Vox AC30tbx Mod/ signal path resistors?

[Wagster]

Can anyone tell me what the pcb identification numbers are for the signal path resistors from looking at this schematic? http://www.schematicheaven.com/voxamps/ac3093pr.pdf

I'd like to replace the carbon films with carbon comps in the signal path. I'm trying to learn how to read a schematic and can't figure out.

Billy

[Wagster]

Anyone

[WaltC]

there's no difference (on most guitar amp schematics anyway) between carbon comp, carbon film, metal film or whatever. The resistors are labeled by value and you can use any kind you want. As for which kind, that a religious discussion and for a different set of folks as far as I'm concerned.

FWIW I'd not do what you're contemplating, *way* more trouble that the end result will be worth and a much greater chance to introduce problems into the bargain.

[donnyjaguar]

Pretty much every resistor can effect tone in this circuit.

[Wagster]

I was told this makes a difference in tone by one of the most knowledgeable Vox techs in the US. Out of respect I want to keep his name private. I also don't want to keeping asking him a lot of questions since he was kind enough to tell me all of his mods for the TBX. Here's what he said to do in regards to the CC resistors.

The other thing I would do is replace all the signal path resistors with
Carbon Comps. That's what Derek Underdown spec'd on all the amps in the late
`50's & `60's. Derek was the head engineer at Vox/JMI.

btw: I've been soldering for years. I'm just not good at reading a schematic. I'm going to search the web for a good tutorial so I can teach myself.

[WaltC]

Well, if you want to do it, just copy the values for all the resistors in the signal path (but even that's a question...<G>) from the schematic (like the input RF stoppers, grid stoppers and plate load resistors), order replacement carbon comps from Antique Electronics Supply, Mojotone, or Hoffman Amps and go to town.

But... desoldering all those pc-board pads, removing the resistors, installing the new ones , not to mention removing the chassis from the cab, getting the pc-board loose and out where you can work on it well, putting it all back together (and then taking it apart again to troubleshoot what went wrong, yes <G> I'm a glass half empty guy) seems like an immense amount of trouble for what I suspect will be very minimal audible difference when it's all said and done given all the underspec'd cathode bypass and filter caps, microscopic polyester tone and coupling caps and other compromise parts (ot, pt, etc.) in there.

Now if you wanted to take all that out, get some new Mercury Mag iron, build a new turret board and populate it with robust, high quality parts, that might be pretty cool.

You take apart a lot of modern boutique, high dollar amps and look inside and you're going to find a *lot* of carbon film and metal film 1% resistors in there and they sound pretty good, so IMHO, YMMV, etc.... and good luck <G>!

[Wagster]

I've already installed a mercury Magnetics Woden and choke, Sozo Vintage caps and Lowered the Filtering to JMI specs. I'm well aware that the CC's will be a subtle change. I found a guy from CA that makes a populated handwired board that drops into the TBX w/o a lot of work. I think a few preamp socket may need to be replace for it to work but nothing major. This is nice to know in case I trash the pcb.

I think I've been bit by the amp tinkering bug

[WaltC]

Oh man, you've done it now <G>!!! Quick, before it's too late give it up and buy a Crate<BG>!

[Wagster]

Help me Oprah!

[Wagster]

I'm still having trouble identifying the signal path resistors. Anyone have a few minutes to help me out?

[WaltC]

Well, at the risk of precipitating a religious war, here's a quick stab at what I could see in the link you supplied (I only see page one of the schematic when I look at it, page two will have more "signal path" resistors it as well). For definition purposes I'll include all the resistors the signal passes through, including in some cases cathode resistors, and including plate load resistors (some could rightly argue this one) but not including the pots of course (which are variable resistors after all):

Vib/Trem

R26, 27, 80, 62, 25, 42, 22 (might as well do 23), 21, 19, 40, 41, 16,15, 39, 37 (and 36 I guess), 38, 17, 18 (Might As Well -- MAW). You could argue for and against including the actual term stuff like R's 11, 12, 13, 63, 66, 43, 44, 64, 65, 66, 67, 68, 49, 48, 47, and 46 as you could argue against most of these just on general priciples.

Normal
R's 30, 31, 58, and 6

Brilliant
R's 34, 33, 57, 55, 56 (MAW), 35, 7, and 5


and the rest of the appropriate PI and power tube stuff on Page 2 I imagine.

(took more that a couple of minutes....)

[Wagster]

Walt, I owe you a beer my friend
I already know where to put the PI and power tube resistors.

Cool looking amps btw. The Argyll looks interesting.

Thanks,
Billy

[WaltC]

My latest amp is called the Highlander and is basically a JTM 45/JMP in a Bassman chassis, either 4x10 or 2x12 so far. Fun to play and doing pretty well so far. I've not taken the time to update the site in a over a year, but maybe this winter...<G>