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Post subject: Weird (for me) amp event
Posted: Sun Dec 23, 2018 8:18 pm
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My Boogie .50 Calibre was making a strange noise intermittently.
I thought I could hear something odd for a few days now but it would stop when I stopped playing.
I wasn't sure if it was real or if I was just imagining it because I had been going through pretty much all of my pedal board settings, tweaking things.
Then today it started to keep on making the noise for a second or two after I stopped.
Ah-hah! I wasn't imagining it.
In a way it was similar to microphonic tube but not really.
The amp wasn't howling/ringing.
It was more like an A/C buzzing sound but heard through a radio from the 30s or 40s.
Real mid-rangey and nasally and thin.
I tried a bunch of different things but couldn't figure out how to duplicate the conditions to make it do it.
It would just decide to make the noise and then quit.
I brought the amp over to the table and pulled the chassis out and laid it on top of the cabinet upside down and plugged in the speaker and powered it up and poked V1 & V2 with a chop stick.
It made some ringing noise but not the same noise.
I replaced V1 and V2 with new EHXs I had on hand because they sounded like they were on the verge.
I reinstalled the chassis and went to work playing for a half hour or so hoping I had it cured.
It was better for a while but once the amp started getting warm it came back.
I pulled the instrument cable out to see if it was a problem with the signal going in but it kept on making the noise with an empty input jack.
Leaving it powered up, I pulled the amp away from the wall and turned it around and I started poking at every tube with the chopstick.
Loewen behold, a power tube made the exact noise.
I've never had a power tube go microphonic in my life.
I've had them wear out and start to sound like crap.
I've had them start eating fuses for lunch.
I've replaced power tubes but never for this reason.
Now I know what a microphonic power tube sounds like.
This old dog learned a new trick today.
Luckily I had a matched quad of Groove Tubes on hand and now the amp is fine.
I just ran it hard for about an hour and no noise at all.
The GTs have a little more top end than the Gold Lions I was running but that's nothing a slight adjustment of the presence control can't fix.
While still good and hot, I pulled the amp away from the wall again and poked every tube with the stick again and it is dead silent.

I tossed the bad tube so now I have a matched trio of EL84s which is of no use to me since I need a quad.

I find this all very interesting.
As I said, I've never encountered a microphonic power tube in my life.
Preamp tubes, yes of course. Who hasn't?

I don't need any help, I just needed to talk about it I guess.

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Post subject: Re: Weird (for me) amp event
Posted: Mon Dec 24, 2018 2:16 am
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Good to know you find it.

It is a good lesson for all of us and we must remember :

1- For most if not all issue with a tube amp, we must start with new good tubes.
2- "similar to microphonic tube ..." ; It is a tube, what else ?


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Post subject: Re: Weird (for me) amp event
Posted: Mon Dec 24, 2018 8:10 am
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May also have been a pre-short condition, it happens.
Glad you noticed it and found it before a major incident occurred.


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Post subject: Re: Weird (for me) amp event
Posted: Mon Dec 24, 2018 8:35 am
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Mesa uses very low gain power tubes in most of their amps.
They do not have a bias adjust so you have to adjust bias by selecting the proper gain power tubes.
Blue rated Groove Tubes will give a reasonable bias on that Mesa. White or Red may be too hot. Check your bias.


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Post subject: Re: Weird (for me) amp event
Posted: Mon Dec 24, 2018 8:51 am
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Or change bias circuit's resistors value.


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Post subject: Re: Weird (for me) amp event
Posted: Mon Dec 24, 2018 9:36 am
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I've been in email conversations with the folks at Boogie several times.
They've told me twice that all Boogies are self biasing.
I'm not an amp tech so I don't know how they achieve that.

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Post subject: Re: Weird (for me) amp event
Posted: Mon Dec 24, 2018 11:05 am
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BMW-KTM wrote:
I've been in email conversations with the folks at Boogie several times.
They've told me twice that all Boogies are self biasing.
I'm not an amp tech so I don't know how they achieve that.


Same old story with all cathode bias amp....from amp builder
This do not mean tubes have the best bias.


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Post subject: Re: Weird (for me) amp event
Posted: Tue Dec 25, 2018 8:24 am
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Changing bias resistors is not a sure thing on Mesa amps.
The bias supply often runs other circuits as well. Changing resistors affects the other circuits.
Best bet is to call Mesa and order power tubes for your model amp. I don't recommend that for any other builder, but with Mesa its the way to go


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Post subject: Re: Weird (for me) amp event
Posted: Tue Dec 25, 2018 9:14 am
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TimAudio,
Looking at schematics I do not see the difficulty but as you proved your great competence in the past, what can I say?


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Post subject: Re: Weird (for me) amp event
Posted: Mon Jan 28, 2019 4:33 pm
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I popped in to see my amp tech today (on an unrelated matter) and I told him of our discussion here.
He gave me this for my future reference and I thought I would share it here.
Not being a tech myself this means very little to me but I thought some people might be as interested as I am to know the relative order of colour codes from cold to hot.

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