Output Impedance error

[desertmouth90]

I lent my amp to my friend and accidentally set the impedance to 4ohm into a 16ohm Cab 240w.
He said he was playing it loud clean as a pedal platform for 2 hours and it just faded to zero volume.
The amp is a Dumble ODS 102 clone 100w.

What are your guesses for what went wrong before I get my hands on it tonight?

[Roe]

I lent my amp to my friend and accidentally set the impedance to 4ohm into a 16ohm Cab 240w.
He said he was playing it loud clean as a pedal platform for 2 hours and it just faded to zero volume.
The amp is a Dumble ODS 102 clone 100w.

What are your guesses for what went wrong before I get my hands on it tonight?

check fuses and outout tubes (this may have overdissipated the screens)

[pdf64]

A slow clean fade out of signal indicates a loss of heater power to one or more valves.
Whereas a distorted fizzling out may be a general loss if mains power eg mains fuse, or a loss of screen grid voltage, eg HT fuse.

[desertmouth90]

I wasnt there so I didnt really witness it, so im hoping it was just a fuse

[Stevem]

If the amp has a HT fuse and it blew I would start with a replacement of 1/2 the normal amperage.

[desertmouth90]

If the amp has a HT fuse and it blew I would start with a replacement of 1/2 the normal amperage.

I have 60w Current Limiting Bulbs,

Thats a good shout

[Stevem]

With an impedance load mismatch like your buddy did your amps output tubes where seeing less of a load then they would with the correct load.

With the mismatch your buddy put on your amp the tubes where seeing about 3 times less then the correct load and such a mismatch would extend there life because there wattage output was so limited.

I gotta say that whatever issue befell your amp would have taken place even with the correct load on any of the secondary taps that your output transformer has.

[Helmholtz]

With an impedance load mismatch like your buddy did your amps output tubes where seeing less of a load then they would with the correct load.

The plates were seeing less load, while screen dissipation was significantly increased (as already said by @Roe).

[Stevem]

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?

Assuming there’s a screen resistor.

[Helmholtz]

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.

[pdf64]

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?

Assuming there’s a screen resistor.

If screen grid current limiting resistors are fitted, it's simple to assess screen grid current when the amp is cranked, and compare it at the nominal load impedance, and at higher and lower load impedances.

[desertmouth90]

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?

Assuming there’s a screen resistor.

If screen grid current limiting resistors are fitted, it's simple to assess screen grid current when the amp is cranked, and compare it at the nominal load impedance, and at higher and lower load impedances.

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

Everything in the amp looks fine which is leading me to believe its the Output Valves. so far

I will need to build a tester for those eventually. But tonight I will set the current limiter with 2x60w Bulbs in Paralell and do some tests with no valves in.

Luckily I have a spare set of Valves!!!

[Roe]

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.

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.

[desertmouth90]

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.

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?

[martin manning]

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??