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valco66 ([email protected])
8/18/2001 7:44 AM
inaudible frequency oscillation
Hi, I'm new to this site (talk about a light in the wilderness) and I could use some help: I know
what parasitic oscillation sounds like. I read about oscillation at inaudible frequencies, and my
question is, how do you know if this is happening? I don't have any special tools beyond a
multimeter and a variac. I imagine this would cause excessive heat in one or both transformers.
Anyone?
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Gene ([email protected])
8/18/2001 9:34 AM
Re: inaudible frequency oscillation
Check this out http://www.geofex.com/ampdbug/ampdebug.htm
Gene
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steven oda ([email protected])
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8/18/2001 9:59 AM
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Parasitic Oscillation
Gene,
That link hadn't finished the section on parasitic oscillation. This can be the inaudible oscillations
that leads to no sound and overheated transformers.
As far as I know when this occurs you usually add a grid resistor in series with the the offending
tube. Or, as in Weber's "Tube Amp" book, he suggests adding a shunt capacitor from the plate load
resistor of V2 (especially in Marshall Plexi's).
Both act as filters or dampeners to reduce high frequency oscillations, in both power and high gain
front end tubes.
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Mark Lavelle ([email protected])
8/18/2001 4:03 PM
Audible?
Would this oscillation sometimes be audible, sounding like a really fast tremolo? And how is it
related to squealing? I'm getting both of these from my recently-assembled preamp...
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steven oda ([email protected])
8/18/2001 11:20 PM
Re: Audible?
Mark,
My skinny on this thing is:
Parasitic or HF oscillations is unique to the tube and chassis setup. It's kinda like a super high
resonance frequency that is inherent with the tube's construction and the EMF surrouding it.
Once you hit this frequency, the tube starts to go ballistic and sucks (I guess this is where the term
"parasitic" came from) all kinds of juice from the PS. Meanwhile, electron flow is altered within the
tube to the point where it's like clamped to ground. No output or output at very high levels above
22 kHz (above the audible range).
Resistors to grids and the bypass cap to present grid resistors adds a dampening factor to the
circuit to try an avoid the tube's acceleration into this damaging oscillations.
It's like when a car's shocks go bad. If you hit a bump at just the right speed (frequency) that is
just the right depth or height (wavelength)---BINGO the car start oscillating out-of-control. And
nothing can stop the up-&-down wild gyrations, except full brakes.
The grid resistor or bypass cap act like a suspension dampener, at this HF frequency to slow down
the oscillations and prevent out-or-control resonance. Seen on the oscilloscope as that spike as
Randall said.
BTW---this added resistor or cap will alter the tone. So you probably want to use the added
component with the smallest value that eliminates the problem.
Now, for your system. Squealing and fast beating sounds are usually a sign of a pre-amp tube
(probably in the first gain stage) going microphonic.
Switch tubes around and see if that eliminates or reduces some of the problem. You may need to
replace one of the pre-amp tubes. I find this happens a lot with Sovtek and Svetlana 12AX7 in the
first (highest gain) stage---in high powered amps.
Sometimes, to get an amp to be more quiet and less prone to microphony, I've used a 5751 (little
less gain than the 12AX7 and more rugged) into the first gain stage.
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steven oda ([email protected])
8/18/2001 11:24 PM
Beating
Mark,
I should note that some amps with tremolo circuits will develop this odd beating tone, with a bad
driver tube. So, you may need to swap tubes around within all of the pre-amp's sockets to isolate
the culprit.
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Mark Lavelle ([email protected])
8/19/2001 6:49 AM
Re: Audible?
1. Not really related to the previous posts in this thread, but I thought it would be educational to
share: You can kill a *lot* of hum by connecting your DC filament ground to B+ ground. Having a
SS bridge & .01F (yes, that's .01 Farad!) filtering on your DC filament supply isn't enough if you let
it float in relation to the rest of your preamp.
2. It really helps if your wiring matches that carefully planned schematic (DOH!). I had swapped
two of the connections on the DPDT switch that selects the high gain 'channel' (4 stages, as
opposed to 2).
3. Having addressed items 1 & 2, I'm getting a nice 2-stage sound, but still have some ugly action
when I have all 4 stages switched in.
Tomorrow I guess I'll try:
A. Looking at the 4 stages with the scope,
B. A 12AT7 or 12AU7 for the extra stages, and
C. Some of the suggestions about resistors & caps.
But now it's time for some sleep...
Thanks to all (and an apology to valco66 for horning in on your thread).
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steven oda ([email protected])
8/19/2001 11:41 AM
0.01 Farad Cap?
Mark,
Where do you find one of these? Is this like one of those 10,000 mfd, 60 volts monster cans that
they use for big momma subwoofer ss car amps?
Boy, that's one big filter for the filament supply. Do those hum adjustment pots off the the heater
taps work (on AC taps for the filaments)?
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steven oda ([email protected])
8/19/2001 11:48 AM
Adding resistors to end of heater line
Mark,
Or adding 100 ohm resistors to the end of the AC heater filament tap line to ground? Like some
Mesa Boogies? Have you try this?
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Mark Lavelle ([email protected])
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8/19/2001 4:20 PM
Subject:
Re: 0.01 Farad Cap?
It's 10,000uF/10V. I probably got it from Jameco or Digi-Key. I assumed it was meant for switching
power supplies, but could be totally off.
It's that big because I built K. O'Connor's TOT "Experimenter's Power Supply" (EPS) and bought
the parts off the list in the back of the book instead of the schematic (which says 2200uF). Kevin
later informed me that the schematic wins in any conflict, but of course you can't really over-filter
a DC filament supply, eh?
The EPS uses a Hammond 270BX and provides both AC and DC filament supplies. In my case it's
built into the same chassis as my power amp (which came out very nicely, indeed). The AC supply
is straight off the tranny with two 100ohm resistors to a center ground, and in my case is feeding a
pair of 6V6s (but can handle two 6L6s). The DC supply goes through a full SS bridge and then gets
filtered by the 10,000uF beast. No adjustments of any kind. My preamp (just two twin triodes - no
trem or anything else fancy) & PI filaments are all using this DC.
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benboom
Date:
8/20/2001 2:16 PM
Subject:
Re: 0.01 Farad Cap?
The EPS uses a Hammond 270BX and provides both AC and DC filament supplies. In
my case it's built into the same chassis as my power amp (which came out very nicely,
indeed). The AC supply is straight off the tranny with two 100ohm resistors to a center
ground, and in my case is feeding a pair of 6V6s (but can handle two 6L6s)
Did I read that right? A 270BX? Here are the specs for a 270BX:
53VA, sec. 275-0-275, DC ma 50, Fil.#1(rct) 5.0v, 2a, Fil.#2(htr) 6.3v, 2.0a
A pair of 6L6's on a PT with a 50ma rating? I must have something wrong here.
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Mark Lavelle ([email protected])
8/20/2001 4:23 PM
re: Hammond 270BX & 6L6s
Good point, but that's what Kevin O'Connor claims it can handle. My guess is that he's assuming
you're not running them at 100% and that the Hammond has significant headroom. I've only used
it with 6V6s, myself.
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Mark Lavelle ([email protected])
8/19/2001 4:25 PM
Re: Audible?
A 12AY7 for the 'extra' gain stages greatly reduces the squealing and other obnoxious effects, but
doesn't totally tame the thing. I guess I get to spend the day tweaking...
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steven oda ([email protected])
8/20/2001 5:48 AM
Re: Audible?
Mark,
Check the dressing of the components and wires to the offending socket. Weber suggests (and it
works) to have the least (zero, if possible) resistor and cap lead from the socket to the component.
Are use using tube shields when testing your unit? Funny as it sounds, sometimes you get some
weird EMF coupling between pre-amp gain tubes that are close together. The shields help to reduce
this.
Good luck.
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Mark Lavelle ([email protected])
8/20/2001 7:45 PM
Re: Audible?
It turns out that using lower-mu tubes in both positions gets rid of all the nasties and still gives me
as much gain as I'm ever likely to want, so I'll stop worrying about that (until I want more gain,
that is
). Also, one of my 12AX7s was definitely microphonic.
Now I've just got to figure out how to get rid of the hum on the supply to the plate of my last stage
(*and* come up with a better tone stack, but that's not really debugging)...
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Stephen Conner ([email protected])
8/18/2001 4:49 PM
Re: inaudible frequency oscillation
Hi
High-frequency (ultrasonic or RF) oscillation can be a hassle. The ideal way to find it is using an
oscilloscope. Even though you don't have one, there are still things you can do.
For a start, you can measure the output tube cathode current and observe how it changes as you
slowly turn down the bias voltage from maximum (ie turning the tubes slowly on) It should ramp
up smoothly. If it suddenly jumps from one value to another, doesn't stay stable, doesn't give the
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value of idle current you expect, or changes as you wave a hand near the circuitry, that's a dead
giveaway.
HF parasitics can also cause lack of power, overheating, and distorted sound, although there are
plenty of other possible causes for these symptoms.
Finally, a trick that radio hams used to use is to wave a neon bulb near the output tubes.
Apparently if there were parasitics, the bulb would glow because of the RF field. I don't know if this
would work on amps as well as it did on radio transmitters, mind you, the fields won't be as strong.
You could make it more sensitive by actually hooking the bulb up to the circuit, e.g. from one
power tube plate to ground. (You'd need a 100K resistor and 100pF 1kV cap in series with the
bulb)
As for low-frequency subsonic oscillations, these are pretty rare. You'd know all about them
though; your speaker cones would pump in and out like crazy.
Steve C.
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valco66 ([email protected])
8/18/2001 5:28 PM
Re: inaudible frequency oscillation
Thanks Steve, those are useful tips. I appreciate the help, and I'm sure I'll be back.
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Wild Bill ([email protected])
8/20/2001 3:10 PM
Re: inaudible frequency oscillation
If you don't have a 'scope try measuring the voltage drop across the plate resistor of the suspected
preamp stage. Figure that you should draw only a ma or two of current if things are working
properly and calculate the voltage drop for the value of plate resistor.
If the stage is oscillating you'll be drawing a lot more current and thus dropping far more voltage.
I've seen a case where the oscillation followed the volume pot and you could see the extra voltage
drop kick in as the pot was raised, even though the frequency was too high to hear anything in the
speaker.
In a properly operating preamp stage the plate voltage should be stable, especially with no signal
applied.
---Wild Bill
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