Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Raybob on November 16, 2011, 01:53:47 pm

Title: Odd 120hz hum in PR build
Post by: Raybob on November 16, 2011, 01:53:47 pm

I built essentially an AA1164 PR but using larger trannies and 6L6 power.  I've built the same circuit many times with very quiet idle results, with JJ 40-20-20-20 cap can.  This one has a very loud (65db) 120hz with volume off.  Hum will pulsate with tremolo.  I tried different power tubes, checked filters, tried different rectifier, and now ready to try different PT.  Using a GZ34 recto, the problem is there.  What's odd is using a Weber CC, I get no noise at all for minute or two, then it 'kicks in'.  Once with Weber CC, I pulled PI while it was silent, put in PI and noise was back.  Using a signal generator through a speaker, I turned it on with CC, had nothing but could faintly hear signal in speaker.  I pulled PI and faint signal noise got louder, plugged in PI and signal was full strength.

Anyone have ideas where to check before trying to swap PT?

Title: Re: Odd 120hz hum in PR build
Post by: stingray_65 on November 17, 2011, 12:00:58 am

Filter caps?

I would suspect a bad cap can first

Magnifying glass to check for cold solder joints, or using shotgun troubleshooting, de solder the joints with braid and re solder them all

120Hz indicates an issue in the B+ rail.

I fail to see how a PT would be at fault here. It's design is to put out many hundreds of volts at 120Hz.

The recto tube and filter caps do their best to bring it down to 0Hz

Does the hum change volume with the volume knob? or is it a constant 65dB though the entire sweep of the volume pot?

Price of cap can vs the price of a PT, I'd swap out the JJ.

Title: Re: Odd 120hz hum in PR build
Post by: Raybob on November 17, 2011, 05:18:34 am

Quote from: stingray_65 on November 17, 2011, 12:00:58 am

Filter caps?

Just tried new cap can, same noise.  Going to check all joints, starting with power supply.

Quote

Does the hum change volume with the volume knob? or is it a constant 65dB though the entire sweep of the volume pot?

With tone controls up, turning volume knob up give normal 'hiss' but turning it down, it should be quiet.  I need to double check all the wiring because it makes no sense a signal at input could even get to the speaker without the PI tube in.

Title: Re: Odd 120hz hum in PR build
Post by: Raybob on November 17, 2011, 06:07:48 am

Duh.. found it.  I forgot to put the ground to reverb pot.  duh  :BangHead:

Title: Re: Odd 120hz hum in PR build
Post by: billcreller on November 21, 2011, 11:48:39 pm

 :icon_biggrin: :icon_biggrin: