Humming amp

suspiciousminds

The sound in my lc30 has been fading in and out so at the weekend i thought i’de try swapping out each valve 1 by 1 to see if it made a difference.
Anyways, swapped out one of the el34’s and the main ht fuse blew (turns out the swapped valve was knackered).
Popped another one in and now theres a loud buzzing noise, which is there even with the volume turned down.
Would a full revalve make any difference?

Keefy

IIRC there is an issue with VC30s where there is a connection to a normally unused pin on the output valve sockets. If you fit a TAD valve (as per stock) it's fine, other valves can cause problems.

Modulus_Amps

you should really swap both power valves at the same time to ensure they are matched, matching ensures you don't get hum from the output section. if it is a fixed bias amp you really should check the bias too.

ICBM

There's also a possibility that the shorted valve blew a component associated with it - probably the screen resistor - as well as the fuse. If so that valve socket now won't work and the amp will now hum.

suspiciousminds

Ahh right, so if i’m looking at all 8 valves, they should all glow as normal except the knackered one?

ICBM

suspiciousminds said:

Ahh right, so if i’m looking at all 8 valves, they should all glow as normal except the knackered one?

Not necessarily - you often can't tell if a valve is conducting or not just from the filament glow, which could be normal. If you look carefully inside the plate structure you can often see blue glow, which is a good indicator if there's none - but it can be tricky to see. Try turning the standby on and off and see if you can see any blue glow come and go.

suspiciousminds

Ordered a full set of valves but Is the screen resistor the one that is actually on the valve socket?