ron_swanson
Well-known member
I have a vintage MkII VOX AC-100 amp ( this is the version of the amp I have... The VOX Showroom - Vox AC-100 MkII Fixed Biased Amp Head - A Look Under the Hood ) on the bench that's exceeded my troubleshooting experience. Hoping someone here with more guitar tube amp repair knowledge might point me in a proper direction.
Initially, the amp was clean and strong until the volume hits about 1/3 - 1/2 after which it goes into an ugly thumping / distorted sound mostly effecting the lower ( guitar ) frequencies. Not quite motor boating sound, but something in that sound range.
After replacing all of the electrolytic caps ( power & preamp which were likely original from the 1960's as they were all UK made ) with new caps, unfortunately, the same issue persists.
Notes...
Someone in the past has re-designed the bias section that incudes a trimmer that allows for the -35V negative bias to be adjusted which the original OEM design did not. Not sure why that was necessary, but it seems to be doing the job.
After measuring numerous test points, all of the noted schematic voltages are within spec with the exception of the voltages around V3 ( phase inverter ) which is WAY off.
Schematic...
Test points ( TP ) are noted in yellow. General area in question within a red dotted rectangle. RED test points are outside of expected voltages by over 100V. The 'M' in each test point box is the actual measured voltages. The voltage below is the schematic voltages. Hope that makes sense...
Most of the resistors in the red dotted rectangle section in question were either resistor values ( by code bands ) not indicated on the schematic ( more redesigning? ) or above value specification when measured physically. I replaced all of these since they all likely original components from the 1960's to match the schematic with modern carbon film 5% resistors.
After the replacement and triple checking my work, the initial thumping / distorted sound remained unchanged and the voltages at TP10A / TP10B only slightly improved - both test points now measure 235V respectively and TP11 remains unchanged. The measured voltages above where those BEFORE replacing the resistors in this area.
Video of scope going from less than 1/2 volume to 1/2 the back down again.
View attachment IMG_2616.MOV
So, with the above in mind, I'm stumped of where to go next. Any pointers will be greatly appreciated.
Thanks in advance.
Initially, the amp was clean and strong until the volume hits about 1/3 - 1/2 after which it goes into an ugly thumping / distorted sound mostly effecting the lower ( guitar ) frequencies. Not quite motor boating sound, but something in that sound range.
After replacing all of the electrolytic caps ( power & preamp which were likely original from the 1960's as they were all UK made ) with new caps, unfortunately, the same issue persists.
Notes...
Someone in the past has re-designed the bias section that incudes a trimmer that allows for the -35V negative bias to be adjusted which the original OEM design did not. Not sure why that was necessary, but it seems to be doing the job.
After measuring numerous test points, all of the noted schematic voltages are within spec with the exception of the voltages around V3 ( phase inverter ) which is WAY off.
Schematic...
Test points ( TP ) are noted in yellow. General area in question within a red dotted rectangle. RED test points are outside of expected voltages by over 100V. The 'M' in each test point box is the actual measured voltages. The voltage below is the schematic voltages. Hope that makes sense...
Most of the resistors in the red dotted rectangle section in question were either resistor values ( by code bands ) not indicated on the schematic ( more redesigning? ) or above value specification when measured physically. I replaced all of these since they all likely original components from the 1960's to match the schematic with modern carbon film 5% resistors.
After the replacement and triple checking my work, the initial thumping / distorted sound remained unchanged and the voltages at TP10A / TP10B only slightly improved - both test points now measure 235V respectively and TP11 remains unchanged. The measured voltages above where those BEFORE replacing the resistors in this area.
Video of scope going from less than 1/2 volume to 1/2 the back down again.
View attachment IMG_2616.MOV
So, with the above in mind, I'm stumped of where to go next. Any pointers will be greatly appreciated.
Thanks in advance.