Vibrochamp Style Tremolo Low Oscillating / Motorboating Problem

[kleyplays]

I'm looking for a little help to troubleshoot my tremolo.  The amp is an AB763 inspired design.  I'm attempting to install a one tube AA164 type vibrochamp preamp bias wiggle tremolo.  When I turn the intensity up I get a bad low oscillating motorboating noise.  Turning the speed control ramps up the frequency of the oscillation. 

Here is the schematic I'm following for the trem insertion.



Here is a video where you can hear the problem.

Here are some pictures.





Here is a layout if the pictures are confusing or hard to see.



Here are my voltages on the left compared to the vibrochamp schematic's listed voltages on the right.



I've tried rolling tubes.  I initially had it installed where the B+ supply was on the same last in line preamp node, but have since moved it up one node to decouple.  It now shares with the phase inverter.  I had some of my B+ wires feeding the capacitor dog house running near this tube, so I have since rerouted them further away.  Nothing has made much impact on the oscillation. 

I've run out of ideas.  Any thoughts?

[glass54]

Silly Question Keyplays.
Did you use shielded cable (and dressed leads) for inputs?

Kind regards
Mirek

[kleyplays]

Silly Question Keyplays.
Did you use shielded cable (and dressed leads) for inputs?

Kind regards
Mirek

I did not use any shielded cable. 

[glass54]

OK. I am also concerned about the earthing Buss as well. It appears that the input/preamp/trem/PI and sections of the PA are coupled together (see attached pix).
This Hoffman site has a good guide for lead dressing (general project assembly/instructions) and includes pix of a Fender chassis towards rear. See https://el34world.com/Hoffman/instructions.htm
Kind regards
Mirek

[sluckey]

Try connecting the INT pot to the cathode of the second preamp triode just like the VC.

[kleyplays]

Try connecting the INT pot to the cathode of the second preamp triode just like the VC.

I have tried inserting the output of the Int pot to both those locations.  No dice. 

[kleyplays]

OK. I am also concerned about the earthing Buss as well. It appears that the input/preamp/trem/PI and sections of the PA are coupled together (see attached pix).
This Hoffman site has a good guide for lead dressing (general project assembly/instructions) and includes pix of a Fender chassis towards rear. See https://el34world.com/Hoffman/instructions.htm
Kind regards
Mirek

Good point.  This was the second amp I built several years ago and it definitely has a lot of gremlins. 

For grounding I currently have several grounding points.  One at the input jack grounding everything connected to the wire running behind the pots.  Then I have a second grounding the caps from the doghouse on a bolt on the PT.  A third grounds the output tubes with two 1 ohm resistors on another bolt on the PT.  Then one for the power cable ground.  The negative DC bias also has a ground on the middle terminal strip.

When you say to decouple, I had seen that suggestion elsewhere.  I thought that was referring to the B+ supply.  On first test with this tremolo I had the B+ supply coming from my last node which is shared by the other preamp tubes.  I have now moved it so it ties in to node C on the schematic which ties with the phase inverter. 

Are you saying to decouple it has to do with grouping ground points?  In some of my more recent amps I have tried implementing many of the grounding strategies from Merlin Blencowe's article - http://www.valvewizard.co.uk/Grounding.html.  Are you suggesting that as well?

[vampwizzard]

We've been talking about ao-29 conversions and its like we summoned you here. HA! big fan dude.

[kleyplays]

We've been talking about ao-29 conversions and its like we summoned you here. HA! big fan dude.

Haha it is wild to me that some of these crazy videos I make have any reception at all.  I will be checking out that AO-29 thread post haste!  I love Hammonds!

[sluckey]

Does everything else work properly? If so, disconnect the INT pot from the preamp cathode. I assume the amp is back to working properly now?

Troubleshoot the trem oscillator. Measure voltages. Plate voltage will be swinging wildly if the oscillator is working. If plate voltage is steady, oscillator is not working. Not too many components. Double check your wiring against the VC schematic.

[kleyplays]

Does everything else work properly? If so, disconnect the INT pot from the preamp cathode. I assume the amp is back to working properly now?

Troubleshoot the trem oscillator. Measure voltages. Plate voltage will be swinging wildly if the oscillator is working. If plate voltage is steady, oscillator is not working. Not too many components. Double check your wiring against the VC schematic.

Yeah the amp is glorious with the trem off or even with the intensity turned off. 

Here are some fresh voltages:

1 - 203 to 208 swinging
2 - 0 to -13 mv
3 - 2.08v
4 - 3.22 vac
5 - 3.22 vac
6 - 424
7 - 200 to 207 swinging
8 - 205 to 210 swinging
9 - 3.22 vac

If I measure for dc voltage on the cathode where it is tied in at V2B I get DC voltage of 2.1 and AC voltage that is swinging of about 0 to 17. 

[kleyplays]

I just experimented with grounding.  I clipped all of the tremolo grounds off the preamp buss and aligator clipped them to a bolt on the PT.  No change.

[vampwizzard]

are you giving the trem 12ax7 too much voltage? I wouldnt think 10% over was too much overvoltage but maybe?

[vampwizzard]

also, I dislike linking off forum but this discusses the swing voltage requirements on that cathode. May need some tweaking to make this work on a non-vibro champ amp: https://www.tdpri.com/threads/understanding-the-vibrochamps-tremolo-circuit-and-how-it-might-be-implemented.993267/

[kleyplays]

are you giving the trem 12ax7 too much voltage? I wouldnt think 10% over was too much overvoltage but maybe?

Hmm interesting.  I have gotten so used to kind of YOLO'ing my 12AX7 voltages that I hadn't really thought about it.

I'm thinking it would be required to add an additional B+ node with its own dropping resistor and filter cap?  The value of the dropping resistor to get me to the target voltage?

[kleyplays]

are you giving the trem 12ax7 too much voltage? I wouldnt think 10% over was too much overvoltage but maybe?

I just tried adding a new B+ node with a 10k dropping resistor and a 27uf filter cap.  B+ on pin 6 got to 345vdc - which is pretty close to the 340vdc I see on the schematic.  Still motorboating...

I'm thinking about ripping it all out and wiring it all up again.

[sluckey]

Before you do that, replace the oscillator cathode R/C with a red LED. The LED's cathode connects to ground. This will give you a visual indication (blinky light) that will let you know for sure the oscillator is working. No blinky, no worky.

[PRR]

> For grounding I currently have several grounding points.

That's rarely a good plan. If only because it is impossible for others to really visualize what-goesta-what.

A general good idea is "Ground Follows Signal". There should not be signal on the chassis, but the circuit normally must tie to chassis as a shield. At ONE place. With metal g-jacks the input is logical. With plastic jacks it may be convenient to run a bus from there to the PT bolt. There ARE in-between ways to skin this cat (preamp at input, power stage at output) but that's trickier to adapt to new builds.

[kleyplays]

Before you do that, replace the oscillator cathode R/C with a red LED. The LED's cathode connects to ground. This will give you a visual indication (blinky light) that will let you know for sure the oscillator is working. No blinky, no worky.

I pulled the 4.7k resistor and 25 uf cap off pin 3 and replaced with a red LED.  The LED illuminates, but it does not blink.  That must mean my oscillator is not oscillating right?  So the problem must lie in the components off pins 1, 2, and 3 right?

[sluckey]

Exactly. That narrows the problem considerably. You're sure you have the LED cathode connected to ground. right?

[kleyplays]

Exactly. That narrows the problem considerably. You're sure you have the LED cathode connected to ground. right?

LOL I have checked it because that is an error I would make after all this debugging.  Long lead is on the cathode.  Short lead to ground. 

[sluckey]

Exactly. That narrows the problem considerably. You're sure you have the LED cathode connected to ground. right?

LOL I have checked it because that is an error I would make after all this debugging.  Long lead is on the cathode.  Short lead to ground.

Now I really have to ask again. There's a better way to identify. Flat side is cathode.

[kleyplays]

Exactly. That narrows the problem considerably. You're sure you have the LED cathode connected to ground. right?

LOL I have checked it because that is an error I would make after all this debugging.  Long lead is on the cathode.  Short lead to ground.

Now I really have to ask again. There's a better way to identify. Flat side is cathode.

Wow I worded that poorly.  I have the anode of the LED on the cathode of the tube and the cathode of the LED on ground.  Thanks.

[sluckey]