Can I do this?

crhyner

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Hi,

Quick question...trying to stay on point. A few years ago I picked up a '66 Vibrolux, which I adore. I put in some new speakers, had the amp overhauled, and it's just great. However, I suspect that when it was overhauled, the tech put in power tubes that might not have been new. When I am gigging these days, occasionally, the volume will drop like one of the power tubes isn't functioning correctly. I've tried cleaning the tube pins, but I have a hunch I need to replace the two JJ 6l6 power tubes.

Can I bias the amp myself? That's my question. I am comfortable opening the amp chassis, but don't know how to properly discharge the caps. Is it as easy as picking up a voltmeter and attaching that to a couple of things inside? Is there a bias pot in these amps? Is this too difficult to do for a novice? I've looked online, but am not finding much on my specific amp.

One of my main reasons for learning to want to do it myself is because there are fewer and fewer good techs in my area, and the ones that exist live a good ways away. Anyway, is there a simple guide to doing this myself? What would you advise?

thanks,

Charlie
 

hepular

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1. HOWDY!
2. I was in your position a couple of years ago. Be humble, willing to try and learn and above all cautious: both you and the thing you want to work on are valuable, so don't be in a hurry.
3. MANY resources are out there on the youtubes & here: I've found rob robinette's site & the uncle doug and colleen fazio videos most useful (especially fazio because she actually shows her soldering technique on occasion).
4. PRACTICE soldering. a lot. (if you don't have an iron & the necessary stuff, you'll need em. doesn't havta be super expensive. proper technique and maintenance are the keys.)
5. put in some time to STUDY the phenomena associated with tube amp performance (& figure out who the Ray & Tom of tube amp repair are so you can learn some of the fun diagnostic usual suspects).
 

crhyner

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Thank you....do you have any links to a specific guide on biasing a '66 Vibrloux, or amp just like it? Some of the videos I've found cover different things, or different amps...totally willing to learn.

OK. Also, here's a picture of my amp's innards. Is the thing I circled the bias pot? So..now that I know how to access it and adjust that with a screwdriver, what are the next steps once the two new power tubes are in? What kind of voltmeter do I need to pick up?
 

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slider313

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The best thing you can do is buy a bias meter to read the current draw. The Eurotubes Bias Probe is a good choice. It only reads one tube at a time but does show the operating voltage. You'll have to switch to the second socket, and tube, to get its reading but it's a cost effective unit.

 

crhyner

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Do I need a separate multimeter and bias probe, or just the bias probe? This seems like a pretty solid video, right? It seems like I could handle it.



Would I be able to get by with these two items?



and

 

slider313

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You can use a multimeter and a single probe but would need to pull the chassis and use a second multimeter to read your plate voltage. You really don't have to pull the chassis with the bias meter. I remove the top back panel and flip the amp upside down. This gives access to the bias pot from the underside.

The bias pot is behind the choke. The adjustment is easy with a small flat head screwdriver.

1669575799627.png
 

crhyner

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Thanks, Slider.

Follow up Qs...

1 - Do you need to bias the rectifier tube in a Vibrolux?
2 - Why is it necessary to check the bias of both power tubes? The bias pot adjusts the bias for both tubes, so is it just to confirm that they are both at the same setting? I assume this is why they are sold in matched pairs?

thanks.
 

Ten Over

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Electrocution Caution Sign.png


Even though that amplifier looks relatively benign, it can actually kill you. It doesn't take that much talent to charge the filter capacitors with no drain path in this particular amplifier. With just a little more talent, a vicious shock can be delivered to the end user even with the power off.

It might be well worth it to spend the money on the eurotubes bias probe once you factor in the threat to the inexperienced amp technician.
 

Ten Over

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I see that the bias supply electrolytic filter capacitor has been changed, but all of the bypass electrolytic capacitors are still original. Those bypass capacitors should be replaced because electrolytic capacitors go bad with time.

What about the filter capacitors underneath the capacitor cover (doghouse) on the top of the chassis? Don't touch those capacitors unless you are dead certain that they are discharged.
 

Ten Over

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I also see that you still have a 2-prong power cord. This should be updated to a modern 3-prong power cord with the chassis properly grounded. The capacitor on the Ground switch should be removed unless the switch is bypassed when a 3-prong power cord is installed.
 

Ten Over

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If you are going to start soldering on this amp yourself, you could install one Ohm resistors between Pin 8 and ground on your power tubes. Then you can easily set your bias without the need for any bias probes.
 

crhyner

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Ten, thanks. The pic I posted was taken by my amp tech before he serviced it. He did in fact put in a 3-prong power cord and he replaced 5 filter caps (checked the invoice from a few years ago).

Excuse my ignorance, but how is the Eurotubes bias probe safer? What other safety precautions do you suggest I take when doing this?

Also, I have no desire to do any soldering, just don't want to drive over an hour and pay someone some money to bias my new tubes each time I replace them in this and my Princeton. Been playing tube amps for over 20 years, but never learned how to do this.

Also, here's more of what was done on the amp by the tech during the overhaul. Not sure how it relates to my question, but here it is...

Screen Shot 2022-11-27 at 7.13.37 PM.png


thanks again,
C
 

crhyner

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Lastly, is it really necessary to check the plate voltage when biasing the amp, or can you just make sure both your tubes are drawing at about 35ma or so?
 

AlbertaGriff

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I am also the proud owner of a '66 Vibrolux Reverb. I have a hunch that the problem you encounter has nothing to do with a potentially faulty tube. However, I could be totally wrong AND it's still worthwhile learning how to change and bias your power tubes.

With that said, you could theoretically change the power tubes and BEFORE biasing run the amp for a few minutes to see if you encounter the same problem. If you do, it isn't the tubes and you don't need to change/bias them.
 

dan40

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Lastly, is it really necessary to check the plate voltage when biasing the amp, or can you just make sure both your tubes are drawing at about 35ma or so?

When biasing an amp for the first time, you must find out the plate voltage so that you can do the necessary math to achieve the correct dissipation. Some people like to set their tubes at a cooler 50% dissipation while others prefer a warmer 70% setting. Once you have taken a plate voltage reading on that particular amp, you won't always need to take a new reading to set the bias on a new pair of tubes.

There are a few different methods that you can use to bias your tubes. Since you don't have a lot of experience working inside of a live tube amp, the safest and easiest method by far would be the bias probe that Slider linked to above. Don't cheap out on probes because many of those cheaper units are poorly built and could damage your amp if they fail. You also want to get a decent multimeter because once you become comfortable with the biasing, you may find yourself wanting to learn other aspects of tube amp troubleshooting and repair.

Here is a great site by one of our fellow members that has an article on biasing along with many other great topics.

 

dan40

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Why is it necessary to check the bias of both power tubes?

When purchasing a new pair of tubes from a reputable dealer, you will hopefully get two tubes that are very close in current draw. Over time these matched tubes can drift and become not so well matched. It's a good idea to always check both tubes just to be sure they are a fairly close match. Even with a matched pair, one tube may be 5ma or so higher in current draw than it's partner. In cases like this you can set the bias to the highest tube or split the difference. Either way, it's helpful to know what both tubes are idling at.
 

birdawesome

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Great advice from everyone here, and yes, you can definitely do it yourself, but cautiously!

Best thing you can buy as well would be a snuffer stick to drain the caps that could potentially be life-threatening, as well as practice the "one hand rule," that is, keeping one hand behind your back if you have to in order to prevent touching a hot component and touching a grounding point with the other, thus completing a circuit passing right through the heart! No bueno.

Linked is a snuffer stick at StewMac:
 




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