Replacing valves in a Marshall

[richard parker]

I've just been looking at the Marshall website and they have this to say about both pre and power amp valves:

'Valves must be replaced with the same type and model. If you use a different type or model this CAN and WILL cause damage to the amp. All work of this nature must be carried out by a qualified engineer.'

So I guess if you changed the a pre-amp tube your self Marshall would say you had invalidated your warranty ? What do you make of this ? Can the wrong type of pre-amp valve damage your amp ?

 

[Warheart]

Re: Replacing valves in a Marshall

not the wrong type of preamp valve, but a power amp valve of the wrong type will more than likely destroy the amp.

12ax7 are replaceable with a variety of other types to change the gain structures of an amp.

if you put a 6L6 in an amp that usually takes EL34's, the screen resistors (I believe?) may not be able to handle it.

You can always put the original tubes back in if you wanted to claim warranty I guess?

 

[Lake Placid Blues]

Re: Replacing valves in a Marshall

They are just covering themselves in cases of novices doing things when they don't know what they are doing.

An amp already wired up for EL34s can take 6L6s without too much trouble. The 6L6 draws less than half the heater current of an EL34 and the screen grid resistors for EL34's are typically at least 1,000 ohm and typical SGR er's for 6L6 are 470 ohms.

But where the novice can get into trouble with the typical Marshall is with the bias. For example, I expermented with 6L6's in one of my Marshalls and even with the bias pot turned all the way down, they were running like 60 ma per tube. It would have required modifications of the bias circuit. There is right way and a wrong way to do it. Marshall will supply techs with very exact intructions on how to convert EL34 to 6550 or vis a vis, or 5881 to EL34... ect...

 

[glassman]

Re: Replacing valves in a Marshall

Yup...that keeps novices from tinkering during the warranty period and sells lots of Marshall replacement tubes.

LPB covered the explanation on the power tubes. The preamp tubes would only come into play in the cathode follower portion of the preamp that most Marshalls have. Certain 12AX7s are less tolerant of the much higher filiment to cathode voltages in this configuration. I've noticed quite a few JJ tubes shorting like this lately...usually in the PI position.