Another valve thread for a simpleton

[Doadman]

I have a Marshall DSL401 that I bought off Ebay and had already been fiited with a full set of JJ Tesla valves but I believe that those valves are currently on their way out as the clean channel has lost a lot of volume and the drive channel doesn't seem to have the same high quality tone it once did. I've read a few threads about replacing valves but I'm afraid when you start talking about biasing and gain stages etc. I just glaze over. When I search for new valves online, what exactly should I be looking for and how do I get them installed? I'm assuming that with all the technical talk I've read, I don't just pop new ones in like lightbulbs! I really need some help with this and I'm guessing it needs to be made simple.

Many thanks

[MartyStrat54]

Originally Posted by Doadman

I have a Marshall DSL401 that I bought off Ebay and had already been fiited with a full set of JJ Tesla valves but I believe that those valves are currently on their way out as the clean channel has lost a lot of volume and the drive channel doesn't seem to have the same high quality tone it once did. I've read a few threads about replacing valves but I'm afraid when you start talking about biasing and gain stages etc. I just glaze over. When I search for new valves online, what exactly should I be looking for and how do I get them installed? I'm assuming that with all the technical talk I've read, I don't just pop new ones in like lightbulbs! I really need some help with this and I'm guessing it needs to be made simple.

Many thanks

Your 401 uses 12AX7 preamp tubes and EL84 power tubes. The four EL84's are about the same diameter as the preamp tubes, but they are much taller. In a 401, there is a retainer that holds the power tubes in. This has to be removed to gain access to the power tubes. To do this, the chassis has to be removed. Once the chassis is removed, there is the danger of high voltage present in the amp. Unless you are qualified, then the tubes need to be biased by a tech.These tubes must be biased according to the year of the amp. The early 401's biased very hot and the units made in late 2004 are biased much lower. The 401 is not a good amp to attempt biasing for the first time, since everything is located inside the chassis. The bias trim pot and access is restricted compared to a DSL/TSL with everything located on the back panel.

The four preamp tubes are located to the right of the power tubes (viewing from the back). The farthest one has a metal shield on it. To remove the shield, carefully twist the shield until the notches line up. This is your V1 tube. The shield is to prevent unwanted RF (radio frequency) from entering the tube.

A preamp tube actually is two complete circuits inside of the tube. They are called triodes. So there is a triode A and a triode B.

V1-First gain stage
V2-Second gain stage
V3-Tone Stack/Cathode follower
V4-Phase Inverter

These tubes can be removed from the amp and replaced. Make sure you are powered down. They do not need to be biased. V1 is the dominant tone tube. Say you stick a real Telefunken in V1. Your amp will take on the flavor of the Tele. Now the V2 adds to the flavor, but it is usually a higher gain tube and allows V1 to take off. In my 401's, I have all NOS tubes, preamp and power. The difference between the stock tubes and the NOS tubes is night and day.

Preamp Tubes? - Who, What, When, Where, Why, and How?

[Doadman]

This is quite frustrating. Every time I think it should be easy enough to do myself, doubts cross my mind!