Power tube extra safety device

[Colas LeGrippa]

Hola amigos como están ?


Quien habla español aqui ?




A good trick to preserve power tubes life is to put a fuse in series with the cathode to gnd.
Let s suppose your bias is set to 70mA,
Fusing the cathodes at 100 mA will avoid a tube to redplate if, by example, the second tube  besomes short or dead. You can put Fuses receptacles next to the line fuse for ease of change. A HV fuse of 500mA is not enough to preserve your 500$ 1960 Gold Lion NOS tube.

[shooter]

hola, mi Espanol esta piccito menos 

[Colas LeGrippa]

No hay problema vamos a hablar inglés entonces !

[kagliostro]

Se voe' parlemo anca venexian 

Franco

[shooter]

I never figured out English, spelling, usage, but I can ask for the Bano 

[jjasilli]

In the good old days 10 Ohm resistors were used for this purpose.  They could also be used as bias sense resistors, if you don't mind a moving over a decimal place.

[DummyLoad]

estoy bien gracias! yo hablo bastante bien, lei y escribo también. vivíamos y fuimos a escuela en las islas canaria. ?de dónde viene ud.   

--pete

[Colas LeGrippa]

Soy canadiense, de montreal. Yo hablo español p que mi novia es cubana y me lo enseñó. Mi primer lenguaje es el francés. Yo hablo inglés porque lo aprendí en un banco donde trabajé unos años. Es muy especial hablar español en este foro !

[Colas LeGrippa]

Kagliostro, I had started to learn italian with my girlfriend, Sylvia Rozzi, but we splitted before I could learn more than : buon giorno, ciao, grazie.....I still have italian books at home.
You are from northern Italy as I can see.  Only went to Torino in a trip I made to France.

[Colas LeGrippa]

Jjasilli, can you explain how a 10ohm resistor could replacee a fuse ? It was opening under stress ?

[jjasilli]

Yes, it will burn out like a fuse in an overcurrent  condition.  It's a bit easier to implement than a fuse - no fuse holder required.  But it will read 540mA bias current instead of 54mA from a 1 Ohm bias sense resistor, which freaked me out at first.  Of course you can add a 1 Ohm R on top of the 10 Ohm.

[Colas LeGrippa]

I would like to have a device installed permanently to monitor the tubes plate current, like an ammeter in series between B+ and each tube plate. A cathode current reading is not as accurate 'cause it shows both plate and screen current. I have found ones in an electronic store in Montreal where I live but they are cheapo and I don't trust them. At 2.99$ from China I don't think they will do the job right. I need a good one, made in USA, 0-125 mA @ 500V.
I would insert them in the front plate, with the controls, like db meters on mic preamps.

[Colas LeGrippa]

.....the look would be awesome !
Lemme know if you find something suitable for that purpose


Colad

[jjasilli]

Then put a 1 Ohm current sense resistor as a "plate R" to the power tube in the B+ supply.  Use a volt meter across that R. 

[kagliostro]

Ciao Colas


Previously I wrote in Venedig slang, not italian,  I live around 35 kilometers far from Venedig, my town Is Treviso


Franco

[Colas LeGrippa]

You gave me a heartache Kagli, I really couldnt understand your italian !


Ciao!

[John]

I'm shocked Colas won't trust a $3 part with 400 volts. He should kiss his wife and get her opinion! 

[Colas LeGrippa]

Already done John ! 

[pdf64]

A problem with fusing the cathode returns of power tubes is that a HT fuse is till necessary, as a cathode fuse won't blow with some common failure modes, eg g2 / plate shorting to the heater.
We know that's common because amps often get buzzy after a power tube short, due to heater balancing resistors blowing.
Of course we could have HT AND cathode fuses, but an amp can end up with lots of fuses.
My preference is, as mentioned by jjasilli, to use 1 ohm 1/2 watt MF cathode resistors. I can vouch that they pop pretty quick if, eg the tubes lose bias. Don't ask me how I know this 

I suggest to avoid any any unnecessary circuitry (eg current meters and the wiring to them) into a power tube plate circuit; it becomes a transmission antennae, creating feedback loops to earlier parts of the circuit, hence it may reduce the amp's margin of stability, perhaps to the point of oscillation.
Such current meters would be much better in the cathode circuit, as the signal voltages are vastly lower, usually effectively zero due to decoupling cap/s, so no likelihood of feedback issues.

[Colas LeGrippa]

I had already hooked up 1/2 watt resistors at the k. One of them went dead open and the other tube started to redplate. What I will do then, if the ammeter can cause problems eventually , is monitor the k current through 5w 1 ohm resistors AND put 100mA or so fuses to protect the tubes. I prefer replacing a fuse than a resistor. Removing the HT 500mA fuse is not a good idea that s for sure.
What do you think of a chassis volt meter that would monitor the voltage drop accross the power tranny secondary ? Knowing the resistance between the secondary ct and the plate leads, we can find the plate current by ohm's law . Then we get the real plate current.


Colas

[pdf64]

...What do you think of a chassis volt meter that would monitor the voltage drop accross the power tranny secondary ?

Sorry, I can't work out what you mean with this? Could you sketch out schematic of your idea, take a photo, upload it to imgur or similar, and provide a link?

[Colas LeGrippa]

The voltage drop between the output tranny primary center tap and one of the plate tap is function of the current drawn by the corresponding tube connected to this tap. If we know the resistaces values between the ct and the plate supply ( blue/ brown) which values stay the same all the time AND the voltage drop between the same taps, we know the current.
Eg; 3 V voltage drop between red and blue leads and resistace of 50 ohm between the same leads then the current drawn is 3 ÷ 50 = 60mA which is the PLATE CURRENT, without screen current. So the voltage drop would give us  the plate current. A 0-20 VDC METER wth a switch to select between the tubes would  give us the plate current. Of course, we would have measured the resistance before and noted it somewhere in the back of the amp, to avoid opening it.

[Colas LeGrippa]

.....I understand now why you got confused.
I had written "power tranny secondary" instead of output tranny primary....
which is not quite the same I must admit

[pdf64]

The switch would have to be a 3 position 'center off' type, and with the switch you're back to adding circuitry to the power tube plate. Plus any regular switch is hopelessly underrated for the kV that a generated between either leg of the OT primary, though I acknowledge that many amps have triode/pentode switches there that seem to survive.
Really though, you may be adding difficulty for no tangible benefit, cathode current measurement is plenty accurate for a tube guitar amp, it's not as if it's a guided missile or precision lab gear, where accuracy is a prerequisite.
At idle plate current levels, screen grid current is a fairly constant, known proportion of it.

[shooter]

is plenty accurate

yups, just put a yellow post-it next to the meter that says  "Minus 5%" 

[Colas LeGrippa]

Any 125V rated switch would do the job. From 400 to 395V, even a 25V rated switch would be good for life.