Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jalmeida on May 23, 2023, 11:35:15 am
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Hello,
I am have worked out most of the bugs on my AC15. The last one was a squeal at high volume. On the EF86 channel. I am using a JMI repro chassis which places the inputs right near the speaker junction block. After I chopsticked the crap out of my amp and reflowed some solder joints for good measure I found the squeal remained unchanged. However I did note that all of the original AC15 and numerous other JMI models had this L-shaped aluminum plate the are attached to the OT upper bolts. So on the off chance this was an RF coupling between the input jacks and the speaker terminals I fabricated one and installed it. To my shock it made a significant improvement to the point my squeal is almost completely gone. I get a very faint feedback at full volume if I am not playing.
But, for good measure I installed a tube damper ring on the EF86. Here is my question:
Is it worth mounting the EF86 tube socket on rubber grommets to eliminate and vibrations that may aggravate microphonic susceptible EF86 tubes?
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What values are you using on your EF86? If you have done the chopsticking with good results maybe you do not need physical decoupling. Nice to hear good results from somebody playing it by the book. I have been using Ef86s with 68k plate load because they where uncomplicated and good sounding but tube roling EF86s made a huge difference. I used ridden in ones all the way, i never would want to give them a bad name in first place. I am guessing you already know Merlinīs small signal pentode article.