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Conrad Johnson Tube Amp - advice needed urgently!

M

MrTAD

New Member
I know this is the place for the best audio advice and I need help! I have a Conrad Johnson PV-5 and MV75-a1 preamp and amp. The MV75 had a serious problem and needs repair! What happened was that I was listening to some vinyl for a few hours, then decided to swap out some speaker cables. I shut off the amp (left the preamp on) and CAREFULLY put a different set of speaker wires on....when I turned on the amp I heard an explosion that sounded like glass exploding! The tubes all look ok - I didn't blow out any of the fuses.

I've played voice mail tag with Conrad Johnson - I LOVE my equipment but trying to talk to a human at C-J is pretty damn near impossible and I'm ready to give up! Any recommendations for places that can repair/upgrade the amp in the mid-atlantic area? I don't want to just take it to the first audio repair shop in the phone book.

I just hope this doesn't kill me financially! I have some serious medical bills that I'm trying to pay off! So maybe if there's an alternative tube amp I can buy instead of getting this repaired I might consider it.

thanks for your help!
 
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Did you try emailing CJ? I've had good luck with that approach. They have always responded within 24 hours to my inquiries. For example, I needed some repairs a few years back to one of my pieces of CJ gear. I emailed them, explained the problem and how it happened. They emailed me back and said to box up the unit, ship it to them with a note inside of what the issues were and how it happened. They then said they would email me after they tore down the amp for repair costs. About two weeks later, I got an email with repair estimates...I called them and gave my credit card number and about a week after that I had the amp back...with brand new shipping cartons!
 
If you're up for it, I'd say pull the bottom cover off and see if anything looks like it blew up. Generally speaking when you hear loud noises coming from piece of electronics, a visual inspection will show something wrong. Also, have a closer look at the tubes. If any have a white chalky look to them instead of the normal silver getter flash, that indicates the tube lost vacuum.
 
Kward,
Ironically last fall I emailed them with some questions about something (I can't recall what it was at the moment) and never heard back. Then like two months later I got an email from someone telling me that the tech had retired and apologized for not responding.

I have a question for you (if you don't mind). I've always turned on my preamp and amp at the same time - the PV5 has a few minute delay before a relay clicks on. I didn't know if there was a proper sequence to powering up the amp/preamp...like which to turn on first. When I blew the amp it was the first time I can remember ever turning on an amp an hour after the preamp was powered up. I'm just trying to figure out if I was responsible for causing the amp to blow...or if it was just a coincidence.

thanks
 
Gadget...I did take the bottom cover plate off but didn't see anything that looked like it got "burned up" - nothing looked unusual. I'll check the tubes in the daylight. The power tubes looked ok...I didn't look at the smaller tubes.

If I said the sound of the "explosion" was like a bottle exploding...would that imply possibly a capacitor blowing? The "explosion" was loud enough that it scared the crap out of me...and I actually expected to see shattered power tubes!
 
I don't see how the order of turn on could have anything to do with it. I've turned on my PV12 and Prem11 in random order for 15 years with no problems. You do hear a slight thump if you turn on the amp first then the preamp..but that's not enough to blow anything.

I'm wondering if you accidentally had the speaker wires touching when you turned on the amp. As Gadget said, it wouldn't hurt to open her up and take a look if you're inclined...it might give you a visual indication of where to start looking. Mine made a loud pop and glass cracking noise also when it blew. It was a bank of Zeners that went out that regulates the filament voltage. In my case, a toddler yanked out one of the output tubes, broke off the plastic key and put it back in with the pins oriented wrong. When I turned it on next...bam!

Hmm..if their chief fix it guy retired that would explain a few things... hopefully they'll get someone to replace him who was a good as he was.

Good luck with it...it's definitely worth fixing..hopefully it will be reasonably priced for repairs.
 
MrTad:


Turning on the amp an hour after the preamp was turned on would not cause the issue. Assuming that you were as carefull as your post indicates around no shorts and properly connected speakers the failure was a coincidence.

Are you at all familure with using a voltmeter and do you know how to stay safe around high voltage electronics? If so, and you are up for it, we might be able to talk you through the troubleshooting for this amp. There are a great many of us who have help a great many relitive novices a repair. IT takes patience on the part of the owner but it is well worth it.

Shelly_D
 
Shelly d....
you've got me curious. I'm not an "electrical guy" but I have a voltmeter and when I was younger I even built some solid state power amps for a car stereo - I know that's a far cry from high voltage stuff....BUT what kind of "troubleshooting" are we talking about? What would I have to take apart? My medical issues didn't involve a pacemaker and I wasn't planning to be standing barefoot in a puddle of water!
 
First you would need either a variac or a dim bulb tester. The dim bulb tester is just a light socket with a light bulb screwed into it wired in series with the amp to 1) limit the current flowing through the amp and 2) reduce the voltage allowed to enter the amp.

Then you would need a schematic and you would need to post it for us to see and be able to read.

Once that is accomplished, we could then have you fire up the amp on the dim bulb tester and take readings at the various points we ask you to, posting the results as you go.

Shelly_D
 
shelly d...so I don't have a variac or dim bulb tester. Let me see what I can come up with tomorrow during the day. I'll post back to you. I do have schematics that I found online - through this forum I'm pretty sure - for the MV75a-1. I'll have to dig those out too.
I'll post back and let you know if I'm able to get the stuff I need.
thanks again
 
If you have a multimeter with the ability to check continuity, you should at least verify that the two KTK-1 protection fuses are intact or have blown. If the fuses blew out and did their job of protecting the output transformers, then just replace them and bring the amp up to power slowly with a variac. You'll need to test your tubes as well. Those fuses do not necessarily protect the output tubes.

Another area to check is the capacitor across the power switch. I understand those can go easily and you can get a nice spark across the power switch if the cap is shorted. C-J was replacing these with something like Sprague 715P Orange Drops. An X2-rated film cap would be better suited for that application.

If you've never worked on a tube amp and safely discharged an amp's power supply capacitors, you should not go under the hood of the amp. 400-500 volts is nothing fool around with.
 
Did you find and check the fuse that's inside the chassis? It is shown on the left side of the attached photo to the left of the grey capacitor. This is the main fuse. Make sure the two fuses on the rear of the amp are the proper fuses. They should be either KTK-1, BBS-1 or BLS-1 fuses. I've read that some of the early "a" and "a1" models would only accept the KTK's which are just a bit longer than the BBS and BLS fuses. I don't know if this is a fact since my early model MV75 does not have these fuses.

Oh yeah.... MAKE SURE THE AMP IS UNPLUGGED BEFORE CHECKING THIS FUSE!!
 

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The way you turned stuff on sometimes has an effect. Audio Research SP3 preamps will cause power amps to fail or blow their fuses during startup if the power amp is on. SP3 actually has two switches in front for that reason, one for the preamp, one for the main amp. Some preamps have muting relays that short outputs during startup. I installed one of these in my SP3. Have you got a good clear picture of the top and bottom? I should be able to help you but I am unfamiliar with Conrad Johnson so I would need pictures. If you can hear the explosion, you can find it.
 
MrTAD said:
shelly d...so I don't have a variac or dim bulb tester. Let me see what I can come up with tomorrow during the day. I'll post back to you. I do have schematics that I found online - through this forum I'm pretty sure - for the MV75a-1. I'll have to dig those out too.
I'll post back and let you know if I'm able to get the stuff I need.
thanks again
Click to expand...

The dim bulb tester is built, not purchased. You need 2 conductor wire, an electric plug, a socket to hold a light bulb, and an electrical outlet. This website will show you a picture of how it all wires together:

http://antiqueradio.org/dimbulb.htm

Shelly_D
 
I have an MV75, and lately I have had to do a few things to it. Because of their age it's about time to take yours off service and have it checked. This is a fairly easy amp to work with. I seriously doubt that you live close to me, as to check it. This is a heavy amp and a pain in the neck to ship, so in what city/state do you live?
 
C-J/McCormack of VA is a small operation. If someone retired, maybe it was the technical go between, Ed,that I dealt with getting a Revision done on a McCormack. If you have to go the manufacturer route, expect it to be put back in great shape with anything that might go wrong replaced. This is not cheap but probably worth the investment.

Since the amp is old, going through it with the help here might be what you want to do. Once repaired, consider a refreshening to get it ready for the next 20 years.

Gadget helped me get some 35 year old tube amps running recently. Good help here.
 
MrTAD said:
Gadget...I did take the bottom cover plate off but didn't see anything that looked like it got "burned up" - nothing looked unusual. I'll check the tubes in the daylight. The power tubes looked ok...I didn't look at the smaller tubes.

If I said the sound of the "explosion" was like a bottle exploding...would that imply possibly a capacitor blowing? The "explosion" was loud enough that it scared the crap out of me...and I actually expected to see shattered power tubes!
Click to expand...

Usually when caps blow up, they make a rather loud pop noise and you get a bad smell and often some smoke. If it was a glass sort of sound with no smell, that kinda sounds like a tube bottle shattering. Tubes rarely break just sitting there but if there was a defect in the glass bottle, it may have finally decided to fail after enough heat cycles.
 
I'm back online! So....I took the bottom panel off and checked the "secret" fuse - that looks like brand new and tests out perfect. Also checked the two fuses on the back and they're perfect too. I'm snooping around underneath to see if anything looks obviously wrong.

I don't recall any smoke or horrible smell...although there may have been some smell that I wasn't paying attention to.
 
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