Ace Tone Bass-6 Motorboating

[Rhodesplyr]

I recently repaired an Ace Tone Bass-6 head for someone, but being more of a tube amp tech than a SS amp tech, I'm not actually sure why my fix worked.

No one seems to have a schematic for this Ace Tone amp, which appeared maybe around 1970, but the output stage is very similar to the Peterson/80W Rhodes amp modules, a fairly standard transformer-coupled SS output stage from this era. Both are rated 40 Watts. The Ace Tone uses Sanken silicon transistors instead of germanium.

The Ace Tone was stable into some speakers, like a generic 12" salvaged from some organ, and was perfectly stable into a resistive dummy load, but would immediately start motorboating at turn-on into other speakers, particularly a JBL E120. On advice from other techs, I checked all the PS filter capacitors both with an ESR meter and by clipping in larger capacitors to swamp any problems, but none of that had any effect on the LF motorboating/oscillation. Then, I remembered the Rhodes schematic and the fact that it includes a 270 Ohm 2W resistor between the output and ground. The Ace Tone had no such resistor. As a last-ditch attempt at a fix, I clipped in a 100 Ohm resistor. The motorboating immediately stopped.

One possibly significant detail is the fact that someone replaced the output jack on the Ace Tone at some point, so it's conceivable that this amp might have had such a resistor mounted on the original output jack, but without a schematic, there's no way to know.

I understand that placing a resistor in parallel with the L and/or C of the speaker load makes the load look flatter to the output stage, but for the sake of my own education, I'd like to know more about why this resistor is necessary and why my fix worked.

I'm attaching a copy of the Rhodes schematic for convenience and a photo of the inside of the Ace Tone Bass-6.

[g1]

Looks like the input jacks are plastic isolated type where speaker jack is metal type.
Wonder if there is any chance the speaker jack is supposed to be insulated from chassis?

[Rhodesplyr]

Looks like the input jacks are plastic isolated type where speaker jack is metal type.
Wonder if there is any chance the speaker jack is supposed to be insulated from chassis?

I did, in fact, try isolating the speaker jack from the chassis, but it made no difference to the motorboating.

[Enzo]

So let it motorboat, and scope the power supply. Is the motorboat evident on the rails? Usually some loop is involved, so see how far back the amp the motor boat extends. You might find for example that an earlier stage has marginal decoupling, and adding some extra filter cappage might be the difference between partial stability and stability at all loads. You have auditory examples of the sound, but did you scope the output to see if bursts of RF are accompanying the audio. Perhaps your extra resistor is acting like a sort of zobel.

[pdf64]

I guess that the amp is oscillating at the bass resonance frequency of the load; at bass resonance, its impedance increases significantly, and there's likely some affect on phase too. It might even be seen that the electronic 'spring' of the speaker resonance feeds energy back into the amp.
This may be changing the feedback loop from being negative to positive.
Different speaker models / different cabs have differing resonance characteristics, hence some may be one side of stability, some the other.
A parallel resistor will act to damp resonance, limiting its effect.
It may be that increasing the value of the feedback resistor / tinkering with its shunt cap value, may increase the margin of stability too, so you could give that a go.
I think that the key issue is that the driver transformer is inside the feedback loop, and may be affecting the frequency / phase response too much for the feedback ratio used.
Also it may help to increase the value of the 0.1uF decoupling cap on the driver supply.

[Rhodesplyr]

Enzo,

In terms of scoping the power supply while the amp was motorboating, I was afraid that the motorboating would potentially damage the JBL E120, which is my own shop speaker. Maybe that wasn't a valid concern, but I didn't want to risk it. I tried putting a resistor in series with the speaker, but that damped and eliminated the motorboating.

The amp wouldn't motorboat into a dummy load, nor would it do it without a load attached.

And I did alligator-clip large capacitors across all the major power supply nodes, one at a time, including 5,000uF across the first +/- 40V filters. Not only did that not stop the motorboating; it had no effect on it at all.

[Enzo]

That is as good as scoping, it answers the question.

If you are willing to boat for a brief moment, you can attach a scope to the output, and watch as you turn it on briefly. I understand if that is not acceptable.

[Rhodesplyr]

In the end, I think we'd have a lot more insight into this question if we actually had a schematic, but it wasn't cost-effective to trace it out and draw it. The owner didn't want to drop too much $$ into it, so he was happy simply to have a fix that worked and a 3-wire power cord.

It is odd to me that no schematic can be found since these were made for 4-5 years and Ace Tone reportedly shared some distribution with Hammond via Hammond's use of the Ace Tone Rhythm units.