How to fix tube amp parasitic distortion Grid stop resistors D-lab electronics
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- čas přidán 12. 09. 2017
- This is a boutique tube amp modeled after a Fender Princeton. All was well until I cranked the volume and treble pots, then all hell broke loose. Loss of power, squealing, rotten performance. Took me for a spin down troubleshooting lane. All is well now.
- Jak na to + styl
@Terry....KI4DBK here, I love D-Labs. Have been watching your hamfest videos for two days while recovering from a sinus infection. Wow. I've watched a few of your amp videos but the fest ones are awesome. Keep up the enthusiasm. Hope to see you at a hamfest soon! Best 73's, thank you so much Terry!
KI4DBK
I love all your vids Terry, thanks for doing what you do!
Cool. Now lets fix that 0 volt point crossover glitch. That would make it the best amp ever. Amazingly clean.
Excellent tutorial Terry!
Once again an excellent job Tery !
Thanks for another informative video. I always enjoy these.
Leo Fender based all his designs out of the old RCA tech manual they evolved and improved over time. Grid stoppers 1.5k or less is sufficient nice article! 👍
That is great! Thanks especially for the disclaimer at the end!
Parasitic predicaments are problematic! Great video!
very cool info . Terry . I do remember this syemtem many yrs. back. I don't believe i really understood why till now. thx.! Cheers!
Another relevant issue identified and rectified to add to the tube amp "symptom list". Thank you once again!
Great video Terry! Keep 'em coming
A beautiful example of parasitic oscillation and the fix. I am having trouble with parasitic oscillation in an RF circuit on an old solid-state FM tuner. I wish I could find its source. Thanks for the great video.
Another great video. Very useful information.
Terry - this helps me with my problem, at least one of them :) I sent you a video today via email of my problems... thank man, will check the Grid Stop stuff out on this amp.
thanks
Ron
Huzzah! I sit down and check out my subscriptions, and lo, here is a D-lab video!
I still don't do anything more than soldering, but I am drawn to these videos. Thanks, Terry! Thumbs up.
Thank you for your support my friend
Thanks so much!! I had built an amp, gone over it many many times, and it just wasn't sounding right. It had parasitic oscillations, and I moved one wire from the OT (since it already had screen resistors) and fixed it!
Thanks for sharing your knowledge.
excellent as always
Thanks Terry. You are the man.
Thanks man!
Great thanks again for another great video.
This was very informative!
I don't normaly believe in fate but last evening I was monitoring my output of my amp with my scope and saw that same oscillation thinking it was an issue with my Treble pot. Then next day your "parasitic distortion" video was in my CZcams Home page to watch. You may just have save my brand new set of tubes!!!! I owe you a case of beer. I ordered a pair of 1.5k 1 watt resistors ( looked like 1 watt that you used) and will let you know if that elliminates my oscillation. I will be watching more of your enlightening videos. Thank you
Well, it itruned out that the 571 Khz oscilation I was getting was in the Tone Stack. Tested the caps, resoldered the joints and repositioned them to space them away from each other better. Not sure exactly what was causing the oscillation but happy it doesn't do it anymore. Left the the Grid Stoppers in as well as added a Screen Stopper. Thanks for giving me the incentive to presue the issue more closely. 73 de Harmen VE3EMA
Brilliant!
good advise
Good lesson
I been fixing tube amplifiers for 40 years, I fix most Audio Research models.
Audio Research is a High End audio manufacturer based in Minessota.
I use digital oscilloscopes and high end fluke multimeters for doing my jobs.
Cheers.
Great job Mr. D Lab, I'm wondering if the length of the grid wires contributed to the oscillation. My teaching is from electronics mags from the 1940's & 1950's & the construction advice was always to keep grid (& plate) wires as short as possible.
good explication thanks
The parasitic oscillation is high frequency above your hearing, so when it happens you can see it on the scope but can't hear it from the speaker.
Hi Terry-thank you very interesting !
Can I ask you ,would it be possible could you do a video on neutralizing
parasitic instability on a tube type transmitter pa.
I have just finished renovating a tube set and I just cannot get the pa circuit to settle
Thanks once again
Thank you
Killer walkthrough. I would like to shoot you a few handmade wire wound resistors to use in upcoming videos, on the house if you are interested. Just let me know and ill get them shipped out. Take care, look forward to future videos.
Terry, does it again!! Totally unrelated = please comment on soldering equipment aka your wisdom on this subject...thanks
Thanks for your videos. I´am from Dominican Republic and I have a Hot Rod Deluxe III, with a white or pink noise....after a repair what could be the problem?
Thanks! I just keep wondering how you know so much.
Terry, you are the man!! I have a drake tr4 that needs attention... do you take on ??
Could you add a ferrite bead on the grids instead?
Great! Any comment/videos re par. osc. @ input stage? Thank you Terry
does it apply to bassman 6G6B which doesn't have grid stop resistors ?
Do you need scope to fix parasitic ocisilation
I'd like to see why the amp clips so asymmetrically.
i have a mesa/boogie S.O.B. (son of boogie) from 1984, when i turn the mid knob, during the turn, it does exactly that, i blew a fuse, opened up the amp, the 2watt 470ohm resistor across the 6L6 jack was split and toasted, no grid stoppers....
i suspect that it will keep roasting powertubes and eating resistors when i turn the mids knob, if i dont remedy the lack of gridstopper resistors, is this correct?
is it safe to use 500ohm 5w in place of the 470ohm 2w? i tested the unburned one, and it read 578ohms!!! that was wayyyy out of spec,
can i just send you the amp? it is a real beauty, and the loudest 60watts ive ever heard inmy life, i am afraid to screw something up and destroy it, usually repairing things is a breeze, this amp however is primitive and identical to the first amps he built for carlos santana, before the Mark1 and Mark2, so looking inside you might scratch your head at some things...
its been laying open now for 3weeks, please help....
I have the same problem but with a single ended KT66 tube on a jazz guitar circuit. At full treble the signal goes almost flat. Although there is a grid stopper, I wonder if the value is sufficient or if it might not be placed close enough to the KT66 grid. Any suggestions?
I've been looking for random noises for a couple weeks, ticking, feedback, all kinds of Halloween sound effects would come and go. UNTIL the power scaling MOFSET gave up the smoke but not before making the amp suddenly shriek at 111dB while I tripped over every cord on the way to the power switch.
Just a short while ago.
I jumped out of my f-ing skin btw and I haven't blinked for the past hour and a half...
All fixed! Jumped right over that mess and it's all a normal power supply. I had no idea all that noise could have been that stupid MOFSET
Had a similar problem and found the solution was the old tubes had much less gain than the new so temp. fix was use old tubes until I could fix the amp correctly for modern tubes. I grew up on tubes and poor gain. LOL
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Grid stoppers are theorical. The best is to always test FIRST w/O them, most of the time, is all good ! Adding grid stopper will makes the sound crispy. And awful. I don't know why, I had to remove them from every single amplidier I've build to achieve the best sound. I'm building HIFI amps and make every calculations myself, and always double check ! Any time I was using grid stoppers, I realize sound was not the best possiblez out of any amplifier. The only case is if an oscillation won't stop by any other means, sometimes, it may helps. But it's seriously a joke to use them systematically, always test first without them !
Hey man, great video. I'm new to your channel, perhaps you could provide some insight? I have an amplifier that I built (yes, grid stoppers included) that when you turn the volume up far enough, the unit starts oscillating in a kinda whistle sounding format around 6.6khz with an odd waveform and the frequency seems to change when you alter the control around that point. When you turn it up even further, your actual guitar signal dies away and goes quiet. The high gain channel is even worse because its always in enough oscillation to make the signal splutter etc. but thats because it has lots of gain stages like modern high gain amps. I've done lots of research on star grounding etc. and applied it but it just doesn't fix the problem, any tips? The only thing I haven't done yet is use shielded cable, but I doubt that will stop it.
Possibly you need a shielded or re-routed input line. A common oscillation source is feedback from the high-gain input stage to some place later in the circuit, often but not always the output stages. It often acts like you are describing.
Was Leo Fender the first person to put grid stop resistors on tube amps like this?
Great tip.
Can you tell me the manufacturer of the work light/magnifying glass articulated arm used on your bench please......
Gee a scope I have had a least one for forty years
Should all output stages have grid stoppers? Single ended and push pull? Do the grid stoppers attenuate any noticeable output single? Great video! I think the essence was the significance of the use of the oscilloscope for trouble shooting. You are so right. If you can't hear it, you can see it! Thankz
In modern amp design, all tubes, preamp and power amp should have grid stoppers. It reduces blocking distortion and parasitic oscillation. The grid of a vacuum tube normally has very low current. The purpose of the grid is to create an electrostatic field that varies the amount of current between the plate and the cathode. There is no downside to using grid stoppers that I know of, beside the cost of the resistor.
The input capacitance of tetrodes and pentodes is very low, so that grid stoppers cause negligible loss of signal.
I did some research (read a book) and ... A grid stopper, in conjunction with the inter-electrode capacitance of a tube, forms a high pass filter. The traditional guitar amp input circuit is designed to work with the (approx.) 100pF inter-electrode capacitance of a 12AX7 to cut frequencies above 20kHz and thus 'stop' radio frequencies getting to the grid of the preamp tube to be demodulated and amplified. (Passing taxi, anyone?)The internal capacitance of a tetrode or pentode is tiny, thanks to its screen grid, so that large value grid stoppers can be used. The filter created has a high cutoff frequency which causes little or no loss of treble, but shunts even higher frequencies to ground, preventing the build-up of hypersonic oscillations.
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Well to start I have a complicated one Peavey classic 50 for 10 early 90s, 91 92 complete gone through everything all new caps tubes new fan it needs works so, then I have a fender Super Champ XD same thing those are the two I want to start on if the money's right and the jobs done right which I don't have any problem with that after I've watched you we could take off from there I have a lot of work will get you over some photos to it so you can see what's going on I'm assuming you have a website or web page? I'm going to look for it stead of leaving names and email addresses online where everybody could see him. Thanks
Terry, great stuff. Hard to tell from the video, but I would think the Grid Stop resistors have to be at least 1W or 2w given their role. It looks like you may have used 1/2w but I am not sure. Either way, excellent job of explaining this and fixing it. Awesome. - Ron
Hello Ron, Screen resistors are normally from 2-5 watt. 1/2 watt for Grid resistors is fine, very little current flows thru them.
You are absolutely correct Terry.... got my Grid and Screen mixed up. !! Thank you
And that, my friends, is why you need an oscilloscope that responds in the MHz region even though you're "only" doing audio to 20kHz!
I have exactly the same problem with my proluxe but it have actually that resistors on it...in my case have to be another problem... : (
Mr. D Lab I am the owner of 5- 6 20 to 30 year old apps that need repair I need to really talk to you or someone about connecting and possibly doing some business, how would you recommend we start ??? P. S. I am in Hawaii period
Would a ferrite bead work just as well?
A ferrite bead would be used for RF parasitic oscillations. Not so much for ultrasonic audio frequencies.
Hi Terry. Scopes are pricey. For amp work can you use a single channel scope or any recommendation for an inexpensive scope that will do a good job?
Yes Sir, Check out basic scopes made by Heathkit/Leader/Hitachi/Iwatsu, etc on ebay. Most can be obtained for around $50.00. 10mhz model is fine.
D-lab Electronics thanks Terry!
Terry, what brand of oscilloscope and model do you use?
Hello, That is a Iwatsu 5702, my 1st scope. Great machine
interesting, i don't know anything about electronics but i came here since i wanted to know what it means that some amps dont take pedals well, a guy mentioned that this could happen
Nice scope...a demonstration of what it ACTUALLY SOUNDS LIKE would have been more helpful to most of us
Way out of human hearing range .......but destructive and prone to intermodulation .
You DID hear what it actually sounds like! You can't hear a megaHertz, but you can hear when it robs the amplifier of audio-frequency power.
Correct me if I'm wrong but you aren't getting full output now. Is there a way to determine the exact resistance needed to back the signal down to the right point?
You could jump in different values of resistors with alligator leads hunting for the min necessary value. Or if you have a dual pot, you could jumper it in circuit, dial up a setting that works & then measure it for the fixed resistances to put in. I doubt there's a formula for calculating the right value in advance.
You will not hear the difference in volume with a 1,5k resistor in the grid circuit, yet it will effectively block oscillation.
You had a speaker connected and you forced the amp to oscillate. I heard a thump when the oscillations started but nothing more. I assume the oscillation is thus ultrasonic and can't hear it. Am I correct?
Yes.
Ultrasonic and also RF
It should also be noted that using carbon composition resistors as grid stoppers would be best.
Why is the NEUTRAL fused?
Take a look at vintage fender amp schematics
Nice job wheres the wine?
Use carbon resistors. High freqs travel the surface of metal film resistors.
Had similar problem with Blackstar Artisan 15 czcams.com/video/zH-6yWqo49A/video.html problem occurred by 2 insulated wires touch each other, I’ve separate them and it’s gone, unfortunately one more thing I can’t get rid off is volume balance between channels, channel 2 is much louder and full of juice while channel 1 is noticeable weaker, its specific built with ch1 on ecc83 and ch2 on ef86, tubes has been checked ok. any ideas ?
Can't help noticing insect flying around first 30 seconds
Save your 6V6s fry your ear drums. (Smile)
Thank you for the response, hopefully it's you, so do I give you my email address, do you give me yours, or do I give you my phone number do you give me yours ! OR do we you send your dove to me with a note or I'll send my dub to you with a note how do you want to work this you are the doctor and plus your drink if you didn't drink wine I wouldn't even associate with you.
Hello Mark, I have not responded, so be careful. What make of amp? What does it need?
No technician worth callng a technician will go without a scope. Choosing to do electronic troubleshooting and repair without a scope is choosing to work blind.