Tube replacing

[mr. torture]

Hello all, I have a 1983 Marshall JCM800 2204 50 watt head. I had this amp modded and serviced about 4 months ago. It has had limited use because I have been rebuilding my studio the past 2 months.

The amp started to exhibit a loud squealing sound when I wasn't playing. If I had the amp on and the volume up on the guitar, without playing anything, it would make squealing and other various weird sounds. Almost sounded like radio interference.

Then, about 15 minutes later the amp quit completely, I have no sound at all. I suspect maybe I have a bad tube or tubes? Anyways, If i purchase new tubes and install, will the amp need to be re-biased?

Thank you!

[Gandalf]

Originally Posted by mr. torture

Hello all, I have a 1983 Marshall JCM800 2204 50 watt head. I had this amp modded and serviced about 4 months ago. It has had limited use because I have been rebuilding my studio the past 2 months.

The amp started to exhibit a loud squealing sound when I wasn't playing. If I had the amp on and the volume up on the guitar, without playing anything, it would make squealing and other various weird sounds. Almost sounded like radio interference.

Then, about 15 minutes later the amp quit completely, I have no sound at all. I suspect maybe I have a bad tube or tubes? Anyways, If i purchase new tubes and install, will the amp need to be re-biased?

Thank you!

If you replace the power tubes you need to re-bias. If not I think this can damage the amp.

If you replace pre-tubes you do not need to re-bias.

[Lt. Bob]

No, not rebiasing will not damage the amp.
But for the amp to work its' best it's a good idea to rebias after changing power tubes. But it's not absolutely necessary.
Actually, in 40 years of playing thru tube amps and changing tubes regularly ( I retube my Mesa Blue Angel yearly) I've only rebiased an amp a couple of times. The dozens of other times I've changed power tubes I haven't bothered with it.
I know I should to get the very best out of the amps but they work acceptably well anyway. So the only times I've bothered with it is when there was an obvious need for it which was only a couple of times when the amp just didn't "come to life" with the new tubes.

[mr. torture]

Thanks guys, do you think it's the power or preamp tubes thats causing my problem?

[nevome]

I had similar sounds/lack of sounds coming out of my Blues Deluxe once, it was a loose 12ax7 socket. It's worth looking into to check that your pre-amp tubes are all working and well seated, beyond that I'd take it back to the amp tech.

[Micter]

It's most likely a bad preamp tube. v1 or v2 would be my suspect. If you turn the amp on with the rear panel off lightly tap on the tubes with a pencil. The culprit wil be obvious. As far as bias goes, you won't burn up the amp by not biasing the amp afte a power tube swap but depending on how it is currently biased you could wear out the new tubes. It's always a good idea to bias the new tubes but not absolutely nesessary.

[dementedchord]

dont know what the mod consisted of but if i were you i'ld take it back... that kinda noise issue almost always comes from the front end... and personally the only way i would skip rebiasing an output is if i were using the same brand tube all the time and buying matched pairs of the same rateing... YMMV....

[Lt. Bob]

Originally Posted by dementedchord

dont know what the mod consisted of but if i were you i'ld take it back... that kinda noise issue almost always comes from the front end... and personally the only way i would skip rebiasing an output is if i were using the same brand tube all the time and buying matched pairs of the same rateing... YMMV....

well, it really depends on the amp. For instance ... ALL my older Ampegs are fixed bias.
Three of them are 70's vintage and the 4th is a reissue .... last time I retubed the reissue I rebiased it ..... but the older ones can't be rebiased unless you want to change components.

[rayc]

what demented said!

[sound125]

Originally Posted by mr. torture

Thanks guys, do you think it's the power or preamp tubes thats causing my problem?

If it's a squealing sound it might be the preamp tubes. Tap each one with the erasor end of a pencil and see if you can drive it into microphonics. The amp needs to be on and you'll need to have someone playing guitar through it. If the tapping results in microphonics, then you'll need to replace the preamp tube causing the problem.

[boingoman]

Originally Posted by Lt. Bob

well, it really depends on the amp. For instance ... ALL my older Ampegs are fixed bias.
Three of them are 70's vintage and the 4th is a reissue .... last time I retubed the reissue I rebiased it ..... but the older ones can't be rebiased unless you want to change components.

Fixed bias means the negative bias voltage comes from the power supply and is fed to the grid. It stays constant, or "fixed", as opposed to cathode bias, which changes with the cathode current when signal hits the tube's input. Whether fixed bias is adjusted by changing resistors or turning a pot doesn't matter, it's still called fixed bias. Your new Ampeg is fixed bias too.

[rokinrandy]

Originally Posted by mr. torture

Hello all, I have a 1983 Marshall JCM800 2204 50 watt head. I had this amp modded and serviced about 4 months ago. It has had limited use because I have been rebuilding my studio the past 2 months.

The amp started to exhibit a loud squealing sound when I wasn't playing. If I had the amp on and the volume up on the guitar, without playing anything, it would make squealing and other various weird sounds. Almost sounded like radio interference.

Then, about 15 minutes later the amp quit completely, I have no sound at all. I suspect maybe I have a bad tube or tubes? Anyways, If i purchase new tubes and install, will the amp need to be re-biased?

Thank you!

It could be the preamp tubes ..the jcm has 5-12ax7 and 2-kt88 in the preamp stage..Or mine did anyways ..

If it was my Amp and it was modded and serviced within 4 months I would be calling the tech back asking him WTF! 4 MONTHS

Did the guy guarantee his work ??????

What did he do when he modded it ???

I wonder If you should find another amp tech

[boingoman]

Originally Posted by sound125

The amp needs to be on and you'll need to have someone playing guitar through it. If the tapping results in microphonics, then you'll need to replace the preamp tube causing the problem.

No need for anyone to be playing through it.

[Lt. Bob]

Originally Posted by boingoman

Fixed bias means the negative bias voltage comes from the power supply and is fed to the grid. It stays constant, or "fixed", as opposed to cathode bias, which changes with the cathode current when signal hits the tube's input. Whether fixed bias is adjusted by changing resistors or turning a pot doesn't matter, it's still called fixed bias. Your new Ampeg is fixed bias too.

then I just used the wrong term because my point was that not all amps have adjustable bias. My older Ampegs are non adjustable although you can certainly change resistors to accomplish that.
And my fairly new Mesa Blue Angel has no adjustable bias either. In fact, there was a section of the owners' manual that specifically addressed this and said that they didn't believe in adjusting the bias ..... they set it where it should be and they preferred that you get tubes from them or get tubes they reccomend because they know what the design called for and didn't want any techs mucking it up.
It also said that for this amp the bias wasn't a critical issue and though I don't know this for sure, the tone of that section led me to think that perhaps all their amps are set up that way.
IT sure sounded like that's how they think amps should be.
Other amps, like Doctor Z's for instance, are nowadays self-biasing which does away with the need for it.

But I very clearly stated in my post above that rebiasing after a power tube swap is preferable .... just becaus I don't bother with it doesn't mean anyone else should make that their mantra. My main point was that not doing it doesn't mean death and destruction to your amp.
Sometimes when people see these things, they get the idea that if you don't rebias, it'll destroy your amp and in fact someone in this thread made a similar comment.
I'm not saying don't do it ..... I'm simply saying that if you don't ..... it won't really destroy anything ..... your amp simply won't be at its' best.
As for why I don't do it ...... it's because I gig virtually every day and often double gigs .... hell, bike week coming up I already have 11 gigs and will probably squeeze in 3 or 4 more. I tend to grab whatever amp's by the door and when I get home I toss them in a corner. After over 40 years of it I just don't really care about equipment and the endless tweaking to get this or that sound I'm obsessing over anymore. I get comments on my sound often and I just do what I've always done since it seems to work well.
But my main point was that it's not an absolutely neccessary thing to do to avoid amp damage.

[dementedchord]

Originally Posted by Lt. Bob

And my fairly new Mesa Blue Angel has no adjustable bias either. In fact, there was a section of the owners' manual that specifically addressed this and said that they didn't believe in adjusting the bias ..... they set it where it should be and they preferred that you get tubes from them or get tubes they reccomend because they know what the design called for and didn't want any techs mucking it up.
It also said that for this amp the bias wasn't a critical issue and though I don't know this for sure, the tone of that section led me to think that perhaps all their amps are set up that way..

just in case you want to confuse yourself with the facts here's the output section of that blue angel....

http://www.tubefreak.com/bluangpw.gif


IMHO.... you do a great disservice to those that are seeking answers and or help when you bloviate half trueths and misunderstoods.... but hey that's me...

[boingoman]

Originally Posted by Lt. Bob

Other amps, like Doctor Z's for instance, are nowadays self-biasing which does away with the need for it.

Doctor Z's are cathode-biased, like preamp tubes. He hasn't done away with the need for biasing, he made a design choice, and it's nothing new. Cathode-biasing yields more distortion and less output power. Not good in high-power designs like an SVT or a Twin, you'd need to almost double the number of output tubes to get the same amount of clean power you get with a fixed-bias design. Good for low-power designs that start distorting when the volume hits 3/4, though.

[Lt. Bob]

Originally Posted by dementedchord

just in case you want to confuse yourself with the facts here's the output section of that blue angel....

http://www.tubefreak.com/bluangpw.gif


IMHO.... you do a great disservice to those that are seeking answers and or help when you bloviate half trueths and misunderstoods.... but hey that's me...

And I think you're a prick sucking on a shaft when you continue to take issue with things I say and insult me with little reason to whenever I simply make a comment based on years of fooling with these things.
All I've said here is that not rebiasing a tube amop won't damage it and there's no way you can show that it will.
Just like when you jumped into that piano thread .... googled a few things and decided you knew more about it than I, with 30 years of dealing with them daily do. You were completely full of shit about every single thing you said about pianos.
Just like the thread when I said that there's no reason that leaving an amp on overnight by itself should damage it and you half read what I said ..... said i was talking bullshit and then never made any effort to explain yourself or respond to any of my questions which I asked very politely and genuinely wanted to know why you thought what you did.
Fact is ..... I know more about pianos than you'll ever know and, judging by the amatuerish clearly goggled answers you give to things, I know at least as much about working with tube amps. I've owned and played thru more of them than you're likely to ever see and the fact that you give no regard whatsoever to my long experience with these things makes me wish so fucking hard that I'd run into you in person.
You're a talentless troll and you can suck my cock.

[jimistone]

I've found that if you take your fist and hammer the top of the amp as hard as you can...It either fixes the problem or the amp goes completely out.

(if it goes all the way out you can sometimes hammer it some more and get it to come back on).

[ggunn]

Originally Posted by jimistone

I've found that if you take your fist and hammer the top of the amp as hard as you can...It either fixes the problem or the amp goes completely out.

(if it goes all the way out you can sometimes hammer it some more and get it to come back on).

Ah, yes. The Principle of Percussive Maintenance. I use it often. Even if it doesn't work, I feel a little better. ;^)

[Zaphod B]

Originally Posted by ggunn

The Principle of Percussive Maintenance.

Hahaha, I hadn't heard that in years.

[mr. torture]

Well, I called the place that modded and went through the head. They said it sounds like a power tube went. I found that one of the mains fuses is blown, which explains why it suddenly lost sound all together.

I am going to take it to them and have them replace the power tubes and re-bias the head.

I have to say, after having it back for a few months and already having problems I was a little peeved. However, I have read that tubes are only made in Russia and China now and they are not really all that dependable. The power tubes in my head are JJ's Kt-77's.

I had the "Jose" mod done on the amp, it sounded awesome, when it worked....

I won't say the name of the place here as they are well known and their customer service has been exceptional. Could have just been a crappy tube giving up the ghost a bit early, like I said, tubes today don't last like they use to and they are not always top quality.

Thanks everyone!

[ggunn]

Originally Posted by Zaphod B

Hahaha, I hadn't heard that in years.

Yeah, you've got to be careful with that percussive maintenance on an amplifier. You can hit it so hard that you open a leak and let all the smoke out, then it's got to go to the shop to have more put in.

[dementedchord]

Originally Posted by Lt. Bob

"All I've said here is that not rebiasing a tube amop won't damage it and there's no way you can show that it will. "


tube life is at the very least shortened.... under biasing tends to make it sound edgy and rough and may cause them to thermal... i've seen it happen and for thankfully so becuase thats how I made MY money for quite a while.... i was warrantee authorized for boogie/fender/marshall/crate/ampeg/yamaha/korg/roland/kurzweil/soundcraft/allen and heath/crown/qsc/jbl harmon/and others....


"Just like when you jumped into that piano thread .... googled a few things and decided you knew more about it than I, with 30 years of dealing with them daily do. You were completely full of shit about every single thing you said about pianos."

never said i knew everything about pianos but i think my sources are pretty good and i never use google in discussions... if i cant come up with a thought on my own i read others opinions and draw from there... incedently my source for the info on pianos was the tech at the university i attended where i was a theory/comp major and my pricipal instrument for most of that time (and still is) piano... one of my favorite things to do when not practising was hang out with the piano tech and pick his brain... he and his brother were both well known tuners.. the brother tuned for the st louis symphony... so i suspect they knew a little....



"Just like the thread when I said that there's no reason that leaving an amp on overnight by itself should damage it and you half read what I said ..... said i was talking bullshit and then never made any effort to explain yourself or respond to any of my questions which I asked very politely and genuinely wanted to know why you thought what you did."


if you go back and reread that thread i wanted to defuse the thing and adressed myself to the other partcipants and the reasons you sought came up later... look at the part where we discussed the difference in the current draw with HT applied...


"Fact is ..... I know more about pianos than you'll ever know "


perhaps in terms oftuning and such....

"and, judging by the amatuerish clearly goggled answers you give to things, I know at least as much about working with tube amps."


amatuerish and googled is laughable.... you didn't bother to check that link to your amp did you.... you see i was not trying to start a pissing contest.... i just thought maybe you'ld understand why boogie told you not to worry about the bias... and i was trying to give you a chance to back away from this shit but NO..... you see that amp IS CATHODE BIASED... or self biased as you incorrectly called it but HEY your the PRO!!!


"I've owned and played thru more of them than you're likely to ever see "


i figure ive worked on conservatively in excess of 300... so you must have quite a collection....

"and the fact that you give no regard whatsoever to my long experience with these things makes me wish so fucking hard that I'd run into you in person."


believe me you dont want this.... i'm the kid your mom wouldn't let you play with....


You're a talentless troll and you can suck my cock.

oooohhhh.... and you think that passes for intellect right????
what a fuckin MORON.... you should not be allowed to participate in the gene pool... FOOL !!!!

[mr. torture]

Originally Posted by dementedchord

oooohhhh.... and you think that passes for intellect right????
what a fuckin MORON.... you should not be allowed to participate in the gene pool... FOOL !!!!

Geeze, lighten up Francis. It's only a forum.

[Charva]

Originally Posted by dementedchord

just in case you want to confuse yourself with the facts here's the output section of that blue angel....

http://www.tubefreak.com/bluangpw.gif


IMHO.... you do a great disservice to those that are seeking answers and or help when you bloviate half trueths and misunderstoods.... but hey that's me...

Originally Posted by Lt. Bob

And I think you're a prick sucking on a shaft when you continue to take issue with things I say and insult me with little reason to whenever I simply make a comment based on years of fooling with these things.
All I've said here is that not rebiasing a tube amop won't damage it and there's no way you can show that it will.
Just like when you jumped into that piano thread .... googled a few things and decided you knew more about it than I, with 30 years of dealing with them daily do. You were completely full of shit about every single thing you said about pianos.
Just like the thread when I said that there's no reason that leaving an amp on overnight by itself should damage it and you half read what I said ..... said i was talking bullshit and then never made any effort to explain yourself or respond to any of my questions which I asked very politely and genuinely wanted to know why you thought what you did.
Fact is ..... I know more about pianos than you'll ever know and, judging by the amatuerish clearly goggled answers you give to things, I know at least as much about working with tube amps. I've owned and played thru more of them than you're likely to ever see and the fact that you give no regard whatsoever to my long experience with these things makes me wish so fucking hard that I'd run into you in person.
You're a talentless troll and you can suck my cock.

Originally Posted by jimistone

I've found that if you take your fist and hammer the top of the amp as hard as you can...It either fixes the problem or the amp goes completely out.

(if it goes all the way out you can sometimes hammer it some more and get it to come back on).

Originally Posted by dementedchord

oooohhhh.... and you think that passes for intellect right????
what a fuckin MORON.... you should not be allowed to participate in the gene pool... FOOL !!!!

Jheezhus!

Get a room you dirty whores!