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Thread: Vox Escort Lead 50 repair/tone problems

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    Vox Escort Lead 50 repair/tone problems

    Hi there,

    I have a VOX escort Lead 50, one of the companies first transistor amps.

    After testing loads of the electrolytics were bad, so I went ahead and replaced them all, as it was cheap and easy. The amp is now super silent but the tone is odd (it was odd before the re cap), and I can't believe it was like it from the factory, but maybe these are just crappy odd sounding amps.. if so I'd love someone who knows to tell me its just a bit of a junky amp!

    Failing that, here are the problems - The low input is super bassy, unusable unless you put the eq to full treble ish. The high input is super bright, crazy bright really. The input resistors are 100k rather than the 68k on the schematic, though I ONLY can find the schematic for the escort Bass 50 (not the lead). My amp looks real similar in general to the bass version, so I'm wondering if those 100k resistors should really be 68k (as per the bass version's schematic, and the usual value).

    Additionally, there is a RC4136DB IC providing quad amplification for bits of the eq circuit and on the input, and some of the transistors are 3055's which I'm led to believe are quite low performing...

    I'm looking for either test advice, or experience with this amp those with or similar components as with where to start. All the transistors and the IC amp are pretty cheap, I could re-do them all for Ģ30 ish, though I'm open to anyone saying that's silly and not that likely to help. I'm a bit stuck though, I know where I'm at with valves more or less but this is a bit of a puzzle. Especially as the only schematic I can find has a couple of different controls (no vol. / master vol. on the schematic, the master vol is replaced with a 'harmonics' knob, and the related circuitry is not in my amp as far as I can see...

    Thanks for any help offered!

    EDIT- I have had the PCB off the chassis and fixed any VISIBLE dry joints, but cool to go in again and do the whole thing if people think its most likely that, similarily I haven't rigorously tested all the resistors for drifting etc.. cool to do that if that's more likely than the transistors.
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    Last edited by OwenM; 06-12-2019 at 12:39 AM.

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    I think upgrading IC's or transistors would be futile. You might get some very small gains, but they would not account for the problems you are having. I would only replace any that you found to be defective.
    And don't change parts to match a schematic you know is not the same model.
    First off I would try to find the correct schematic. Try posting in the 'schematic requests' section.

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    Thanks, I'll give it a go, in the past I've searched high and low and only came up with the Lead 30 and Bass 50 models but it must exist somewhere!

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    Assuming your amp uses the same input circuit as shown in the schematic, this could be an explanation for the bad sound. The opamp is wired as an inverting amp. This makes the gain depend on source impedance. In result it will load the PUs with very low 34k when plugged into "Normal" (dull sound) and will overemphasize the resonant peak when using the "Bright" input.
    Can you try a buffer between the guitar and the amp?

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    Thanks, great, that gives me something to investigate. The input resistors on my amp are actually 100k rather than 68k, might that have been a design feature to load the PU's with a higher 50k resistance?

    Also, I am very probably misunderstanding, but from what I read on the schematic the tip of the 'bright' input jack socket is not grounded when a jack is inserted in the 'normal' socket, it looks like the second (bright) 68k resistor circuit would just float doing nothing. Those dashed lines going along the jacks, does that mean it's all grounded, tip to sleeve, when nothing is inserted? If not, then wouldn't the resistance the PU's see be just the 'normal' channel 68k resistor, as the second 68k resistor would essentially be out of circuit? If the whole socket is grounded along the dashed line when nothing plugged in I can see why we'd end up with a 34k resistance on the input (with the two 68k resistors being seen by the input signal, the second going to ground). Sorry if this is basic, the way I read the schematic was the lower (bright) 68k resistor would only do something if the jack was in the 'bright' socket...


    EDIT - I'll try a buffer tomorrow!

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    Yup, using a buffer it sounds great, for what it is. And you can clearly hear, shifting from buffered to not-buffered on the 'High' (mines labelled high and low rather than bright and normal) input you can clearly hear it getting a lot more resonant and nasal without the buffer. The low is still not that defined and a bit too much bass, but sounds pretty great on single coils or with a bit of eq, so I think its much more believable that that's how it left the factory, or closer at least...

    So now gotta tweak those input resistors...

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    Also, I am very probably misunderstanding, but from what I read on the schematic the tip of the 'bright' input jack socket is not grounded when a jack is inserted in the 'normal' socket, it looks like the second (bright) 68k resistor circuit would just float doing nothing. Those dashed lines going along the jacks, does that mean it's all grounded, tip to sleeve, when nothing is inserted? If not, then wouldn't the resistance the PU's see be just the 'normal' channel 68k resistor, as the second 68k resistor would essentially be out of circuit? If the whole socket is grounded along the dashed line when nothing plugged in I can see why we'd end up with a 34k resistance on the input (with the two 68k resistors being seen by the input signal, the second going to ground). Sorry if this is basic, the way I read the schematic was the lower (bright) 68k resistor would only do something if the jack was in the 'bright' socket...

    Don't know what the dashed lines mean, maybe some shielding, outline or mechanical thing. And the input switching arrangement as shown isn't quite clear to me either, doesn't seem to make sense. I would have expected R2 to be connected to ground when using the Normal input. You can find out using an Ohmmeter. But maybe you are right and it's 68k not 34k. But even 68k or 100k are too low. Most guitars sound best when connected to an input >500k.

    Don't expect much difference with 100k instead of 68k except a little less gain. You will need the buffer anyway.

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    Last edited by Helmholtz; 06-13-2019 at 02:36 PM.
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    Quote Originally Posted by Helmholtz View Post
    Don't know what the dashed lines....
    Ok, great thanks a lot, I'll get some 1 meg's out and start there I guess, they re really easily accessible and I can switch them with the amp assembled so should be easy enough to try a few values and see what works!

    Thanks a tonne for your help.

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    I'll get some 1 meg's out and start there I guess, they re really easily accessible and I can switch them with the amp assembled so should be easy enough to try a few values and see what works!
    Using something like 1M instead of 68k will result in much less gain and increased noise.
    An inverting opamp is just no good solution for a guitar input stage.

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    Last edited by Helmholtz; 06-13-2019 at 02:49 PM.
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    It seems like the designers at Vox didn't understand op-amps in those early days. Look at the Supertwin schematic:
    https://music-electronics-forum.com/...1&d=1560433139
    R1 is 220K but the input impedance is set by R2 = 22K.

    On the Escort Bass 50 Normal input is 68K, Bright is 136K
    On a regular Fender amp 1 is 1M, 2 is 136K
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    and will overemphasize the resonant peak when using the "Bright" input.
    I apologize, my interpretation above was wrong. As the gain of the Inverting opamp decreases with higher source impedance, the high PU impedance around its resonance will result in a strong mid scoop instead.

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    R1 is 220K but the input impedance is set by R2 = 22K.
    If C1 in the Supertwin circuit is actually 10ĩ (!??), the AC input impedance for the Brilliant channel would be close to zero.

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    Quote Originally Posted by Helmholtz View Post
    If C1 in the Supertwin circuit is actually 10ĩ (!??), the AC input impedance for the Brilliant channel would be close to zero.
    I didn't notice that. Maybe they meant 10PF.

    Here is a simple mod to change input to 1 Meg.
    https://music-electronics-forum.com/...1&d=1560435613
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    Thanks for the help and useful discussion, glad to know it is just a bit of a 'mad old design' thing, makes me less worried about fiddling around with it trying to get it better, too. They're not worth much at all, even mint!

    Thanks for the mod dmeek, I'll give it a go and post back the results!

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    One minor problem with the mod is that the volume will not go to zero. With the Gain control at minimum the op-amp will be at unity or gain of 1. Not sure how quiet that will be in real life.
    Just keep it at 11 and you'll be OK.

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    Ok, digging around my amp I can't see a op amp near the input (I can't see one at all, actually), and now I bring up the escort lead 30 schematic I'm wondering if the preamp is closer to what's in that... My input resistors are certainly not 470k though, they are 100k on both high and low inputs.. however they DO both go from tip to ground of their respective jack sockets as per the Escort Lead 30 schematic. It looks like I have a 10n cap on the Low and 470n cap on the High rather than both 100n like on the escort lead 30 schem.

    Still, its 'High and low' on mine, not 'normal and bright', so I'm guessing the Lead 30 design gets the sonic difference in channels later with those two inputs going to the base of 2 different transistors with their own respective 'normal and bright' pots. Mine just has a 'master volume' and 'volume' (which works like 'gain') which effect both channels equally, so I'm guessing those two different caps on mine are what gives the difference in sound between the high and low channels. Both those caps are chicklets, so TMU not likely to be bad but I certainly could change them...

    Worth trying 470k resistors on the inputs? Or would 100k with this design be fine and so I should look further into the signal path?

    Dammit I wish I could find a schematic for this amp!

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    Last edited by OwenM; 06-14-2019 at 12:06 AM.

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    470K would probably be better but it depends on the rest of the input circuit. Can you post some close-up photos of the circuit board , both component side and foil side?

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    Anyway, if the sound improves with the buffer, the input impedance of the amp is suboptimal. What's wrong with using the buffer? You might even build in a simple input buffer.

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    Quote Originally Posted by dmeek View Post
    470K would probably be better but it depends on the rest of the input circuit. Can you post some close-up photos of the circuit board , both component side and foil side?
    I will do, I'll get the PCB out tomorrow and see where it's going after the input!

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    Quote Originally Posted by Helmholtz View Post
    Anyway, if the sound improves with the buffer, the input impedance of the amp is suboptimal. What's wrong with using the buffer? You might even build in a simple input buffer.

    The buffer I used was feeding it from an aux output of a desk (chastise me if that isn't the same thing, I thought as it was a way of isolating the PU from the circuit it would work like a buffer). Definitely into the idea of building a buffer into the amp though!

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    Here's some shots, the wires and 100k resistor on the 'low' pot are off on these photos.The 'high' tip goes to the 10n chicklet and the 'low' tip goes to the 470n chicklet.

    The 'high' channel seems to go through the 10n chicklet followed by the .022 green cap, the 'low' through a 470n chicklet then a 22k (I think?) resistor... then they meet...

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    Can you take one more photo of the foil side? Its too overexposed on the left. Also a shot of the whole component side.

    I see the 4136 quad op-amp there which suggests it's more like the Bass 50


    Here's the input section. It's getting late here, I'll work up a mod tomorrow,

    https://music-electronics-forum.com/...1&d=1560479296
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    Last edited by dmeek; 06-14-2019 at 03:29 AM.

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    I am assuming that the circuit board picture was before you re-flowed any questionable solder joints.

    Edit: it's sometimes tough to tell in photos but some of those joints look like they need a touch up.

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    Last edited by DrGonz78; 06-14-2019 at 10:19 AM.

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    Quote Originally Posted by OwenM View Post
    The buffer I used was feeding it from an aux output of a desk (chastise me if that isn't the same thing, I thought as it was a way of isolating the PU from the circuit it would work like a buffer). Definitely into the idea of building a buffer into the amp though!
    You can use any pedal with a buffered bypass, like most Ibanez and Boss effects.

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    Vox really LOVED inverting input Op Amps, itīs all the SuprTwin uses: input preamp - reverb drive - reverb recovery - Baxandall tone control



    by the same token, hereīs the AC30SS:



    either itīs an obsession or the Designer read only up to chapter 2 on the Op Amp Design book and didnīt go any further.

    An I am only half kidding here.

    Problem is that inverting Op Amps measure beautifully and in modern times they also simulate beautifully ... if driven by a 600 ohm output generator that is.
    And they suck when driven by ... oh .... a pesky passive electric Guitar or Bass

    But who would plug such things there?

    Remember VOX went through **8** owners, probably about same amount of Factories all over the place (including USA based Vox Thomas, Marshall subcontracts and Korg Japan ) and only revered ones are the original ones from the Dick Denney era: AC4/15/30 .... but the others? ... not so much.

    Compared to these poor British design ones, the USA Vox Thomas designs are WAY more resonable.

    Including higher impedance inputs and the power amp preclipper.

    Just check input preamps in, for example, US VOX Pathfinder V1011

    https://music-electronics-forum.com/...1&d=1560433139

    168k or 236k into an emitter follower transistor, not "tube perfect 470k or 1M" but WAY more reasonable than 22k , or even worse 22k in parallel with a "bright" capacitor which lowers input impedance even more and can only be driven from a bench/lab generator.

    Iīm certain that besides the original ones which were sold on quality, sound and stunning looks, to you-know-who .... most others were sold on MOJO - stunning looks - Guitar Gods shoulder rubbing.

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    Quote Originally Posted by dmeek View Post
    Can you take one more photo of the foil side? Its too overexposed on the left. Also a shot of the whole component side...
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    Here you go, so much easier in daylight The ? pot is a 1 Meg Log, the EQ pots are 47k Lin and the master volume on the power side of the board is a 47k log.

    Thanks so much for your time! Its really educational for me going through this process as well!

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    Quote Originally Posted by DrGonz78 View Post
    I am assuming that the circuit board picture was before you re-flowed any questionable solder joints.

    Edit: it's sometimes tough to tell in photos but some of those joints look like they need a touch up.
    Yeah I have given it a once over, they aren't as bad as they look in that darker photo!

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    Here's the mod. First remove all the yellow wires and there's one trace to cut under the IC. If you don't have 68Ks you can use the 100Ks-not much difference.

    https://music-electronics-forum.com/...1&d=1560521824
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    Is the cabinet cool looking?

    I'd probably gut it and turn it into a tube amp, doesn't help, just sayin'

    That's what I did with an early 70's SS Marshall 2x12 combo.
    Now it's a 1987/2204 flame thrower.



    EDIT: WOW! They are great looking, it would make a perfect AC15!

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    Quote Originally Posted by dmeek View Post
    Here's the mod....
    Thanks so much, I'm so grateful for your time! I need to buy a couple of bits but I'll do it and report back how I get on.

    Just double checking, the dot next to the old .01 and the .022 / 47k caps and resistor, is that left unconnected? I mean, it looks like it is from the sketch, I'm just idiot-checking myself that its not meant to go to ground as well (not that I can see a reason why it would!).

    I know almost anything will work, but any preference for what type the 0.01uf cap is?

    So this is basically taking that first op-amp from the chip and putting it into a non-inverting config.. am I right in saying the only purpose of that op amp is making the master vol work? As the signal is continuing down that 1 meg resistor and the rest of that first op-amp stage is concerned with how much of the signal goes to ground? Don't worry about a thorough breakdown though, I have to get my head around op amps and their uses yet anyway! As a hobbyist I've only really sorted out tube amps till now and the odd transistor that turns up on a newer valve amp board is about as far as I can say I understand with solid state

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    Last edited by OwenM; 06-15-2019 at 01:00 AM.

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    Quote Originally Posted by drewl View Post
    Is the cabinet cool looking?

    EDIT: WOW! They are great looking, it would make a perfect AC15!
    Hehe, well, you may say that It was very much a sad garage find when I got it, someone had put a hole through the speaker and the amp was noisy as I've ever heard. The grill cloth was basically tatters so I've taken it as a learning amp. I've went over it all and it now works well, albeit with the crappy sound from the factory design. But along the way, rather than get a Vox grill cloth I just used some leftover parrot tropical fabric to practice my upholstery on , so depending on your stance its now either a unique, fun oddity or an absolute abomination!!

    Actually, making the cabinet and tolexing it to look like an old Vox is something I'm a lot better at, I have access to a full woodwork facility etc. so the value of the box is less for me..but building an amp to put in it is probs out of my ability right now, so I want to use this as a learner for the electronics side. But I love the idea, in future, of making a great little amp and re-appropriating old shells!

    The badge is long gone...

    BTW, does anybody know what the name of that gold edging strip is called for the purposes of purchase? Would like to get a new, nicer one, but any word combo I can think of for the name just gets a tonne of carpeting strips..
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    Quote Originally Posted by OwenM View Post

    BTW, does anybody know what the name of that gold edging strip is called for the purposes of purchase?
    Like this? Some terms you can search for anyway, https://www.tubeampdoctor.com/en/sho...X_AC30_size_43
    (gold fascia strip, vox)

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    Quote Originally Posted by OwenM View Post

    Just double checking, the dot next to the old .01 and the .022 / 47k caps and resistor, is that left unconnected?

    I know almost anything will work, but any preference for what type the 0.01uf cap is?
    That was the old bright input. Those components aren't needed anymore. For a future experiment you could connect a footswitch to short that terminal to ground. It would be some kind of treble boost.

    Or you can steal the .01 cap for the mod. Those yellow caps are good quality, at least from my experience.

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    Last edited by dmeek; 06-15-2019 at 03:20 AM.

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    Quote Originally Posted by dmeek View Post
    That was the old bright input. Those components aren't needed anymore...
    I got the bits and did the mod today, and it turned out really nicely! Its softened all the weirdness in the sound, its fairly flat on neutral eq and you've got a fair bit of beef and a nice bit of extra treble if needed, the mid control is really useable. Whatever funkiness was going on in the impedance mismatch was meaning the eq circuits were pushing and pulling around some really odd shaped signal by the time it got to them... Now it just works exactly as you'd expect, nothing to write home about, but certainly a nice little amp to have around!

    Thanks so much for your help Dmeek and everyone else, been a good learning project for me!

    Just one thing I'm not too sure of, and would like to ask, what's the purpose of the 0.01uf cap added? Does it form part of a radio frequency filter circuit? I had to fiddle around with the position of that one for the least hum and ended up with it siliconed to the underside of the board, though I may go in with a small bit of universal PCB and make a slightly more robust board if I find the time.

    I assume the 68k resistors at the input jacks are best kept as close as possible to those jacks though, am I right?

    All the best

    Owen

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    Last edited by OwenM; 06-19-2019 at 11:31 PM.

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    There is DC voltage on Pin 2 (and pin 1 and 3) of the op-amp. I think about 15 Volts. The .01 cap isolates the DC from the input, but lets the audio pass through.
    Without the cap there would be 15 Volts on the input jacks which would get shorted out by the guitar pickup. That would throw the amp into a non-working state.

    The connection from the input jack to the op-amp might have to be shielded to reduce hum because the impedance is now very high at 1Meg.
    Keeping the wires as short as possible may be good enough. If not, one trick is to wrap the input jacks, wire and .01 cap with masking tape,
    then wrap tinfoil over that and ground the tinfoil.

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    Last edited by dmeek; 06-19-2019 at 11:50 PM.

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