MV60 transformer hum

[Redver]

Hi RJ, I have 4 kt120 fitted as output tube. The guy I purchased from said it went to Caxton Audio for the triode conversion shortly after purchase, board is dated 1/95 is this the manufacture date? Anyway looking at the board. IMHO It was a terrible job done.

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Maybe this is normal for a board 28 years old? I really don’t know. Looks like it’s been very hot? Do you have a good source for those busman bbs 3/4 750ms fuses? Has anyone found an Aussie equivalent and converted the fuses, $50 is a lot for a fuse isn’t it? You would have thought the prices would come down over the years.

Thanks for your help I’ll let you all know what transpires.

Kind regards, Nick

[admin]

Although circuit boards 30 years ago certainly did not look as "clean" as they do today, the burn marks on the board do concern me. And the soldering on those small resistors is worrisome. The soldering in general is a little "questionable" although may not be a problem.

[AnotherJohnson]

Hi RJ, I have 4 kt120 fitted as output tube. The guy I purchased from said it went to Caxton Audio for the triode conversion shortly after purchase, board is dated 1/95 is this the manufacture date? Anyway looking at the board. IMHO It was a terrible job done.

Maybe this is normal for a board 28 years old? I really don’t know. Looks like it’s been very hot? Do you have a good source for those busman bbs 3/4 750ms fuses? Has anyone found an Aussie equivalent and converted the fuses, $50 is a lot for a fuse isn’t it? You would have thought the prices would come down over the years.

This looks like it went to Caxton Audio for salvage operations.

If this is an “after repair” photo, there’s still at least one burned out resistor, so it’s not done yet.

It is not normal appearance for a 30 year old unit unless that 30 year old unit were driven to failure by a careless user and repaired by an incompetent technician.

Normal is “dusty” inside with some obvious color aging.

The big plate fuses are not overpriced at $50AU. We pay $25US. They’re not ubiquitous like the little glass tube Bus fuses.

These photos are evidence to support the view that it is risky to buy vintage gear … and, in spite of those who claim that the PVx or Premierx reached the pinnacle , they did not. They might fit within a certain “I love this sound” context. But advances in this century are real.

Good luck sorting this out. Looks to me like a new board, which is probably not available, will be needed.

I’m sure Roberto has seen worse … and he may be more encouraging. I hope so. I feel your pain.

[AnotherJohnson]

Another fuse story in support of getting the correct fuse.

I bought a used MV-52 once.

I hooked up speakers, put it in the system, turned it on, and started toward the kitchen. It started motorboating, I saw a cherry red plate, and dove for the power cord. I was just in time to save things.

I pulled the plate fuses which iirc were supposed to be KTK-2s. The fuses were the right geometric size, but were rated for 20 amps …. WAAAYYY over the specified value for the amp.

The fellow who sold it to me had claimed that it was all perfect after recently being restored. I learned that restored often means “fixed by the nearest guy with a soldering iron after having failed.”

The first thing I do when I buy any used gear is to check the fuses. Do not run gear with fuses that don’t meet the OEM specs.

[roberto]

Hola,
Well, definitively you have to change those diodes for a more watt dissipation value. I don't know also, if those diodes are zener type diodes. When changing them, leave a good separation between the PBC and the diodes. Think of how you could make them to dissipate all that heat.

Also, you have to drill down all the burned parts of the PCB because it could gotten to be conductive. When the PCB is burned, the isolation capacity of the PCB material (usually fibre glass) is lost and it might be not doing a good isolation for the PCB conductive traces there. Carbon is a high resistive material, but it is not as higher resistive value enough when the fibre glass is not burned. You can do the job with a Dremel hand drill or similar hand tool. When doing it, you must watch the conductive foils. They should be remain untouched by the file bit. Also watch for a double side conductive foils. The burned material must be removed as much as possible. You could be using a galvanic isolation property here (lol).

You have a quite job to do there. But it will pay you back in return with the Conrad Johnson signature sound quality. Have a lot of patience while doing the job. Also with a hand tool is an easy task. I looked at Amazon a not too expensive hand tools with good bits for doing the job right and fast.

Happy listening!

[Big Dog RJ]

Hey Nick,

I can see the photos pretty clearly now... definitely not a good job done. Bit more work needed. Also, I'm quite surprised that David (owner of Caxton Audio & CJ importer) would do half a job like this. I'm thinking it's gone to someone else other than David at Caxton. He would never do a half-baked cake, no chance!

That date on the board is the date of manufacture.

***The other thing is, you mentioned that the original owner sent the amp to Caxton Audio to be converted to the Triode version***

***Note*** in which case, if it was converted to the Triode version them you cannot use KT120'S!!! In Triode version you can only use EL34 tubes. Hence, KT120's are the wrong type of Output tubes for this amplifier.

If the amp was NOT modified to Triode and remained as the SE version, only then can you use KT120's. No wonder things are burning up... I'm surprised the amp didn't blow the roof off! What a disaster.

Whoever is doing the job, just get them to verify which version this is, whether Triode or SE (pentode). You need to determine that first! Once that's been identified then check on all the fuses, including the two large ceramic plate fuses.
Those fuses are $50 - $60 each, there's no other supplier who will get them cheaper.

You can try The Valve Store, Neil in Melb, He has these fuses and can supply other types if required but he's not going to sell them below $50...

Once you've determined which version the amp is and everything is nicely cleaned up then use the Output tubes corresponding to that version. You can't mix Output tubes on different amp versions, that's the biggest cause of damaging the amplifier.

Best, RJ

[roberto]

I think that those 14 diodes are zener diodes. Their value are 1 watt power dissipation. I don't have their exact zener point. You can change them for a higher power dissipation and you can avoid to connect them in the burned PCB. They are connected in series. You can make a floating series connection and only two solder joints in the PCB. Just thinking loud.

[Redver]

Hi Roberto, I was thinking about this last night, like a sub board on fly leads bypassing the existing mess. If I could pay someone to make this I would be extremely grateful, then I could get that soldered in and I should be in business. Thanks for your input Roberto, now would anyone be able to make this little board up for me please. I would also cover postage etc all costs in fact.

Kind regards, Nick

[Big Dog RJ]

So, I had a good discussion with two of my techies; one says it's not too shabby to use the KT120's in a triode version amplifier, as long as the bias holds steady and does not blow the plate fuses. The other was on the same premise but both agreed that's it's far safer to use EL34's, which are closer to spec on a triode conversion amp.

Also, both agreed that if your MV60SE was not modified to the Triode version then sticking with either 6550's or KT120's would benefit from the SE version, since pentode ultra- linear is always better in all aspects; power, better control and plenty of headroom from the larger tubes.

Just thought I'd forward this info to you, for what it's worth. Hope you can get the amp back running fine. Speaking of which, one tech suggested to convert it back to the original SE version, that way you'll get superior results from the KT120's.

Cheers and all the best,
RJ

[Redver]

Hi Big Dog, the MV60SE amp was bought brand new in 1995 approx. wth KT88’S in it, definitely sent to Caxton for the triode upgrade and returned with KT88 valves. It’s been running for years, the correct fuses have always been used and fitted when I brought it.

Can anyone tell me the voltage of the 1 watt zener diode? With a diagram please, just measuring at the existing zener diodes may not be the voltage they should be. Of course someone may already know the voltage, I would appreciate the info before I get the area rebuilt.

Kind regards, Nick

[Big Dog RJ]

Ok, that sounds like something David would do and certify. Installing KT88's is quite different to KT120's. Anyway, since the amp worked fine for years that's good to know. Very glad to hear that the plate fuses are good and of correct type.

However, at this stage I'm not quite sure as to what caused the amp to burn out one channel... could be a bad output tube, output tramny, resistors, caps... hopefully your tech is able to diagnose and fix the problem.

Are you confident that your tech can do the job? And if so, make sure it's properly done and not half-way, you'll have further issues down the line.

So purchased in 95, wow. I was in Naperville IL in 95, that's where my journey began with Martin Logan stats! Great memories.

Cheers mate, hope all goes well and you'll be enjoying fine tunes in no time.
Best, RJ

[roberto]

Hola,
You can replace direct the KT88s with the KT120s, you can not direct replace the KT77/6CA7/EL34 with the KT120s.The pin's array are here:

IMG_7372.pdf

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[roberto]

The 7AC is for the KT88, 6550, KT120, KT90...
The 8ET is for the EL34, KT77, 6CA7...


The connection pin might looks about the same, but are not.

Happy listening!

[roberto]

Hi Roberto, I was thinking about this last night, like a sub board on fly leads bypassing the existing mess. If I could pay someone to make this I would be extremely grateful, then I could get that soldered in and I should be in business. Thanks for your input Roberto, now would anyone be able to make this little board up for me please. I would also cover postage etc all costs in fact.

Kind regards, Nick

Hi Nick,
Because the diode's array are in series connection, you don't need to have a fancy PCB, if you could look for the diode number and I can search for their zener point. Also, I don't have any information for your amp model. I wish I could help you more.

Happy listening!

[Tommd]

in my situation the hum eventually turned out to be due to power supply capacitors that had had their day. After I had them replaced (by heavier ones) it didn't suddenly sound better and tighter, but the hum had also completely disappeared. You can simply measure the value; if the capacity has dropped below 70% replace them.. A real win win situation..