microphonic preamp

[lumox0013]

I just recently got brave and started building after a few franks , I built a liver pool sounds great but the first pre amp tube is miccrophonic no matter what i put in there (good tubes out of other amps ,au at ax,)if I use a 12ax the whole chassis becomes microphonic.now I am experiencing the same thing on a 5e3 I've just finished. I was hoping somone here might have some idea what is going on? any help would be greatly appriciated!!! thanxxxxx lumox

BTW: thank you to every one on this forum I never would have atempted any of this without your help.ampgarage is the greatest amp resource ever!!!!

[collinsamps]

It's a grounding issue. You've either got a ground loop or to many ground references. Unless running a star ground scheme, typically The first few filter caps should be tied to a ground lug near the transformer along with the PT secondary centertaps and power cord ground. Other board grounds should go to a buss that ties all pots and input jacks together. If you have shielded cable on the input or V1 to volume pot, make sure that only one end is grounded.

I've had repairs which have had microphonic chassis that take hours to figure out and it can be frustrating.

[pinkmarkos]

I had some success taming the microphonics in my express and liverpool by putting a small value resistor (grid stopper) in between the shielded input wire and the grid of V1a (I used 33k). This seemed to help a lot with the microphonics- not totally gone, but I can use a lot more tubes in V1 now. Also, I believe there are some other suggestions in the build guide if you are using that as a reference. Good luck. MP

[paulster]

I've had exactly the same thing that was attributable to the shielded cable between the input jack and V1 grid.

I deliberately chose an ultra-low capacitance cable, but it was so microphonic as to be unusable in these amps.

A reel of RG174/U sorted that out.

It's also likely to be a cold solder joint.

What type of coupling caps are you using? Some of these can be quite microphonic and a blob of silicone may be your answer.

If you're getting nowhere after all that, take a look at this thread:
https://tubeamparchive.com/viewtopic.php?t=8561

[collinsamps]

I've found that the location the shield ground on the input line makes a huge difference in noise and can easily cause ground loops if not grounded at the same location and the input jacks and pots(ie brass plate, ground buss, chassis tie point). Moving it only an inch or so can make the difference in dead quiet or insane hum.

[paulster]

I've found that the location the shield ground on the input line makes a huge difference in noise and can easily cause ground loops if not grounded at the same location and the input jacks and pots(ie brass plate, ground buss, chassis tie point). Moving it only an inch or so can make the difference in dead quiet or insane hum.

I agree, but the original poster is complaining of a microphonic chassis, not hum or noise.

[collinsamps]

Yea...........so I said hum & noise in this one and not microphonics. I was simply stating issues with shielded input lines since three other posts said it might be the source of his microphonic chassis. If the guy is dealing with it and wants to move it around, knowing where to ground it might come into play ............ya think?

What time should I muster for the firing squad Mr Mod?

[paulster]

What time should I muster for the firing squad Mr Mod?

There's no need to be a jerk.

The poster complained of a microphonic chassis, which you immediately attributed to a grounding issue as an absolute. I'm sorry, but there are all sorts of reasons why you can get microphony on a chassis and grounding issues are not the be-all and end-all. That kind of statement could easily send him down a long and fruitless path when there are other things that could and should be evaluated as well.

Your subsequent post clarified that grounding issues can be a source of hum and noise, which I absolutely agree with, but my point was this wasn't what the OP was complaining about, so it looked as though you'd actually mis-read the OP's problem. Going back and editing the post doesn't help either.

You've already posted in another thread that you haven't dabbled in Trainwrecks, which have a very well documented layout and grounding scheme that works. It isn't the world's best, granted, but it works and so long as you follow it you can have an amp that doesn't end up any problems that necessitate a redesign. Provided you've followed it you shouldn't need to change it, although there is some residual hiss that can be removed by a few tweaks, as Roe has found if you've been following, but that's not relevant here.

How about being a little less sensitive and directing some of that energy into something more constructive, and maybe this guy can get his amp working properly?

[Structo]

Did you see this thread in the Trainwreck section?

He talks about isolating V1 and using a new type tube dampener.

https://tubeamparchive.com/viewtopic.php?t=8561

[Andy Le Blanc]

Jeez....... relax......

Take you aggression and give the amp a few good thumps, If it makes funny
noises it didn't pass. Very precise. Start with the tubes, tap and wiggle.
You've got more than one amp displaying the same issue, it could be bench parts.
I've suffered that. Check all the tubes. Repeat thump test. Inside the amp
wiggle wires, tap components, chop stick, look for bad joins. Repeat thump.
Ive had bad wire, it can happen, jiggle the signal path. Tap the caps, if you
unfamiliar with hand building you can break the end of the leads inside a cap.
Flex the component board. Try and localize a source, it could be your fault,
it might not. If you cant easily do this, you need to reduce the amp to the
powerside, from the power supply to inverter, review the circuit, add one
stage at a time until the issue happens, and then you'll know where generally
the fault is. Repeat thump test. Trouble shooting is process, the real cause
of the problem might be a surprise, don't jump to a conclusion, especially
in the face of so many so called expert opinions.

[collinsamps]

Your subsequent post clarified that grounding issues can be a source of hum and noise, which I absolutely agree with, but my point was this wasn't what the OP was complaining about, so it looked as though you'd actually mis-read the OP's problem. Going back and editing the post doesn't help either.

Like I would edit my post because you yelled from the heavens and said I screwed up? Is this Paulsters Amp Garage?

How could I have misread it if I addressed it in the first post...........you got your panties in a wad because I said hum & niose and not microphonics in my second post.............are you also an english Prof on the side?

You've already posted in another thread that you haven't dabbled in Trainwrecks, which have a very well documented layout and grounding scheme that works. It isn't the world's best, granted, but it works and so long as you follow it you can have an amp that doesn't end up any problems that necessitate a redesign. Provided you've followed it you shouldn't need to change it, although there is some residual hiss that can be removed by a few tweaks, as Roe has found if you've been following, but that's not relevant here.

How about being a little less sensitive and directing some of that energy into something more constructive, and maybe this guy can get his amp working properly?

And I'm the sensitive one? ...............I didn't realize that there is only ONE way to do things in a trainwreck.......Kens and Yours.........Is that two ways or did you just take everything he did as gospel and yell at anyone who mentioned that there may be another way? Do you have thoughts and designs of your own or do you just pound out trainwrecks all day?

I can see that you are so worried about the guy getting his amp going that you post two topics to tell me I'm I jerk..........thats gotta help him a lot.

PM me if you want to yell at me directly please

[collinsamps]

especially
in the face of so many so called expert opinions.

Where did anyone call themselves an expert? I take it by "so called" you mean that someone did?

[Andy Le Blanc]

Seriously, If your having a bad day, its ok. But save the crap slinging for the county fair.
Its the poor guys first post, WELCOME, yes we like to throw poo, but most of us also like to build amps,
and, if you couldn't tell, are passionate about it.

[ampdoc1]

I'm no expert, but I believe everyone is over thinking this. I just finished a D'Lite a few weeks ago, and had microphonics and feedback that no change of tubes would rectify.

I finally had a "vision", and I put a small rubber band around each preamp tube. The glass in the tubes was resonating.

I hadn't run into this before only because I usually install grommets on the tube bases, but I forgot on this amp.

a'doc

[Kregg]

I built an amp that crackled and thumped when ever I adjusted the volume. I fixed it by eventually replacing the pot ... or so I thought. It sounded great so I moved it into my jam room. I plugged into a different wall outlet (the one without the power conditioner) and the volume began to cut out when I adjusted volume or tone. It was all over the place. Next I noticed the indicator light would dim, then brighten, and if I punched in the tremolo the amp would play louder; kind of like a booster.
I decided it was time to crack the amp open, and get surgical with my trusty ole chopsticks. Nothing, I wired that amp perfectly. I decided to rechecked the schematic and lo what did I find? Oops ... I didn't have a proper ground wire to ground lug at the first speaker out jack. Now it's perfect!