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VOx AC-50...component drift, and mojo factor...???

10/16/2004 12:22 AM
Nick	VOx AC-50...component drift, and mojo factor...??? 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 componets 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 12:22 AM

Nick

VOx AC-50...component drift, and mojo factor...???

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 componets 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 1:59 PM
loudthud	Many people will disagree about this subject so let the Bar-B-Que begin. It's been my experience that carbon comp's usually drift to the high side. One can assume that the more heat a resistor receives the higher and faster the drift. All the NOS parts I have obtained over the last couple of years have meeaured higher than nominal value, most out of spec. Those Sprague caps can pass a phenomenal amount of current. I used them on a project to tune a 133KHZ antenna and they were passing 10 amps P-P. Most other caps I put in that circuit burned out. They tend to sound a little harsh. I have tried 100K 1/2w CC's for plate loads and they subjectively add a little sweetness. I've also tried 1/4W 330K's in the second stage ( with appropriate cathode resistor ) and they sounded as good if not better. Operating them beyond their voltage rating makes a sweet sound. I suggest replacing those 716's and the plate loads first. If you're not there yet, replace the rest of the resistors in the signal path. Check the resistors in the B+ decoupling. They may have reduced the B+ some and that could also affect the sound. As to what component went where, I'd put the high ones in the plates but I don't think that's as important as the type of part. I hope you will report you results here for us all to see.

10/16/2004 1:59 PM

loudthud

Many people will disagree about this subject so let the Bar-B-Que begin.

It's been my experience that carbon comp's usually drift to the high side. One can assume that the more heat a resistor receives the higher and faster the drift. All the NOS parts I have obtained over the last couple of years have meeaured higher than nominal value, most out of spec.

Those Sprague caps can pass a phenomenal amount of current. I used them on a project to tune a 133KHZ antenna and they were passing 10 amps P-P. Most other caps I put in that circuit burned out. They tend to sound a little harsh.

I have tried 100K 1/2w CC's for plate loads and they subjectively add a little sweetness. I've also tried 1/4W 330K's in the second stage ( with appropriate cathode resistor ) and they sounded as good if not better. Operating them beyond their voltage rating makes a sweet sound.

I suggest replacing those 716's and the plate loads first. If you're not there yet, replace the rest of the resistors in the signal path. Check the resistors in the B+ decoupling. They may have reduced the B+ some and that could also affect the sound. As to what component went where, I'd put the high ones in the plates but I don't think that's as important as the type of part.

I hope you will report you results here for us all to see.

10/16/2004 2:49 PM
Nick	Thanks for the reply. Here's a schematic of the amp BTW: http://www1.korksoft.com/~schem/voxamps/ac50.pdf I should mention that I don't think the original R's were carbon comps. They're old ceramics. At least as far as I can tell. They're all white...I could be wrong, who knows. Yeah, I think i'll start by replacing the 716's and putting the Mustard caps back in there. He also put a bunch of ceramic discs in for the pf range stuff. So I want the old ones back in there (silver mics, i'm assuming) The R's in the B+ line are 22K's, and the 2 I have are reading right on (22.14K, and 22.45K, respectively), surprisingly. I never measured the pre-amp voltages before, as I saw no need to...the amp sounded GREAT. So i'm not sure it would matter if I put them back in, or left them out. The thing that's confusing me the most are the 100K's that I have. I've got 7 of them. 5 were plate R's, and 2 were the slope R's for the tone stacks. I've got one that's reading about 99K, and the others are drifted up a bit. Anywhere from 109K to 114K. I'm sure that's part of what made the amp sound like it did, but would you recommend against putting something that far drifted back in the amp, just to regain some of the vintage component mojo? Is it safe to assume that the R's drifted farther from their value are the ones that were further up the B+? That 114K, for instance, was probably one of my PI plates, huh? And the 99K is probably one of my slope R's? It sounds very good. But it doesn't have the same tonal coloration that it did. It's much brighter and harsher, and it doesn't have the nice overdriven grind it had before. There's more white noise in the tone, something that wasn't there before, and a lot of the warmth isn't there anymore. It sounds good...but it sounds like a brand new amp, which it is. I mean, I had a '71 Marshall Superlead (GREAT sounding amp), which I sold because the AC-50 had a better clean, AND overdrive. You simply could not mess with the tone of this amp. It was fantastic! I want my '64 AC-50, which had been well cared for, and aged into the best sounding head I own...not a clone of the amp. I'm hoping I can piece this puzzle back together and get the amp sounding like it was. Thanks again for the help. -Nick

10/16/2004 2:49 PM

Nick

Thanks for the reply.

Here's a schematic of the amp BTW:

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

I should mention that I don't think the original R's were carbon comps. They're old ceramics. At least as far as I can tell. They're all white...I could be wrong, who knows.

Yeah, I think i'll start by replacing the 716's and putting the Mustard caps back in there. He also put a bunch of ceramic discs in for the pf range stuff. So I want the old ones back in there (silver mics, i'm assuming)

The R's in the B+ line are 22K's, and the 2 I have are reading right on (22.14K, and 22.45K, respectively), surprisingly. I never measured the pre-amp voltages before, as I saw no need to...the amp sounded GREAT. So i'm not sure it would matter if I put them back in, or left them out.

The thing that's confusing me the most are the 100K's that I have.

I've got 7 of them. 5 were plate R's, and 2 were the slope R's for the tone stacks.

I've got one that's reading about 99K, and the others are drifted up a bit. Anywhere from 109K to 114K. I'm sure that's part of what made the amp sound like it did, but would you recommend against putting something that far drifted back in the amp, just to regain some of the vintage component mojo?

Is it safe to assume that the R's drifted farther from their value are the ones that were further up the B+? That 114K, for instance, was probably one of my PI plates, huh? And the 99K is probably one of my slope R's?

It sounds very good. But it doesn't have the same tonal coloration that it did. It's much brighter and harsher, and it doesn't have the nice overdriven grind it had before. There's more white noise in the tone, something that wasn't there before, and a lot of the warmth isn't there anymore. It sounds good...but it sounds like a brand new amp, which it is.

I mean, I had a '71 Marshall Superlead (GREAT sounding amp), which I sold because the AC-50 had a better clean, AND overdrive. You simply could not mess with the tone of this amp. It was fantastic!

I want my '64 AC-50, which had been well cared for, and aged into the best sounding head I own...not a clone of the amp. I'm hoping I can piece this puzzle back together and get the amp sounding like it was.

Thanks again for the help.

-Nick

10/16/2004 3:27 PM
loudthud	Your're probably making a good guess on the resistors. Maybe you should try just replacing the caps first to evaluate their sound in isolation. The 22K's aren't far enough off to worry about, it's just something I wouldn't forget to look at when trying to get the mojo back.

10/16/2004 3:53 PM
Nick	Thanks...yeah, I definitely want to do it in stages. If not to evaluate the sonic differences, to be on the safe side with what i'm putting back in there. The amp DID fall off a 4x12 cab onto a concrete floor. I guess i'll put all the caps back in, then the R's directly in the signal path (slope R's and such). Then i'd do one gain stage at a time, replacing the plate/cathode R's. Thanks again for the help... -Nick

10/16/2004 3:53 PM

Nick

Thanks...yeah, I definitely want to do it in stages. If not to evaluate the sonic differences, to be on the safe side with what i'm putting back in there.

The amp DID fall off a 4x12 cab onto a concrete floor.

I guess i'll put all the caps back in, then the R's directly in the signal path (slope R's and such). Then i'd do one gain stage at a time, replacing the plate/cathode R's.

Thanks again for the help...

-Nick

10/16/2004 5:27 PM
Satamax	Hi nick, just 100K resistor wise, you have 49 different solutions! Good luck! Best regards. Max.

10/17/2004 2:26 PM
Ian Anderson	I would start with replacing those Orange Drops, they do sound different than Mustards. I like those litte yellow Vishay polyester caps, they always sound great and Mustard'esque™. There are also Sozo caps which some people swear by. ... Ian

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