VF14 tube recovery question

[o3misha]

Hi guys . I have a few questions that haunts me for several years, because no one can give me a clear answer . It is believed that the normal plate voltage of VF14 in U47 should not be lower than 32 volts. Otherwise , the tube is considered to be spoiled . Klaus  said absolutely incomprehensible things for me here:
http://repforums.prosoundweb.com/index.php?topic=35516.0
1. Why emission is considered bad if plate voltage is lower than 32VDC? Logically , if less plate voltage means more current - and therefore emissions contrary better!
2.Usually when the VF14 loses emissions, plate voltage increases above 34 volts , and not below, because the emission is less and the current is less. When performed revival of VF14 , the heating voltage for some time is maximized to ~55VDC  , it leads to the purification of the cathode and improve emissions. As a consequence , there is a reduction in plate voltage at several volts, not increasing!. When I restored one of my tube, I had  plate voltage  39 volts before recovery , and after recovery - 36 volts. Klaus says the opposite: after the recovery procedure voltage grows! Where is the truth?
3. Why is deemed unacceptable plate voltage lower 30-31VDC? I have a lamp that does not make noise, but has a plate voltage of 29 volts.
Thanks for answers and opinions!

 

[granger.frederic]

http://www.john-a-harper.com/tubes201/

 

[o3misha]

???

 

[granger.frederic]

you're welcome !

 

[o3misha]

I read such literature on my native language, but thanks anyway for the link.
Honestly, I did not found the answer on my question about : why two used  tubes  recovered in different ways? With one tube plate votage increased after regeneration (Klaus describe this), and in another tube plate voltage decreased after regeneration (usual situation, as I described)? I can't find the answer also, why exactly plate voltage lower than 32 VDC  is undesirable for U47? Noise? Also I'd  like to understand: why there are two opposing views on the plate voltage changes during aging lamps. I have always believed that the plate voltage increase and the current  decrease in the aging process of  lamp . However, I heard from Klaus, on the contrary, reducing the  plate voltage is the sign worn or even a faulty tube.

 

[granger.frederic]

plate voltage says nothing but the pate voltage!
you can't say if a tube is shot, just by measuring the plate voltage
if you read carefully the article, you'll understand that a tube is complex to measure, because each element interacts with all the others...
to know if a tube is shot, you'll have to draw the curves and see if they are at original specs
OR you have a very accurate tube meter ...
OR you hear it ...

i remember a ac701 successful regeneration ( awful discharges): 1000V with 50Mohms between anode/cathode during one week, and then 4.7V heather during 5h without HT

 

[o3misha]

Fred, I've read such articles a long time before, when I started to learn tube technology. I can't understand Klaus's point of view and he did not gave an answer.
In this case, I'm wondering example from your personal experience ,guys, which helped me to understand other points of view or definitively sure of mine.

 

[granger.frederic]

Mr KH, says that a lower anode voltage correspond to a tube weakness.

IMHO, it's totally wrong and absurd

anode voltage says nothing but the anode voltage.

emission is another parameter, like inter-electrode leakage,transconductance, vacuum leakage,inter-electrode capacitance,internal resistance...etc

you have to measure ALL the parameters to know if a tube is shot, then draw the curves and then compare to the original specs.

Some very accurate tube tester can evaluate a tube health for you, but only few for VF14 in triode mode...

VF14 ( multiple ) rejuvenation is quite complex and as i never done it before, i can't explain exactly the method, but it shouldn't be far from the ac701's method.
The most probable issue is the cathode caking due to the under-heating, thus you have to put around 65V without the HT during 5 to 8h, and see if it's better.But don't trust the anode voltage, trust only your ears...
If you have some discharges, then you have to try a re-ionization with 1000V (through a 50Mohms min resistor) between anode/cathode (during one week!), and then the cathode during 5h (after another week off).

However, if you want to do a static measure , you can re-create a fixed bias circuit ant then measure the cathode current.
But it does not tell if the tube is completely good, for that you'll have to do dynamic measurements and it's quite easy to do if you have some time.

 

[o3misha]

Fred, I totally agree that there are too many other parameters, besides plate voltage, which are more important. KH is very popular mic guru, but I don't agree with him in many aspects of technology. So, I decided to ask other guys. Thank you for posting your opinion. I hope that anyone else will post some info, because I realy interested to learn more about VF14's regeneration experience. I regenerated  several used VF14s from Ebay with full heating during 10h. It reduced noise and increased emission. I had never got increased plate voltage after regeneration,- just a bit decreased.

 

[bezen4uk]

There is my best recipe of how to reanimate contaminated tube (it worked 100% with two VF14 tubes, as well as many others).

Connect the tube in triode configuration (G2,G3 to plate).
Connect G1 to cathode via 1kOhm resistor.
Connect current meter between plate and plate and plate voltage source (or use 1 Ohm cathode resistor as current sensor).
Apply full heater voltage conform tube's datasheet (55V for VF14).

Cathode and G1 heating:
Find in tube's datasheet maximum allowable plate current (<18mA for EF14 and so for VF14).
Apply 0V plate voltage and rise it until plate current reaches maximum (<18mA for VF14).
Check if plate current is stable and doesn't drift to higher value.
Keep this process for 4-8 hours.

Plate and G2,G3 heating:
Apply datasheet's voltage values for heater, G1 and plate(+G2and G3) (for VF14: 55v, -4.5V and 250V respectively).
Check if plate current is in datasheet's range (12mA for VF14).
Keep this process for 4-8 hours.

G1 and cathode heating is needed because these parts tend to poisoning.
Plate heating is needed to run the whole tube at high temperature.

Sometimes you need more time to detoxicate the tube.

 

[paulcheeba]

Does anyone do this regeneration in the U.K. ? Thanks!

 

[paulcheeba]

This was posted on once by @bezen4uk

“There is my best recipe of how to reanimate contaminated tube (it worked 100% with two VF14 tubes, as well as many others).

Connect the tube in triode configuration (G2,G3 to plate).
Connect G1 to cathode via 1kOhm resistor.
Connect current meter between plate and plate and plate voltage source (or use 1 Ohm cathode resistor as current sensor).
Apply full heater voltage conform tube's datasheet (55V for VF14).

Cathode and G1 heating:
Find in tube's datasheet maximum allowable plate current (<18mA for EF14 and so for VF14).
Apply 0V plate voltage and rise it until plate current reaches maximum (<18mA for VF14).
Check if plate current is stable and doesn't drift to higher value.
Keep this process for 4-8 hours.

Plate and G2,G3 heating:
Apply datasheet's voltage values for heater, G1 and plate(+G2and G3) (for VF14: 55v, -4.5V and 250V respectively).
Check if plate current is in datasheet's range (12mA for VF14).
Keep this process for 4-8 hours.

G1 and cathode heating is needed because these parts tend to poisoning.
Plate heating is needed to run the whole tube at high temperature.

Sometimes you need more time to detoxicate the tube.”

 

[kingkorg]

The "guru" title is absolutelly appropriate. Thank lord we have engineers. Do everyone a favor, never re-post anything from Klaus Heyne. He has his heavily moderated (read North Korea moderation level) forum for spreading his disinformation.

 

[adc004]

VF14 ( multiple ) rejuvenation is quite complex and as i never done it before, i can't explain exactly the method, but it shouldn't be far from the ac701's method.
The most probable issue is the cathode caking due to the under-heating, thus you have to put around 65V without the HT during 5 to 8h, and see if it's better.But don't trust the anode voltage, trust only your ears...
If you have some discharges, then you have to try a re-ionization with 1000V (through a 50Mohms min resistor) between anode/cathode (during one week!), and then the cathode during 5h (after another week off).

Hi Frederic,
Sorry for replying to an old thread, but can you explain the AC701 rejuvenation technique to me? I have a noisy AC701, and since they are so expensive, I'd like to try these two techniques out. I haven't worked with tubes very often though, aside from changing them out when needed, so I'm not so familiar with the terminology.

- For the first one (cathode caking), you said to put 65V without HT for 5-8hrs. Where do you apply the 65V (what pin...heater)? Are there any resistors that you need for this process?

- For the re-ionization, what pins are these (anode/cathode)? You mentioned in an earlier post that the second step was applying 4.7V at the heater for 5hrs without HT...is that the cathode? Hopefully, I don't have to do this one, as I don't have a 1000VDC power supply.

Thanks in advance for your help!

 

[granger.frederic]

Hi,
65Vcc is for a VF14 , be carefull !!!
For AC701, try first 4,2Vcc (max 4,7) for a couples of hrs without HT, only on heater.
See if it's better...
then 1000Vcc or even more (with 50Mohms min, in serie) between plate and grid-cathode (together), for a couples of hrs, without heater. Use google for a voltage multiplier, quite easy but warning: dangerous voltage !!! Use gloves...
See if it's better.
Keep in mind that it's a torture test with no warranty of success.
Regards
Fred

 

[adc004]

Great, thanks so much! I thought that was too high for an ac701 I’ll give the first procedure a try through my regular mic power supply with the HT disconnected. Hopefully that helps with the noise.

But just to confirm, for the second procedure, when you say 1000V between plate and grid/cathode (together), do you mean this setup like this?

Plate (red dot pin 4) connected to a 50Mohm resistor then to 1000VDC

Grid-Cathode (pin 2 tied to the top pin) connected to the ground of the power supply

I looked up some circuits for the 1000VDC…some people say they have a decent amount of ripple…is that a concern?

Thanks for all your help! I’m sure this is great information for others with old AC701 tubes that might need a little rejuvenation.

 

[granger.frederic]

Hi,
yes that's a correct wiring, a little bit of ripple is not critical
One more time, this process is based on science theory (low rate of success) and potentially dangerous (high voltage)
regards
Fred

 

[adc004]

Hi,
yes that's a correct wiring, a little bit of ripple is not critical
One more time, this process is based on science theory (low rate of success) and potentially dangerous (high voltage)
regards
Fred

Great thanks! Yeah, I figure chances of success aren’t very high, but hopefully it helps a little. Will be an interesting experiment. I hope it helps others with noisy ac701 tubes…please post your experiences if it does!

 

[nielsk]

Can anyone supply instructions for this on an EF14?

 

[granger.frederic]

Hi,
Same process applies to all kind of tubes (triodes,pentodes,etc) as long as you don't over-volt too much the filament...