Wizard MCII 50W Tubes and Biasing?

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bkemp23

Member
I think my 2018 MCII needs new power tubes. When I turn it on, it makes an intermittent popping noise. I've never changed tubes or biased an amp before but my background is controls engineering so I have knowledge of circuits and the dangers associated with high voltage (however typically no more than 240V). My plan is to replace the stock tubes with the same JJ EL34L. From what I've read, getting a matched pair seems to be ideal.

1) How does a stock 50W MCII come biased in mA?
2) Does anyone know if the caps self discharge after power down or will they require discharging to GND? (wise to verify either way)
3) Do you have to measure mA AND V on both tubes at the same time?
4) Given my background, do you think this is a good idea or would it be wise to observe/get help from someone the first time?
 
I called a local shop to see about tubes and got to talking about the amp and it's symptoms. They think it could be a capacitor going out. Does this make sense with the popping noise it is making? The pops do seem to come at a fairly consistent interval.
 
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bkemp23 said:
I think my 2018 MCII needs new power tubes. When I turn it on, it makes an intermittent popping noise. I've never changed tubes or biased an amp before but my background is controls engineering so I have knowledge of circuits and the dangers associated with high voltage (however typically no more than 240V). My plan is to replace the stock tubes with the same JJ EL34L. From what I've read, getting a matched pair seems to be ideal.

1) How does a stock 50W MCII come biased in mA?
2) Does anyone know if the caps self discharge after power down or will they require discharging to GND? (wise to verify either way)
3) Do you have to measure mA AND V on both tubes at the same time?
4) Given my background, do you think this is a good idea or would it be wise to observe/get help from someone the first time?
Click to expand...
4) If you have to ask, please don't do it yourself. Take it to a qualified tech.
 
bkemp23 said:
I called a local shop to see about tubes and got to talking about the amp and it's symptoms. They think it could be a capacitor going out. Does this make sense with the popping noise it is making? The pops do seem to come at a fairly consistent interval.
Click to expand...

I would doubt there's a capacitor issue for a 2018. Could be, but I'd say it's unlikely. Change the tubes and see if that fixes it. Most of the time tube amp issues are related to a tube.

Checking bias is fairly easy if you get something like a Weber bias rite or the Eurotubes bias meter. Then just watch some youtube videos on how to bias. Just don't touch anything inside the amp. Use an insulated screw-driver to turn the bias pot.
 
The popping could be a preamp tube. Does it pop with the volume at zero?

Get a chopstick and lightly tap each and everytube, with the volume up. You'll find crackles and pops from tapping.
 
Thanks for the feedback everyone. After reading some more on the issue, I am also leaning towards a preamp tube. I can pull one from another amp and roll it through the Wizard tomorrow. I will report my findings. I will also try and get a video of the issue.
 
Update: I've played 4 days in a row and no issues with the amp. I guess i will keep playing and if it does it again, start trying to diagnose from the above suggestions. Thanks again.
 
bkemp23 said:
Update: I've played 4 days in a row and no issues with the amp. I guess i will keep playing and if it does it again, start trying to diagnose from the above suggestions. Thanks again.
Click to expand...
I would still replace the power tubes at that point. One could be slightly arc'ing which could cause a lot of damage.
 
glip22 said:
I would still replace the power tubes at that point. One could be slightly arc'ing which could cause a lot of damage.
Click to expand...
Thanks for the heads-up. I will go ahead and do that.

Is that something that is visible if looking at the tubes?
 
Not necessarily. It could have been a dirty connection on one of the pins in a socket also causing a pop sound. The tubes might be ok but with a nice amp I always err on the caution side and just replace the tubes.
 
I agree with everything said so far.
Looks like you are squared away.
I was going to recommend spraying the PINS on the preamp tubes and then sliding them in and out of the socket to clean.
I don't have a Wizard but I thought this biasing tutorial I made a log time ago might help. It is very long and very detailed but I did it in case it can help someone else who was in my shoes.

https://splawnfans.proboards.com/thread/7786/amp-biasing-tutorial
Good luck.

Tubes are a crap shoot. I've had tubes that have lasted for over 10 years and I've tubes last a month.

And yeah - get something decent like a Weber or better. I have a Weber Bias Rite. The cheap $20 probe I had before didn't last.
 
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311splawndude said:
I agree with everything said so far.
Looks like you are squared away.
I was going to recommend spraying the PINS on the preamp tubes and then sliding them in and out of the socket to clean.
I don't have a Wizard but I thought this biasing tutorial I made a log time ago might help. It is very long and very detailed but I did it case it can help someone else who was in my shoes.

https://splawnfans.proboards.com/thread/7786/amp-biasing-tutorial
Good luck.

Tubes are a crap shoot. I've had tubes that have lasted for over 10 years and I've tubes last a month.

And yeah - get something decent like a Weber or better. I have a Weber Bias Rite. The cheap $20 probe I had before didn't last.
Click to expand...
That is exactly what I was looking for. Thanks a ton. Not sure when I will get around to changing the tubes but I will update this thread and let everyone know how it turned out.
 
Update: Thanks for all of the advice from everyone in the thread. I was able to install the new power tubes without issue! It was extremely easy thanks to the tutorial above from @311splawndude . I haven't had a chance to crank it yet so hopefully nothing changes. Loud volumes will commence tomorrow!
 
One last question/observation, the previous tubes measured 488VDC and 31.9mA DC which puts them at about 62% plate dissipation. I installed the new tubes and they measured 50mA DC initially so I was able to reduce that down to 32mA. I will leave it there for now. Is that roughly the current that others are seeing in their MCii amps or was there some bias drift?
 
bkemp23 said:
One last question/observation, the previous tubes measured 488VDC and 31.9mA DC which puts them at about 62% plate dissipation. I installed the new tubes and they measured 50mA DC initially so I was able to reduce that down to 32mA. I will leave it there for now. Is that roughly the current that others are seeing in their MCii amps or was there some bias drift?
Click to expand...
Every set of tubes is going to be a different setting, that is why you must bias.
 
bkemp23 said:
Thanks @Geo. I didn't word my question properly. Is 32mA roughly the stock bias current that Rick ships his MCiis out with?
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I am sure it is, the components are probably built with tight specifications. Not a real concern, sounds like you have them set OK.
 
Finally got a chance to play at volume. Everything seems to be functioning correctly! I’m glad I did the biasing myself. It ended up being pretty easy.
 
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Reactions: Geo
311splawndude said:
I agree with everything said so far.
Looks like you are squared away.
I was going to recommend spraying the PINS on the preamp tubes and then sliding them in and out of the socket to clean.
I don't have a Wizard but I thought this biasing tutorial I made a log time ago might help. It is very long and very detailed but I did it in case it can help someone else who was in my shoes.

https://splawnfans.proboards.com/thread/7786/amp-biasing-tutorial
Good luck.

Tubes are a crap shoot. I've had tubes that have lasted for over 10 years and I've tubes last a month.

And yeah - get something decent like a Weber or better. I have a Weber Bias Rite. The cheap $20 probe I had before didn't last.
Click to expand...
Thanks for taking the time. That is awesome.
 
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