how about bad caps in the altec crossover?
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I replaced the xover caps and whats wierd is that 4 other amps work great with the Altecs and the dynaclone works great with any other speakers save those Altecs.. Got a set of Valencias in storage id like to pull and see how they behave since the voice coil/mag structure is the same betwenn the Santanas and Valencias. Mybe it is some inductive quality that only is present in Altec drivers..Thanks for the thought - best regards
Well, it really could be anything, right? Two additional impeding mechanisms have to be taken into account when talking about inductance besides the normal resistance of DC circuits: the induction of voltages in conductors self-induced by the magnetic fields of currents (inductance), and the electrostatic storage of charge induced by voltages between conductors (capacitance). The impedance caused by these two effects is collectively referred to as reactance and forms the imaginary part of complex impedance whereas resistance forms the real part.
You are dealing with transformers in your amp output circuit, right? Maybe somehow they are getting saturated?
This Wikipedia writeup seems relevant.
My suspicion is that there is nothing wrong with the speakers, and that some marginal instability in the amplifier is being brought out by the particular impedance characteristics of the Santana.
The following statements in the writeup particularly grab my attention, especially given that you have modified the original design to use a tube high voltage rectifier:
Low frequency oscillations like motorboating indicate that some device or circuit with a large time constant is involved, such as ... the filter capacitors and supply transformer winding.Regards,
Hey Guys, Its solved. Most of us were on the right track with the various inductive properties mismatch theory and it turns out, after studying crossover theory that the cure seems to be a "Zobel" series RC network in paralell with the amps output. Many crossover designs actually incorporate these in the LF driver portion of the crossover to make the amp "feel" a constant impeadance and thusly make it a happy camper with "bangy" high inductance, super reactive unstable feeling alnicos.
I started with a 14uf + 10ohm resistor which stopped the motorboating but seemed to take away some of the "umph". I reduced to 8uf + 6 ohms and it sounds smashing. Installed a switch and those components in my amp so I can add or delete the Zobel depending on my speaker choice with a flick of the switch. Looking forward to trying out on some of the other speakers that were non-problemaic to see how fidelity does in comparison with in or out of the Zobel.
Interesting to note; only the big alnico altecs seem to have the massive ammount of inductive kickback to cause the issue. Thanks to all of you for the input and and thanks to Otto Zobel up there in the big laboratory in the sky!
If it were me I would look at the filter caps on the plate supply for the power tubes, even though this otherwise seems to be solved.
Motorboating is a classic problem of defective/inadequate power supply filtration. My guess is the amp is right on the verge of having troubles even if it seems to sound fine.
If the capacitance for the power tubes is inadequate, the amp will sound cleaner and more gutsy if this problem is corrected and it won't be subtle!