Musicians Roadhouse

alright all!
i swopped my marshall el34's for some mesa boogie el34,is it ok to run it without getting the bias done,i still reading up on dave hunters valve amp book,and still getting my head around how to do it,so is it ok to try the amp unbiased??.till i get it sorted myself.

please don't... you're putting the amp at risk...
powertubes in a fixed bias amp should ALWAYS be biased...

in this case it might even be more damaging if you place those mesa's unbiased in your amp (what model is it, by the way?)
mesa selects tubes, so that they perform good in their amps... in other words, mesa amps can't be biased, because instead of a trimpot they use a fixed resistor... this resistor keeps the majority of tubes biased cold, so they select tube that runs rather hot on a given biasvoltage... so i think these mesa would run seriously hot in your marshall and very likely will die immediatly... in a worst case scenario they could take the output transformer with them...

Hi dooley
thanks for the advice ( i hope we can start over )i tried them for about 1 min and they sounded very aggresive, added a lot more gain, pulled them out did'nt think something was right sounded all wrong put the marshall 34's back and lucky for me all seems fine, only picked em up as they go for £25 each new' these are new boxed and i got 2 boxes with 2 in each for £20 so ,a good deal

don't worry, we're cool...

20 pounds seems like a good deal if those tubes are matched.. fortunatly mesa use codes for their tubes, so if those 4 tubes have the same code they are matched...
biasing the amp yourself is not that difficult once you understand how... but i also like to know WHY tubes must be biased, as the background knowledge gives you a better insight of what you're doing exactly...

the TSL is known for it's possibility, that it can be biased without pulling the chassis, which provides some safety... however, plate voltage is also known to be all over the place on these amps, and it's pretty important to know where it's at...and to be able to get a reading on plate voltage you have to pull the chassis anyway

after reading a lot on various forums i found that the voltage shunt method across the OT is very accurate and you only need a DMM to do it... no bias probes needed and it works on almost every amp marshall has put out, including the TSL, the JCM900 and the JTM45 you have

If the power tubes are not red plating you should be alright but make sure you keep a close eye on them and for hum that seems to get louder.

The bias on my 2466 Vintage Modern was 79mv which is factory spec and they red plated prior to biasing at 92mv. So in theory as long as your tubes don't red plate and the noise floor seems acceptable you should be ok. One sign of a tube red plating is when the noise floor rapidly gains noise in which case you should turn the amp off. Now the down side to this is that without biasing you won't get the full potential out of your tubes for tone, so comparing various types of EL-34s may be redundant.

I ran my Bandmaster for years without biasing just slapping new JJ's in every 3 years, I was lucky to have had cooler running tubes during that time, by the time your tubes shit the bed a tube running unbiased cold sounds better than a tube thats going out so it always improved the tone and I never thought much about biasing early in my playing because I figured there was a big mystery surrounding it (myth!). My first year playing guitar using that Bandmaster I used to hit the top of the amp with my fist when noise would come followed by the tube red plating and that actually helped those 30-40 year old tubes stop red plating a few times till one day they just got unplayable and I switched them out.

I now bias all of my amplifiers :)