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Someone hacked up my Vox...need some advice.

10/16/2004 8:54 AM
Nick	Someone hacked up my Vox...need some advice. (This is a cross post from general discussion) I have an old '64 Vox AC-50 head. I've owned it for a few years, and it's always been one of, if not the best sounding head i've ever heard. I love this amp dearly. But a few months back, I lent it to a buddy of mine. He knocked it off the top of a 4x12, and it got banged up pretty bad when a concrete floor ended its journey. The chassis got bent pretty good, and whatever electrical problems it had were a bit over my head. I decided to send it out to a pretty reputable tech in the midwest. To make a long story short, he did a FANTASTIC job fixing it. He bent the chassis back and relaquered it...really cleaned the whole amp up and rewired the entire thing. One slight problem. When I got the amp back, I didn't have a '64 AC-50 anymore. He replaced EVERY SINGLE component in the pre-amp, and most of the components in the bias and output sections. 95% of the components are new. I now have a clone of a '64 Vox AC-50. Needless to say, the amp sounds a bit different than it did before. I have all the original components, as he sent them back to me in a bag. I have a few questions... I obviously want to put the essentials back in, like the Mustard caps. But i'm wondering where I should draw the line, and what components will get my head sounding the way it did before. Should I worry about things like pre-tube cathode resistors or plate resistors? Secondly... I obviously have no way of knowing which part went where. For instance...I've got 7 100K resistors. Some of them are plate resistors. Some of them are slope resistors in the tone stacks. Some are drifted up a bit (112K, 114K)...If I put the drifted ones in the wrong place, it's a moot point putting them back in the amp. I'm assuming the plate R's are the ones that are drifted up. But i'm wondering if I can figure things out by trends that components have historically. Do plate R's drift up usually? Do slope R's usually stay stable? Sorry to write a novel on this...but i'm just wondering if you guys (who largely have much more experience than I ), can point me in the right direction as to what I should worry about putting back in the head to get it sounding like it did...and if you have any tips on trying to figure out which components (of the same value) went where. He's replaced all the caps with Sprague 716P's BTW. Resistors seem to be 1W carbon comps, newer style ones. Thanks in advance for dealing with my longwindedness. -Nick

10/16/2004 8:54 AM

Nick

Someone hacked up my Vox...need some advice.

(This is a cross post from general discussion)

I have an old '64 Vox AC-50 head. I've owned it for a few years, and it's always been one of, if not the best sounding head i've ever heard. I love this amp dearly.

But a few months back, I lent it to a buddy of mine. He knocked it off the top of a 4x12, and it got banged up pretty bad when a concrete floor ended its journey.

The chassis got bent pretty good, and whatever electrical problems it had were a bit over my head. I decided to send it out to a pretty reputable tech in the midwest.

To make a long story short, he did a FANTASTIC job fixing it. He bent the chassis back and relaquered it...really cleaned the whole amp up and rewired the entire thing.

One slight problem. When I got the amp back, I didn't have a '64 AC-50 anymore. He replaced EVERY SINGLE component in the pre-amp, and most of the components in the bias and output sections. 95% of the components are new.

I now have a clone of a '64 Vox AC-50. Needless to say, the amp sounds a bit different than it did before.

I have all the original components, as he sent them back to me in a bag.

I have a few questions...

I obviously want to put the essentials back in, like the Mustard caps. But i'm wondering where I should draw the line, and what components will get my head sounding the way it did before. Should I worry about things like pre-tube cathode resistors or plate resistors?

Secondly...

I obviously have no way of knowing which part went where. For instance...I've got 7 100K resistors. Some of them are plate resistors. Some of them are slope resistors in the tone stacks. Some are drifted up a bit (112K, 114K)...If I put the drifted ones in the wrong place, it's a moot point putting them back in the amp. I'm assuming the plate R's are the ones that are drifted up. But i'm wondering if I can figure things out by trends that components have historically. Do plate R's drift up usually? Do slope R's usually stay stable?

Sorry to write a novel on this...but i'm just wondering if you guys (who largely have much more experience than I ), can point me in the right direction as to what I should worry about putting back in the head to get it sounding like it did...and if you have any tips on trying to figure out which components (of the same value) went where.

He's replaced all the caps with Sprague 716P's BTW. Resistors seem to be 1W carbon comps, newer style ones.

Thanks in advance for dealing with my longwindedness.

-Nick

10/18/2004 4:43 PM
anon	Your first mistake was to not fix it yourself.

10/18/2004 5:23 PM
Bruce /Mission Amps	Can you post a digital picture of the two circuit boards and the pots wiring? I doubt he would do anything but replace parts one at a time but, there are three different AC50s, of which two look the same but have different phase inverters and mixing schems for the two channels. Plus, I've seen a few bone stock AC30s and AC100s with the tone controls wired differently and they sound different because of it. Bruce

10/18/2004 5:23 PM

Bruce /Mission Amps

Can you post a digital picture of the two circuit boards and the pots wiring?
I doubt he would do anything but replace parts one at a time but, there are three different AC50s, of which two look the same but have different phase inverters and mixing schems for the two channels.
Plus, I've seen a few bone stock AC30s and AC100s with the tone controls wired differently and they sound different because of it.

Bruce

10/18/2004 8:41 PM
Nick	Thank you Anon, for your contribution. Bruce, I'll snap some pics of the head tomorrow and post them. I guess he took a high-res pick of the pre-amp tag board before he ripped it out, for reference. So he's gonna email me that, which I can post as well. The amp is identical to Joe Piazza's redraw on the Schematic Heaven site: http://www1.korksoft.com/~schem/voxamps/ac50.pdf But...I never looked too close at the tone stacks to see if they were wired with or without the 'mistake' on the bass pot (I think that's what you're referring to). But they definitely act differently now. I'm not sure if that's due to the new parts, or if he wired them up according to a schematic, as opposed to how they were in the amp originally. What gets me is we spoke on the phone numerous time about him not touching the circuit except for replacing any bad components, or ones that were drifted very far off. I was pretty upset when I popped it open. I sat down with my meter, and this schematic, and I was able to place about 70% of the parts in the amp. I haven't wired them in yet, as i'm waiting for him to email me that chassis pic so I can make sure I orient everything in the same direction it was originally (coupling caps with the same side tied to plate, plate R's with the same side tie to B+...). I'm not sure what I want to do about my 100K R's (7 total- 5 plate R's, and 2 slope R's for the tone stacks)...as they're all drifted to different degrees. I'm also not sure what to do about my 56K's, because one is reading 62K, and the other 67K. They're both cathode resistors for the direct coupled stage before the TS. The mustards are all dead on tolerance, so no worries there. The PF range caps are drifted all over the place...though I can place them visually when I get that picture, as they're different sizes and the leads are cut different lengths. I know a few things have to be changed, as there wasn't any free real-estate on the tag board originally...there still isn't, but where the cathode bypass caps were stacked over their respective R's, they're in their own slots on the board now. I've got to trace out exactly what he did in there. What a nighmare. -Nick

10/18/2004 8:41 PM

Nick

Thank you Anon, for your contribution.

Bruce,

I'll snap some pics of the head tomorrow and post them. I guess he took a high-res pick of the pre-amp tag board before he ripped it out, for reference. So he's gonna email me that, which I can post as well.

The amp is identical to Joe Piazza's redraw on the Schematic Heaven site:

http://www1.korksoft.com/~schem/voxamps/ac50.pdf

But...I never looked too close at the tone stacks to see if they were wired with or without the 'mistake' on the bass pot (I think that's what you're referring to). But they definitely act differently now. I'm not sure if that's due to the new parts, or if he wired them up according to a schematic, as opposed to how they were in the amp originally.

What gets me is we spoke on the phone numerous time about him not touching the circuit except for replacing any bad components, or ones that were drifted very far off. I was pretty upset when I popped it open.

I sat down with my meter, and this schematic, and I was able to place about 70% of the parts in the amp. I haven't wired them in yet, as i'm waiting for him to email me that chassis pic so I can make sure I orient everything in the same direction it was originally (coupling caps with the same side tied to plate, plate R's with the same side tie to B+...).

I'm not sure what I want to do about my 100K R's (7 total- 5 plate R's, and 2 slope R's for the tone stacks)...as they're all drifted to different degrees. I'm also not sure what to do about my 56K's, because one is reading 62K, and the other 67K. They're both cathode resistors for the direct coupled stage before the TS.

The mustards are all dead on tolerance, so no worries there. The PF range caps are drifted all over the place...though I can place them visually when I get that picture, as they're different sizes and the leads are cut different lengths.

I know a few things have to be changed, as there wasn't any free real-estate on the tag board originally...there still isn't, but where the cathode bypass caps were stacked over their respective R's, they're in their own slots on the board now. I've got to trace out exactly what he did in there.

What a nighmare.

-Nick

10/19/2004 7:30 AM
Wild Bill	Nick, things might not be as bad as they seem! Remember that those resistors never were precisely the marked value - that's why there's a gold or silver tolerance band on 'em. Most likely they were 10%, that was pretty standard. Anyhow, the values you measured for the most part are within tolerance. I doubt if 10-15% would make any noticeable tone difference. More than 20% and if you have good ears you'll start to notice it. Some areas of the circuit have almost zero tone effect anyway, like cathode followers. Unless the resistor drifted so far as to make the stage clip or blow up! The caps of course ARE a big deal! If you liked mustards before - put 'em back. As for the pf caps - what kind of meter did you use? It's very difficult to measure such low values with a DMM that has a capacitance scale. Move the test leads around and the reading will swing all over the place. You need a unit designed to be accurate in this range. The good news is that such caps rarely actually drift much to affect audio circuits. Different story in a radio circuit, of course. My point is that you should worry about specific areas rather than everything. It's like if your vintage car had some work done by a poor mechanic and lost horsepower. You don't worry about the tolerance on the window seals when your problem is the engine power. I think Bruce pointed out the single most important point - that tone stack wiring! Nail that question down, replace the caps and fiddle with what's left just on general principles. I'll bet you a beer (Canadian!) that by this time your amp will be its old self. .02 ---Wild Bill

10/19/2004 7:30 AM

Wild Bill

Nick, things might not be as bad as they seem!

Remember that those resistors never were precisely the marked value - that's why there's a gold or silver tolerance band on 'em. Most likely they were 10%, that was pretty standard.

Anyhow, the values you measured for the most part are within tolerance. I doubt if 10-15% would make any noticeable tone difference. More than 20% and if you have good ears you'll start to notice it. Some areas of the circuit have almost zero tone effect anyway, like cathode followers. Unless the resistor drifted so far as to make the stage clip or blow up!

The caps of course ARE a big deal! If you liked mustards before - put 'em back. As for the pf caps - what kind of meter did you use? It's very difficult to measure such low values with a DMM that has a capacitance scale. Move the test leads around and the reading will swing all over the place. You need a unit designed to be accurate in this range. The good news is that such caps rarely actually drift much to affect audio circuits. Different story in a radio circuit, of course.

My point is that you should worry about specific areas rather than everything. It's like if your vintage car had some work done by a poor mechanic and lost horsepower. You don't worry about the tolerance on the window seals when your problem is the engine power.

I think Bruce pointed out the single most important point - that tone stack wiring! Nail that question down, replace the caps and fiddle with what's left just on general principles. I'll bet you a beer (Canadian!) that by this time your amp will be its old self.

.02

---Wild Bill

10/19/2004 8:01 PM
SpeedRacer	just want to second Bruce and Wild Bill's vote.. make sure you don't have a wiring issue #1 #2 If you are certain that you are not "hearing things" i would put the old coupling caps back in and see where that gets you. I would not waste time on the resistors. Those values are probably where they always were and you're not going to hear a 5 or 10% difference. A lot gets made of drifting values, but a lot of the time they simply are not that audble. Pop an R-substitution box into a working amp sometime and try out some different values. It's not the kind of 'night and day' thing that some folks make it out to be. Don't make yourself crazy over it. Also keep in mind also that simply swapping out the tubes could have completely changed the sound of the amp and the components he put in may sound just as good as the ones that came out. There are a lot of variables here. Again, I would check out the tone stack first, play with tubes second and then replace the tone caps (tone stack caps, bright cap and then coupling caps) in that order to see where it got me. JM2C

10/19/2004 8:01 PM

SpeedRacer

just want to second Bruce and Wild Bill's vote.. make sure you don't have a wiring issue #1
#2 If you are certain that you are not "hearing things" i would put the old coupling caps back in and see where that gets you.
I would not waste time on the resistors. Those values are probably where they always were and you're not going to hear a 5 or 10% difference. A lot gets made of drifting values, but a lot of the time they simply are not that audble. Pop an R-substitution box into a working amp sometime and try out some different values. It's not the kind of 'night and day' thing that some folks make it out to be. Don't make yourself crazy over it.
Also keep in mind also that simply swapping out the tubes could have completely changed the sound of the amp and the components he put in may sound just as good as the ones that came out. There are a lot of variables here.
Again, I would check out the tone stack first, play with tubes second and then replace the tone caps (tone stack caps, bright cap and then coupling caps) in that order to see where it got me.
JM2C

10/21/2004 3:09 PM
anoinimois	You quit smoking yet wild bill?

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