• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

end of tube

hi,

Darn, i think the filament broke....
I used an 12ax7-a which gave a bright flash the first time i used it..
worked fine though, next two days nothing was wrong...just a bit of extra glow at startup (a lot of tubes seem to do this...)
but this morning, it flashed brightly again...and now it doesn't work anymore...

Now i use an ecc83, at startup one of the filaments will give quite a bright orange glow, after 0,5s or so it will resume startup.

It has to do with the filament being cold thus representing a low resistance...

Is the flash a product of aging???
Is the effect of tubes being quite bright at startup harmful, as in do they need current limiters? (though i almost never see them in amps)

one of my py500A tubes also flashes a little at startup...i'm getting worried....
 
Actually, IIRC it was simply omitting the heater-cathode insulation layer on the very end. Sausage effect, with the loop "squeezed" by the heavier layer of insulation over it, the thin lengths near the base connections heat up rapidly, until the resistance of the portion inside the cathode increases enough (positive tempco) that it levels out.

Tim
 
I bet it is one of the first tube in the heather chain... so it's getting the whole heater current capacity of the PT, instead the last tube, having a higher resistance because it's cold (and mostly because heater wires do have a finite resistance), will get less current.

I suggest some kind of limiting resistor: althought it shouldn't be needed... (given that the tranny puts out 6.3VAC and the line voltage is stable...)

Bye!
 
I bet it is one of the first tube in the heather chain... so it's getting the whole heater current capacity of the PT, instead the last tube, having a higher resistance because it's cold (and mostly because heater wires do have a finite resistance), will get less current.

I don't follow. "First tube" in what sense? And don't cold heaters have lower resistance?
 
I'll try to explain... sorry for my english... and sorry for saiyng wrong things...

With first tube I mean the nearer to the PT for filaments: you usually connect the filament wires to a tube, then from here to another, etc... So the first tube, because of heater wire resistance, is getting (at startup, as you said, heaters looks like shorts) more current than other tubes.

Don't blame me, I read it somewere! :confused:
Sorry for the other thing I confused the two things. Heather DO increase their resistance when hot :smash:

Bye!
 
Giaime said:
With first tube I mean the nearer to the PT for filaments: you usually connect the filament wires to a tube, then from here to another, etc... So the first tube, because of heater wire resistance, is getting (at startup, as you said, heaters looks like shorts) more current than other tubes.

Oh, y'mean parallel circuit current hogging?

The first tube(s) will pull more current due to the low "cold" resistance pushing wiring resistance to a visible percentage, but only for a split second and only very slightly (if your wiring hasn't melted yet, it's less than 5% difference).

Tim
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi kathodyne,

If the heater circuit is operating within design limits, it is not common for heaters to fail in a parallel setup. These tubes normally last over 10 yrs without heater failure. Heater failure can happen, but it's an isolated instance unless there is a bad production run on those tubes.

I mentioned that you can see heater flash more on 12AX7's only because often the heater wire extends outside the cathode sleeve and you can actually see it. This happens with all tubes but you may not be able to see the heater element clearly.

-Chris
 
Hello,

There was only one tube in the circuit... bu t i think i found out why....(probably)

The amp has to run on a 6v battery....
The heater-wiring is connected directly to the switch which connects/deconnects the battery...

But at home i have it running from a transformer with rectification... it gives 9v unloaded and (exactly!!!) 6,3v with the circuit switched on...

Now, my guess is that sometimes at an unlucky moment the peak of the switch being switched and the fact that it takes time for the voltage to lower to 6,3v (although my meter is not fast enough to read this time)

At such an occurence the filament could get somewhere between 9and 6v while it is still cold.... and break....

Well, it's the only solution i can think of because it seems to be very uncommon for filaments to break...
(but why has the ecc83 which also flashes lasted alreaydo for 3 weeks of everyday use???)

Or maybe it was just a freak-accident :)
Greetz,
Henk

Thnx for some thoughts about filaments!
 

EC8010

Ex-Moderator
2003-01-18 7:57 am
Near London. UK
Even if your transformer produces 9V open-circuit, so long as it produces 6.3V on load, then the valve will be as happy as it will ever be. If you had a digital oscilloscope across the valve and measured the voltage from a fraction before power was applied to the mains transformer until a minute after, all you would see would be a step from 0V to about 6V at the instant of applying power, and then a quite fast rise to 6.3V as the valves heater warm up and consume less current (allowing the power supply to achieve its on-load voltage).
 
But at home i have it running from a transformer with rectification... it gives 9v unloaded and (exactly!!!) 6,3v with the circuit switched on...

Now, my guess is that sometimes at an unlucky moment the peak of the switch being switched and the fact that it takes time for the voltage to lower to 6,3v (although my meter is not fast enough to read this time)

Mmmmh, and how do you obtain the HV supply ?
Inverter from 6.3V DC or fully separated ?
If from an inverter on the same DC supply, the load will be low until the tubes be hot enough to draw some plate current.
And you tell about PY500 ? Are there in the same amp ?

Yves.
 
The hv is extracted from the 6vdc by means of feeding it into the center tap of a 6-0-6v prim. 230-0sec transformer. At the outer legs of the 6v windings there is a converter circuit (lot like an astable multi-vibrator..) The hv is then rectified and filtered.....

The conversion takes up most of the power :)

The py500A is in another amp
As rectifying diode in the psu for my 7027a PP amp
It does the 'normal' flashing

Well the 12ax7 tube must have been wrong...because the flash was REAL bright, not like the 'soft flash' which other tubes give during startup....