Vox AC-30 JMI Vib / Trem Channel Doesn't Work (Vox AC30)

[Butler_B73]

Background:

- JMI Vox AC-30 TB
- Both brilliant and normal channels work without issue
- Vib/Trem channel, no sound besides standard hiss from speakers. Can hear very faint signal if the volume is turned up full and signal loaded with additional preamp boost
- Footswitch does nothing
- Voltage on Pins 1 and 6 for V7, V9, V8 are next to nil (obviously a problem), Voltage on Pins 1 and 6 on the other V1, V2, TB are fine at ~200V.
- Checked all resistors, and they all appear to be in the ballpark

I figure either a failed cap or resistor earlier in the vib/trem circuit as it's taking out all three vib/trem preamps, but I can't seem to pinpoint. Any help would be greatly appreciated.

Schematic attached.

[Jon Snell]

If it is original, I would firstly check the ECC83, (anode voltages)and check the 22k feed resistor. Nine times out of ten, the wire has broken off the valve holder lug because it is solid wire.
Otherwise the anode voltages will tell you where the issue is.

[Butler_B73]

Again, thanks Jon. Really appreciate your helping me sort out these issues with the Vox. Just to confirm, would the anode voltage for the ECC83 be pins 1 and 6, which i've already measured as having very low/nil measurements? Not sure where to go from there? Again, all the 3 preamp tubes connected to the vib/trem circuit have nil 1 and 6 pin values. The 22k feed resistor on the other side of the chassis looks to still be intact with accurate measurements.

[g1]

What is the DC voltage at either end of that 22K ?

[Butler_B73]

Closing the loop on this thread. Happy to report the culprit was the 22K resistor after all. Although I thought I was getting in range readings, the resistor physically looked like it had its day (as you can see in the original photo), so I replaced it. Turns out when removed and tested again, it was indeed fried. Got a brand new 22k 14W installed and the vib/trem is back in action in all its glory. Jon, g1, much thanks for pointing me in the correct direction!

Status on repairs/restoration for the Vox AC-30:
1) Solid state to GZ34 tube rectification rebuild - DONE
2) Fix broken vib/trem channel - DONE

Now what's left is getting the amp to "chime" like my other JMI. For some reason, this one is darker, with less dynamic (perhaps also touch sensitivity/feel), even on the Top Boost channel and no matter the treble/bass/cut settings. I'll head over to the "Vox Voicing" thread and see if I can find my answers there. Wondering if it could also simply be due to different components (i.e. wodens vs haddons, wimas vs lemcos, carbon film replacements vs carbon comps, cathode bias ohm ratings, late 60's celestion alnicos vs mid 60's celestion alnicos). Both have NOS Mullards so think i've got that variable neutralized. Realizing this one may not be that easy.... I'll look to take measurements on all resistors and capacitors and see drift from original JMI spec and go from there (unless anyone has any shortcuts).

[Helmholtz]

Could well be the speakers.
The original silver alnico speakers in my 1970 AC30 sounded midrangey, lacked chime but were incredibly efficient/loud.
I later could change them to earlier Vox silver alnicos, which I liked much better.

There's some rumour that Celestion changed the voice coil (and probably cone) with later alnicos to increase power handling.

Try connecting the speakers from the older amp.

[Butler_B73]

Thanks, Helmholtz. To be honest, I was thinking the same. Even though the ones in mine are considered coveted celestion alnico "silver bells", they are later production line (like yours). They simply don't have that "sparkle", sounds like a blanket covering the speakers with sort of a nasal midrange....can't really put into words. It's not "that" bad on its own, but when I A/B it with the other JMI, it's night and day. I was hoping it was some resistor or capacitor in the tone signal chain that's drifted...but yeah, maybe i'll try hooking up the cab from my other JMI to this chassis. Great you found earlier ones to swap in on yours. Did you ever dabble with current production celestion blues? I presume they are also don't quite capture the "magic" as the early-mid 60's alnicos.

[Helmholtz]

Did you ever dabble with current production celestion blues? I presume they are also don't quite capture the "magic" as the early-mid 60's alnicos.

No chance yet to directly compare with new alnico blue's. But I've heard them with AC30 reissues and the sound was very good.
I mean at that price the new blue's should be really good.

Also 20W (not so much 25W) greenbacks sound very good in an AC30, but lower efficiency.

[Butler_B73]

Thanks, will need to take a closer look at all the variables. Still sorta hopeful I can tweak some of the voicing caps/resistors, but sounds like you've already gone down that rabbit hole. But yeah, will look to swap the cabs with my other JMI to confirm first.