What has to be done if motorboating occurs? Increase or decrease the coupling RC time constant(s)? Which one(s)? Increase or decrease the filter capacitors? The stereo amplifier in question as part of a radio console shows the following structure per channel: Input stage ½ 12AX7, followed by a passive Baxandall tone control stack, next stage ½ 12AX7, to whose cathode NFB is applied off the OPT's secondary, 12AU7 LTP, QQE03/12 (aka 6360) final tube in fixed bias class AB. PSU is: Bridge rectifier, 200 µF 1st filter cap that feeds the power tubes, 2.200 Ω filter resistor, 2nd filter cap 50 µF that feeds the LTP's and the 2nd stages, 27 kΩ filter resistor, 3rd filter cap 10 µF that feeds the input stages.
Best regards!
Best regards!
Quite often is happens because several coupling cap RC times are similar leading to 2nd order or more responses. It often is enough to have couopling can RC times that are a factor of 5 different from each other.
Jan
Jan
The power supply must have much greater LF bandwidth than the audio circuit in tube amps.
Low frequency stability depends on the relationship of both.
Low frequency stability depends on the relationship of both.
Yes, because bad capacitors often decrease in uF value, which could trigger the LF instability.
Also, low frequency motor boating can be the result of oscillation at super sonic frequencies. This could happen since hollow state doesn't self destruct like solid state.
That is called “squegging”, or “zzzzhmppp, zzzzhmppp, zzzzhmp”. It’s a type of relaxation oscillator. Yes, typically high frequency, and doesn’t usually happen with tubes. When tubes oscillate ultrasonically, current consumption comes DOWN with respect to full power in-band. With solid state push pull amplifiers the opposite happens - at high frequency you get common mode conduction which can blow the output stage up. It start out stable at low Vcc, starts oscillating when it gets high enough, drags the supply down enough to make it stop, the voltage comes back up, and the cycle starts all over again. Why you hear a “zzzzhmp zzzhmp” is the amplifier gets noisy in band, many times with a lot of power supply ripple noise due to the current demand when it is oscillating at high frequency, and quiet when it quits. If a similar phenomenon happens in a tube amp, you may see bursts of oscillation on a scope, and hear small disturbances, but not with the cone excursions you get with real motorboating and is safe for the equipment. Well, it won’t blow the amp up. Tweeters could be smoked if the frequency isn’t high enough for the transformer to roll off or the VC inductance is still pretty low.
I have seen a variant happen with tube stages, where the cathode voltage gets pumped by the h-k voltage, and the oscillation kicks in and out at a 120 Hz rate. Along with a bias shift. Never seen a power stage do it, because those are almost never operated at elevated h-k. Happens a lot in Cathodynes or direct drive followers because the signal voltage on the cathode can be large. Might work fine at low audio levels and kick in at a certain point. Thats either a bad tube or just too high an h-k. It can sound positively nasty when it happens, but the only damage is usually to the speaker if it can’t handle full RMS power of the amp at 120 Hz square-ish wave. It took me forever to figure out what was going on.
I have seen a variant happen with tube stages, where the cathode voltage gets pumped by the h-k voltage, and the oscillation kicks in and out at a 120 Hz rate. Along with a bias shift. Never seen a power stage do it, because those are almost never operated at elevated h-k. Happens a lot in Cathodynes or direct drive followers because the signal voltage on the cathode can be large. Might work fine at low audio levels and kick in at a certain point. Thats either a bad tube or just too high an h-k. It can sound positively nasty when it happens, but the only damage is usually to the speaker if it can’t handle full RMS power of the amp at 120 Hz square-ish wave. It took me forever to figure out what was going on.
Sometimes, a complete and accurate schematic has been known to aid in troubleshooting over the internet.
Full wave rectification of 50Hz power mains (Germany's mains frequency?) is 100Hz.
The capacitive reactance of 200uF @ 100Hz = 8 Ohms.
60Hz mains, Xc is even lower.
Imagine a high voltage secondary, and a solid state rectifier trying to repeatedly charge up 8 Ohms capacitive reactance.
Forget about asking most tube rectifiers to do that.
And, I bet the power transformer gets hot.
Adding a first capacitor of from 20 to 40uF, and from there, a 20 Ohm series power resistor to the 200uF capacitor would probably only reduce the B+ voltage by a couple of Volts, but would allow the power transformer to run a little cooler.
Full wave rectification of 50Hz power mains (Germany's mains frequency?) is 100Hz.
The capacitive reactance of 200uF @ 100Hz = 8 Ohms.
60Hz mains, Xc is even lower.
Imagine a high voltage secondary, and a solid state rectifier trying to repeatedly charge up 8 Ohms capacitive reactance.
Forget about asking most tube rectifiers to do that.
And, I bet the power transformer gets hot.
Adding a first capacitor of from 20 to 40uF, and from there, a 20 Ohm series power resistor to the 200uF capacitor would probably only reduce the B+ voltage by a couple of Volts, but would allow the power transformer to run a little cooler.
Wavebourn,
Reminds me of the Buttler Oscillator, not very similar, but close enough.
I always enjoy when someone tells me their cathodes are not coupled together, both with real world parts, and also with parasitics . . . Really?
Example: self bias resistors, electrolytic bypass caps, and plastic caps in parallel with the electrolytics (to bypass at HF, VHF, and UHF).
All those parts connected together to "Ground".
"Grounds are Commonly Mis-understood"
So are the wires that connect to ground.
The Parasitics are Dead.
Long Live the New Parasitics!
I supported testing and analysis of a prototype PCB. A DAC drove a SOT89 NPN Emitter Follower. The terminating emitter resistor was on another PCB, they were connected with a Flying Lead Wire. Wonderful inductor . . . that wire. With the stray capacitance, the SOT89 NPN oscillated at near the its ft frequency. And, as the DAC ran through its range, the load current varied, so the ft, and oscillation frequency varied.
Solution: Put the terminating emitter resistor right at the emitter. The flying lead's purpose was to transmit DC voltage, not intended to be an inductor for a resonator.
Reminds me of the Buttler Oscillator, not very similar, but close enough.
I always enjoy when someone tells me their cathodes are not coupled together, both with real world parts, and also with parasitics . . . Really?
Example: self bias resistors, electrolytic bypass caps, and plastic caps in parallel with the electrolytics (to bypass at HF, VHF, and UHF).
All those parts connected together to "Ground".
"Grounds are Commonly Mis-understood"
So are the wires that connect to ground.
The Parasitics are Dead.
Long Live the New Parasitics!
I supported testing and analysis of a prototype PCB. A DAC drove a SOT89 NPN Emitter Follower. The terminating emitter resistor was on another PCB, they were connected with a Flying Lead Wire. Wonderful inductor . . . that wire. With the stray capacitance, the SOT89 NPN oscillated at near the its ft frequency. And, as the DAC ran through its range, the load current varied, so the ft, and oscillation frequency varied.
Solution: Put the terminating emitter resistor right at the emitter. The flying lead's purpose was to transmit DC voltage, not intended to be an inductor for a resonator.
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I had a pair of K-followers direct driving a pair of outputs, located about 12” away i thought I’d be cute and twist the outputs going from cathodes to grids together, cancel out any common mode pickup, right? Well, it did that but the few pF cathode to cathode created oscillations that were too high up to see on a 100 or 300 MHz scope. Separate them and all is “well”, except for the hum pickup I was trying to cancel. Ended up redrilling the damn chassis to put the drivers closer.
Wavebourn, wg_ski, and All,
I just added edits to my Post # 392, After you both responded.
It might be fun for the re-read.
Thanks for the reminder! . . . filament to cathode capacitance.
One of the reasons I do not like the Concertina Phase Inverter.
more . . .
My favorite JFET oscillator (intended), was used in Ultrasound dopplers. I modified the oscillator for more tuning stability.
The non buffered oscillator's load was a tuned NPN amplifier. I changed to a Colpitts circuit, so the LC ratio had lots of C and little L.
When the NPN collector was tuned, the reverse trans-admittance barely tickled the oscillator C.
As an additional feature, as the ultrasound probe was moved, pressured harder and lighter, its impedance changes; that effect still changed the NPN base impedance, but the stable Colpitts did not care anymore.
$0.03
adjusted for inflation
I just added edits to my Post # 392, After you both responded.
It might be fun for the re-read.
Thanks for the reminder! . . . filament to cathode capacitance.
One of the reasons I do not like the Concertina Phase Inverter.
more . . .
My favorite JFET oscillator (intended), was used in Ultrasound dopplers. I modified the oscillator for more tuning stability.
The non buffered oscillator's load was a tuned NPN amplifier. I changed to a Colpitts circuit, so the LC ratio had lots of C and little L.
When the NPN collector was tuned, the reverse trans-admittance barely tickled the oscillator C.
As an additional feature, as the ultrasound probe was moved, pressured harder and lighter, its impedance changes; that effect still changed the NPN base impedance, but the stable Colpitts did not care anymore.
$0.03
adjusted for inflation
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6A3´s quote "One of the reasons I do not like the Concertina Phase Inverter."
I would also add, another annoying trait is the often 100V max between heater and cathode requiring an elevated heater voltage off "0V" which conflicts when low noise pentode and triode are in same envelope. Compromises ! Only solution.... run at 50/50.
I nearly always buffer the concertina phase splitter with a CCs tail diff driver, which is a good tactic to balance up any output inequality, unless only a few watts is required. It also avoids any instability from an output stage class A into the B region using high gm output stages operating at high levels and high voltages which or can aggravate instability......I´ve never experienced issues after using a CCS tail diff stage, though it sure helps to keep interwiring as short and symmetrical as possible.
This used to be a common problem with some early SS power amps using output stages in class B with peak currents with long speaker leads just entering nasty clipping which oscillated and permeated back into the supply. A sure way to destroy tweeters.
Viva Zobel ! but it didn´t always solve it.
Stay safe
Bench Baron
I would also add, another annoying trait is the often 100V max between heater and cathode requiring an elevated heater voltage off "0V" which conflicts when low noise pentode and triode are in same envelope. Compromises ! Only solution.... run at 50/50.
I nearly always buffer the concertina phase splitter with a CCs tail diff driver, which is a good tactic to balance up any output inequality, unless only a few watts is required. It also avoids any instability from an output stage class A into the B region using high gm output stages operating at high levels and high voltages which or can aggravate instability......I´ve never experienced issues after using a CCS tail diff stage, though it sure helps to keep interwiring as short and symmetrical as possible.
This used to be a common problem with some early SS power amps using output stages in class B with peak currents with long speaker leads just entering nasty clipping which oscillated and permeated back into the supply. A sure way to destroy tweeters.
Viva Zobel ! but it didn´t always solve it.
Stay safe
Bench Baron
The you can omit the Concertina as well, as it doesn't provide any gain or do any different than a LTP phase inverter.
Best regards!
Best regards!
Ah, yes but I like pentode 1st stages.....if the gain is too high there is always the option of leaving the cathode resistor without a bypass cap, i.e degenerative feedback, or strap tube as a triode but the drawback with pentodes is that unpredictable g2 screen. That resistor value plays a big part when it comes to thd and sound quality. Don´t forget some early pentode amp designs used 30dB global feedback to keep it quiet which I know many affectionados wouldn´t dare tolerate that value. I´ll stick with 20dB.
Viva concertina !
Bench Baron