Noise troubleshooting

[jimbones]

So today i just switched back to my P12's after having used my NAD M23 for the past 7 months. I had not turned on the P12 for 7 months so I assume the caps have to reform. I didn't have this level of noise when I shut them down for summer. I am troubleshooting it is a mix of a buzz and tube rush (low). Not 60 hz hum. I suspect tubes or caps. Tubes are Tung Sol reissue with less than 500 hours on them. The input tubes are not new I am unsure of the hours on them. I suspect the 6FQ7 or 5751 as a first place to look. What do you guys and gals think? Oh, it is slightly stronger in one channel than the other. After that it could be caps (Ugh) costly!. I dont mind doing the work myself but I dont want to change every cap in the unit lol. I think someone sent me a schematic at some point.....Thanks for your help.

[admin]

I would start with swapping the tubes. This is a possible cause and obviously the most easy one to fix. Having a backup set of tubes is not a bad idea so if you don't already have a set, it may be worthwhile to buy now for both the backup and diagnostic purposes.

A visual inspection of the insides would be my next step. Looking for bulging caps, burn marks, etc.... After that it starts becoming more complex.

[roberto]

Well said Admin. Usually that noise is coming from a small signal tube. Just swap the suspect and have a listen. It is always wise to turn off the preamp while doing the swapping. Yes I know it takes more time...this is the safer way to do this kind of job.

[admin]

Yes, everything should be off (and I unplug everything as well) when changing tubes. If you do an internal inspection, make sure it is unplugged and be careful as caps can hold their charge so do not touch anything with your fingers or conductive material. Safety first.

I also like to wear vinyl gloves when changing tubes. Some say that the oil on your skin can degrade the lifespan of the tube,... I don't know. At least it keeps them smudge free!

[jimbones]

Oh definitely everything turned off lol. I am a former tech/engineer. The only thing I hope it isnt one of the 4 large caps (or Xformer) in the PS because I dont know where I would get them and would have to send them back to the factory $$$$$$.

[jimbones]

BTW the small signal tubes are old stock. I want to buy new production. What are the better new production 6FQ7 and 5751? I have toms of power tubes.

[admin]

You can play it safe and order tubes directly from CJ. They will obviously cost a little more than a 3rd party supplier but you know you are getting the best. Smaller input tubes are typically not that expensive. I don't know if there really is a "better new production?" I will often talk to some of the better known tube suppliers, they often can make some good recommendations. There has been topics on recommended tube suppliers here on CJO so I would maybe look at those posts.

[SolderSlinger]

Tubes shouldn't go bad from nonuse. If that was the case, 70 YO vintage tubes wouldn't demand a premium.

I would look at mechanical connections. Try inserting the tubes a number of times (10?) to reestablish a good connection. If that doesn't work, try cleaning the RCA jack and the tube sockets with a good contact cleaner. If spray type, be careful around the socket pins. Some cleaners use harsh chemicals. Simple household products like vinegar or lemon juice work well.

[admin]

Tubes shouldn't go bad from nonuse. If that was the case, 70 YO vintage tubes wouldn't demand a premium.

Shouldn't but that is exactly what happened with my PV-14L. The seller advised that it was in storage for some time but it had been working well before. I didn't have any reason to doubt him and he covered the cost of the tube replacement. I don't have a good explanation either but I have experienced the situation. Good point about cleaning the contacts however. Those can oxidize with time as well.

[SolderSlinger]

Agree that tubes can fail at anytime. Especially when shipped as filliments become brittle from use. Recently I dropped a 6DJ8 Holland Bugle Boy with about 2,000 hours from about waist high onto carpet. It ruined the tube.

[jimbones]

Thank you all. I left the amp on for 24hrs to "burn in" and the noise floor is lowered but not as much as I would like. I did measure the frequency of the buzz and it is 480hz. Just curious if anyone has pics of the underside of the amp, might be a cap but I dont even want to attempt unless I know what I am in for.

[admin]

I found this with a fair number of pictures of the circuit board: https://ginotrifiro.it/portfolio-artico ... er-twelve/

I can't read the text but it looks like perhaps some modifications are being made. Regardless, you can do a visual comparison. With faulty caps I look for bulging or discoloration. Beyond that you would have to test the values manually.

[jimbones]

TY. I saw that too, plenty of photo's but I cant really figure out the 4 main PS caps. I assume they are not something I can just buy off the shelf.

[admin]

Not sure if those are proprietary caps or not but you should be able to find their values from the schematics (which we do have in our database). I would probably make sure they are faulty before replacing them as they are probably pretty expensive and I would replace all of them and not just one if I were going to do a replacement.

[jimbones]

Yes I have the schematic. I believe they are 4700 uf 35 volt electrolytics. However the drawing also shows a 47pp. not sure what "pp" stands for. I also agree change all. In fact if Im under the hood i would also change the four 0.15uf 600v caps in the signal path to some premium cap.

[SolderSlinger]

@admin thanks for the ginotrifro link. Good stuff.

@jimbones Are you asking for pictures of the bottom of the amps or for a component placement diagrams? I've never seen a component placement guide for the Premier 12. A long time ago I asked CJ for one and Jeff said it's proprietary. Because you have 2 amplifiers, when you get ready to work on them, keep one as the "gold sample" while you work on the second. Have a physical reference is infinitely better than a picture. I always like to record key voltages and waveforms before making any changes.

The 47 pp caps are 47uF polypropylene. The are the black round metal case parts sticking out of the top of the amp. No voltage is given so plan on 600V because the 0.15uF bypass cap is 600V, and B+ is ~512Vdc at nominal line voltage. The challenge if finding a capacitor in the same package. Obbligato makes a metal case cap, but I think it's too large to fit. Definitely not a direct replacement. It's always worth sending an email to CJ asking for availability and pricing. If you don't ask the answer is always no.
https://partsconnexion.com/products/obb ... 14b1&_ss=r

Personally, I would want to measure ESR and dissipation factor before replacing. Not that I have a lot of experience with CJ gear, but I have yet to find a bad PP filter cap.

The 4,700uF 35V capacitor is the small signal tube DC voltage supply filter cap. I have seen those fail in CJ amps. And when they do, they typically present a weak short failure, taking the rectifier diodes with it. Very similar to what Roberto found when trouble shooting the Classic 60. The capacitor failure more is a current leak that is low enough to keep it from rupturing/venting, but large enough to cause a lot of ripple or the "DC voltage rail. If the leak is large enough it will cause the diode to fail over time. I personally replace the cap and bridge (early Premier amps) or bridge diodes while I have it open. Once the amp is up to temp you should have a fairly stable 6.3-ish volts with some AC ripple. You can do a google for power supply filter cap ripple voltage calculator to get a close enough value to confirm if the cap is leaky.

Many comments on this forum about replacing parts with non-CJ components. I won't judge you if you do. My $0.02 is that if you are going to replace the 0.15uF caps, consider using TEFLON and doing a 1 for 1 substitution for all 0.15uF caps in the audio and power supply sections. Solen is making a reasonably priced (when compared to V-Cap) capacitor. I would opt for the foil vs. metalized construction.
https://partsconnexion.com/search?type= ... +capacitor*

The Premier 12 has 8 of the large black Vishay resistors. The Good Stuff. 7 are around V1 and the 8 is the global loop feedback resistor. If you want to make additional improvements V2 and V3 resistors with lower noise offerings. Odds are you will not find the same Vishay "52", "53", or "42" style resistor at any electronic supplier. However, Digikey carries a wide selection of Dale (owns Vishay) resistors that have a very low noise level approaching Vishay. From memory the lowest noise varients are available in a maximum 1/2 watt package size. You'll need to use 2 to substitute for a 1 watt part. And you need to wire them in series to maintain correct voltage rating.

Last is to replace the sockets.

Average voltage measurements from warm units at 115Vac.
C 512V
OA 416V
OB 370V
OC -73V
OE 1.2V (for 6550), 1.0V (for EL34)

Have fun!

[jimbones]

Holy Crap @solderslinger I should just send it to you!! LOL!!

[jimbones]

@admin thanks for the ginotrifro link. Good stuff.

@jimbones Are you asking for pictures of the bottom of the amps or for a component placement diagrams? I've never seen a component placement guide for the Premier 12. A long time ago I asked CJ for one and Jeff said it's proprietary. Because you have 2 amplifiers, when you get ready to work on them, keep one as the "gold sample" while you work on the second. Have a physical reference is infinitely better than a picture. I always like to record key voltages and waveforms before making any changes.

The 47 pp caps are 47uF polypropylene. The are the black round metal case parts sticking out of the top of the amp. No voltage is given so plan on 600V because the 0.15uF bypass cap is 600V, and B+ is ~512Vdc at nominal line voltage. The challenge if finding a capacitor in the same package. Obbligato makes a metal case cap, but I think it's too large to fit. Definitely not a direct replacement. It's always worth sending an email to CJ asking for availability and pricing. If you don't ask the answer is always no.
https://partsconnexion.com/products/obb ... 14b1&_ss=r

Personally, I would want to measure ESR and dissipation factor before replacing. Not that I have a lot of experience with CJ gear, but I have yet to find a bad PP filter cap.

The 4,700uF 35V capacitor is the small signal tube DC voltage supply filter cap. I have seen those fail in CJ amps. And when they do, they typically present a weak short failure, taking the rectifier diodes with it. Very similar to what Roberto found when trouble shooting the Classic 60. The capacitor failure more is a current leak that is low enough to keep it from rupturing/venting, but large enough to cause a lot of ripple or the "DC voltage rail. If the leak is large enough it will cause the diode to fail over time. I personally replace the cap and bridge (early Premier amps) or bridge diodes while I have it open. Once the amp is up to temp you should have a fairly stable 6.3-ish volts with some AC ripple. You can do a google for power supply filter cap ripple voltage calculator to get a close enough value to confirm if the cap is leaky.

Many comments on this forum about replacing parts with non-CJ components. I won't judge you if you do. My $0.02 is that if you are going to replace the 0.15uF caps, consider using TEFLON and doing a 1 for 1 substitution for all 0.15uF caps in the audio and power supply sections. Solen is making a reasonably priced (when compared to V-Cap) capacitor. I would opt for the foil vs. metalized construction.
https://partsconnexion.com/search?type= ... +capacitor*

The Premier 12 has 8 of the large black Vishay resistors. The Good Stuff. 7 are around V1 and the 8 is the global loop feedback resistor. If you want to make additional improvements V2 and V3 resistors with lower noise offerings. Odds are you will not find the same Vishay "52", "53", or "42" style resistor at any electronic supplier. However, Digikey carries a wide selection of Dale (owns Vishay) resistors that have a very low noise level approaching Vishay. From memory the lowest noise varients are available in a maximum 1/2 watt package size. You'll need to use 2 to substitute for a 1 watt part. And you need to wire them in series to maintain correct voltage rating.

Last is to replace the sockets.

Average voltage measurements from warm units at 115Vac.
C 512V
OA 416V
OB 370V
OC -73V
OE 1.2V (for 6550), 1.0V (for EL34)

Have fun!

Couple of questions/Thoughts.
1) Im glad you said that you never seen the 47uf PP caps go bad. I agree that PP is fairly reliable, I would expect an electrolytic to fail before a PP. My concern here was where i was going to find a cap of the same electrical parameters and form factor. Whew!
2) On the schematic there are four 0.15 uf PS caps (connected to 10k pots) that I have marked on the schematic as some one else recommended as having the "biggest" affect on sonic quality if I dont want to go and replace all the caps willy nilly.I assume PS is polystyrene?? Can you confirm that this is true? or are there other caps I should focus on?
3) I am not interested in installing resistors in series to make up the 1 watt power handling. I like a clean install and I believe reliability and clean assembly would be compromised. I'll play it safe.
4) ANY component that is upgraded must NOT compromise reliability. That is just a rule of mine. I came from the defense industry and I am mil spec certified at soldering and assembly so I was trained that way. Any caps must not be so big such that they can not be installed properly and safely.

Really great thoughts everyone. TY all for the input I'll let you know how I do.

Jim

[jimbones]

Hold the Phone!!!

Tonight I was going to prepare to work on the amp when I decided I would check one last thing. It was a crazy idea but what the hell. I plugged one amp into one duplex and moved the other amp to another duplex on a different circuit. The amp that I moved to a different circuit is now quiet. The other still had noise. I put a Gnd cheater on it and it got quieter. Now the other day I did put the Gnd cheater on, but it did not have any effect because I had them both plugged into the same duplex outlet.
So now the only thing that remains is some tube rush. I will start with the signal tubes tomorrow.

I guess this is good news but bizarre.
Stay tuned.

[jimbones]

OK so changed the input driver tubes; still have tube rush. On to the power tubes!! Damn they are nearly new