Ampeg Jet II J12R issue

[ehacker]

Greetings all...

More or less an electronics noob here. Researched this for 3 hours last night and 2 more this morning with no success.

I have an Ampeg J12R reissue I picked up a few months ago. Great little amp (once I got all the vibration issues taken care of). I installed a new matched set of EL84s two weeks ago (with the understanding that this is a 'self biasing' amp). Amp sounded great. Everything was fine. Had a preamp tube (12AX7A/7025) go microphonic on me last week. Ordered a matched pair of 12AX7WBs (with the understanding that the WBs are a direct replacement). Installed the tubes. Fired up the amp and let it warm up for a few minutes. Started playing. Sounded grittier than stock (as I assumed it would based off the WB reviews I'd read), but considerably grittier than I'd expected. About 2 minutes into playing, the amp made a quick squeaking sound (like air coming out of a pinched balloon neck), lost guitar signal, and smoke began coming out of the accordion input jack. Took a quick look at the tubes and they were still glowing 'normally.' Shut the amp down. The chassis was notably warmer than usual. Nasty smell. After it cooled I pulled the chassis and found no obvious signs of burning, either on components or component sides of the boards (didn't pull the boards to check solder side). Made sure the accordion jack was free of all debris (none came out). Fired the amp back up and got a hum for about 20 seconds, then some quick static and a lower volume hum. Did not attempt to run a signal through it.

Ideas on what may have gone wrong? Was I misinformed about the compatibility between the 7A and 7WB? If so, what components would have been affected by incompatibility?

Note: About a week after installing the EL84s, I was getting some intermittent noises out of the amp. Ran a jack in and out of the accordion input thinking it may have been dirty from lack of use, and the noises cleared right up.

Thanks in advance for any assistance!

[phsyconoodler]

You may have gotten some bad EL84's and one took out a screen resistor.That could explain the noises and smell.
   Burned resistors smell.Get a schematic and locate the screen resistors and measure with am ohmmeter.
 Do not attempt to play the amp anymore intil you find the issue.
  If you don't feel comfortable inside the amp,take it to a good tech.

[stratele52]

  I installed a new matched set of EL84s two weeks ago (with the understanding that this is a 'self biasing' amp) 

And you think no bias required ?  Wrong , a bias check is a must for any kind of amp each time you replace output tubes. Check that first .

[ehacker]

You may have gotten some bad EL84's and one took out a screen resistor.That could explain the noises and smell.
   Burned resistors smell.Get a schematic and locate the screen resistors and measure with am ohmmeter.
 Do not attempt to play the amp anymore intil you find the issue.
  If you don't feel comfortable inside the amp,take it to a good tech.

I got in touch with the tube supplier to let him know what happened and made an appointment with an authorized Ampeg tech for this coming week. Figured that was best for any potential reimbursements from tube supplier.

Thanks for the quick response!

[ehacker]

And you think no bias required ?  Wrong , a bias check is a must for any kind of amp each time you replace output tubes. Check that first .

I was told otherwise by 2 (what I assume are) knowledgeable techs, including the one I made the appointment with to look at the Ampeg later this week, who's worked on several units for me in the past and not let me down - the J12R is plug and play tube wise.

(No wonder amp techs get such a bad rap. Like doctors... 5 different answers from 5 different people.)

Thanks for the quick response!

[worth]

Ampeg has been using 1/4 watt screen resistors... I had the same issue a couple of years ago. Replaced them with 2 watt.. solved the problem.

[phsyconoodler]

1/4 watt screen resistors?An invitation to failures.

 At least 1 watt.It does however,save the amp from damge in the event a tube is bad.Just frustrating when they pop with a little bit of extra screen current from a set of tubes that draw a bit more than the stock ones did.

1/4 watt resistors belong in computers not tube amps.

[stratele52]

And you think no bias required ?  Wrong , a bias check is a must for any kind of amp each time you replace output tubes. Check that first .

I was told otherwise by 2 (what I assume are) knowledgeable techs, including the one I made the appointment with to look at the Ampeg later this week, who's worked on several units for me in the past and not let me down - the J12R is plug and play tube wise.

(No wonder amp techs get such a bad rap. Like doctors... 5 different answers from 5 different people.)

Thanks for the quick response!

Do the test yourself , take bias mesurement with few output tubes ( different brand or year ) and you'll see bias will not be the same or you are very very lucky. This is my experience and electronic theory too.
I don't know what is the knowledge of "your "2 tech , but many of them don't are electic /electronic  engineer .Just have many year in working in amp , sorry this not enough.
A good tech like Gerald Weber ,(  you know him ? ) write the same information in his book.

But your are not the first and the last to think class A amp or self bias never need a bias check.