Tube amplifier oscillation As one of this page's opening epigrams reveals, its principle designer, Otto Schade of the Radio Corporation of America, knew from lab tests that the tube was capable of secondary-emission phenomena—but only under conditions not normally It isn’t safe to poke around in a high-power vacuum tube amplifier while looking at voltages, but a simple demonstration circuit can be constructed. Top . The reverb is really powerful and becomes almost unusable past about 1-2. They're either descriptive at a lower level than I understand now, or the MIT Vacuum Tube electrical engineering book from the 1950's that starts out with physics of trajectories of charged particles in high voltage fields. 1A is shown a triode having electrode supply voltages. ) The 6146 worked well as a straight through amplifier on 6 meters and could be used with reduced power on 2 meters with careful circuit design and lead dress. p Watch TI Precision Labs the electronics industry’s first comprehensive online classroom for analog engineers. ! The amp is D3a 45 300B tubes. . Here's the part of the circuit that most likely causes the problem (deduced by checking what pots change this weird effect): (yes I use power tube in preamp, because I like power tube distortion and I don't like playing very loud for most of the time). Those could be creating an impedance or cross-talk issue. Mar 22, 2015 · I’ve got a couple amp builds under my belt with satisfied customers and I want to revisit this first amp build of mine (which I’m keeping for myself) to iron out the kinks, namely the oscillation issue at high volume knob settings as well as possibly replacing some of the ugly wiring job, possibly with cloth wire as I’ve really taken a Mar 25, 2018 · The 6BQ5 screen shouldn't be connected to the same power supply node as the 12AX7s - it draws more current than they do, so variation in screen current makes the supply voltage change, which makes the plate voltage on the first stage change, which gets amplified by the following two stages. The problem occurs notably in RF, [1] audio, and other electronic amplifiers [2] as well as in digital signal processing. Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through your heart. The cathode can't magically produce more current as an oscillator than the saturated emission would permit in any other service. 1114 or –19dB. A parasitic oscillation is shown in (b), this occurring at one point of the sine wave only. Nov 19, 2015 · Even worse, a marginally stable circuit may be functional during testing but useless when the amplifier is exposed to different operational or environmental conditions. Class AB1 Operation Class AB1 amplifiers are roughly 60% efficient. Check Cracks; First, you must inspect the amp’s tube because the damaged and cracked tubes are the most common reason behind oscillations. However, by properly designing the entire magnetic circuit, Ceramics can be made to behave quite well for desirable guitar amp tone and dynamics. I have all grounds star connected to the first power supply cap. In the context of a limited dynamic range system like the phonograph, recordings made with vacuum-tube pre-amplifiers will have more apparent level and a greater signal to system noise ratio than recordings made with transistors or operational amplifiers. The latter would be more common with multistage amplifiers with global feedback but it won’t hurt to rule it Nov 9, 2023 · Re: Wierd self-oscillation issue in tube amplifier « Reply #11 on: November 10, 2023, 09:50:41 pm » Put a voltmeter or scope on B+ and see if it is also moving up and down with the oscillations - it needs to be quite solid. Hoffman Amplifiers Tube Amp Forum, Transformer orientation/location considerations. Get another one. 812 Tube. Its operating point being at a on the E g-I p characteristic curve shown at B. Layout: Here, you must decide if you want an "all-in-one, desk top" amp, or a "floor console", or a "two-piece" unit. If you're not a tech with the right tools, or familiar with amp layouts, you're fighting some tough odds. Add feedback from output to input, that is phase shifted by 180 degrees. That is a circuit that occurs yet is not physically on the schematic. Turning the Reverb past 5 and knocking the tank causes a bad oscillation at about 5khz or so. (Having worked with vacuum tube amplifiers, which due completely shit noise rejection are very picky about grounding, I can vouch for the design of the circuit ground. Some comments / remarks: - oscillation happens only with a speaker connected, NOT with a dummy load connected. also one must be sure about that there is no positive feedback applied, it is easy to measure with the oscilloscope, output on the speaker with Jun 1, 2019 · Hey, I just built a deluxe micro 5e3. In working on the amp I noticed that the power transformer gets hot pretty quickly and the OT gets a little warm also. The Class A amplifier can be compared with a gas turbine engine. Look for big no-nos like OT plate leads going anywhere near small-signal stages. For an amp with DC heaters, it's very quiet. With 6e6p tube with less amplification factor ( u =35 ) i think NEVER oscillate. 99% of guitar amps use the same power amp circuit and values and negative feedback around the PA. The scope and output voltmeter had a steady jump. ) Do this while the amp is in oscillation to see if you can get it to stop--if it's ultrasonic oscillation then feed a signal into the amp so you'll know when the oscillation stops. i have no any problem at all, but i'm using 0-4-8-16 ohms technique where 4 ohms is GND, 0 ohms goes to first cathode, ¸16 ohms to the second. By Rob Robinette Jun 5, 2020 · Well, that can be a number of things. Jun 2, 2016 · This type of oscillation is usually in the hundreds of Megahertz or Gigahertz range, and is frequently caused by the input or output device parasitically oscillating. Designing PCB's for Tube circuits concerns me regarding the trace properties- specifically accommodating the higher voltages and current demands for the tube amplifier. Help support this channel on Patreon https://www. Under these conditions, the output can be connected to the 12AX7 grid, creating positive feedback that can force the circuit into oscillation. Maybe a small amount of feedback will tighten it up, but I am not going to try that now. Dec 17, 2024 · I’m using separate lab supplies for these test runs. Please This chapter contains sections titled: Types of oscillators, Conditions for self-excitation in feedback oscillators, Conditions for oscillation in the tuned-plate oscillator, Build-up of grid-bias voltage in the grid-leak-and capacitor circuit, Design of the tuned-plate oscillator, Other feedback-oscillator circuits, Oscillation in amplifiers, Resistance-capacitance oscillators, Negative the loop will be at about 35MHz, where the amplifier has a lag of –261° and the feedback network lags –79°. It could be a cold solder joint somewhere. Welcome To the Hoffman Amplifiers Forum April 01, 2025, 02:57:45 pm: Welcome, Jan 16, 2022 · Could be Oscillation with D3a tube and not oscillation with 6e6p tube in the same tube amp. Welcome To the Hoffman Amplifiers Forum February 04, 2025, 05:32:57 pm: Welcome, Guest. The first beam power tube introduced to market—in the States, at least—was RCA's 6L6, intended for use as a class A 1 or AB 1 AF power amplifier. Other article which cover this subject in some degree: Dec 9, 2017 · Coming back to the problem of the soft buzzing/oscillation from this amp, I have found that it comes from having other equipment plugged in to extension cords in the same room. > what causes oscillation Gain and sneak-back. This is an (It doubled to 10 meters from a 20 meter signal on the 807 grids. Use just one hand when working on a powered amp. tube. Just having another tube amp plugged (but switched off) into an extension socket causes the sound. Feb 20, 2024 · The amp employs a pair of 6E5P in a class A PP design with input transformers. An amplifier may be required to develop as much as 1500 W of RF output power, the legal maximum in the United States. The dirtyness cannot be seen due to my low bandwidth, but its amplitude is approx. If your amp is oscillating, you should immediately be able to tell by listening. The amp has respectable distortion and frequency response: I'm sure Doug will drop in to answer questions. I wouldn't pay attention to this matter until I drove vacuum tube DUT into small oscillation under heavy overload at 20 - 30 Hz. When I max out both the master volume and the preamp volume the amp slowly begins to oscillate just like a tremolo would. . See more tube amplifier safety info here. Cons for vacuum tube amps for the RF bands. Thus, it is critical that we thoroughly understand why negative feedback can lead to oscillation and how to ensure that the amplifier will amplify, not oscillate. The company I am most comfortable with describes their traces as: Aug 19, 2013 · The same rule of correlation between stability and quality of sound seem to be applicable to tube amplifiers. All was well until I cranked the volume and treble pots, then all hell broke loose. There is no evidence of high frequency oscillation. Jul 21, 2020 · Hoffman Amplifiers Tube Amp Forum, 5e3 New Build Troubleshooting Hum/Oscillation. Jun 1, 2009 · The amp sounds nice but the bass is a little loose. Compared with the crystal amplifier, the relative life of the tube amplifier is determined by the electron tube. g. Feb 5, 2018 · I've been picking around the web to understand oscillation in vacuum tube audio amps, but haven't found a good source yet. The rectifier is near the transformer, the voltage gets via (2) to the capacitors. The driver tube is EL802 with 1k CC grid stopper and the power tube is 6P45S with no grid stopper. I tried swapping the AT7s as well as the recovery tube. I have swapped cables and pans so I ruled that out. and the screen current changes some more. Welcome To the Hoffman Amplifiers Forum March 25, 2025, 11:08:52 pm Jan 14, 2022 · +1 on post upper i'm doing it regulary, PP with CFB. But that may not even be possible with a triode output stage - triodes take much more drive Jun 1, 2009 · If it’s a problem with layout parasitics, you would get a high frequency oscillation (well above audio). Perhaps the builder would like to build a mono-band amp first, to gain experience and confidence, before proceeding to a more complex multi-band design. Generally speaking, it is only a few thousand hours, but Jun 1, 2009 · When my tube SET idles and I touch the output tube with a coax cable with 2 inches stripped, I get a dirty sinewave of 500kHz and 10-15mV amplitude. You can hear the ring with the amp reverb turned to 0. For this example, we will assume that it is desired to make a three-stage vacuum tube phase shift oscillator with a nominal oscillation frequency of 7Hz. You can make a simple RF detector for your meter. Do this while the amp is in oscillation to see if you can get it to stop--if it's ultrasonic oscillation then feed a signal into the amp so you'll know when the oscillation stops. The trade-off for increased efficiency is slightly more distortion--roughly 1/10,000 part, or minus 40db. While going thru the Heathkit manual during the porcess of rebuilding two 1956 W-5M amps I was thumbing through the troubleshooting section and came across a way that I had never heard of to prove out if a amp is oscillating with out the use of a O scope, I will type it out here as printed in the manual for the most part. What frequency is your oscillation with the tone stack at "neutral"? Parasitic oscillation is an undesirable electronic oscillation (cyclic variation in output voltage or current) in an electronic or digital device. At the stock capacitor size, it needs to be provoked as described above. In this video I show how to find out in a “hands-on” way where a parasitic oscillation in an electronic circuit is prese Jan 23, 2014 · The amp overall sounds a bit too bright and hi fi. 2mV. Tubes with long thin single grid leads are the most unstable at VHF, while tubes with thick wide multiple grid leads provide significantly higher grid self-resonant frequencies. This indicates there is an unwanted Mar 28, 2021 · Switching tube doesn't help, so I would rule out bad tube. A VHF parasitic oscillation begins in the anode's VHF resonant parasitic circuit. These tube amps are designed for softer genres, so they don’t perform well under extreme pressure. If you hear pinging, or a change in this noise, then that tube is suspect. Following are the steps necessary to design a vacuum tube phase shift oscillator. Im having a strange oscillation issue. Building your own tube-type HF amplifier, doing it "The Right way". So yes, tube guitar amps do sound better than solid state. You might compare the two magnetic circuits to solid state amps versus tube amps, where the solid state amp gives it all its got then clips hard, while a tube amp compresses nice and smooth. So perhaps I am being hurt by this electrical issue, but I'd like to use the scope to track oscillation better. Hope that helps I didn't say to omite grid stoppers at all , I usually use 1K in all my circuits either it is a line preamp or RIAA preamp or Power amp , even with ECC83 and with ECC85 ( which is closer to ECC81 ) the same thing 1K grid stopper , the reason is better to be sure , only in one line preamp with the PCF200 I use 5K6 because of it's pentode high transconductance ! . Neutralization can go a long way, but it cannot fully fix a crappy tube. ti. I was given a 60s Farfisa full tube guitar-organ amp from a good friend and after a long restoration and repair project got this beautiful amp working great again, except for reverb. Welcome To the Hoffman Amplifiers Forum April 02, 2025, 01:48:59 pm Jun 1, 2009 · The amp stays clean, if under-powered, then starts acting like a proper 5W tube amp loud yet busted fuzz pedal before it goes into oscillation. This may be the amplifier itself or components and leads outside the amplifier. Oct 9, 2014 · For your application, depending on the frequency of oscillation, you might hear whistling, whining, a sound like rushing air, or "motorboating", which is kinda like a slow "wub-wub-wub" noise. Feb 12, 2022 · I've probed the supply rails with a signal tracer and they have practically no interfeering signals on there, I've tried removing all the preamp tubes, and the PI tube, which didnt change it, I removed one of the 6P3S output tubes and that reduced the magnitude but it's still there when I remove both output tubes the oscillation stops. May 19, 2011 · When the amplifier is turned up past 4 on the reverb channel, the amplifier breaks into a horrible parasitic oscillation which causes distortion and a crackling/sputtering sound to the notes. Jul 1, 2023 · Do not test new power tube without getting that oscillation fixed because that might stress it! One test is to test power tube as Triode operating moving Screen feed to Anode. com/ti-precision-labs-op-amp May 9, 2020 · The active life of tube amp In principle, the life of the tube amplifier is semi-permanent. The theoretical life of the electron tube is not too long. Both have a smooth, continuous power stroke--and neither one is very efficient. It is often caused by feedback in an amplifying device. The clue that told me that it was NOT oscillation is that only the amplitude changed, not the frequency. In Fig. WEBER:"Your amp has a parasitic oscillation. VHF instability is almost always caused by a high impedance (or even parallel resonant) path from grid-to-ground. What ideas do you guys have? I have googled endlessly but finding any tips on tube amp oscilloscoping is pretty much a needle in a haystack event. That shown in (a) illustrates the effect given due to noise introduced by a faulty component. It is like a phantom circuit. Sep 14, 2023 · Driving an amp into clipping with no load can create very high voltages as a tube abruptly cuts off (and the energy stored in the output transformer's inductance has to go somewhere) - it can arc over the output transformer, tube socket, or tube itself. Jul 15, 2022 · Hoffman Amplifiers Tube Amp Forum, Negative Feedback Causing Oscillation? Welcome To the Hoffman Amplifiers Forum April 01, 2025, 03:04:41 pm: Welcome, Today, a "high-Mu triode" usually indicates an amplification factor of 100 to 240. Sep 15, 2020 · - Poked around with chopstick: basically tried to dampen every component inside of the amp - Dampened each transformer and tube - I even played the exact loop through another tube amp and verified this amp did not ring Here is a great demo video from today. Dick Ehrhorn, W4ETO, contributed materially to this section. 5/ 1+( )f 8MHz 2 =0. 12ax7 into parrallel 12at7 output stage. This excludes the amplifier’s B+ and bias supply as a source for the oscillation. 5w input , this tiny amplifier is providing about 50w output. The parasitic oscillation may only occur at some input signal levels. Jan 1, 2012 · While there may be exceptions, neutralization of a high frequency PA (power amplifier) vacuum tube amplifier has little to do with VHF or UHF stability. I tested with several speakers and all speakers cause this oscillation. Add feedback from output to input that is phase shifted 0 degrees. EL519/6P45S (6П45C)Tube Amplifier Tube 6P45S is a EL519 equivalent 300w amplifier 4w input on 20m This following one using a single 6P45S and provide about 130 up to 150w 150W amplifier with one tube with N7DDC ATU display on the front panel With about 1. Outside of the reverb oscillation, the amp behaves like a deluxe reverb should, and sounds good. If your vintage audio amplifier is oscillating, this video shares a few tips on how to determine the cause. Loss of pow Feb 20, 2010 · Basically, high frequency oscillation (high-pitched squeal) can occur within a tube based on certain factors even if the circuit the tube is in was designed in such a way to eliminate them (which Nov 21, 2023 · Measured B+ with Oscillation present: Barely fluctuates (900V +/- 5V, with 50mA Bias per tube which somewhat keeps oscillation under control between 10 and 120mA so not to stress the tubes too much) Measured Bias Voltage with Oscillation present: Barely fluctuates (+/- 1V) Measured DC Offset on Grids: None apart from the Bias Voltage To check preamp tubes, there’s an old trick of using a chopstick or other softish, nonconducting material, to gently tap on each tube while the amp is on. The negative feedback for the power amplifier is normally applied from the loudspeaker to the input tube of the power section, after the tone stack. This is the target frequency where we want to apply dampening, and of course it varies a great deal with amplifier construction and the construction of the tube and socket. Vacuum Tube Evolution. I've checked the wiring of this stages and everything looks fine. Aug 5, 2022 · Dealing With Oscillation Problem In Tube Amp. In effect, the entire amplifier becomes a relaxation oscillator. C1 is driven with a signal generator, L1 is selected to match the FET output to a 50-ohm load. This looked like a constant 30 KHz haze over the amp's output which changed when you moved stuff or got your hand near a tube, just like a real oscillation would. If it's a bad tube though, which it could very well be, well then you're in luck. Hoffman Amplifiers Tube Amp Forum, KT66 SE amp NFB loop oscillation problem. The amplifier’s 22dB times the feedback –19dB yield a Apr 5, 2008 · The first two gain stages are at the middle tube, then left side of the right tube, both left tube stages, master volume, last stage and power amp. Before application of global NFB, first use a feedback resistor of say 50 - 100 times the given value. Lethal supply voltages in the range of a few 100 Volts up to several kV are used. May 30, 2006 · If it is a parastic oscillation, send it back. It could also be the extra gain pushing the tube into self oscillation. For a 1kW amplifier 100 to 150 MHz is typical. 1 his chapter describes the design and construction of power RF amplifiers for use in an Amateur Radio station. Jun 1, 2009 · Take an amplifier that is inverting from input to output (180 degrees). The 812 was a moderately low mu tube, primarily intended for grid driven modulator service. My oscilloscope is dead and I'm off work or I'd borrow one from there, but testing shows the issue is 100% coming from the gain stage (ignore my comment in the video, I've never noticed a power tube Dec 24, 2023 · If the reverb pot was at 5+, the amp would break into an oscillation. I also can’t quite tell from the picture where you have the twisted black and white wires that cut across the amp. It should be easy to wire tubes up to an OPT the correct way. This is a boutique tube amp modeled after a Fender Princeton. Amplifiers oscillate and oscillators don't. The AL811 tube is a descendent of the 812 tube. I think the amp with D3a triode mode ( u =77) sometimes oscillate. … RF Power Amplifiers and Projects 13. It could be another issue as others have described, but I’ve seen this exact issue from preamp tubes going faulty. Replace the tubes and bias the amp. what Ive If I move the amp into my dining room, the amp has no such whine when dimed. Page 3 of Mar 13, 2014 · Since the tube continues to operate as an amplifier even though it is in an oscillatory circuit let us study some of the important characteristics of vacuum tube amplifiers. Similarly, this pressure can result in cracks. I played some different music while turning the knobs. Here the power supply circuitry is of my own design and probably not quite as good. https://training. Feb 9, 2006 · When a power amp goes into HF oscillation - going RF as we say - it will usually still pass audio, but it does have a certain weak, washed out sound to it, and it is noisy in a hummy sort of way. The input txs are vintage JBLs with a number of taps that Doug has used to include switchable gain. The feedback can compensate for an underbiased amplifier at low frequency, but not for a high frequency signal like 10kHZ. if is oscillating i'm changing g1 and g2 input wires. I am suspecting a parasitic oscillation issue probably in the reverb section of the circuit. The 572B is an indirect descendent of the 811 tube. The fact there is oscillation can in itself be construed as a "a good thing" inasmuch as it indicates there is gain in the circuit. Conceivably, if the 3 RC filters are driven by a 12AX7 amplifier, the output will be in phase with the amplifier's input and will have a greater amplitude than the input. That said, amps with four power tubes typically pair two sets of class-AB-operating tubes, working much like a two-tube amp but adding power to each half of the sine wave. The reasons for this parasitic-oscillation problem are the same as the reasons for the parasitic-oscillation problem in the Trio-Kenwood TL-922 amplifier, which was discussed at length in an article that begins on page 36 in the October 1988 issue of QST Magazine. This "smoothen" how power tube operate Jul 7, 2004 · M. Jun 1, 2009 · Oscillation caused that way can overload the amplifier, thus causing extra peaks/spikes, which can spike through/blow an OPT. That means our AC4 is class A, too. When the signal crosses the zero line, the amp attempted to compensate for the bias but was too slow. Apr 19, 2019 · I noticed what appeared to be a slight LF oscillation in amp, roughly at 2hz. Jul 19, 2017 · If you have not been trained to work with high voltage then have an amp technician service your amp. It can be shown that as the Jul 15, 2013 · Any amp with a single power tube (aka "single-ended" amps) will always be class A—that single tube must handle the entire wave. Aug 26, 2021 · "This is an oscillation at about 1 or 2 Hz, and is invariably caused by unintentional feedback via the power supply, due to the rising impedance of filter capacitors at low frequencies. Jul 7, 2008 · Have used the likes of expressPCB and others with success- but strictly for solid state circuits. A fifty+ year-old parasitic-suppressor design that was usually successful at preventing oscillation in an amplifier-tube with an amplification of 40, may not be as successful on a modern amplifier-tube that has much more gain. e. At this phase and frequency, the amplifier still has 22dB of gain while the divider gain is 0. Launch. [3] Apr 2, 2023 · Hoffman Amplifiers Tube Amp Forum, Blackface-15 Amp Feedback / Oscillation (Solved) Welcome To the Hoffman Amplifiers Forum February 05, 2025, 04:27:38 am Jun 1, 2009 · It took me a day to find a bad clip lead on a bench top prototype. Use a "filter cap on a stick" (500pF cap with a ground clip on one end) to filter all the tube grids to see if you can kill the oscillation. It doesn't change much with the first stages at the left tube. Welcome To the Hoffman Amplifiers Forum April 05, 2025, 12:37:31 am Feb 7, 2021 · Hoffman Amplifiers Tube Amp Forum, 65 Bassman oscillation problem. (sub-detail of Murphy's Law) Theory is little help. A tech will have to look at it. Mar 15, 2025 · Amplifier Oscillation. All of the voltages are spot on. Jan 15, 2022 · Ordinarily, one doesn't include the tone stack within the overall feedback loop. I’ve got a couple amp builds under my belt with satisfied customers and I want to revisit this first amp build of mine (which I’m keeping for myself) to iron out the kinks, namely the oscillation issue at high volume knob settings as well as possibly replacing some of the ugly wiring job, possibly with cloth wire as I’ve really taken a As with all electronic oscillation, motorboating occurs when some of the output energy from an amplifying device like a transistor or vacuum tube gets coupled back into the input circuit of the device (or possibly into an earlier stage of the amplifier circuit) with the proper phase for positive feedback. Amplifier design: An amplifier with a gain of greater than 29 must first be designed. When it is large, the amplifier can breaks into a high amplitude oscillation. Tone controls introduce substantial phase shift. " (page 467, Valve Amplifiers, 4th edition, 2012) Jun 1, 2009 · 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 Feb 20, 2024 · The amp employs a pair of 6E5P in a class A PP design with input transformers. CHECK THE BASIC AMPLIFIER Mar 1, 2022 · Please read the description/textbox. Any suggestions to troubleshoot this? Hoffman Amplifiers Tube Amplifier Forum » ; Amp Stuff » ; Tube Amp Building - Tweaks - Repairs » ; need some help with nasty parasitic oscillation Sep 1, 2016 · Re: I need some help troubleshooting a low frequency oscillation in a tube amp « Reply #22 on: September 01, 2016, 11:11:30 pm » Hi Something like a 0. Might want to scope it to see if the “53BPM“ is a low frequency oscillation, or high frequency bursts, about a second apart. To find the VHF parasitic resonant frequency yourself, unplug the amplifier from the mains, and couple a dip-meter to either side of the DC blocking C. Hoffman Amplifiers Tube Amp Forum, Question About Parasitic Oscillation-Resolved, Thank You. Take an amplifier that is non-inverting from input to output (0 degrees). May 13, 2016 · Oscillation in any amp is caused by poor layout, which in PTP tube amps means porr grounding and poor lead dress. Page 3 of 3 [ 28 posts ] LM380 Bridged Guitar Amp, Oatley K301 Phono Pre-amp, Oatley K272 Headphone Amp, Tube proto-board. I found that the amp sounded better by reducing the B+ from 380 to 350 volts and turning the tube current up to 75 mA each, yeah way over the line. If there is 470 ohm screen resistor move its feed from B+2 to tube Anode. It's not getting its feedback through that. ECL86 (triode+pentode) was missing from chassis and driver transformer was dead (primary windings open reading). Throughout history, to speed development and reduce tooling costs, newer tubes use tooling from older tubes. May 31, 2019 · The size of the cathode bypass capacitor on the input tube seems to affect the amplitude of the oscillations. Sep 1, 2016 · Re: I need some help troubleshooting a low frequency oscillation in a tube amp « Reply #24 on: September 02, 2016, 12:00:14 am » Hi The ceramic bypass goes in parallel with the electrolytic. 1uf ceramic from the cathode to ground may help. Welcome To the Hoffman Amplifiers Forum April 03, 2025, 07:46:21 pm: Welcome For a 10kW amplifier 60MHz is typical. Too much Any claim an amplifier tube that saturates at a few amperes cathode current can provide 50 or more amperes of "big-bang" current from accidental oscillation is profoundly ridiculous. I spent about half an hour poking around in the amp trying to figure out what was going on until I happened to notice the voltmeter on the variac was dancing at the same rhythm the scope was. Jun 2, 2009 · Hi Wavebourn, I removed the tube from the first two stages (12AU7) and turned the amp on, the problem doesn't occur. (c) shows the clipping given by too low a screen-grid voltage in a voltage amplifier. That is an oscillator. To solve oscillation problems, the source of the oscillation must be located. Hoffman Amplifiers Tube Amp Forum, Gibson GA-5T Oscillation 1961? 6AQ5 6EU7 Version - How to Fix. Apr 15, 2010 · This amp was underbiased, clearly visible from the zero crossing on the second picture. “Lethal” is no joke; if you touch the high voltage side of a tube amp, you are lucky to survive this. qxvpz llnj sjy gbuh popjtb zsyd zzobdam mxwfkb frkwl pgp tuxy gtyh apma nsami vigo