Design flaw for Line Magnetic's LM-508IA.

For me at least its difficult to take the LM-508 to a tech guy for a cap replacement as there arent many around here with the expertise for it, so I'm just trying to figure out the odds of this event to happen as I wouldnt like mine to blow up. The 805 tubes work with high voltage and it can be dangerous. So if it's something very common to happen, I will have no choice but to find a tech to do it. Every LM-508 owner should be aware of and check it on his amp.

For instance, yesterday I used my LM-508 for about 7 hours, at different volumen levels, and, at all time, even before I shut it down, I could lay my hand over the grill next to the cap. It was always pretty warm (no hot). So even its not an exact measuring procedure, I think it shouldnt be over 55/60 degress celcius and the cap its rated to work up to 85 degrees. If the temperature near the cap were 85 degrees, I think I couldnt touch the grill at all.

Well, just my experience. I would like to hear others so we all can learn about this.

 

I know the type. You want to be convinced and everything I presented including 3 people's accounts still isn't enough.

Up to you to take the chance. I've done way more than my part to relay the information to others. If you decide to stay put, that's a decision you'll need to take accountability for in the event you encounter the same issue.

Hopefully, you won't.

 

You can't find a tech who can replace a few capacitors? Unless you're in a town of a few hundred people, that's hard to imagine.

 

If it's preventative, he technically only needs to replace a single cap.

 

Hi Timind
Thank you for the answer.

I live in a medium to large city, but I don't know why, but most technicians I asked don't have experience with valve equipment and don't want to put the hands on it. I guess they don't like valves working with high voltage and here valve based gear it's rare.

Before the LM-508, I've had an EL-34 Yaqin valve integrated that I wanted to upgrade the caps and I couldn't find a tech to do it. Incredible, but true. I've been told that there's a valve tech in a bigger town, about 100 miles away.

Hi Strat_Mangler,
Thank you very much about warning us, but I'm just trying to figure out the chances of failure collecting information from another LM-508 owners and dealers because carrying an almost 100 pounds valve amplifier to another city (100 miles away) and then come back later to pick it up, after the cap change it's done, it would take a LOT of effort for me.

Why do you get upset about my inquire for information?

Happy listening

 

Not upset in the least. I just don't see the point of this exercise. You have the info. Up to you to decide whether to take a chance by doing nothing or to do some work to be proactive about a possible issue.

 

Hej,
One more with this problem. My amp stopped working the other day...
I’ve been following lm508 thread for some time so I have been aware...
So I opened the amp and found that the capacitor was blown in the same way as showen above.
I have a picture on it but I haven’t figured out how to post it. (I’m new to this forum).
Stratmangler you mentioned in a post that there are few other capacitors that might be broken, do you have any more info?
Apart from this I’m extremly happy with this amp!
I’ve rolled some of the tubes pretty much basen on experiences shared in this forum.
I’ve also changed the coupling capacitors which I feel improved the sound quite a bit.

 

@Sandasnickaren , how long have you had the amp?

 

Did you get the amp repaired and running again? Because blown caps in tube amps have been known to take out output transformers, which is the second worst-case scenario. (The worst case being a fire, something I had happen in an amp I once owned. Just after a very loud "pop!")

Strat-mangler didn't reply the way he did because he's personally upset. He's taking a serious tone because he's trying to warn you. Tube amps are hazardous in ways that most transistor amps are not. A blown power cap in a solid state amp is merely expensive, even if it takes out other components. A blown cap in a tube amp can take out everything downstream of the signal path, including the output transformer. And the repair cost of that problem can very often exceed the price of a new amp. Tube gear is also liable to catch fire. and the risk of this is greatest for the power amp section. The more powerful the amp, the bigger the risk. The trouble usually starts with a blown cap.

I advise all tube amp audio owners to have their amps plugged into a variac (or in bypass mode, if one if provided), and to keep just a few volts flowing through the amp at all times. Leaving one on at full voltage all the time risks a fire flaring up when you aren't in range to shut down the gear. Turning tube amps on and off always involves the caps warming up, which means the sound can be a little off until they're recharged. And if the gear hasn't been used in a while, the caps can discharge to the point where a sudden input of full AC voltage can blow them up. (Not so much a problem for new gear. Big problem for any gear with the old paper capacitors, which eventually dry out and go bad.) Also, a good variac prevents voltage spikes, which can be a real problem with some AC lines. Apartments are a special problem in that regard, because the load tends to shift around.

Replacing a capacitor or adding a diode in the right place is not difficult for someone who knows how to use a soldering iron. You don't have to diagnose a problem in order to do that. But it's extremely important to make sure that the capacitors in the circuit are discharged, because even unplugged tube gear can pack a wallop. If you aren't sure that you know how to discharge the caps in the circuit safely, leave the lid on.

 

One year. But I bought it used so it’s at least more than 2 years old. There was no warranty left when I bought it.

 

Not yet, still researching and checking before ordering parts.
Yepp, I’m aware of the hazards with tube amps.

 

the main issue is the negative voltage across the capacitor in question. the cap is a electrolytic cap which has a polarity. during the power up standby, according to Strat-Mangler's tech, a large negative voltage is applied across it momentarily. This cause electrolysis to occur inside the cap which dissolve some of the oxide layer inside the cap. the standby time is only 30 seconds so it is not big of a deal but over time it may eventually dissolve enough oxide and causes short inside the cap and then boil off the electrolyte and causes the cap to explode.

So while having a cap with higher temp rating doesn't hurt but not the main solution. the main fix is adding the protection diode which will prevent any large negative voltage drop across the cap.

Any 508IA out there the cap in question will have deteriorated to some extent already. Depending on how much power on/off cycles the amp has. At minimal this will decrease the capacity, at worst, it will blow up.

So for anyone who has the means, make the suggested mod: replace the already deteriorated capacitor, and add a protection diode.

a more detailed article on the topic:

Does an electrolytic capacitor degrade each time it receives reverse voltage?

 

Sorry you also experienced the problem.

Glad my info was at least useful to someone.

You need to use an image hosting service like Imgur where you upload the picture to. Then, you look at your picture on that website, right-click on it, select "Copy" or "Copy Image" (depending on your browser), then right-click in the reply box in this forum, and select Paste.

I don't have specifics but my tech told me the other affected capacitors were the ones connected to the dead capacitor I talked about in the first post.

Only additional information I can provide is a schematic for the amp @jmpsmash posted a few days ago in the other thread. Here it is below.

Dropbox - LM-508IA Schematic.pdf - Simplify your life

Which caps did you remove and which ones did you replace them with?

 

Ok, I’ll try to post pictures later on.
Excellent News that we now have a schematic!!!! Makes it much easier!
I replaced C 102 and C103 ”multicap” with U-Cap hybrid teflon CU.

 

My 508 blew when doing tube swapping. Did any of you do tube swapping close to when it happened?

 

This is a simple repair/modification. If anyone lives in Central Florida and wants it done, buy the parts and I'll do it for you for free.

 

It maybe a coincidence, or not. were you swapping the 300B? that cap is the supply for the 300b and 6SN7/6SL7. The cap was about to blow and a change in operating point just triggered it.

 

It occurred to me but it was merely a coincidence for reasons explained in this and the general 508IA thread. Bottom line is it's a bad design decision by LM to have installed the stock cap which creates this problem over time.

Prior to the implosion, I was getting hum through the right speaker which was getting worse and worse. While troubleshooting the issue, I did switch tubes around as I wanted to eliminate the possibility a tube was the cause for the problem. After switching them around and lowering the bias screw, a couple of loud pops and white smoke came out of the amp after I powered it on.

My tech confirmed it wasn't the direct cause for the issue, although the side effect of this problem is one tube died and another was affected to the point of it being defective. It was a costly repair but I'm glad my info is proving useful to some.

Too bad LM won't ever admit to being responsible for that poor design decision.

 

WOW!

This is why this place rocks! Regardless of whether anyone takes you up on it, props to you for your generosity.

 

I have a 508IA and this thread has gotten me quite nervous. If I want to take this to a tech to get it done, I would like to do some upgrades to improve the sonics as well, namely other capacitors in the amp and possibly wires, binding posts and pots as well. Does anyone know of a definitive resource or post where I can find which components I should be upgrading with pictures on where they are in the amp?

Also does anyone know of a skilled tech in Houston, Texas?

 

There's a schematic a few posts up that was posted. You can grab that. As for which ones should be upgraded and to what, that is entirely subjective. You could ask 20 people who've done it and they'd likely give you all different answers.

Personally, I don't believe it's necessary to upgrade the rest of the components but some have made some changes and have reported back positive results. Just my opinion, but it's tough to gauge how much of a change it actually does as you'd need two of the same amp to really tell. And I'm not entirely sold on the idea that musical memory is as such that one can remember how something sounded from the same unit hours or days ago. Just not a good comparison method, IMHO.

Aside from that, it can get quite expensive to buy all these parts and have someone perform the work.

Having said that, the best I can offer is a website where caps were tested for sound. Again, completely subjective and there's no way for me to know if it's impartial or if there's some sort of affiliation with any of these capacitor brand names. With that out of the way, happy reading!

Humble Homemade Hifi - Cap Test

 

I am one of those who believe and are able to hear difference between caps and resistors.

IMHO, the components that makes the most difference are: (in decreasing order)

- tube
- (de)coupling caps
- power supply caps
- cathode bypass caps
- resistors
- wires
- connectors

They are also decreasing order of cost. unfortunately.

I have prepared the following to be swapped into my 508IA

- decoupling cap between 6SL7->6SN7 : Jupiter Copper 0.1uF
- decoupling cap between 6SN7->300B: Miflex DCPU 0.1uF
- 300B power supply cap: Mundorf HV 680uF Mlytic
- 6SL7 power supply cap: Mundorf EVO oil 22uF
- cathode bypass: ELNA Silmic 220uF (wanted to use Audio Note Kaisei but they are sold out)
- various resistors: Takman REY

I will report back my findings afterwards.

 

Thanks gents. Have I gone crazy or do I remember someone else who have already done all the mods and was talking about it in a thread? Either way @jmpsmash I'm looking forward to your reports.

 

@Strat-Mangler For the replacement cap, I need the same 470uf but higher voltage, temperature and ripple current rating correct?

 

I will definitely post my findings. It might take a while as it is a lot of components and the holiday is coming up so I won't get to it until probably mid-January.