Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: varaso on November 05, 2018, 05:53:11 am

Title: 90's Vibrolux thunderous pop when intensity over halfway.
Post by: varaso on November 05, 2018, 05:53:11 am

Hi,

a friend brought me his vibrolux, after upgrading some caps, and moding the reverb to original blackface specs there is a problem when you turn the intensity pot up.

The issue happens when the intensity pot is over 4 and only when you dig in with the guitar with volume at about 2+. It's a thunderous pop sound that is not synced with the speed, and it is not always continuous, some times it stops. It happens to both channels.

So far I tested the circuit for continuity, fixed some bad traces, rechanged some caps, changed all the tubes except the 6l6s and nothing changed. I guess it's a post PI issue? or an issue in the tremolo circuit.. ? Could it be a bad new cap? (caps changed: all the filtering caps, all the cathode bypass caps, C37(only checked this one so far), C30 and the tonestack caps)

What would you suggest me to do next? Should I try the trick with the diode on the intensity pot, so the bias to the output tubes only decrease, because as a dummy I don't remember exactly how to wire it, as I can't find any info online?

I've attached the schematic bellow.

Thanks in advance!!

Title: Re: 90's Vibrolux thunderous pop when intensity over halfway.
Post by: pompeiisneaks on November 05, 2018, 01:39:28 pm

Do you have a scope?  I'd try watching the signal and playing until it pops and see if at each triode it happens or not, to find the source. Have you chopsticked around those components to look for bad resistors/coupling caps/etc. or any other kind of odd noises on pressure?  It could be reverb or vibrato I'd guess.  But the fact that it seems to follow intensity seems to peek at the vibrato.  Also those roaches do go bad, have you checked to ensure it's not doing some kind of odd thing causing that pop? 

There are some pretty sharp people on here that likely know the right answers, I'm just thinking out loud :D

~Phil

Title: Re: 90's Vibrolux thunderous pop when intensity over halfway.
Post by: sluckey on November 05, 2018, 02:27:32 pm

Connect your meter to pin 5 of one of the 6L6s to monitor the negative bias voltage. What happens to the voltage when you hear the popping?

Title: Re: 90's Vibrolux thunderous pop when intensity over halfway.
Post by: varaso on November 06, 2018, 01:31:00 pm

Thanks guys!

Saddly, I don't have a scope, I'll chopstick the components and monitor the negative bias voltage for now.

I'll keep you updated!