Quantcast
Recording tonight, Quick tube question - Gearspace.com
The No.1 Website for Pro Audio
Recording tonight, Quick tube question
Old 13th March 2014
  #1
Gear Nut
 
🎧 5 years
Recording tonight, Quick tube question

So im gonna be recording in a few hours. Last week my amp blew 3 fuses and possibly a tube (I checked and one was cracked).. I have a Marshall JCM900 dual reverb combo. I bought a matching set of tungsol tubes and replaced tube slots 3 and 4. Now I am getting a loud humming sound and to me the volume isn't what it should be (LOUD).

Could I have put the matching tubes in mismatched slots? I don't know if the tube pairs are outside and inside or slot 1,2 and 3,4. Any help would be GREAT!
Old 14th March 2014
  #2
Gear Addict
 
dented42ford's Avatar
 
1 Review written
🎧 5 years
1) Yes, you put the matching tubes in mismatched slots. The tubes are in pairs 1+4 and 2+3. The inside tubes are a pair and the outside tubes are a pair.

2) The loud humming is likely just a bias issue. If you don't know how to bias your amp, look it up - but if you don't like working with high voltages, take it to a pro. Mucking around inside a tube amp can kill you - highly unlikely, but it CAN happen. Much more likely is you causing damage to the amp.

3) If you actually CRACKED a tube, there is a possibility that your other pair might be off as well. In a 4-tube Marshall, the bias supply is effectively shared, meaning that if the inside and outside pairs are not reasonably close to each other (in bias point), it can cause issues. This didn't used to be a problem, but new production tubes can be pretty far outside the spec. That's why it is generally recommended to replace tubes as full sets. It isn't strictly speaking necessary, but it is advisable.

4) If you take it to a tech, don't let them tell you out of hand that you've blown your transformers, or that you need a "recap", or any other serious issue. Those issues are extremely rare - the likeliest thing you need is a new quad of [biased] EL34's, and the worst [usual] case is a couple of blown resistors.

This is just a PSA - I consult for a guitar amp company, and you'd be absolutely shocked how many respected techs call us with dumb ideas (like wanting to recap a 2-year-old amp). Generally speaking, 99% of issues with tube amps are simple tube and/or related circuitry (resistors, usually) related problems. Blown Xformers, especially on newer amps, are exceedingly rare, and re-caps are generally only advisable on amps that are VERY old and VERY underplayed...
Old 16th March 2014 | Show parent
  #3
Lives for gear
 
Hendyamps's Avatar
 
🎧 5 years
Quote:
Originally Posted by dented42ford ➡️
1)
In a 4-tube Marshall, the bias supply is effectively shared, meaning that if the inside and outside pairs are not reasonably close to each other (in bias point), it can cause issues. This didn't used to be a problem, but new production tubes can be pretty far outside the spec. That's why it is generally recommended to replace tubes as full sets. It isn't strictly speaking necessary, but it is advisable.
Man, replacing all four EL34's because one went down really financially sucks. I actually have an amp at the studio right now that currently has three bias supplies...I kept adding them over the years because when an EL34 went south I didn't want to shell out the cash for a new matched quad set. I wanted to just use whatever I had laying around. Way cheaper to just drill some new holes and add a diode, couple of caps, resistors and pot...haha
Old 16th March 2014 | Show parent
  #4
Gear Addict
 
dented42ford's Avatar
 
1 Review written
🎧 5 years
Quote:
Originally Posted by Hendyamps ➡️
Man, replacing all four EL34's because one went down really financially sucks. I actually have an amp at the studio right now that currently has three bias supplies...I kept adding them over the years because when an EL34 went south I didn't want to shell out the cash for a new matched quad set. I wanted to just use whatever I had laying around. Way cheaper to just drill some new holes and add a diode, couple of caps, resistors and pot...haha
That's a little beyond the scope of what your average player is willing to do. Not to mention the fact that it will inevitably have sonic consequences. If you're a big fan of tweaking, it is possible to minimize the unwelcome asymmetries caused by mis-matched tubes - but it requires a fair amount of tweaking, most conveniently done with an oscilloscope and a signal generator...

I stand by my original statement - if you don't want to bother with tweaking, and you want the cleanest, easiest, least invasive path to "stock" amp tones, replace quads as quads - or, at the very least, with two reasonably-close-to-matched pairs. Given that you will only have to do it once every few years, it is cheaper than the time investment required to have the knowledge and experience to do what Hendyamps is suggesting...

Or get the amp modded by someone you trust, and get them to show you the procedure. Just be forewarned that it might not have the best results!
Old 16th March 2014 | Show parent
  #5
Lives for gear
 
Hendyamps's Avatar
 
🎧 5 years
Quote:
Originally Posted by dented42ford ➡️
That's a little beyond the scope of what your average player is willing to do. Not to mention the fact that it will inevitably have sonic consequences. If you're a big fan of tweaking, it is possible to minimize the unwelcome asymmetries caused by mis-matched tubes - but it requires a fair amount of tweaking, most conveniently done with an oscilloscope and a signal generator...

I stand by my original statement - if you don't want to bother with tweaking, and you want the cleanest, easiest, least invasive path to "stock" amp tones, replace quads as quads - or, at the very least, with two reasonably-close-to-matched pairs. Given that you will only have to do it once every few years, it is cheaper than the time investment required to have the knowledge and experience to do what Hendyamps is suggesting...

Or get the amp modded by someone you trust, and get them to show you the procedure. Just be forewarned that it might not have the best results!
I think you misunderstood my post. I was not suggesting that he gets himself killed trying to create another bias circuit, nor was I suggesting my methodology was an altogether good idea. I was simply bemoaning the fact that a matched quad of 34's costs a lot of money and how much I hate replacing all four when only one needs replacing.

I actually thought your original reply was spot on good advice for the OP, hence why I "liked" your comment.
Old 16th March 2014 | Show parent
  #6
Gear Addict
 
dented42ford's Avatar
 
1 Review written
🎧 5 years
Quote:
Originally Posted by Hendyamps ➡️
I think you misunderstood my post. I was not suggesting that he gets himself killed trying to create another bias circuit, nor was I suggesting my methodology was an altogether good idea. I was simply bemoaning the fact that a matched quad of 34's costs a lot of money and how much I hate replacing all four when only one needs replacing.

I actually thought your original reply was spot on good advice for the OP, hence why I "liked" your comment.
Ah, sorry for the misunderstanding - and I agree with you, power tubes are a bit of a downer, from a financial point of view. Try retubing a SVT (6x6550) or Ultra Lead (4xKT88), then complain, though!

They really need a FB-like "like" function for these sorts of things...

Old 17th March 2014 | Show parent
  #7
Lives for gear
 
Hendyamps's Avatar
 
🎧 5 years
Quote:
Originally Posted by dented42ford ➡️
Ah, sorry for the misunderstanding - and I agree with you, power tubes are a bit of a downer, from a financial point of view. Try retubing a SVT (6x6550) or Ultra Lead (4xKT88), then complain, though!

They really need a FB-like "like" function for these sorts of things...

no worries...and yes, that does suck. My 4xKT88 bass amps always have at least two bias circuits.
📝 Reply
Topic:
Post Reply

Welcome to the Gearspace Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…

Forum Jump
Forum Jump