Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TubeGeek on May 19, 2015, 11:31:45 pm
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I currently have an ampeg b15-s on the bench for a tune-up.
So far I have:
replaced all electrolytic caps
replaced faulty channel 2 12ax7 tube
ordered new 7027A tubes
repaired the power lamp
re-wired the on/off switching...The on/off switching wiring was a rats nest from a previous job and the power light wasn't working. The power switch was not original and it was inexpensive and did not look proper on the amp. After looking at the wiring for a while i started noticing it was not the same as the schematic. I cleaned all that up and removed the polarity wiring from the amp.
Question: Can I remove the polarity switch and associated parts? There is a 3 prong grounded mains cord on the amp.
The reason I am asking is because I have a problematic 120Hz hum I need to troubleshoot. I have gone over my wiring several times now and I am stuck.
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Question: Can I remove the polarity switch and associated parts? There is a 3 prong grounded mains cord on the amp.
Yes, it's just a SPDT switch across the PT primary AC lines with a cap to ground, just like a Fender and others.
The courtesy power socket, you might want to disconnect that as it has no safety ground.
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Ok I thought it was no big deal. Wanted someone else to confirm.
I'm tracing wiring out in hopes to find why I have the 120Hz hum.
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How close are the transformers to each other?
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Here is some debugging tips:
http://www.geofex.com/ampdbug/hum.htm (http://www.geofex.com/ampdbug/hum.htm)
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There are a couple of things to check related to ground loops.
The ext spkr jack normally has a set of isolation washers on the jack. If the 4-pin speaker connector was converted to a 1/4" jack on the chassis, it should have a set of isolation washers as well.
The cap can should be isolated from the chassis. Normally there is a fiber washer. Don't ground the cap can case to the chassis, it should be part of a ground bus that terminates at the input jack.
The polarity cap could be leaking. Most people simply remove it. If you decide to leave it in and retain the polarity circuit (it filters line noise), replace it with a type Y safety capacitor.
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How close are the transformers to each other?
The OT and PT are at the ends of the chassis. Proximity wouldn't be an issue.
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There are a couple of things to check related to ground loops.
The ext spkr jack normally has a set of isolation washers on the jack. If the 4-pin speaker connector was converted to a 1/4" jack on the chassis, it should have a set of isolation washers as well.
The cap can should be isolated from the chassis. Normally there is a fiber washer. Don't ground the cap can case to the chassis, it should be part of a ground bus that terminates at the input jack.
The polarity cap could be leaking. Most people simply remove it. If you decide to leave it in and retain the polarity circuit (it filters line noise), replace it with a type Y safety capacitor.
This amp still has the 4 pin speaker connector on it. I do see the fibre washer installed on the ext speaker jack….
Aha figured it out! :headbang:
Your advice got me thinking… there should be a wire connecting from the tip/ring of the ext speaker jack to the multi cap ground. That was not reconnected…as soon as I reconnected that, the problematic hum disappeared. Thanks for steering me in the right direction.
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Aha figured it out! :headbang:
Your advice got me thinking… there should be a wire connecting from the tip/ring of the ext speaker jack to the multi cap ground. That was not reconnected…as soon as I reconnected that, the problematic hum disappeared. Thanks for steering me in the right direction.
Good job guys! :icon_biggrin:
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Aha figured it out! :headbang:
Pat yourself on the back. These amps are normally pretty quiet.