Old amp sounds "tired". What do you do?

[cthulhu]

I have an old ss Vox amp. A '67 JMI Vox Defiant. A really cool beastie that sounds like a lot of old recordings. The thing is, it sounds "tired". It doesn't have the hair, crispness or edge of the origional. For example, listen to "Hey Bulldog"-Beatles. It's not the speakers, because I use weber "blues" and the origionals used vox blues.

So... AFAIK, transistors don't get "tired" or old like tubes. All the supply voltages and transistor biasing holds up, and the caps cosmetically look good. But they are old, origional equipment.

Should I just go thru and replace all the caps? I already had to change the inductor (old Midrange boost pre-wah effect). Will new caps resurrect it? I am probably going to build a replacement distortion channel board using the same type components as the origional and drop it in and see if it makes a difference. Could it be the power transistors? I don't know here. For transistors I was always told either it works or it doesn't.

anyone?
-Darren

[Ray Ivers]

Darren,

I've worked on one Defiant, one Supreme, and two Conquerors (my fave of the 3, FWIW), and found a number of bad caps IIRC, along with one bad transistor (do yours have the different colored drops of paint?) and a bad "reverb drive transducer" (AKA a ceramic phono cartridge :)).

You might want to measure for DC leakage across the caps and replace as needed, but IMO a shotgun replacement of all electrolytic caps in general - and a replacement of e'lytic coupling caps with new bipolar units (from Mouser or wherever) in particular - might make your amp come back to life, at least a bit. No harm in keeping the old ones in a plastic bag fastened inside the amp chassis just in case; I've had plenty of customers over the years insist on it, so I do it as a matter of course now.

Since the transistors apparently all work, I'd try the caps first before replacing any of them. Regarding transistor life span and failure modes... R.G. Keen's Geofex web site has a great paper entitled 'When Good Opamps Go Bad' (it covers transistors as well; link below) that I read for the first time yesterday:

http://www.geofex.com/circuits/when_good_opamps_go_bad.htm

Ray

[cthulhu]

Darren,

I've worked on one Defiant, one Supreme, and two Conquerors (my fave of the 3, FWIW), and found a number of bad caps IIRC, along with one bad transistor (do yours have the different colored drops of paint?) and a bad "reverb drive transducer" (AKA a ceramic phono cartridge :)).

You might want to measure for DC leakage across the caps and replace as needed, but IMO a shotgun replacement of all electrolytic caps in general - and a replacement of e'lytic coupling caps with new bipolar units (from Mouser or wherever) in particular - might make your amp come back to life, at least a bit. No harm in keeping the old ones in a plastic bag fastened inside the amp chassis just in case; I've had plenty of customers over the years insist on it, so I do it as a matter of course now.

Ray

Thanks for the input, Ray. Yes, there are the paint dots all over. How did you find the bad transistor? Did the amp still work?

My amp also has the same problem with the reverb. How did you fix that? Those use old sonotone mono phono cartridges which are impossible to find these days. Did you fix that?

BTW: These early ss Amps are pretty easy to work on, considering the stuff that's out there now. I don't mind at all working on ss when it uses discrete components. And yes, I keep all the old parts too.

And all 3 of those amp models should sound the same, no? The preamps are identical and the power sections, though different, should all be simply clean boosts to appropriate levels, correct? I'm assuming same speakers (though I know they didn't all come with the same speakers).
-Darren

[Ray Ivers]

Darren

Yes, there are the paint dots all over. How did you find the bad transistor? Did the amp still work?

Yes, the amp still worked - but IIRC there was a problem involving the MRB circuit; I guess in yours it was the inductor, in mine it was a leaky cap toasting a transistor, I think (this repair was some time ago - I believe I found the bad transistor using my 'scope and tracing the signal).

My amp also has the same problem with the reverb. How did you fix that? Those use old sonotone mono phono cartridges which are impossible to find these days. Did you fix that?

The amp's owner found a source for ceramic phono cartridges, and we just threw the one he found in there and it worked fine. I also increased the gain on the reverb return stage by (I believe) adding an emitter resistor bypass cap - it increased the reverb level all right, but it also showcased the 'springiness' of that single heavy-gauge spring too.

BTW: These early ss Amps are pretty easy to work on, considering the stuff that's out there now. I don't mind at all working on ss when it uses discrete components. And yes, I keep all the old parts too.

Yes, they weren't too bad, although to me all those old Voxes have a 'brittleness factor' that should be kept in mind. I remember that after lifting that Swiss-cheese circuit board a few times, several connecting wires snapped and had to be stripped back/resoldered, clipping the 'scope probe on several cap leads caused them to pull out of the cap bodies, etc.

And all 3 of those amp models should sound the same, no? The preamps are identical and the power sections, though different, should all be simply clean boosts to appropriate levels, correct? I'm assuming same speakers (though I know they didn't all come with the same speakers).

Yes and no... the Conqueror was the only one I really liked the sound of when dimed, but out of respect for the original output transistors I didn't really crank any of them for very long, just enough to see that they were OK. I love that transformer driver - IMO that's a big part of why I like these amps.

Ray

[cthulhu]

Darren

Yes and no... the Conqueror was the only one I really liked the sound of when dimed, but out of respect for the original output transistors I didn't really crank any of them for very long, just enough to see that they were OK. I love that transformer driver - IMO that's a big part of why I like these amps.

Ray


The only other thing of note is that the Normal channel is a LOT louder than the distortion channel on my amp. I'm not sure if this is correct or not.

Maybe what you didn't like about the others was the midax horn. I don't use one.

The 7120, now that's a nice amp too, rarer than hen's teeth.

[Ray Ivers]

Darren,

All the amps were played thru my Fender Bandmaster cab (giant ported 2-12) with JBL D120's - but I know what you mean about the horns, 'piercing' just doesn't describe it. :o

The distortion channel has all that clipping-transistor weirdness after the last gain stage, then goes thru the MRB - it's kind of amazing IMO that any signal gets thru at all... ;) If you were really motivated, you could always fiddle with the mixing resistors (R30 for reverb return, R34 for distortion, R36 for clean/vibrato - not that they're labeled or anything :)) and then compensate by raising the gain of that final preamp 'Mixer/Driver' section if desired.

The 4- and 7-series Beatles-at-Shea-Stadium amps! Believe it or not, I've worked on two 7120's and a 4120, along with a 760 and 730 (this guy LOVES these SS/hybrid Voxes!). The 120's seem to have hum/grounding issues - at least these 3 did - but were pretty cool amps nonetheless. The only choke-input guitar-amp power supplies I've ever worked on - and what a mother of a choke, too!

I think I'd better post this now while I'm still (relatively) young - all sorts of website-not-found/gateway-timeout/etc. stuff going on, only at this site.

Ray

[cthulhu]

Darren,

All the amps were played thru my Fender Bandmaster cab (giant ported 2-12) with JBL D120's - but I know what you mean about the horns, 'piercing' just doesn't describe it. :o

The distortion channel has all that clipping-transistor weirdness after the last gain stage, then goes thru the MRB - it's kind of amazing IMO that any signal gets thru at all... ;) If you were really motivated, you could always fiddle with the mixing resistors (R30 for reverb return, R34 for distortion, R36 for clean/vibrato - not that they're labeled or anything :)) and then compensate by raising the gain of that final preamp 'Mixer/Driver' section if desired.

The 4- and 7-series Beatles-at-Shea-Stadium amps! Believe it or not, I've worked on two 7120's and a 4120, along with a 760 and 730 (this guy LOVES these SS/hybrid Voxes!). The 120's seem to have hum/grounding issues - at least these 3 did - but were pretty cool amps nonetheless. The only choke-input guitar-amp power supplies I've ever worked on - and what a mother of a choke, too!

I think I'd better post this now while I'm still (relatively) young - all sorts of website-not-found/gateway-timeout/etc. stuff going on, only at this site.

Ray


Cheers Ray!

Great info. I can't believe you've worked on 7120s. I love those early _JMI_ British ss/hybrid Vox amps. They don't exist here in the states. I had to import my Defiant, VAT and all.:(

Thanks for the resistor info, I'll take a look at it! The beauty of these amps is that once you get the speakers working the volume doesn't change the tone. I think if all these amps were ss preamp and tube power section they would be fantastic. I'd love to try and marry the Conquerer/Defiant preamp section with an EL34 power section. I just don't know how to do it. transistors= high current low voltage and tubes the opposite. Use a transformer to interface ss and tube sections?

-Darren

[Ray Ivers]

Darren,

I'd love to try and marry the Conquerer/Defiant preamp section with an EL34 power section. I just don't know how to do it. transistors= high current low voltage and tubes the opposite. Use a transformer to interface ss and tube sections?

Sure! How about this - leave your Defiant bone-stock, but use it to drive the primary of a small custom-wound driver transformer - IMO you'd want Heyboer to wind you one with 16R primary resistance and impedance - with a CT step-up secondary for connection to the EL34 grids, CT to bias? With a 50W "driver" available, adequate grid drive shouldn't be a problem ;) - all you'd need would be a small PA/PS subchassis (possibly purchased "ready-made" on eBay in the form of an old tube P.A. amp or inexpensive tube guitar amp chassis) with PT, PS components, two EL34's, and an OT - and with that driver transformer it should be safe for use with just about any tube or SS guitar amp; you could even have Heyboer add additional taps so it would work with practically ANY amp.

I know - wacky idea (it isn't mine BTW, much as I wish it was - well, the custom DT is my own little twist, I guess) - but I wonder how many people out there are happy with the basic tone of their amp, they just wish it was louder? I can't recall ever seeing any posts on the subject... :D

Ray

[cthulhu]

Darren,



Sure! How about this - leave your Defiant bone-stock, but use it to drive the primary of a small custom-wound driver transformer - IMO you'd want Heyboer to wind you one with 16R primary resistance and impedance - with a CT step-up secondary for connection to the EL34 grids, CT to bias? With a 50W "driver" available, adequate grid drive shouldn't be a problem ;) - all you'd need would be a small PA/PS subchassis (possibly purchased "ready-made" on eBay in the form of an old tube P.A. amp or inexpensive tube guitar amp chassis) with PT, PS components, two EL34's, and an OT - and with that driver transformer it should be safe for use with just about any tube or SS guitar amp; you could even have Heyboer add additional taps so it would work with practically ANY amp.

I know - wacky idea (it isn't mine BTW, much as I wish it was - well, the custom DT is my own little twist, I guess) - but I wonder how many people out there are happy with the basic tone of their amp, they just wish it was louder? I can't recall ever seeing any posts on the subject... :D

Ray


Very interesting Ray. Is that similar to how Vox did it for the 7 series? I thought those used EL84s to drive the ouput section. Do you know where I can take a look at a schem of a 7120?

As far as the tone is concerned, I think the tone of the Conqueror/Defiant/Supreme series is fantastic, but it's the SS nature of the power section that gives the unpleasant piercing highs if one is not careful. I think using a tube power section would be a great sonic improvement with it's compression and more natural sounding distortion.

[Ray Ivers]

Darren,

The 4120/7120 have paraphase PI's, implemented with an EL84 and half an ECL86. An AC-balance control uses local NFB around the 2nd PI stage.

I just figured you'd want to keep your Defiant stock as it's quite valuable to some people. A tube output stage might physically fit into the chassis, but putting it in would be no picnic; IMO you'd be much better off driving a dummy load with the the amp and sending a line out to another tube PA.

Ray

[cthulhu]

Darren,

The 4120/7120 have paraphase PI's, implemented with an EL84 and half an ECL86. An AC-balance control uses local NFB around the 2nd PI stage.

I just figured you'd want to keep your Defiant stock as it's quite valuable to some people. A tube output stage might physically fit into the chassis, but putting it in would be no picnic; IMO you'd be much better off driving a dummy load with the the amp and sending a line out to another tube PA.

Ray

No worries, I wouldn't mod the Defiant. I'd make a new amp, using boards cloned from the Defiant. Then figure out how to do the tube PI and power section.

[drewl]

There's a great tutorial on fixing SS vox amps on ....Blue Guitar (I think).
They also recommend replacing all the caps in these amps for they are a big problem.
I've also had the coils go bad in the tone networks....

[andrewwyld]

If you were really motivated, you could always fiddle with the mixing resistors (R30 for reverb return, R34 for distortion, R36 for clean/vibrato - not that they're labeled or anything :)) and then compensate by raising the gain of that final preamp 'Mixer/Driver' section if desired.

I have just bought a Conqueror on eBay and I have the opposite problem—the normal channel is almost unusably quiet while the brilliant channel is lovely and loud. Would an analogous solution apply here?

Also, does anyone happen to have the footswitch pinout? I am going to have to build my own, I think. The schematic I have doesn't seem to mention the footswitch pinout, though it does tanalizingly mention a schematic for the power stage for something called the Virtuoso, an amp of which I have never heard ....

Thanks!

[Steve Conner]

I've not posted any new work in ages, but I got real interested in those old Vox/JMIs too and I'm currently working on a new hybrid topology inspired by them.

It's basically the same thing, but with the output stage optimized for modern silicon transistors, and the preamp and driver stage all tubes. So exactly the opposite of what Ray is suggesting :rolleyes:

I have a prototype running that makes 3w "barefoot" (the tube driver stage and transformer driving the speaker directly) and 30w with its transistor "boots" on. The idea is that you have the 3w setting for practice and recording, and use the 30w for gigging, or just any time you want more clean headroom.

The transistor finals do indeed bring a lot of extra high end, but I think I know what causes it and how to tame it.

[Princeton]

My princeton is sounding crackly. I just biased it and put in brand new tubes but it still has this crunch to it? any diagnosis?

[Enzo]

Hi, welcome to the forum.

May I suggest you start a fresh thread for your amp instead of tacking onto the end of this old one. More people will see it and so you will get better response.

[Princeton]

yeah, I'll do that.