Valve advice / info

I have just replaced the valves in my Icon Stereo 40 , swapped EL34
for KT77 .

Biased them up as per spec 35mA , and they work fine in triode mode , but in ultra linear they crackle and hum .

Swaping back to EL34's and everythings fine in both settings so it's not an amp problem .

The amp spends most of it's time in triode so it's not a problem ,
but does anyone know why it does this .

 

are you certain the kt77 is a drop in for the el34 ?

6ca7's yes....oops just checked and they are .....

how about there over running the psu ....as the drawing more power ?

 

I think that might be it.

how old are your rectifiers?
my ones have caused crackling problems when they are really old,
maybe try one of those solid state replacements, if you want a bit of a power boost, I don't like the way most of them sound mind but it might help if you really want to use the kt77's.
I would check the ht if I were you before and after swapping valves.
(make sure the pins are clean too btw)

 

All valves are new and of the same manufacture , JJ .

I did some research last night and it seems valves can make these kind of noises when under biased , I uped the bias in 5mA steps and eventually got a clean sound ( ultralinear ) at 60mA , still works perfectly in triode too , but the transformers getting a hotter .

I've now put the bias back to the recommended 35mA and am keeping it it triode mode .

still unsure why this is happening , but I'm happy with the sound in triode so I'll keep it this way .

 

that does sound as if your pulling too much power [more than the unit is designed to cope with]....

probabley best to do as you've done!