How to fix tube amp parasitic distortion Grid stop resistors D-lab electronics 

@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

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! 👍

Cool. Now lets fix that 0 volt point crossover glitch. That would make it the best amp ever. Amazingly clean.

very cool info . Terry . I do remember this syemtem many yrs. back. I don't believe i really understood why till now. thx.! Cheers!

Parasitic predicaments are problematic! Great video!

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 RU-vid 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

Another relevant issue identified and rectified to add to the tube amp "symptom list". Thank you once again!

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.

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!

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.

I love all your vids Terry, thanks for doing what you do!

That is great! Thanks especially for the disclaimer at the end!

Excellent tutorial Terry!

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.

Thanks for another informative video. I always enjoy these.

Thanks Terry. You are the man.

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.

Another great video. Very useful information.

Once again an excellent job Tery !

Great video Terry! Keep 'em coming

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

Thanks for sharing your knowledge.

Terry, does it again!! Totally unrelated = please comment on soldering equipment aka your wisdom on this subject...thanks

good advise

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.

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

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 !

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.

excellent as always

Great thanks again for another great video.

Thanks! I just keep wondering how you know so much.

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

Thank you

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

This was very informative!

Brilliant!

Good lesson

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

good explication thanks

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

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!

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?

does it apply to bassman 6G6B which doesn't have grid stop resistors ?

Nice scope...a demonstration of what it ACTUALLY SOUNDS LIKE would have been more helpful to most of us

I'd like to see why the amp clips so asymmetrically.

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

It should also be noted that using carbon composition resistors as grid stoppers would be best.

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... : (

Terry, you are the man!! I have a drake tr4 that needs attention... do you take on ??

Great! Any comment/videos re par. osc. @ input stage? Thank you Terry

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?

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

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....

Great tip. Can you tell me the manufacturer of the work light/magnifying glass articulated arm used on your bench please......

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.

Do you need scope to fix parasitic ocisilation

Was Leo Fender the first person to put grid stop resistors on tube amps like this?

Could you add a ferrite bead on the grids instead?

Use carbon resistors. High freqs travel the surface of metal film resistors.

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?

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?

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?

👍👍

Terry, what brand of oscilloscope and model do you use?

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

Nice job wheres the wine?

👍

Why is the NEUTRAL fused?

Would a ferrite bead work just as well?

Can't help noticing insect flying around first 30 seconds

Had similar problem with Blackstar Artisan 15 ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-zH-6yWqo49A.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 ?

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.

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.