Replacing Valves in Marshall amp

Idle Bull

Active Member
Hi all,

How easy is it to replace the valves on an amp? I have a Marshall DSL40C, and it looks straightforward until it comes to Biasing them. (there’s 4 preamp and 2 power amp valves. )

I have a couple of questions:-

Is biasing absolutely necessary?

Am I correct in thinking that only the 2 power valves need biasing?

What areas do I need to avoid to prevent electric shock from charged capacitors, is it any part of the circuit or just areas around the capacitors?

Thanks

Nick
 

hyperfish

Distinguished Member
Physically changing the valves is quite easy. You just have to ensure that the pins are aligned properly. I once managed to bend a pin when swapping a preamp valve. Your EL34 power valves have a keyway in the centre which should help.

I read up on biasing some time ago out of interest and yes it is necessary although some amp designs like my Orange do not need it. It's needed to ensure each valve is working equally and optimally. This affects both the sound and longevity of the valves and components. Biasing is adjusted with the power on and the voltages involved are potentially lethal, so something that should be done professionally.

You are correct, just the poweramp valves need biasing. Those in the preamp do not.

Just a quickie. Why are you replacing the power valves?
 

Idle Bull

Active Member
Physically changing the valves is quite easy. You just have to ensure that the pins are aligned properly. I once managed to bend a pin when swapping a preamp valve. Your EL34 power valves have a keyway in the centre which should help.

I read up on biasing some time ago out of interest and yes it is necessary although some amp designs like my Orange do not need it. It's needed to ensure each valve is working equally and optimally. This affects both the sound and longevity of the valves and components. Biasing is adjusted with the power on and the voltages involved are potentially lethal, so something that should be done professionally.

You are correct, just the poweramp valves need biasing. Those in the preamp do not.

Just a quickie. Why are you replacing the power valves?


I'm basically replacing all the valves as I bought a full set. I'm having issues with the sound sometimes getting muddy, then brightning up again, the getting muddy again, and rather than take it to an amp Tech (like I shoukld) I thought I'd just try replacing the valves.

On further googling, looks like the DSL40 has some dedicated pins for the Vu meter, on the board, to make biasing easier, but I like the sound of getting someone else to do it. No point having new valves if I'm not around to play it :)
 

Christian 71

Distinguished Member
If you find the right guy it shouldn’t be expensive. My JCM started acting up a few years ago guy figured out it was one of the big bottle caps, replaced it and the power tubes and charged sub 50 quid including the biasing. Said it took him 20 minutes all in.

From what I’ve read generally the voltage in the same brand is usually pretty close so you can swap one out in a gig, it’s just long term it puts stress on transformer if they are way off (ie expensive). Think if you know the bias you can buy balanced sets but given it’s a five minute job to bias most people don’t bother.
 

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