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Old 7th June 2012, 06:56 PM   #1
Niquel is offline Niquel  Spain
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Default VALVEKING ROYAL8 with 6p14p-EV

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.
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Old 7th June 2012, 07:21 PM   #2
7N7 is offline 7N7  United Kingdom
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Originally Posted by Niquel View Post
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.
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
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Old 7th June 2012, 07:34 PM   #3
12E1 is offline 12E1  United Kingdom
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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).

Last edited by 12E1; 7th June 2012 at 07:37 PM.
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Old 7th June 2012, 07:38 PM   #4
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Quote:
Originally Posted by Niquel View Post
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.
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.
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Old 7th June 2012, 07:58 PM   #5
DF96 is offline DF96  England
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Is anything connected to pin 1? Some valves use this for an internal connection, others leave it isolated. Some circuits use it as spare tag, assuming it will be isolated, and will fail if a valve is inserted which has pin 1 connected to something inside.
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Old 8th June 2012, 04:06 AM   #6
6J6 is offline 6J6  Australia
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Hmmmm. I bought some regular military Nos 6P14P's from Ebay and over half of them were used and had black burn marks on the glass where the plate holes are. So you never quite know what you are getting until it arrives. Perhaps the ones you have are borked, or relabelled 6P15's??......
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Old 8th June 2012, 05:16 AM   #7
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Something is wrong.
What do you measure on cathode when you ground first grid (pin2)? Does it change?
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Old 8th June 2012, 10:28 AM   #8
Niquel is offline Niquel  Spain
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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 ChernobylClick the image to open in full size..

Now I'm wondering how this plate tube will last after this stress (Ipmax=65mA).

Billions of thanks to all
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Old 8th June 2012, 10:38 AM   #9
7N7 is offline 7N7  United Kingdom
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Good news.

As for the valve that was cooked, try plugging in one of the others and compare voltages; if there is no substantial difference then you have probably got away with it.

Paul
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Old 8th June 2012, 01:05 PM   #10
12E1 is offline 12E1  United Kingdom
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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.
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