VOX AC30 / 6 Vibe Channel 'ain't right'

[Masco]

I should have looked at the schematic yesterday. The only gain stage is the first triode for the V/T channel, the second is just a splitter. There will be some loss there, probably more on the plate than the cathode.
That channel in general has always been the weakest from my experience and there is a tonal difference between the trem and vibe settings because of the one input to the modulator that gets grounded in the trem setting and the fact that different filter networks/coupling caps are used for the EQ'ing "vibe" effect.

[Buddha's Guitar Tech]

Reeltarded, you're probably talking good sense.
It's amazing how fixated we can get when faced with what we think is an issue.

Has to be said though, in its current state, the VT channel just has no oomph compared to the normal channel. And this is with either the trem or vibe on or off. Just sounds... blah.

And thanks Masco for the heads up on that second stage, it sounds like i'm not nuts!

What i'm going to do is replace a bunch more stuff. The caps and resistors in that divider on the cathode of that splitter you mention, the .0047s before the PI, and the filter caps - they are ancient too, so it's probably something that should be done anyway. Busy busy busy...

Cheers

[Reeltarded]

Uhoh! I forgot the pic, but I did the test. It was less than I expected. 12 normal, 12 and 2 pointer widths vib. Teeny.

Phone rang and people from 25 years ago like to talk about the old days like they matter. Sorry!

[crbowman]

I had an early 80's AC30 for awhile. the Vib/Trem channel was definitely the weakest of the 3 channels, and on mine, very midrangy and just weird. The only real way to use that channel was to jumper it with either the normal or top-boost channel. Doing this actually sounded pretty good, but the vib/trem channel by itself was just pretty unusable on mine.
And yes, I did try a number of different preamp tubes.
I love AC30's but they can be a real headache.

[Buddha's Guitar Tech]

Crbowman, you are my new hero.
i have heard a few times now, that the VT channel was the weakest, and your experience sounds closest to what's going on with the beast here.

i'm still going to replace a load of components (as this is not the most mind-bending of skills) and maybe something good will come of it.

[Reeltarded]

It's an AC-30. Somethig good will ALWAYS become of it!

[crbowman]

Thanks Buddha.
Thinking back, I recall working on a friends early 60's JMI AC30. His Vib/Trem channel sounded better than mine, but was still clearly the weakest of the 3 channels, and still just didn't sound 'right'. Roughly half of the AC30's components are in the vib/trem circuit and, as I'm sure you've noticed, there are A LOT of components squeezed into these thing. So there's lots of room for potential problems and discrepancies there. Lead dress is of course, very critical in these beasts.
Jon Bessent, an amp tech here in Austin had a rule 'No SVT's and no AC30's'!

But MAN! Early Rory Gallagher, back when he used the Strat-> Rangemaster -> normal channel on an old AC30 cranked.........what a sound!

[Buddha's Guitar Tech]

As promised.....

[img:640:480]https://lh5.googleusercontent.com/-EUtx ... G_1874.JPG[/img]

[img:640:480]https://lh4.googleusercontent.com/-50BQ ... G_1875.JPG[/img]

[img:640:480]https://lh4.googleusercontent.com/-k3cX ... G_1876.JPG[/img]

[img:640:480]https://lh4.googleusercontent.com/--Y_y ... G_1877.JPG[/img]

[Reeltarded]

AHA! You need to put more holes in the cloth!

Easiest amp ever to work on, huh? I mean with the chassis all in the tray.. lol

I have no idea why they all smell like new toasters up to 50 years later. I guess they forget they aren't new?

I am not familiar with that TB version. What is it?

[Buddha's Guitar Tech]

i know nothing of tuberculosis, sorry.

Tell you what, i worked on an AC30 CC2 a while back and hated it. (i'm a sensitive soul, you see.) At least with this 30/6, you can easily disable the speaker connection. To get the the CC2 on the bench, you have UNSOLDER the speaker and reverb connections. Grr...
This amp has screw terminals, also known as 'idiot connectors.' You might be able to see them at the far left of the first pic.

[M Fowler]

I hope you two were just having fun about that TB version.

Top Boost.

[Masco]

Hey Buddha, that's a "T" (for treble) version AC-30.
They sound thinner and brighter than the "N" (for normal)versions. Notice the 330 ohm/220pf cap network at each volume pot. That's the "T" mod. The "T" model also has a split cathode first stage w/ .1uF on the brilliant channel's triode, unlike the shared stage w/ 1k5/25uF in the "N" model.
If you remove the 330/220pF network(s), as done in the "N", you'll get less attenuated lows and overall more gain.

[Buddha's Guitar Tech]

Ah, ok, TOP BOOST, gotcha. (as far as i know, this isn't a top-boost, as the TBs had an extra stage of EQ, right?)

Yes, i noticed those differences from the schematic i had. i thought perhaps they were ancient mods.
The Normal and Brilliant channel don't need anything done to their 'oomph', they have it in spades. But removing the 330/220s on the VT channel pot, there's an idea.

i resume work on it tomorrow, replacing the power supply caps, and a load of coupling/tone caps. If after that, it's not what i expect, then we just live with it.

i'm really impressed with the input y'all, thanks a ton.

[crbowman]

Ah! A thing of beauty!!
No, really.
I really do love the old JMI's.
If it's really a TB it'll have separate treble and bass controls either on the panel or on the back cover in addition to the 'Cut' control.
Please keep us posted as to how it comes out Buddha!

[Roe]

ac30s are a thing of beauty. I built myself one with the 1960 ef86 channel instead of tremolo. it works well, only problem is a slight hum - something that seems ordinary with these amps