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Oscillation in tube amps
Oscillation in tube amps
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Old 28th November 2014, 02:51 PM   #11
hego is offline hego  Europe
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Good try: it works

" Cats (max 64khz) or dogs (40-60khz) act weird around the amp, when they don't around others or playing similar music/sounds"

Cheers
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Old 30th November 2014, 03:01 AM   #12
JamesG is offline JamesG  United States
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In my experience, oscillation is usually a function of layout and proper placement of grid resistors, which should always be soldered directly to the tube socket. For similar reasons, I have never used tag board construction for high gain guitar amps. I am a proponent of point to point construction. Of course, I don't build 10,000 amps. I build just one.
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Old 30th November 2014, 10:59 AM   #13
bayermar is offline bayermar  Germany
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Could this become a thread for the sticky section?
As George said: A lot of information about oscillation is usually scattered across a lot of different threads, some of th em quite long, therefore hard to find.

Martin
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Old 30th November 2014, 03:50 PM   #14
DF96 is offline DF96  England
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Quote:
Originally Posted by robertaudio
oscillation can happen if the Amp has ALL the following conditions;
1. Gain too high
2. Positive feedback path.

Eliminate any of the conditions and oscillations will go away.
OK, provided that you remember that these criteria apply to each stage separately as well as the whole amp. (And any pair of stages in a three stage amp).
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Old 30th November 2014, 06:23 PM   #15
Celsius is offline Celsius  Spain
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Same time is not only one cause.
Im my case I have change all rectifier bridge by HER 207 diodes and ferrite on the anode of the preamp valve, now the oscillation have been reduced a lot, but follow with a little, tomorrow will mount a snubber in the 211 filament line, I think 99% that the noise residue that have is provide by the switching power supply, maybe the snubber circuit help to reduce in other case will change by coil transformer.
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Old 2nd December 2014, 04:20 AM   #16
wa2ise is offline wa2ise  United States
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Quote:
Originally Posted by JamesG View Post
In my experience, oscillation is usually a function of layout and proper placement of grid resistors, which should always be soldered directly to the tube socket. ...
That's called a "grid stopper". Idea being that the resistor combines with the stray capacitance inside the tube to create a low pass filter that kills the tube's ability to go into supersonic oscillation. I've fixed strange hum problems that turned out to be byproducts of supersonic oscillations.
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Old 2nd December 2014, 11:03 AM   #17
luckythedog is offline luckythedog  United Kingdom
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In small signal pentode circuits, frequencies at which grid stoppers become effective is at high rf because miller C is so low. When such circuits do oscillate, it can be at very high frequencies indeed. As in super high GBW op-amps, it's worth intentionally restricting stage bandwidth by design to avoid self-oscillation. Small signal pentodes seem to have a reputation for this, but really it's generally the circuit design and layout which often sets up the opportune circumstances for pentodes to do what they do best, having terrific GBW by nature. Just my 2p worth.
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Old 2nd December 2014, 06:14 PM   #18
kevinkr is offline kevinkr  United States
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Oscillation in tube amps
Made the thread a sticky since it seemed like a good idea and was also suggested by another member.
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Old 8th December 2014, 12:06 PM   #19
nl3prc is offline nl3prc  Netherlands
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Quote:
Originally Posted by celsius235 View Post
is possible that in the first 20 minutes after switch on the tube donīt oscillate and later begin, I rebuilt my amp and now in the first 20 minutes the listen in perfect but later begin as Tubelab said, high piano notes begin to be horrible.
Is it maybe possible that in the beginning he oscilates above the 20000 Hz and you can't hear it
maybe its better to look at this with a scoop

salutions look at the feedback your using and also look at your OPT so that the input is shielded from it
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Old 16th December 2014, 08:11 PM   #20
Miles Prower is offline Miles Prower  United States
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Quote:
Originally Posted by luckythedog View Post
In small signal pentode circuits, frequencies at which grid stoppers become effective is at high rf because miller C is so low. When such circuits do oscillate, it can be at very high frequencies indeed.
This isn't the case. Grid stoppers don't prevent oscillation by acting as LPFs. Stoppers act by loading down parasitic resonant circuits to drop their Q-factors below the point where they can sustain oscillation. Your best bet for grid stoppers are C-comp resistors. 4K7 for control grids; 1K for screen grids (if using pents) are good. For power pentodes, screen stoppers from 470R -- 1K5 are good for stopping/preventing snivets. Some types are worse than others for this. 807s and other 6L6-oids like to make snivets, while others (6V6-oids, 6BQ6GA don't seem to have this problem) These values are high enough to de-Q parasitic LC tuners, low enough so's not to excessively roll off treble frequencies.

When doing high gain circuits (small signal pents, cascodes, BJTs and MOSFETs) it's always a good idea to treat 'em like RF circuits: keep all leads short as possible, and include stoppers. Especially with pents/cascodes, look for sockets with a central pin, and connect it to the circuit ground. Screen bypasses should be installed last, and installed so's they span the socket between the plate and control grid, with the "outside foil" connected to the ground plane. That way, it does double duty as a bypass and electrostatic shield. even if you need a big electrolyric to handle low audio frequencies here, you can always parallel it with a smaller capacitor as a high frequency bypass.

That'll keep the oscillation gremlins at bay.
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