Help Request: Vibro Champ Voltage, Bias and Tremolo Fixes

Check all the caps and resistors in the trem circuit. The ceramic discs can go bad. One resistor on mine came apart. You have to lift one side to measure. You didn't spray deoxit in the trem jack? The guy who bought my 68 did that, turned it into a 14mohm resistor. Took an EE to sort that.

The trem on these is quite shimmery and not strong. Usually needs to be on 6 to be really noticeable.

I used a 110 volt primary step down on my 68 and 78 VCs and the plate voltage and B+ at the first cap can junction were 20% high.

If you drop bias a lot of the tone falls off IMO.

Your bypass caps are good?

 

^^^ this for sure. Hang in, you'll get this.

I know you have pics in your build thread, lots if I recall. But it helps the sharp-eyed pros here to put a few inline in this thread too. Just a few specific shots framed to show max detail. I'm thinking clear shots of the trem jack, the trem array on the board, and the wiring to the pots. Since the pot leads are hard to see, I'd do as much DMM continuity/resistance/ground testing as possible. Bad solder joint? Bad pot? Bad component? Bad jack? Etc...

Specifics I wonder about:

Note what @Dacious says about cool bias. Are you saying your bias is 75%? Most folks want at or even above 100% in these cathode-biased amps. I'm not sure it'll affect your trem, but it's important. And I don't always see great results when folks increase their cathode resistance a great deal. Did you do this to get bias down??

 

Oh, and regarding your target voltages, it's important to look at the voltages on both the original VibroChamp schematic:



and the later Bronco AB764, which in fact was identical to many (? most) VibroChamps.



Which Fender are ya gonna believe: B+ 355, or B+ 410? Yes, the transformer changed (sorry, I used to know when) but the 410 B+ was notoriously accurate for many years. There's a lot I don't know, but I'm not sure the 355 was ever accurate, even with the 125P1B PTs.

So. Note *both amps have a 470 bias resistor.* So your 405 B+ isn't quite a bit too high -- it may be almost perfectly vintage. Given the PT you're using, it's what you'd expect. So one more argument to run a 470 bias resistor.

Now, full disclosure -- my 1967 VC ran the notorious ≥ 400 B+ on modern wall voltage, and sounded a bit too bright, clean, hard-edged, so I put a dropping / sag resistor between the rectifier and the reservoir. But I'm still running a 470 bias resistor and dissipating above 100%. And yes, again, this may well have nothing to do with your trem problem, and I may have missed something obvious that means I'm all wet. But FWIW...

 

Your voltages are a touch higher but nothing unusual. The cooler bias on the 6v6 will raise voltages throughout the amp. I don't have much experience with the Vibrochamp circuit but here is a great thread from TGP with some good info.

https://www.thegearpage.net/board/index.php?threads/vibro-champ-clone-vibrato-not-working.1921018/

 

Have you tried a continuity check using layout and all components involved in the trem circuit? Even with hot bias I could notice trem on or off in my vc

Sorry to throw this in but it can happen- have you tried foot switch turning trem on /off?

 

Any progress here?

I would recommend at least halving your 6v6 1K bias resistor. I would use the stock value. YMMV

Your voltage chart indicates some oscillation happening in the trem circuit. On your board, I cant' tell where the yellow wire from the intensity pot terminates. it should go to v1b cathode. Ohm from the trem pot yellow wire lug to v1p8.

 

Try the stock cap values in the trem first, then raise them one at a time. It may be going REALLY slow and you don't notice it. Also, is the oscillator oscillating?

 

Does the speed pot have effect on the oscillation? If you plug in the footswitch and cycle the switch, does the oscillation stop/start as expected. If yes we may assume V2a is functioning as designed so we can focus on V2b and the injection point.

 

It looks like your using an isolated jack for your trem switch. I think that needs to be grounded since it is isolated from the chassis. I think the way it works is that the switch interrupts the ground connection to turn off the trem.

Nice looking build otherwise.
John

 

I think the trem jack is the one on the left here.

 

I’ll let @D'tar and the other smart folks think about the electronics here. Trem circuits have several sticking points, like starting up the oscillation, governing its amplitude, keeping it stable....

If it was mine, tho, I’d be tempted to rebuild/rewire that whole part of the board. It may be perfect — but that's anybody’s guess. The ODs block our view and have zero advantage here. It’s easier to see what’s where if it looks more like the original. Ceramic discs are fine, or use nice small axial caps.



And without pulling the board (unless you need to) you could use the visibility of a rebuild to triple check the wires are routed right and don’t short to each other. This is a particularly gnarly area of the layout.



For sure it’d be tedious and might well make no difference. So maybe only if the smart, directed, electronics approach doesn’t work.

 

Your oscillator is working, it shows in the voltage swings in V2, however it is not effecting the the V1b cathode voltage to create the effect. Your V1 voltage should swing also when the trem is engaged. Seams like your tremolo circuit is isolated from the rest of the amp, maybe your intensity pot?

Keep at it, my two most difficult builds were with trem circuits.

John

 

To confirm proper wiring and conponent values. IMO, this is how I would proceed.

We have multiple paths to ground so I recommend disconnecting a couple things to minimize confusion. 1, Disconnect the NFB feed at a convenient location. 2, Remove the yellow wire from the intensity pot.


Can you give us the ohm readings for

V1p8 to the intensity pot lug3(yellow wire disconneted end)

V1p8 to ground

V2p8 to the intensity pot wiper(brown wire)

Also v2p8 to ground with intensity pot max/min

This should give an idea of whats happening.

 

This is with amp off, unplugged, caps drained right?

The v1p8 to lug 3, i should clarify. Ohm from v1p8 to the end of the yellow wire that was connected to lug 3. So, to the wire, not the pot.