Can I Kill My Champ 600 With Pedals?

disaster

Member
Messages
869
I'm a bit apprehensive with my little champion 600 these days- after the JJ 6v6 I put in it went bad, I took it to an amp tech who said that apparently the the (comparatively) high output of the JJ also fried the the "resistor that controls the tube bias" (his words).
He replaced that 1 watt resistor with a 2 watt, and said to keep an eye on it.
If I crank the little beast and/or hit it with high output pedals like my Fulldrive (on Comp-Cut) and my Fuzz Factory, am I putting it at increased risk?
What can an input overload (if there is such a thing- though I've heard of Sonic Youth killing amps with pedals) do to an amp?
 

John Phillips

Member
Messages
13,040
Possibly a risk, but the amp should be well-enough built to take anything that can be put into the input.

I agree with your tech though, a 1W resistor is not enough for a cathode bias resistor in a 6V6 amp. Assuming a plate voltage of around 300-350V, and a dissipation of 12W, that means a current of 35 to 40mA, which through the typical 470-ohm resistor for a 6V6 could give up to .75W - not enough safety margin for running inside a hot amp and to cope with an overload. If the plate voltage and dissipation are a bit higher it could even be close to 1W. 2:1 is usually considered the minimum safe margin for this sort of job, so a 2W is the smallest the designer should have used anyway.

I've blown up a couple of amps with pedals too - but I'm fairly sure they were close to it anyway... one was an amp not renowned for its reliability and the other just blew a (very old) power tube.
 

John Phillips

Member
Messages
13,040
It depends what the pedal does to the maximum power condition of the amp and certain components.

Some fuzzes (especially) and other effects can produce frequency distributions which could overload some components, or produce current demand on the power section, that would never occur with any normal input signal, no matter how loud.

The two amps I blew up were with a combination of fuzz-distortion and octave divider, and I think a resonant filter on one of them.
 

67super

Member
Messages
1,505
I found a fender schematic online which indicates the 470 ohm cathode resistor is already 2 watts.
 

disaster

Member
Messages
869
Thanks guys. Good info, as always.

I wonder wether he was just mistaken about the resistor or the value was actually different from the the schem.
I suppose I'll have to be careful of the little guy and use the half stacks for noise rock freakouts.
 




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