Davy's KEL84 Grid Voltages

[Richard]

Originally Posted by David Mccallum

Hi Richard,i was wondering if you had tried another type/make of el84 to see if it gives the same results.I dont have any spare so couldn't try changing them.

Cheers

Good thinking Batman Yes, found 2 sets tested and marked;

5 Mullards, all very stained and used-looking with various currents but all good slopes. One I had also marked "insulation US" so that looked interesting.

1 set 4 boxed JJs which are very closely matched according to my AVO with only 2mA between any of them and all same slopes.

In with the JJs and the results are impressive; all valves showing 11.0V at the cathodes and 0V on the grids after 15 mins.

Then the Mullards, paired reasonably on currents and not using the "insulation US" one,

Cathodes were a little varied but quite OK in the 10.6V to 11.3V range and closer between pairs. Grids were all 0V except one which was 0.07V. A tiny amount but definitely different to the others.

Now the one marked "insulation US" was paired with another which had the same test current and I monitored voltages closely after switch-on;

Cathode and grid voltages steadily rose and I thought the thing was going to run away. Just as I was about to switch off it started levelling out and a glance at the plate showed no redness so I left it on. 7.4V on the grid and 17.1V on the cathode Monitored it for 20 mins and it stayed there and the plate didn't glow red.

From the outside you wouldn't know there was a problem but I can't see that valve lasting much longer. There was no output so the amp would still be silent. Suggests grid voltage will start rising with age but Davy's valves may well be fine for a long time yet. Check them again in a while.

Rich

[David Mccallum]

Hi Richard.

Interesting results indead,i think i'll have to monitor this when i get the amp back in use(im missing that bass at low volume) so i think it will replace the icon amp, for now at least .I will change my speakers at some point so i'll compare them again then and use the amp i like the most at the time,the KEL84 will probably win again.
I might have to get some spare EL84's though,thank's again for all the help.

[David Mccallum]

I found this and wondered if it could help regarding G1 voltage.http://www.eierc.com/rc/EL84.htm

[powertriode]

Wouldn't be a leaking coupling cap would it?

[NickG]

It follows the valve.

[Richard]

All this has reminded me of my Kit88 which had small differences on the op valve grids despite being fed half its bias as fixed type. Grid current must have been the reason.

That lead me to think of the failures of those JJ KT88s in the early days. Plate disippation was reduced, and helped, but if we look at the gridleak values against the data sheets we find they should have been a max of 1/2 the fitted value of 330K.

Checking that amp's predecessor, Kel34, suggests it may simply have inherited the 330K value from that design. El34s are fine with up to 500K whereas KT88s aren't....

Checking 6550 valve data against Kit6550 shows a similar situation; the gridleaks fitted are 560K whereas the data sheet says 250K max. OK, perhaps not a problem for new valves but if they do start to pass grid current as they age those gridleaks are going to double the amount of bias created and hence double the increase in current passed.

That said, Kit6550 was supplied with Svet 6550C which I found were very reliable in Kit88, and I don't remember any catastrophic failures, though I wore a couple of sets out.

I wonder if the Svet 6550C "gold-plated grids" help lower grid current. If so there may well be some truth in makes such as Mullard using better materials than lesser brands as those EL84s I checked in Davy's were well-used (they looked more than the 350 hours of Davy's JJs) but showed little grid current - except that very-tired one.

Rich

[Richard]

HI Nick

I came across this Russian data sheet for the valvey things thread,

http://www.tubes.ru/techinfo/HiFiAudio/6p3ce.html

The valve is a Russian equiv 6L6 but what intrigues me is the differences in left hand and right hand data columns. The RH one for the E spec valve seems mostly to be a de-rating of the LH one in voltages to allow greater hours life.

A ruggedness spec is also included but the interesting thing is how the grid resistance has been lowered. I read this as helping it cope with grid current after long hours. What do you think?

Rich

[NickG]

Yes, interesting, the E version is spec'd has haing lower grid current anyway, but the lower grid resistance may be a belt and braces thing as you say.

I was thinking about this. The cause of grid current (when the grid is negative of the cathode) must be due to emmision from the grid (I also assume that if the space charge area and the grid coincide then current will flow). So all it takes is some of the cathode coating to find its way to the grid and it would emit.

I will se if I can find any papers on this, I expect its a well covered subject in the old texts.

[BOONDI]

Hi Nick, all...

I post the following as an extract from here...

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

To quote:
Grid Current
Since the grid has no physical connection to anything (as the circuit symbol shows), it is natural to think of it as being electrically isolated. In fact, this is not the case, because of the flow of electrons and ions inside the tube.
Current flow to and from the grid arises for two reasons. The most obvious is when the grid is positive, which causes it to attract electrons. Less obviously, when the grid is negative, it attracts positive ions resulting from collisions between electrons and gas molecules. These cause the grid to become less negative. It is because of this effect that the grid must never be left truly floating but must always be connected through a resistance, typically for a small tube not more than 1MW. Without this, the voltage of the grid will gradually creep up, reducing negative bias and increasing current through the tube and leading to a runaway which, at worst, will destroy it through overheating. Tube gassiness is measured, in tube testers, by measuring this small current (typically a few nA), which is in direct proportion to the amount of gas in the tube.
At the right voltage, these two currents cancel out. This normally occurs at around –0.5V (depending slightly on plate voltage), which is slightly positive relative to the virtual cathode. If the grid is simply left disconnected, it will float at this voltage.
Positive grid current is deliberately used in some cases, for example in high-µ transmitting tubes. When it occurs, Child’s Law (or the more accurate formula taking account of initial electron velocity) gives, not the plate current, but the sum of the plate and grid current. This is because total current is controlled by the field in the immediate vicinity of the cathode, which is due to the combination of the two other electrodes. To calculate the actual plate current (and hence also grid current), it is necessary to know how much of the electron stream flows to each of the two electrodes. This is given from the following formula:
where:d = current division factor
The current division factor is slightly greater than the shielding ratio of the grid. If grid current curves are available for a tube, it is easy to determine its value from them.
Unfortunately this tidy formula only gives the value for the primary current. Once the grid is more positive than the plate, secondary emission will start to occur, resulting in a secondary current flow from the plate to the grid and reducing the effective grid current. This is, for all practical purposes, unpredictable. Depending on the tube, the secondary current may even exceed the primary current, typically at around 30-50V, resulting in a second stable voltage for a disconnected grid at the point where the primary and secondary currents exactly balance.
Positive grid current results in heating of the grid, for the same reason that the plate gets hot (i.e. due to the kinetic energy of the arriving electrons). Small tubes are not generally designed for this, but power tubes and especially tubes designed for positive grid operation have substantial dissipators attached to the grid structure. Overheating of the grid is bad for two reasons. Firstly, since it is not designed to run at a high temperature, it does not have any way to retain its tension as the metal expands. This results in changing geometry and in the worst case melting or a short to the cathode, which is instant death. Secondly, the grid is generally contaminated by oxide from the cathode, and if it gets hot then it will start to emit electrons like the cathode, resulting in a substantial secondary current

[Richard]

Thanks for that Boondi. Yes I'm sure it was all known by the old timers but it's good to think it through and try to understand.

Grid emission suggests gold plated grids may be an effort to lower the effect. It also reminds me of a set of Sovtek KT88 I bought several years ago.

When tested hot and switched back through the insulation tests they showed low resistance, that is 5 meg or so on a new valve. I had several email comms with New Sensor in US about it and they explained it was the tester reading "secondary emissions" and here we see that phrase again.

In their case they said the emissions were due to a new valve and would decrease with use. I didn't keep them long enough to check that I'm afraid but suppose it could be so if something is on the grid.

Rich