Valve Problems

[benjaturner]

I've been having some trouble with my amp and I think it's to do with the valves. I installed a matched pair of EL34's about 4 months ago, though didn't bias them. I only heard about biasing recently and still don't really understand what it is. Now they're glowing with a blue-ish glint on the bottom. Is this a normal sign that the valves are wearing out?

I've also tried replacing the pre-amp valves tonight with ECC83's tonight but this didn't help at all. The valve types are the same as the ones I took out.

Basically, the main problem is the volume disappearing, though not completely, and coming back again randomly. There's also a really nasty distortion type sound, even on the clean channel. It sounds really weak.

Anybody got any ideas about what else I could try before taking it to a tech?

Thanks for reading,
Benja.

 

[milkbone]

Re: Valve Problems

Sounds like your power tubes are taking a dump. If you see blue flashes in them, best bet is to shut her down and take her in if you don't know how to service it. I wouldn't run it with the bad tubes in her. Is this you JTM?

 

[benjaturner]

Re: Valve Problems

Yeah, it the JTM. If amp problems were a country then this amp would be the capital. I love the sound and I think I'd have to spend a lot of money to get something that sounded better but the reliability issues are getting beyond a joke now.

It's not blue flashes, the valve constantly has a blue gleam amongst the normal orange glow. I'm guessing that this is a result of me not biasing the valves when i put them in, I didn't think I needed to as they were already matched but have learnt since that I still should have done.

What should I do? Should I just not try to do anything myself and take it to someone who knows what they're doing? Or is there something else I could try? The first thing I did was to put the old power-amp valves back into the amp but this had no impact what-so-ever. I've been told that biasing amps isn't difficult and have been meaning to learn how to do it myself. Do I need to buy an oscilliator or something?

Thanks,
Benja

 

[Jeff Seal]

Re: Valve Problems

Benja, there is an "actual fix" for these, but it required some redesigning of the amp. The good news is no new parts are needed and it's resonably simple, so long as you can solder. The bad news is it doesn't really sound like you're that confident messing about in the amp...and I wouldn't advise you to.

But, I'll let you decide that....
The inherent problems these amps have is the preamp tubes have their heater voltage "recified" to run DC to them, as opposed to the conventional AC fed heaters that 99% of the rest of the amps ever made use. The DC circuit is used to eliminate preamp tube "hum" which would a good idea if the components used didn't heat up to point of actually "unsoldering" themselves. If you look inside the chassis, at the board where all the tubes plug in, you'll see a "black box" looking component..(should be between the EL34's and the 12AX7's) It's about 1 inch by 1 inch and about 3/8 in. thick. It's component designation is DB1. Although the conventional repair would be to "resolder" the legs of this component, it will only hold up for a short period of time. (the solder connections will usually look "burnt" to start with.)

If you remove this component entirely, as well as capacitors C134,C135, C143 and C144 (important!) and simply "jumper" (with a short piece of wire) the connections where the "black box" looking thing was. so the top two holes are connected together and another "jumper" wire across the bottom two holes...but don't connect the two jumpers together!!, you will convert the heater supply from DC to AC...! (don't worry about the connections where the caps were...)
You should never have this problem ever again.

The volume drop you're noticing is because the heater voltage is being cut in half while one of the connections is lost. This also explains why the tone gets fuzzy everywhere...The 12AX7's aren't hot enough for proper thermionic emission to occur.

I have done this to hundreds of these amps and I've never had one come back, nor have I even noticed any more "hum" than the amp had to begin with... None of my customers have ever noticed it either....go figure..

Anyway, I'm not sure how much that helped (or confused)
But, there it is!

Jeff Seal

 

[benjaturner]

Re: Valve Problems

Jeff, where have you been all my life? Seriously, this problem has been the bain of my entire existance for about three years. I'm definately going to do this some time this week. I'm pretty happy with everything you've described in terms of understanding what it is I need to do, and am confident with soldering.

Is this the cause of the blue gleam in the EL34's as well? I figured it was because I hadn't biased the amp when I replaced the valves.

Can I just throw the caps out once I've taken them out? I guess I won't be needing them anymore, hehe.

Once again Jeff, from the bottom of my heart, thank you! Could you please explain why Marshall chose to run the pre-amp valves on a DC current if it meant the components were so vunrable to heat exposure. I guess with hindsight it's easy to critisise but you would have thought any engineer would have foreseen this!

If you could also clarify that this problem is a likely cause of the blue gleam in the EL34's than I think this would probably be the best day of the year. If not could you please suggest any other possible causes of this. Is it just that the valves have expired.

Many, many thanks,
Benja

 

[Jeff Seal]

Re: Valve Problems

Jeff, where have you been all my life? Seriously, this problem has been the bain of my entire existance for about three years. I'm definately going to do this some time this week. I'm pretty happy with everything you've described in terms of understanding what it is I need to do, and am confident with soldering.

Is this the cause of the blue gleam in the EL34's as well? I figured it was because I hadn't biased the amp when I replaced the valves.

Can I just throw the caps out once I've taken them out? I guess I won't be needing them anymore, hehe.

Once again Jeff, from the bottom of my heart, thank you! Could you please explain why Marshall chose to run the pre-amp valves on a DC current if it meant the components were so vunrable to heat exposure. I guess with hindsight it's easy to critisise but you would have thought any engineer would have foreseen this!

If you could also clarify that this problem is a likely cause of the blue gleam in the EL34's than I think this would probably be the best day of the year. If not could you please suggest any other possible causes of this. Is it just that the valves have expired.

Many, many thanks,
Benja

The blue glow is "normal" and doesn't indicate "improper" bias at all...(almost all output tubes have this glow, it's just more apparent in some design's than others) However you might notice it "dissapearing" if you reduce the bias (provide more negative voltage to the grids) In any case I would verify the bias just to be on the safe side. Blue glow is good, Red glow in the middle of the tube is very bad!

Good luck with it....

Jeff Seal

 

[benjaturner]

Re: Valve Problems

I think I know what the 'Black Box' is as well. I've had the amp apart that many times to replace a fuse that I think I could draw this insides of this amp without even seeing it.

Will the valves be okay running on an AC current or do I need to do anything to them? Or have I totally missed something? Will the valves somehow still run on a DC current? Please help.

 

[gordon_39422]

Re: Valve Problems

Benja, here is a link to a site with in depth descriptions of what biasing an amp is, and how to actually do it! Of course, you will notice right off that it is done step-by-step on a Fender Hot Rod Deluxe, but from there, you should be able to do it on your JTM...

Hope this at least gives some insight... If you get the time, explore the rest of the links from the home page of the same site. There are MANY in depth explanations of basic electronics as they relate to tube amps. I still revisist the page from time to time to pick up on things I may have overlooked or forgotten..

Enjoy!
Allen

 

[benjaturner]

Re: Valve Problems

Hi Jeff,
I hate to be a pest as I'm incredibly grateful for the help you've already given me but if you think I'll be able to keep up will you please explain why Marshall chose to convert the current to DC for the pre-amp valves. Is it to cut out hum?

 

[Jeff Seal]

Re: Valve Problems

benja, running the tubes on AC instead of DC is absoltulety fine. There are only 4 current production amps that use DC heaters.

Marshall DSL/TSL
Diezel VH4
Framus Cobra...(if memory serves me..)

Exactly why Marshall decided to do this makes little if any sense, whatsoever. Basically, it's just more trouble than its worth.... Usually you'll find this in high end Stereo gear and lots of older jukeboxes....where cost isn't or wasn't an issue.

The caps and the bridge rectifier (black box) can be tossed if you want...up to you..

let us know...

Jeff Seal

 

[benjaturner]

Re: Valve Problems

Hi Allen/Gordon,
Sounds really useful. Would like to look into it. Am I missing something, you don't seem to have posted the link.

Thanks,
Benja