Re: Changing the tubes of a Marshall JCM900 (EL34)
Hi ! I have some tube questions :
1) The tubes of my 1990's Marshall JCM900 need to be changed. I never did it by myself before but I would really like to try. I could be dangerous (I know a soundtech who got once shocked). Is it OK if the amp have not been used for one or two days ?
2) Currently I have JJ EL34 and the three pre-amp tubes, all made in Slovakia and changed by a shop in 2016.
The sound is crackling horribly if I boost the clean channel "A" with a some powerfull vintage booster (Sola Sound Colorsound PowerBoost but even with the pedal gain at 9 o'clock if you really hit with the pick). Is that the power tubes of the pre-amp ? I change first the 4 EL34 and see ? Or the other way round ?
3) I am an dummy for amp circuitry : does both channels uses the 7 tubes ? I suppose so but am I wrong ?
4) And lastly, the most important question : Any recommendation ? For me the JJ EL34 sound was OK when new. Mullar are expensive and I share the studio with a band that will always crank the master twice a week ... (They emulate Messhuggah). And I'm myself also guilty as I add a MXR 10-band EQ in the loop. So maybe I should avoid deluxe/pricey tubes... But this is also my recording amp.
So any nice sounding tubes robust enough to hold their course ?
1) Swapping tubes only is a safe activity -just only touch the tubes and the tubes covers/springs etc. Those 2 blue circular components are the deadly things -but on a JCM900 they are easy to avoid -especially considering the bottom side is where they dissipate energy -just avoid them. Also wear a glove or wipe the new tubes down before turning on -you don't want the oils from you hand on the glass once they warm up. If you expose the electronics under the chassis to bias or change the electronics -then electrocution is a risk if you don't know what you are doing. If you lest an amp sit, yes over time the capacitors will bleed off the energy stored -but nobody is going to make that calculation of when they are no longer deadly for you -the safer approach is to bleed them off with a giant resisitor before going in the chassis (there are plenty of videos online of safe ways to do this)
However, when you swap tubes -unless the values of the new tubes are very very similar to the old ones -a rebias of the circuit is performed. This requires getting inside the chassis and turning a pot while measuring plate current or voltage and converting on a JCM900 -which is best done by a professional unless you want to learn how and buy a voltmeter and bias probe. But swapping the tubes and listening is fine too, as a hot bias is preferred by some as is a slightly lower bias -so you can just swap tubes and see how your amp performs -if it sounds bad -like it's starving or way to hot -then take it to a professional. This is why companies like Ruby and Groove Tube rate their tubes with a number system-so you can just swap a set in with the same value without a re-bias.
Here's a video if you are interested in learning how to rebias your amp -it's simple to learn
https://www.youtube.com/watch?v=Sw2JQqwYjvE
2) That cracklin is likely one of the 3 small tubes. V1 is the preamp tube V2 is the Reverb V3 is the Phase Inverter tube. V1 is the furthest away from the EL34s. Swap and experiment with different tubes in those positions. I'd do this first to see if you solve the issue before messing with the EL34s, that way you know your EL34s are good to save as backups. also save the 12Ax7s that aren't causing an issue.
3) Yeas both channels use all 7 -they share the preamp, reverb and phase tubes and all 4 power EL34 tubes.
4) Original Mullards don't actually exist anymore (unless you pay a mint for an old set) -if you are thinking you want to play the tubes Hendrix played -Sovtek just bought the name and charges more and build them in the Reflector factory with Svetlana, Tung Sols, and EHX branded tubes
JJs are great tubes in my experience -they are from the TESLA tube factory in Slovakia -if it rains, they all go pick mushrooms in the hills -so no tubes get made on rain days.
I traditionally like Svetlana brand in my Marshalls -which Sovtek also bought and still produces but I don't believe they are marketed under the Svetlana name any more -they are the Saratov Russia mades tubes offered under several names from Sovtek -but they are essentially American RCA style tubes -you want the ones with a C logo on them -but S are fine too.
Outfits like Groove Tubes and Ruby are not tubes manufacturers -they buy Russian, Slovakian and Chinese tubes and stress test and rate them for quality and consistency and to get that number rating I was speaking about. It's a great way to know you're getting quality and to avoid biasing when you swap tubes.
If you amp is getting hammered by other people in your practice space, I don't know -I wouldn't spend for big money on tubes -I'd probably buy some chinese rated Groove tubes and keep a Russian version on the side with the same number rating to swap in for recording -I don't like the idea of another man touching my woman. But that's extreme for most people.
I like big Power tube distortion with Speaker distortion -so for me the El34s were the important part, but for many people playing around with different 12Ax7s is where they fool with their tone -12ax7s are inexpensive, I'd buy several different brands and experiment with your tone -you can swap them in seconds -it's a lot of fun.
Also remember your JCM 900 100 Watt can be changed into a ferocious 50 Watt amp in seconds. just pull out either the outer 2 EL34s or inner 2 -just know that when you do -the EL34s pairs start to get different age on them. You might prefer 50Watt for recording or playing without a band.
