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Post by doctorice on May 8, 2011 5:34:03 GMT -7
Feels like it's time to put some new power tubes in the Z-28. I know that requires a bias check and possible adjustment.
I am not afraid to get inside the amp, but I am very mindful of the need to discharge the capacitors. Let's start with that.
I've read about and seen several approaches, including:
1. Use a screwdriver between the chassis ground and the capacitor lead (this does not appeal to me!)
2. Run a wire, with alligator clips, from ground to various capacitor leads or junctions.
3. Same as 2 with the addition of a resistor in the jumper wire.
4. Take a suitable resistor and attach it to the + and - leads from each capacitor to drain any charge.
5. Use the alligator clip jump wire but connect to chassis ground through a resistor that's part of the input circuit.
Now, safety rules also include having the amp unplugged, using the "one hand in pocket" technique and checking the caps for any remaining voltage before proceeding.
However, in order to make the adjustment, I'm going to have to power up the amp and then start adjusting. In a sense, doesn't this sort of render the safety step moot?
Anything else on the safety front?
I have a digital multimeter and plan to get a bias probe. Any other necessary items? Any tips?
Dave Hunter's amp book has a photo of the innards of the Z-28. I assume the bias adjustment is done via the trim pot on the inside.
I suppose I can check the bias on the new tubes without pulling the chassis and if the readings are good, then I can skip the re-bias. Yes?
Thanks, guys.
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Post by benttop (Steve) on May 8, 2011 7:30:32 GMT -7
I'd be very surprised if you put new tubes in there and find that the bias is correct. That would be a stroke of luck, but in my experience it is pretty rare. Everyone has their favorite method. Mine is adding a couple of 1 ohm 1% resistors to the cathode so I can read milliamps directly using the volts range on my meter. There are problems with this method (you have to be good at soldering, and you have to admit it is not exact for some circuits because of screen currents) but I use it for a good starting point and then my ears for the final dial in. The shorting method is probably more exact, but I am a scaredy cat. I think most of the plug in bias probes use some version of the cathode current method. I have one of those but I've never used it - no amps that need biasing here any more. But those rigs are very handy and would be a good way to go in my view. Good luck!
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Post by Phil (aka Phil) on May 8, 2011 8:23:40 GMT -7
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Post by Deleted on May 8, 2011 14:37:47 GMT -7
You shouldn't need to discharge filter caps to do a retube and bias. Just stay away from touching the +/- end of the filter caps. I used the shunt method to bias my Route 66. There was a red/white wire soldered to the cap on the top of the chassis this should be the center tap. The + tester wire goes there. The - tester wire goes to pin 3 of the output tube. Do not touch either lead when you have the other hooked up. You will get 480 volts or whatever the Z28 puts out, running into you. Don't ask me how I know this. Also make sure your tester is set to mA or you will blow up your tester or maybe just a fuse if you have a good tester. Don't ask how I know this either. Make sure when you retake the plate voltage that you switch back to DCV on your tester.
Like Steve said if you have a bias probe it is definitely safer. You're still inside the chassis to adjust the bias pot. Just avoid touching things that you don't know what they are. Still no need to discharge caps though. As you stated when you fire up the amp to bias it the caps will be fully charged.
Good luck and be safe.
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Post by benttop (Steve) on May 8, 2011 20:42:37 GMT -7
+1 for the above. The good news is that if you accidently get a charge from a cap it is not ususally fatal. It will knock you on your azz and get your attention for sure........you will get up and want a beer! I did so don't ask hahaha. You make that mistake once, I was being careless and messed up. Good luck with the revalving. Peace, Jeff Well if you have one hand in your pocket, you're right, it isn't usually fatal (but it is very annoying). But if you have your left hand on the chassis and touch the B+ with your right, the current path is right through your heart. That's not a good thing....
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Post by bluzman on May 8, 2011 21:02:13 GMT -7
Then...
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Post by dixiechicken on May 9, 2011 14:41:58 GMT -7
DC here! The easy way to discharge your capacitors is this: - Start amp normally
- Play it just as loud a usual for a minute or two.
- Pull the power plug right out of the socket in the wall
- Keep playing untill all the sound has gone away (horrible spluttering)
- Open the amp - for safetys sake and do the screwdriver dance
Thats it - requires absolutely no extra probes or anything. Cheers: Dixiechicken!
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Post by Phil (aka Phil) on May 10, 2011 5:08:35 GMT -7
DC here! The easy way to discharge your capacitors is this: - Start amp normally
- Play it just as loud a usual for a minute or two.
- Pull the power plug right out of the socket in the wall
- Keep playing untill all the sound has gone away (horrible spluttering)
- Open the amp - for safetys sake and do the screwdriver dance
Thats it - requires absolutely no extra probes or anything. Cheers: Dixiechicken! Or...instead of pulling the plug out of the wall you can play the amp while turning off the power switch and leaving the standby switch in the "on" position. The same result will occur.
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Post by Deleted on May 10, 2011 14:41:15 GMT -7
DC here! The easy way to discharge your capacitors is this: - Start amp normally
- Play it just as loud a usual for a minute or two.
- Pull the power plug right out of the socket in the wall
- Keep playing untill all the sound has gone away (horrible spluttering)
- Open the amp - for safetys sake and do the screwdriver dance
Thats it - requires absolutely no extra probes or anything. Cheers: Dixiechicken! Or...instead of pulling the plug out of the wall you can play the amp while turning off the power switch and leaving the standby switch in the "on" position. The same result will occur. What does playing the guitar have to do with anything? Not trying to be a smarta$$. Just never understood the science behind the idea.
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Post by Ed M. on May 13, 2011 9:09:37 GMT -7
Or...instead of pulling the plug out of the wall you can play the amp while turning off the power switch and leaving the standby switch in the "on" position. The same result will occur. What does playing the guitar have to do with anything? Not trying to be a smarta$$. Just never understood the science behind the idea. Make sure you play the right notes ;D You play just so you can hear the sound go away which is an indication that the high voltage has bled off. I just turn off my amp normally and wait a few minutes and the voltage has always drained off. I always check it before I start poking around though.
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Post by oldwoodak on May 13, 2011 10:04:28 GMT -7
Be careful when re-installing the chasis. I scratched the faceplate on my Z-28 on the staples that hold the grill cloth on. Next time I take it out I'm going to flatten them and maybe add some padding??
What tubes are you putting in?? I'm running Brimars and they have smooth woody tone compared to stock.
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Post by Deleted on May 13, 2011 13:07:03 GMT -7
Make sure you play the right notes ;D You play just so you can hear the sound go away which is an indication that the high voltage has bled off. Is there some kind of secret pattern that has to be played I just turn off my amp normally and wait a few minutes and the voltage has always drained off. I always check it before I start poking around though. This doesn't work on all amp designs. So it is definitely best to check the caps before getting in there.
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Post by Phil (aka Phil) on May 13, 2011 14:26:45 GMT -7
Make sure you play the right notes ;D You play just so you can hear the sound go away which is an indication that the high voltage has bled off. Is there some kind of secret pattern that has to be played I just turn off my amp normally and wait a few minutes and the voltage has always drained off. I always check it before I start poking around though. This doesn't work on all amp designs. So it is definitely best to check the caps before getting in there. This is true. Also, in one of the cooler phenomena in electronics, electrolytic caps can "rebound" after you discharge them and partially recharge over time with no voltage applied. I've seen 50V on the bigger ones. It's not enough to kill you but it will still shock you and scare the crap out of you. The only way to truly prevent this is to ground them while you're working on them.
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Post by dixiechicken on May 15, 2011 4:03:21 GMT -7
DC here!
Playing your guitar will make you feeeeel good - absolutely essential! ;D ;D ;D
Cheers: Dixiechicken!
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Post by Deleted on May 15, 2011 6:41:46 GMT -7
DC here! Playing your guitar will make you feeeeel good - absolutely essential! ;D ;D ;D Cheers: Dixiechicken! I like it K for you DC
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Post by jimmysmith on May 15, 2011 13:18:04 GMT -7
i dont know if i have ever heard anyone say to discharge the caps before you bias.. its stuff like this that causes people to back away from a simple tube change/w bias,,, biasing has to be the simplest, but, seemingly misunderstood task that an amp owner can do. right up there with taking the bolts out of the chassis,,, i used to think that amp techs liked to keep this stuff confusing to keep you in the dark so they would have more work to do,, but i think it is just because there is not a definitive source on the internet with a well thought out study of cathode, fixed, individual tube watts and bias settings and why,,, they are there but just not very popular it seems.
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Post by Deleted on May 15, 2011 14:19:39 GMT -7
Jimmy, I've never seen anything about discharging caps to bias an amp either. I have found a lot of great sights. I knew nothing about tube amps before 2001. I did a lot of research before my first purchase, which was a Maz 38 in 2004. Once this site came online in 2005 I learned even more. I then bought a Soldano HR-50 and that pushed me even more. I wanted to be able to do my own repairs and tube swaps.
Duncan Amps has a lot of great tools and knowledge to impart. R.G. Keen also offers some great tools and info. Both sites explain very clearly the different ways to safely bias an adjustable bias amp, along with safety concerns. Ignorance can be fatal. Any time you remove the chassis there is a chance to touch something that can kill you. Its more than just avoiding filter caps. There is also B+ voltage. Other areas of the amp have enough current to kill someone several times over.
It would be nice if this info was in one location. I think you would be hard pressed to find even one book that contained all the info. You would also probably pay a great deal for such a book. I've collected a few of those as well. One thing that has stuck out in all my research is the differing opinions on numerous topics. Its been both eye opening and frustrating at the same time. Looks to me like I'll never stop learning new things about tube amps.
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Post by Phil (aka Phil) on May 16, 2011 6:54:48 GMT -7
Sorry about the confusion jimmy. I think we added the "how to discharge caps" info in response to that question being brought up without explaining its role in the biasing process. You're correct that you do not discharge the caps to bias an amp. The reason we all preach safety around these amps is that to bias most amps you of course have to take the chassis out of the cabinet, install bias probes in between the tube and socket (if you're using that method), and then with a proper load applied turn the amp "on" and access the bias pot loacted among all the "hot" components. This of course renders any discharging of the caps a moot point, as you have now just re-energized them As we all know putting both hands in or around a device with 300V+ in it can be fatal. The actual act of biasing an amp (if you know what you're doing LOL ) is relatively easy in the grand scheme of electronics but the fact is that the vast majority of musicians are not trained in electronics, and even fewer are really knowledgeable about the theory and operation of the "outdated" vacuum tube. It's not quite the same thing as telling someone who has no mechanical training how to change their car's air filter. Biasing is kind of as much an art as a science; there's not one number that sounds best with every amp even though there are ballpark figures. There's a few factors to take into account electrical theory-wise and not every musician is going to understand them. I've been doing "amp teching" on the side (when I'm not engineering at the power company, doing photography, taking my kids all over town to swimming and karate, mowing the yard, doing the laundry...etc ) for various local bands and musicians for quite a while. (Before I had kids I used to build my own amps and restore vacuum tube radios.) I will tell you that in my opinion the VAST majority of musicians I know should not come anywhere near the inside of an energized tube amp. This does not mean they're stupid. Most of them are better musicans than I'll ever be. They probably could, with instructions off of the Internet, open their amps up and set the bias to a figure like 60-70% ID with some simple math calcualtions or in a ballpark range for a particular tube, and hope things like the plate voltage are "normal". They will then function adequately and not be over- or under- biased, even if they are not tuned for optimal performance. However, these "laymen" are not trained to work around high voltage circuits and the safety precautions that are *mandatory* if you want to live. One slip, even for a second, can kill you. I guess an analogy is this: I'm not a trained zoologist. I'm sure you could show me how to feed the lions once. Seems pretty simple. However, unless you are brought up with the kind of training zookeepers get and live it every day, with the constant vigilance it takes to be around these predators, eventually you are going to screw up and become lunch So, I don't think that the vast majority of amp techs are shrouding the biasing process in mystery to inflate their fees. I think it's a lack of understanding of the complete story of the biasing process and its seemingly simple nature that leads to that assumption. Cathode biased amps and amps that have a pre-installed 1 ohm resistors and external test points and bias pots are *so* much easier to bias
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Post by benttop (Steve) on May 16, 2011 7:55:11 GMT -7
Yeah, what Phil said. That was one of the things I really liked about my THD Flexi-50. It had the 1 ohm resistors, and separate bias pots for the push and pull sides of the power amp, so you could even run one EL34 and one KT66 if that's what floated your boat. Very cool design when it comes to biasing. I discharge the mains caps because I like putting that 1 ohm resistor in the circuit, so I'm physically touching the circuitry. Things have to be quite dead before I'll do that.
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Post by Deleted on May 16, 2011 12:28:17 GMT -7
I discharge the mains caps because I like putting that 1 ohm resistor in the circuit, so I'm physically touching the circuitry. Things have to be quite dead before I'll do that. Good point Steve.
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Post by Deleted on Jun 4, 2011 17:59:39 GMT -7
I plan to ship mine down to Dave's when the time comes. 'lectricity scares me.
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Post by dixiechicken on Jun 5, 2011 14:44:39 GMT -7
DC here! If you dont feel comfortable poking around inside your amp - and/or certain you know what you are doing - then dont. The trick is knowing what you can/should do yourself - and what you should leave to more knowledgeable people. I'm comfortable doing electrical stuff around the house - but I wouldn't mess around with the plumbing. Cheers: Dixiechicken!
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Post by Jacques Belanger on Jun 7, 2011 3:55:34 GMT -7
Yeah, what Phil said. Things have to be quite dead before I'll do that. Yup, 'cause if not, YOU will be!!
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Post by doctorice on Jul 14, 2011 15:13:00 GMT -7
OK I finally found time to put new 6V6s in my Z-28 today. I splurged and bought a Compu-Bias. It made things easy. Hardest work was removing the amp chassis from the combo box! Here's the gadget www.compu-bias.com/
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Post by Deleted on Jul 14, 2011 15:40:13 GMT -7
Mike, which is the best one to get for Dr Z amps - "octal" probe or "noval" probe Compu-Bias? Thanks
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Post by Deleted on Jul 14, 2011 16:07:16 GMT -7
Tom,
I would say "octal". If you have a Z with EL84's, it's cathode biased, so the "noval" would be a waist of money.
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Post by Deleted on Jul 14, 2011 16:30:30 GMT -7
Thanks Doug...
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Post by doctorice on Jul 14, 2011 19:29:08 GMT -7
Yes, the Z EL84 amps do not need a bias adjustment after new tubes are installed, so there's no need for the noval probes.
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Post by benttop (Steve) on Jul 14, 2011 19:39:53 GMT -7
So tell us how it sounds!
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Post by Deleted on Jul 15, 2011 10:09:38 GMT -7
You're welcome Tom. I think the Remedy is cathode biased as well so the octal would be moot if you own one.
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