v1 tube keeps going microphonic help! 5e3 build

[fenderkid]

this is my firs time posting so bare with me . i am a college student majoring in electronics and have been playing with tube amp's for a while now . i have been using fourms since day one and have never had to post till now. i cant find the answer! i built a 5e3 head out of a hammond ao29 organ amp . used 90% of it (build on a budget ) . all new capacitiors& resistors tho . my b+ is 345v with the 5u4 rect . i changed the cathode bias resistor to a 300 ohm for proper bias , used a grounding buss instead of brass plate , heaters had center tap so there was no need for an artificial ground, added standby , 3pron power cord, and used 120v lamp . these are the changes i made from the orig circuit . i am using 12au7 in v1 and 12ax7 in v2 . im using 12au7's do to the fact i dont have 12ay7's . v1 keeps going microphonic ( tap on chassis and hear it ) . i know its v1 b/c i replace it and it goes away (for a day or two) . i tried adding pictures but i dont know how so if some one can lead me in the right direction it would help . these forms are great and would be lost with out them . thanks

think i got the pictures figured out( maybe) . i allso forgot that i used 22uf's in the power filter section .

[GloBug]

What kind of tubes are you replacing it with?

Microphonics is a mechanical problem with the tube. Some tubes go microphonic from repeated heating and cooling cycles, some are bad from the start.

Now a guitar amp is not the nicest environment for tubes, but I doubt it would cause a tube to go bad in a few days.

What brand of tube did you replace it with? Maybe an N.O.S. or vintage tube will solve your problems.


-Pictures - there is a "manage attachments" button just down below where you were typing. Click on that then "Browse" to find your pic on your computer, then "upload"
Resize your pics to around 800 pixels wide before you upload, it will be faster.

[fenderkid]

they are all nos tubes from the hammonds i have been dealing with . i have hade 4 go microphonic ! all ge or rca from the 60's . im just stumped . its a head unit with an open back so the amp is not seeing hardly any abuse . im wondering if my b+ is to high for the 5e3 preamp bias . idk

[SemperFi]

A long shot, but...is the heater voltage for V1 correct?

[trobbins]

Are you saying you 'know' it is microphonic because you hear the 'ding' from the speaker when you hit the chassis ? Do you hear any howling or strange noises when the amp is on, but you are not hitting the chassis ?

[Printer2]

Stop hitting the chassis!

Just having some fun,sorry. If you do not have any issues when the amp is running normally don't worry about it.

[Enzo]

First, I am unsure what YOU mean by NOS... You say they are "from" old Hammonds. NOS means "new, old stock". In other words brand new tubes from the past, still in the box, just were never used and are now old. That is not at all the same thing as a good tube pulled from equipment.

But NOS or new current production, ANY tube can be microphonic.

The first tube in most guitar amps is a high gain stage, and is normal for it to be microphonic to some extent.

As GloBug said, microphonics is a result of mechanical action in the tube. It has nothing to do with bias or voltage or where the grounds are. Not any more than a rattle in your car could be due to the brand of gas you use or the voltage in the battery.And as the last two guys suggested, is there an actual problem? Does it feed back or cause noises? Or is it just that you note you can tap on it and hear the sound from the speaker? In which case, so what?

[SY]

Sometimes, when a circuit is oscillating, you get very high microphonics. Stop the oscillation, the microphonics go away (I think this was what trobbins was driving at). This may or may not be the case here, but it's something I'd check using a wideband scope and a 10x probe.

[fenderkid]

ok well lets not call them nos tubes but the tubes all worked great in my other amps when i tested them(befor i put them in the amp we are talking about ) . its like my hole chassis goes micro . i put new v1 tube in and its gone . the heaters are all hooked up right . how woud i go about checking for oscillation with my scope . where should i starts and what should i look for . im kinda new with using the scope on my amps . in school we dont play with tube amps . i have a function gen with the scope so im all set to start testing just dont know how to go about it . or i can just wait till i start school aging in a couple weeks

[trobbins]

If the microphonics is from a bad joint/connection, or cracked resistor, or ceramic capacitor, then 'chopsticking' wires and pins and parts may identify the problem spot. If it's just the input valve then you have a number of control techniques that could be deployed. There are many references on the topic.
eg:
Microphonics