Vibro Champ with minor issues

[Potato Cakes]

Hello, everyone,

In a previous thread I mentioned that I finished building a Vibro Champ and had the joy of troubleshooting what was a bad output transformer. Now it's done and it sounds really great. It does have a couple of small things that make my OCD flare up. One is the foot switch sometimes pop. It's infrequent, but does occasionally happen when disengaging the tremolo. I've gone through all of the things one might do to fix a switch that pops and even found a thread on another forum where the guy said he got it to completely go away by soldering the ground side of the switch to the chassis of the switch and then add a 47k resistor to the tremolo switch and heatsink it so it stays together. That part didn't make too much sense to me but I think this pertains to a two channel foot switch. (The ultimate Fender style footswitch - The Amp Garage). There is kind of an explanation at the bottom of the thread but it also didn't make too much sense what was the purpose of the 47k resistor. I don't know of another way to bleed DC to ground without altering the function of the tremolo itself unless I can strap something like a 1n4004 across the contacts. Not a big deal but a mystery to me nonetheless and I don't like mysteries.

The other thing is that there is a slight amount of distortion when using the tremolo. At the height of the amplitude wave there is audible breakup. Nothing too crazy but enough to want to investigate the cause. I tried pulling the cathode bypass cap on V2 of the tremolo circuit thinking the gain on that tube needed to be brought down but it did not fix the issue. I even tried different speakers as I saw that solved a similar distortion issue for someone else. It just sounds like the second gain stage of V1 is breaking up because the tremolo is hitting it with too much signal at the peaks. This happens even at lower intensity levels. I also tried turning down the bass to see if minimizing the low frequencies would help but it was to no effect. I did go through a fun tube swapping exercise for both of the 12AX7s.

This may but just the way the circuit is. I have a friend who as a Vibro Champ reissue (solid state rectifier and possible tremolo section as well) and his amp does not have this break up characteristic so he even noticed the slight break up on the tremolo of the amp I built. This may just be normal for an amp built with the original circuit, but I can't stop thinking that there must be a way to fix this without completely changing the schematic.

Thanks!

Paul

 

[AnalogPackrat]

VCs don't have a lot of headroom. What guitar/pickup are you using? What are your rough amp control settings? How does the amp sound when the trem is off (not oscillating) while you vary the intensity manually?

On my original 70s models the intensity control shifts the operating point of the second triode and the amp sounds thin with the trem off. I usually set the intensity to zero unless I'm planning to use the tremolo.

 

[Matador]

That part didn't make too much sense to me but I think this pertains to a two channel foot switch. (The ultimate Fender style footswitch - The Amp Garage). There is kind of an explanation at the bottom of the thread but it also didn't make too much sense what was the purpose of the 47k resistor.

If memory serves, the oscillator is defeated by lifting a ground reference in one of the RC delay networks, which stops the oscillation. The3 vibrato effect is activated by re-grounding that node, which is done by the switch.

The later re-issues have added a 10K resistor in series with the switch (which is likely what the 47K referenced above was doing), as well as a 4.7uF cap on the switched node to ground. Some vibrato networks are biased with a negative voltage from the bias tap (-50V or close), and some are referenced from the cathode of the driver tube, so you need to pay attention to the polarity of the cap.

The RC filter on the switch node should slow it down enough to stop the loud pop if it is caused by the abrupt changing of the voltage on that switched node.

 

[AnalogPackrat]

Schematic for reference. No negative bias node in these little SE amps.

 

[Potato Cakes]

VCs don't have a lot of headroom. What guitar/pickup are you using? What are your rough amp control settings? How does the amp sound when the trem is off (not oscillating) while you vary the intensity manually?

On my original 70s models the intensity control shifts the operating point of the second triode and the amp sounds thin with the trem off. I usually set the intensity to zero unless I'm planning to use the tremolo.

This happens with humbuckers or single coils and regardless of the amp settings. There is no change in the sound of the amp with the tremolo disengages from the foot switch. The amp itself sounds much bigger than my friend's reissue.

Thanks!

Paul

 

[Potato Cakes]

If memory serves, the oscillator is defeated by lifting a ground reference in one of the RC delay networks, which stops the oscillation. The3 vibrato effect is activated by re-grounding that node, which is done by the switch.

The later re-issues have added a 10K resistor in series with the switch (which is likely what the 47K referenced above was doing), as well as a 4.7uF cap on the switched node to ground. Some vibrato networks are biased with a negative voltage from the bias tap (-50V or close), and some are referenced from the cathode of the driver tube, so you need to pay attention to the polarity of the cap.

The RC filter on the switch node should slow it down enough to stop the loud pop if it is caused by the abrupt changing of the voltage on that switched node.

I did think about the 47k being in series with the switched node but I thought with the resistance in line it wouldn't make proper contact to ground thus the tremolo would still be active. Now that I say that out loud in my head the shrink tubing part makes more sense. I may try to through that in for one more test. However I am getting to the point that I may break something as I already had torn a fair amount out of it just to find the original issue with it having a bad transformer.

As for distortion in the tremolo, it is what it is unless I find a solution that is proven and is known to fix this very issue. Again, I'm afraid I'm going to pass the point of the soldering tolerance of some of these components.

Thanks!

Paul

 

[CJ]

If you can post a sound bite,

 

[Matador]

The other thing is that there is a slight amount of distortion when using the tremolo. At the height of the amplitude wave there is audible breakup. Nothing too crazy but enough to want to investigate the cause. I tried pulling the cathode bypass cap on V2 of the tremolo circuit thinking the gain on that tube needed to be brought down but it did not fix the issue. I even tried different speakers as I saw that solved a similar distortion issue for someone else. It just sounds like the second gain stage of V1 is breaking up because the tremolo is hitting it with too much signal at the peaks. This happens even at lower intensity levels. I also tried turning down the bass to see if minimizing the low frequencies would help but it was to no effect. I did go through a fun tube swapping exercise for both of the 12AX7s.

If the intensity pot is at zero, the distortion disappears?

This is the AA764 circuit?

 

[dmp]

What are the voltages on the V2 cathode (1.5v on schematic) and the vibrato tube / intensity pot?
Seems the bias of V2 is thrown off.

 

[AnalogPackrat]

I'd crack one of mine open to check, but I have house guests and my bench is occupied by fuzz face experimentation clutter. Maybe later this week.

To be clear (again), on this amp the intensity control affects the second amp stage (driver triode) bias point even when the tremolo is "off." Off just means not oscillating. The amp can sound like crap with the intensity above 3 or 4 and trem "off." If you set the intensity to zero the cathode of the second audio stage triode should be around 1.5-2V.

 

[Potato Cakes]

If the intensity pot is at zero, the distortion disappears?

This is the AA764 circuit?

This is the AA764 circuit. When there is no tremolo there is no distortion artifact. It only happens when there is tremolo and only at the peak of each oscillation.

Also, I delivered the amp off to the client. He loves it and as soon as he added effects from his pedal board it was no longer an issue. Point is that I do not have the amp anymore to take measurements. I know I was getting the correct voltages at the plates but neglected to check at the cathode.

What I'm hearing could very well be how the tremolo in this amp sounds. All my other references are solid state versions of this effect, which I'm sure is how it is in the '68 Champ reissue my friend has that was used for comparison as it was perfectly clean.

Also, added the 47k resistor inline with the switch node and only now get a very faint mechanical noise some of the time with the foot switch.

Talked to the guy about possibly adding an external speaker connection for the 4 ohm tap on the amp. If I do I will try to get a recording of the noise then.

The bias being off on V2 seems to be a reasonable cause of the tremolo distortion. If I do get a chance to get it back on my bench I'll be sure to check this measurement.

Thanks!

Paul

 

[Matador]

The bias being off on V2 seems to be a reasonable cause of the tremolo distortion. If I do get a chance to get it back on my bench I'll be sure to check this measurement.

It's very likely the bypassed 1.5K cathode resistor may not be optimal for the operating point of that specific tube. But if the customer isn't complaining, then by definition it's working correctly.

 

[Potato Cakes]

I did put in a switch that takes the 10uF/25V bypass cap out of circuit with reduces the overall gain and bass response, but even with the lower gain it is still there.

The customer is definitely not complaining, but I am wondering now if it does need the 4 ohm 10" Jensen speaker that this circuit originally used. Maybe next time.

Thanks!

Paul

 

[AnalogPackrat]

The original Vibro Champ speaker is a 4ohm 8" small magnet alnico unit. And Matador was referring the the bias resistor (1.5k) perhaps not being optimal. I think the effect you describe may just be a consequence of the design. Shifting the bias point up and down by several volts puts the second triode in non-optimal conditions during the extremes (to the point of passing no signal at one extreme).

When my house guests have left I'll try my two units out. I've never noticed the distortion you mention, but then I'm used to the dirty and imperfect "practice amp" tone the VC makes at my usual volume settings of 5-8 and I don't often use the tremolo (shame on me).

 

[Potato Cakes]

Very interested to see if you find something similar when you do get a chance to do some tests. I've never owned a Vibro Champ or any of the other Champ iterations before so I don't have any reference of what to expect. Overall I did like the one I built as there was a surprising wide palette of sounds to be had depending on the amplitude and frequency of the guitar settings being used.

I did find someone's mod (I made the two red line corrections) that incorporates a 6L6 output tube which I've attached. If the client decides he wants more volume I will offer to do this. I may just build another Vibro Champ myself with this schematic even though I said I probably just stick to push pull designs.

Thanks!

Paul

 

[CJ]

i got this guy for Christmas back in 64, what kind of voltages do you need?

 

[Potato Cakes]

Oh, very nice!

I personally don't need any voltages at this point. If anything maybe the voltages at the cathodes. The plate voltages I was getting are close to what is shown on the schematic.

Also, I sent you that bad transformer today. Please let us know what you find.

Thanks!

Paul

 

[AnalogPackrat]

Very interested to see if you find something similar when you do get a chance to do some tests. I've never owned a Vibro Champ or any of the other Champ iterations before so I don't have any reference of what to expect. Overall I did like the one I built as there was a surprising wide palette of sounds to be had depending on the amplitude and frequency of the guitar settings being used.

No distortion noted when using tremolo on my main VC. I'd forgotten how nice the bias varying type sounds. Maybe after I get my fuzz face sorted I'll pull the chassis and check other stuff. I believe I have some notes somewhere from when I recapped this one. I also did the slow mod to the oscillator and maybe tweaked a few other things.

My second VC is unmolested except for (maybe) a recap. Also no distortion present when using the tremolo. Seems like something must be off on yours. Either the oscillator or around the cathode connection to the driver triode.

I did find someone's mod (I made the two red line corrections) that incorporates a 6L6 output tube which I've attached. If the client decides he wants more volume I will offer to do this. I may just build another Vibro Champ myself with this schematic even though I said I probably just stick to push pull designs.

Thanks!

Paul

 

[Martan]

Forgive me if I missed it, but your issues exist without the foot switch plugged in?

 

[CJ]

Check the polarity of your bypass cap on that second stage preamp, if reversed it could through your bias off,