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Old 6th November 2005, 01:59 AM   #1
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Default 6550 making a Popping sound?

I just worked on a BIG Sunn bass amp 300 watt, six 6550 tubes. the customer was complaining that after the amp warmed up, it would start making a popping sound.

and sure enough it does! and i noticed that if i touched the metal spring hold down on one of the tubes, the popping would go away. but only that one tube! We replaced that tube with an old tube the client had on hand and the popping went away.

it works well enough to get him through his gig tonight but im curious if this is a common thing with 6550's??

It was and Electro Harmonix brand tube and less then 1 month old.

The spring hold downs are fasten to the chasssis, so i am making an assumption that they are grounded? If i removed the spring hold down device it doesnt pop.

So i am wondering if the tube is arcing through the glass? or building up a charge on the glass or what would cause this??

Zc
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Old 6th November 2005, 02:40 AM   #2
Trout is offline Trout  United States
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I Would be suspect of a vibration issue. Those spring hold downs do resonate a bit, Deadening them by touching them my be all it needed to kill the vibration. I had a 5Y3 rectifier tube that was doing this, Took me hours to figure out the source.

EH tubes can be fussy sometimes, many early run valves had issues later corrected. I had a few 12AX7EH's that blew the filaments in less than 2 hours of operation. They however replaced them under warranty and admitted the issue had been cured.
I Still feel they are great tubes!
Try Putting a couple silicon "O" ring dampeners on them. Might Help, But ultimately once A tube starts possibly shorting internally they are doomed to a sudden and often destructive death in a guitar amp.
Trout
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Old 6th November 2005, 03:03 PM   #3
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Default Re: 6550 making a Popping sound?

Quote:
Originally posted by Zero Cool


So i am wondering if the tube is arcing through the glass? or building up a charge on the glass or what would cause this??

Zc


Nup.......no way....possible without a complete duff tube.

On stage I had this problem with 807's on pwr-up I noticed in true tetrode mode the screen current was actually appearing secs' before the anode juice came up. (defect in Sep B+)
SOmetimes one can view what's going on by peeping through the holes in the anode plate of power tubes on pwr-up. The interelectrode blue glow (noticeable in dark env) should be even on all and gradually fade when hot. Sometimes with Siemens EL34'sone would see glowing stars on the g2 (dirty tube assembly).

The int octal holder is mechanically up on limit with the 6550 size without a J strap.
I've had 88's jump out of the sockets with amp placed on a Peavy Bass stack.


richj
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Old 6th November 2005, 07:43 PM   #4
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On further testing, i found that if i connect an alligator clip lead from a solid chassis ground to the spring, the popping goes away. if i touch the metal band around the base of any of the tubes to ground, they pop!

I did not have time to test any further. we replaced the tube and the popping stopped, but i did not have any time to test any more.


Zc
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Old 7th November 2005, 07:10 AM   #5
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Quote:
Originally posted by Zero Cool
On further testing, i found that if i connect an alligator clip lead from a solid chassis ground to the spring, the popping goes away. if i touch the metal band around the base of any of the tubes to ground, they pop!

Zc

That looks like an instability problem....(a scope is best to hunt this out).........Grounding ?...the metal ring on tube bases should always be connected to ground/chassis within the octal base. On high mu tubes i.e 6550/KT series .....Pin 1 should be connected to chassis.

Double check the grounding status for missing/defect tag loose bolt....if you have an amp with a switchable ground (to avoid equipment earth loops) this is even more important.

Another........Are you using ceramic tweeters in your system ? Alot of Tube amps don't like negative Z and will oscillate.
Either way, inevitably you will have to up-end the chassis and investigate the innards.

What is your tube line up.......?
More later
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