Solving Tube Amplifier Motorboating with B+ Decoupling Resistors (Demonstration)
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- Опубліковано 15 лип 2024
- A quick demonstration of the role power supply decoupling resistance and capacitance plays in suppressing low-frequency oscillations (motorboating).
Video on this amplifier: • Designing a Balanced, ... - Наука та технологія
After further testing with signals, I've upped the resistor to 4.7k. The 1.5k would oscillate with a sine wave applied.
A clear explanation and demonstration of the problem.
Excellent explanation of the phenomenon. Very innovative and cool breadboarding technique.
Totally love this mess! Congratulations!
Clever of you to use octal relay sockets for prototyping. I've taken a much more traditional approach with a wooden board and sockets mounted on L-brackets.
It's a very nice explanation of motorboating :) and it will occur in class AB or V amps with high-Z power supplies (e.g. tube rectifiers, thin wire on power transformer) if the voltage sags under load.
When you have fluctuating voltages manually set the range, on the DVM to eliminate auto-ranging annoyances.
I see you use relay sockets for prototyping like I do. Super cool.
How can you do any analogue design without an oscilloscope?
Hi. what company made this 1.5k resistor?
Would this work with the 400w 6550/kt88 ppp williamson design?
Just 2c... You could induce oscillations just by the fact you have everything out on a benchtop without a chassis, with leads running every which way. Sitting transformers one way or moving them a couple of degrees can upset stuff too.
The very audible slow, 'blat, blat blat' motorboating noise coming out the speaker Is most often caused by a bad ground connection on the negative end of the power supply filter caps. Dry solder joints or a ground connector lug not tightly or cleanly bolted to the chassis. The filter caps can't pass sufficient amperage to ground, they begin to charge up but then collapse so setup a regular, high voltage (intrusively loud) pulsing. This seems like a lower frequency parasitic oscillation. Lead dress can cause it, it might have been interesting to try suspending some grounded tinfoil over the wires sprawled all over the table to see if the oscillations stopped. I can't see enough to read your finely drawn circuit diagram and component choices so pls excuse me if my comment is unrelated or unhelpful.
Those 200 ohm resistors should be fed from the same source. Feeding one from the others output can increase your problems.
I had the same thought, however I tried it in both configurations, and there was seemingly no difference in stability. But feeding one from the other gives lower ripple since the two filters are in series, so I chose that.
@@messingwithelectronics7462 i learned that practical experience often trumps theory decades ago!
I've never seen so low value resistors in tube amp power supplies. 1.5k is quite ordinary, up to 20k easily ...
But it's not only resistors, this is caused quite probably with too much of capacitance (you made a big slow oscillator). I've first heard of motorboating when somebody wrote that you should avoid too high capacitors because of it ... But I am too ignorant to be sure about it.
That is a metric butt-ton of filtering for what you have going on. Too much of a good thing is still too much... There is a mathematical solution to this. More or less, X capacitance requires X resistance for X filtering. It is not an arbitrary number. I can't give you a number off the top of my head,, but, it is an easy-to-find number on the interwebs.
Yeah, I did model it in PSU Designer II and it's very overkill. This design is kind of nostalgic for me because it's based on an old Capehart amplifier I restored that also had a huge choke, so I wanted to replicate that 1940s look and feel. Cheers!