-pentode switch. Good amp to experiment with (low power, small and cheap).
Regards!
In fact, I have seen a very small crack at the end of the carbone crack in the Vr3 pot which cut the circuit to the ground pin. Due to the lack of space inside the amp, the pin was bended against the last coupling cap in the preamp section. It was a Peavey Nanovalve converted to ValveKing8. They are almost the same amp but with tone and master vol. pots and I added the standby and triode-pentode switch. Good amp to experiment with (low power, small and cheap).
Regards!
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