MV-55 output issues

[chromatic-third]

Greetings all,

I'm having some problems with my MV-55 and was hoping that some of the expertise here might help provide a solution. I've documented the issue and the troubleshooting steps I've undertaken as best as I could below.

Below I refer to the left and right channels as being the left and right speakers when facing the system.

Original Issue: No sound out of the right channel.

Troubleshooting Steps Taken:

• Swapped Sources: Switched between various input sources to rule out any issues with a particular source, but the right channel remained silent.
• Swapped Left and Right Inputs from Pre-Amp: Exchanged the cables from the left and right inputs to make sure the issue wasn't with the pre-amp. The right channel remained silent.[
• Swapped Input Tubes Between Left and Right Channels: no sound from the right channel while the left continued to perform normally
• Swapped Output Tubes Between Left and Right Channels:
  • V5 output tube brightness increased
  • Shortly thereafter the "Fuse out" light came on for the left channel so presumably the fuse blew.
  • The "Fuse out" light is not illuminated for the right channel.

My suspicion is that the tube that is now in the V5 slot is faulty in some way and has simply blown the output fuses, although the fuse out light did not come on for the right channel, but I figured someone here might have better insight before I start doing anything that could damage the amp further.

Any advice on how to approach this or suggestions for additional troubleshooting would be immensely helpful.

Thank you for your time and assistance!

[admin]

First, welcome to CJO. Great to have you with us.

Fellow MV-55 owner. Sounds like you have a bad output tube that was originally in the right channel. I would get a fresh set of output tubes, replace the blown fuses, and try again.

[Truth71]

What output tubes were originally supplied with the MV-55?

[Joe Appierto]

The tube replacement set (RT55) on the Conrad Johnson website lists 6CA7/EL34 for the output tubes for the MV-55.

[Truth71]

The tube replacement set (RT55) on the Conrad Johnson website lists 6CA7/EL34 for the output tubes for the MV-55.

Let me re-phrase:

What BRAND of Output Tubes (ie. Svetlana Winged C, Eletroharmonix, GE) ?

[chromatic-third]

Thanks so much! My concern was that one of the fuse out lights wasn’t lit. I suppose I should just take out the fuse and check it.

[admin]

Thanks so much! My concern was that one of the fuse out lights wasn’t lit. I suppose I should just take out the fuse and check it.

Checking all the fuses is a good idea and easy to do. Low hanging fruit. But I still think that the tube(s) might be bad.

[RJKflyer]

I have been an MV-55 owner for 30 years and have rolled many tubes.

I recall the original CJ fitment in mine were Sovteks EL34G.

I have used:
1. ElectroHarmonix 6CA7 'big bottles'
2. Sovtek EL34WXT
3. Modern Tungsol EL34B
4. Mullard reissue EL34 - found these to be poor quality and poor longevity
5. Tesla/Svetlana - poor longevity - the only two brands where one actually blew the fuse and failed (or the other way around!)
6. Harma EL34 'retro'

I'd rate 1 and 3 as the best for me. YMMV. I buy quad matched although not necessary in theory.

Buy top quality fuses - Bussman BBS.

[chromatic-third]

Sorry for the long delay. An update and further questions.

I went ahead and replaced all of the output tubes and the fuses. The left channel now works again, but the right channel is still out. Is there anything else that would be worthwhile for debugging purposes before having a technician take a look at the lamp?

Thanks again!

[admin]

Is it completely silent in the right channel?

[AnotherJohnson]

Are you sure the plate fuse is good?

The plate fuses are typically large KTKs. Ask Jeff. Probably KTK2s. MAKE SURE YOU GET THE RIGHT SIZE. I bought an MV52 once and the prior owner had put a fuse 10 times too large in. I discovered it when the amp went red plate and started motorboating.

[RJKflyer]

The MV-55 was specified with Bussmann BBS-3/4 (i.e. 750mA fast blow).

An alternative is the Littelfuse BLS.750 - same rating and behaviour.

They're both 0.406" dia. by 1.375" long.

[AnotherJohnson]

From the manual

IMG_1297.png (353.83 KiB) Viewed 568 times

Interesting that he went back to a small glass tube fuse.

[RJKflyer]

They're neither 'small' nor 'glass'!

They're ceramic, industrial, high voltage (600V) High Breaking Capacity (10,000A) fuses.

[AnotherJohnson]

Well, whatever they’re protecting, it’s surely been fried.

If you recently acquired it and it was represented as good condition, I’d take it up with the seller. In my mv52 case, they were 20 amp and, as the tube failed, I was present and was able to dive for the power plug, saving the other components from failure. I was able to negotiate the seller into paying for the repair since he’d advertised it as excellent.

[roberto]

Are you sure the plate fuse is good?

The plate fuses are typically large KTKs. Ask Jeff. Probably KTK2s. MAKE SURE YOU GET THE RIGHT SIZE. I bought an MV52 once and the prior owner had put a fuse 10 times too large in. I discovered it when the amp went red plate and started motorboating.

Hola Chicos,
This is the correct value for that particular fuse. Is it a KTK type fuse.

https://uk.rs-online.com/web/p/cartridge-fuses/4588497
https://www.amazon.com/Fast-Acting-Midg ... 167&sr=8-4

[AnotherJohnson]

Are you sure the plate fuse is good?

The plate fuses are typically large KTKs. Ask Jeff. Probably KTK2s. MAKE SURE YOU GET THE RIGHT SIZE. I bought an MV52 once and the prior owner had put a fuse 10 times too large in. I discovered it when the amp went red plate and started motorboating.

Hola Chicos,
This is the correct value for that particular fuse. Is it a KTK type fuse.

https://uk.rs-online.com/web/p/cartridge-fuses/4588497
https://www.amazon.com/Fast-Acting-Midg ... 167&sr=8-4

These KTK style fuses make more sense, as noted in posts above.

Too bad the fuses seem to have been replaced with what amounts to no protection at all.

Maybe ARCs old approach of using resistors was based in keeping idiot owners from replacing plate fuses with whatever was handy.

CJ’s approach requires a certain amount of knowledge on the part of the owner. I like that, but there are sad examples like this one where an owner wasn’t sufficiently knowledgeable.