Announcement

Collapse
No announcement yet.

Vox Escort Lead 50 repair/tone problems

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • 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 £30 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.
    Attached Files
    Last edited by OwenM; 06-11-2019, 11:39 PM.

  • #2
    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.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    Comment


    • #3
      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!

      Comment


      • #4
        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?
        - Own Opinions Only -

        Comment


        • #5
          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!

          Comment


          • #6
            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...

            Comment


            • #7
              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.
              Last edited by Helmholtz; 06-13-2019, 01:36 PM.
              - Own Opinions Only -

              Comment


              • #8
                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.

                Comment


                • #9
                  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.
                  Last edited by Helmholtz; 06-13-2019, 01:49 PM.
                  - Own Opinions Only -

                  Comment


                  • #10
                    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
                    Attached Files

                    Comment


                    • #11
                      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.
                      - Own Opinions Only -

                      Comment


                      • #12
                        R1 is 220K but the input impedance is set by R2 = 22K.
                        If C1 in the Supertwin circuit is actually 10µ (!??), the AC input impedance for the Brilliant channel would be close to zero.
                        - Own Opinions Only -

                        Comment


                        • #13
                          Originally posted by Helmholtz View Post
                          If C1 in the Supertwin circuit is actually 10µ (!??), 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
                          Attached Files

                          Comment


                          • #14
                            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!

                            Comment


                            • #15
                              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.

                              Comment

                              Working...
                              X