Is this a dodgy preamp valve or something more sinister?

rico

At rehearsal last night my DSL401 was making a substantial buzzing noise on the clean channel only (I only use this channel) that increased as the master volume is cranked. It’s not single coil hum and it’s nothing to do with my pedalboard, cables or guitars. 

I am planning on using this amp for a NYE gig and would like to try and solve it ASAP. 

Dodgy preamp valve or something else? 

ecc83

No doubt ICBM will be along shortly. My guess would be the PI bottle. But! You would be a pretty daft bunny to do a NYE gig without a backup amp! You could get lynched!

Dave.

rico

I’ll take the 2204 in the car as well. 

@ICBM your wisdom would be appreciated. 

ICBM

Replace each preamp valve in turn with a replacement and see if it stops.

I did have one in a while back that had something like this, but unfortunately I can't remember exactly what the cause was now - it *wasn't* a valve, it was something much more difficult to find. And not one of the 'usual faults' with these.

DJH83004

preamp valve substitute as per ICs recommendations, starting with V1 (with the aluminium shroud). My experience with the 401, has mainly been volume fade associated with the preamp heater bridge rectifier, excessive mains hum, which was filter cap related, transformer swaps and output valve bases, non of which sound like your symptoms.   

rico

@DJH83004 what would be symptoms of excess mains hum and what is the solution? 

DJH83004

rico said:

@DJH83004 what would be symptoms of excess mains hum and what is the solution? 

From memory, it was one of the 10uF / 450v power supply caps in the preamp stage (there are quite a few - at least 4 or 5 dotted about). But this was causing more of a continuous mains hum in the background (actually 100Hz ripple) but was not channel related. It  was a pig to track down, as I recall.  

clarkefan

DJH83004 said:

rico said:

@DJH83004 what would be symptoms of excess mains hum and what is the solution? 

From memory, it was one of the 10uF / 450v power supply caps in the preamp stage (there are quite a few - at least 4 or 5 dotted about). But this was causing more of a continuous mains hum in the background (actually 100Hz ripple) but was not channel related. It  was a pig to track down, as I recall.  

Thank you, I'm having a very similar problem with my DSL 50.  Faint but there.  Will examine caps, though to be honest I replaced the main PCB and it's still there.  

Also the attenuator I bought recently kills it

ICBM

clarkefan said:

Thank you, I'm having a very similar problem with my DSL 50.  Faint but there.  Will examine caps, though to be honest I replaced the main PCB and it's still there.  

Also the attenuator I bought recently kills it

Just make sure that isn't too close to the truth... what impedance is the attenuator?

The TSL100 I had did something similar too - I was going to trace the fault but in the end there was so much else wrong with it that I just scrapped the amp.