Just a few quick notes on a revived thread that is useful for people troubleshooting this part of the engine/tractor:
1. A little quick background, RMS stands for Root Mean Squared and is a mathematical conversion of the AC sine wave to a value that would produce the same power dissipation/heat in a perfect resistor as a DC source of the same value. Peak voltage is 1.41 x RMS and Peak to Peak is 2.82 x RMS.
In the U.S., nominal AC voltage at your household outlet is ~120 volts AC and this is the RMS value. Peak voltage from your AC outlet is 169.2 volt, peak to peak is the absolute difference between positive and negative peaks or 338.4 volts nominal.
2. The common DMM (digital multi-meters) that are available today are referred to as RMS and they will provide the RMS, not peak or peak to peak value, when measuring AC voltage.
As a minor distinction, the cheaper meters are not "true RMS" and will read slightly off on the somewhat to heavily distorted sine wave produced by some sources. The commercial power coming into your house will be a very pure sine wave and unless something in your home (i.e. light dimmers and SCR controlled space heaters) is causing distortion, a cheap DMM and a true RMS DMM will read identically (within the calibration tolerance of the two units) when measuring a clean sine wave. But a lot of AC sources don't produce a clean sine wave output (low cost 12 VDC to 120 VAC inverters, some portable generators) and a lower cost DMM will often read slightly high because of the distorted and dirty non-perfect sine wave produced by these units. For several years, I used a portable 10KW gas generator for power outages and it had a fairly dirty output compared to commercial but the 40KW diesel generator I installed has a very clean sine wave output.
3. When converting AC to DC via the typical full wave bridge rectifier, the output will be a series of pulses with the peak voltage of the pulse equal to just under the peak (NOT peak to peak) AC input from the source. The slight reduction from absolute peak is due to a very small voltage drop across the diodes in the bridge rectifier. So a dynamo with 20 VAC RMS output will produce slightly under 28 volt peak DC voltage pulses but the smoothed DC value under load would be less than that dropping to slightly under 20 volts at heavy load.
Or in short, the DC output voltage from a bridge rectifier in a heavily loaded circuit will be very close to the RMS AC voltage input to the bridge. Under light load it can increase to about 1.41 times the RMS AC input.
If the Kubota dynamo is producing a very high AC voltage under load, then it sounds like their vintage rectifier/regulator circuit is using a single diode half wave rectifier circuit which produces a useful output under heavy load of about .45 x AC input.
Hammond transformer company provides a nice design sheet for their transformers explaining expected output under different rectifier configurations and load conditions:
http://www.hammondmfg.com/pdf/5c007.pdf
4. The output pulse repetition rate is double that of the AC input frequency with a full wave bridge rectifier (i.e. 60 hertz AC input results in 120 DC pulses per second at the output node of the bridge rectifier). This is important when you need to convert the AC to near pure DC because the capacitance value needed for filtering is reduced when fed with a higher pulse rate.
And one important caveat, especially with higher tech designs including your current car and truck BUT ALSO recent agricultural equipment built with a lot of sophisticated electronics. The equipment battery also serves as a filter and voltage surge limiter for the 12 volt power bus and you can create a lot of issues when "jumping" a defective battery in modern equipment.
A heavily discharged battery is fairly safe to "jump" but if you try to jump equipment with a defective battery (open cell or open interconnect between cells) or equipment where the starting fault is due to a defective battery cable or connector then the equipment battery no longer serves to absorb spikes that occur when connecting and disconnecting a jump pack, charger/starter, or other external source. This problem is greatly compounded by people who are nervous about jumping a vehicle and don't quickly and firmly attach the final negative lead between the external source and vehicle. When you are creating sparks when making that final connection, you are effectively connecting and disconnecting the external source at a rapid rate and this is what creates damaging voltage spikes. The best jump packs have a switch that allows you to make the connections and then turn the pack on, it avoids the issue of the nervous connecting operator.
Rodger