Output Impedance error

[desertmouth90]

Do you think the screen grid resistors have gone and the valves are fine?

Do they look burned? If you measure them are they still within spec??

Will measure tonight, but no they dont look burned

[Roe]

The higher the load impedance, the lower the plate voltage swings. When at large plate current the plate voltage swings much lower than the screen voltage, the screen current (and thus screen dissipation) shoots up, because the screen now is more attractive to the electrons than the plate.

exactly, with this kind of mismatching you get a 8.8k load on a quad of tubes, instead of 2.2k. On a pair you get 17.6k instead of 4.4k.

with 470R screen resistors and 2 hours of loud clean, you can over-dissipate the screens easily.

Do you think the screen grid resistors have gone and the valves are fine?

I am more worried for the tubes than the resistors.

[desertmouth90]

exactly, with this kind of mismatching you get a 8.8k load on a quad of tubes, instead of 2.2k. On a pair you get 17.6k instead of 4.4k.

with 470R screen resistors and 2 hours of loud clean, you can over-dissipate the screens easily.

Do you think the screen grid resistors have gone and the valves are fine?

I am more worried for the tubes than the resistors.

Same, £180 down the drain, I need to build a tester as the ones on the market are a rip off in the UK

[pdf64]

... I have 470ohm 6W Screed Grids. Which I will bump up to 1k I think. ...

Does your amp have a HT fuse?
If not, 6W seems excessive, as it allows a shorted screen grid to stress the whole HT supply, until the mains fuse blows. Which can take awhile.
The Fender 6L6 screen grid resistor spec has always been 470R 1W.
If going up to 1k, they should be 2W
Metal oxide with a flameproof coating are a good choice for this application.

[pdf64]

... I need to build a tester as the ones on the market are a rip off in the UK

A fixed bias amp with a stiff HT supply, together with a lightbulb limiter and selection of bulb wattages, and regular scope, sig gen and dummy load, can be used to check and grade output valves.
Probably just 1 of the 6L6 has shorted.
Probably now it's a hard fault and will be obvious as soon as it powers up, but sometimes they can be intermittent shorts, only flashing over under heavy usage.

[martin manning]

Ok so why would the screens be called upon to dissipate any more wattage then they would if the output stage was running on a correct load and at maximum possible output wattage.

The higher the load impedance, the lower the plate voltage swings.
When at large plate current the plate voltage swings much lower than the screen voltage, the screen current (and thus screen dissipation) shoots up, because the screen now is more attractive to the electrons than the plate.

Doubling Zaa more than doubles screen dissipation, assuming no screen grid resistor. Screen resistors will help (reducing screen voltage and lowering the plate curves), but the screen grids are still at risk if the load line crosses much below the Vg1=0 knee.

[desertmouth90]

... I need to build a tester as the ones on the market are a rip off in the UK

A fixed bias amp with a stiff HT supply, together with a lightbulb limiter and selection of bulb wattages, and regular scope, sig gen and dummy load, can be used to check and grade output valves.
Probably just 1 of the 6L6 has shorted.
Probably now it's a hard fault and will be obvious as soon as it powers up, but sometimes they can be intermittent shorts, only flashing over under heavy usage.

So you think there is a chance the valves are all fine?

[desertmouth90]

... I have 470ohm 6W Screed Grids. Which I will bump up to 1k I think. ...

Does your amp have a HT fuse?
If not, 6W seems excessive, as it allows a shorted screen grid to stress the whole HT supply, until the mains fuse blows. Which can take awhile.
The Fender 6L6 screen grid resistor spec has always been 470R 1W.
If going up to 1k, they should be 2W
Metal oxide with a flameproof coating are a good choice for this application.

Yeah I put two HT fuses in

[martin manning]

Yeah I put two HT fuses in

Good! The obvious question is are they blown? How about the mains fuse? A fade to silence sounds more like a mains fuse, with diminishing volume as the filters drain, but I suppose the same thing could happen if you just lost the HT.

[desertmouth90]

Yeah I put two HT fuses in

Good! The obvious question is are they blown? How about the mains fuse? A fade to silence sounds more like a mains fuse, with diminishing volume as the filters drain, but I suppose the same thing could happen if you just lost the HT.

all fuses are fine, I dont get it... HT and Mains (and I know what the burnt ones look like as I kept them haha!)

[martin manning]

Ok, the place to start is remove all tubes and check voltages at all of the power supply nodes including the bias supply.

[pdf64]

... all fuses are fine, I dont get it... HT and Mains (and I know what the burnt ones look like as I kept them haha!)

A failed fuse can appear fine.
Please check them with a meter.

[pdf64]

...Probably just 1 of the 6L6 has shorted. ...

So you think there is a chance the valves are all fine?

Perhaps 3 of the 6L6 are fine and only 1 may be dodgy.

[desertmouth90]

...Probably just 1 of the 6L6 has shorted. ...

So you think there is a chance the valves are all fine?

Perhaps 3 of the 6L6 are fine and only 1 may be dodgy.

Im very silly, I forgot I added 1ohm biasing resistors so this should be easy to work out.

Ill post my Voltages, mVs (mAs) later tonight. im still at work obsessing over it.

[pdf64]

So you think there is a chance the valves are all fine?

Perhaps 3 of the 6L6 are fine and only 1 may be dodgy.

Im very silly, I forgot I added 1ohm biasing resistors so this should be easy to work out.

Ill post my Voltages, mVs (mAs) later tonight. im still at work obsessing over it.

Once good fuses are installed, and after you've done Martin's checks, I suggest you power up via a lightbulb limiter and watch the 6L6 carefully for flashover.