Gnarly noise problem

[donzoid]

Thanks Matt, see the attachment - yes its the coupler after 2nd stage. I think the plate R for third stage is 100K but that's from memory so ... what you're saying is:

Check V2 pin 1 for stray DC. Would that show as spiking on my meter, or do I really need an O-scope for that???

Then I'll pull V2, and the PI to see if it stops the noise.

If not, then we are getting back to the PS caps, and output which IS progress.

Thanks!

[Stevem]

Yup, continue yanking until it stops!

[matt h]

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[donzoid]

Well it stopped cold on the PI tube.

I put in another, also have the standby switch jumpered, but that alone made no diff (good suggestion though) so far she's quiet as a mouse.

Much to my chagrin, that would mean that the PI tube (and the last replacement swap) are both bad. This one (tube #3 and FYI I have almost always used 12aT7's here) and no noise a'tall. What...I ask ya are the chances...two tubes in a row? geeeeez. I hope that's all it is, I just got through looking over the whole PI area with my magnifying headset WAY too up close.

[matt h]

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[donzoid]

I'm going to start it up in the am, and let it run all day, before I plug a signal in just to see if that's really "all" it is.

Dang, that's gonna be a tragic loss...the last noisy one was a favorite for this amp! 12aT7WA blackplate military with extra support rods even! Makes me wonder if this isn't eating tubes in the PI position. The heater tap on this Edcor has always run a bit high at 6.8...hmmm...*scratches head*

Also odd that I just did a complete re-check of voltages, and they have dropped about 15v across all the preamp plates since March when I started running it. Is that typical for a break-in/burn in? *scratches head again* 30+volt drop at the PI, but the original measurements were probably done with 12ax7 in there.

[hans-jörg]

but the original measurements were probably done with 12ax7 in there.

Hi,
you gave yourself the answer
12AT7 -more current draw- less voltage.

Probably it´s not the tube but the socket. Maybe loose pins. Try to bend the holes with a strong needle or something like that, to make them for the tube pins smaller, tighter.
Such thermic noise happens if the tube pin is wiggling in the socket.
Or change the socket for a new one, if you have one at home. Than you are on the save side

There is also one brutal method to check: wiggle the tube in the socket when amp is on and listen at the speaker if the noise shows up. Could be, but hasn´t to be.
But please with care!

Hans-Jörg

[matt h]

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[donzoid]

Hahah! OK Matt, got it. We are building amps, not baking cakes, yeah?

Hey Hans, thanks for the tips, all good ones. These were brand new Beltons, and in my opinion, they might be TOO tight (Just my opinion but these things are very hard to insert/pull tubes from). I did take a look at each pin clip with the tube out, using my extreme closeup headset (geek magnifier) and it still looks like new, with no carbon tracing or evidence of arcing, no stretched clips like I have seen and fixed with a dental pick on my old marshalls and fenders for example). I've checked and re-flowed the solder points underneath almost every time (on each socket, if needed or "even maybe").

Anyway, I "did" carefully wiggle (more like swivel gently) all three (V1,2,3) after I got them all back in. It's been on (standby switch too Matt...) for at least 8 hrs and so far no noise, so I guess my bad luck was picking two tubes in a row that are either bad or "not happy with their neighbors". It's an RFT in there now, and it sounds pretty darn crisp. I'm OK with that, I installed a cut control. Thanks to all for the coaching.

[matt h]

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[hans-jörg]

Hi,
good news "Donzoid".

About tubes: I have a bunch of "used" tubes in a box which I use during build up, and especially in Trainwreck kinda amps they behave all different as in other circuits. It´s interesting to watch which tube likes what kind of environment . But never throw them away - in an other amp they are probably very smooth.

Best

Hans-Jörg

[Stevem]

I have found that many NOS tubes have crummy dull to almost corroded pins on them and I have cured issues like yours by polishing the pins with a small 1/2" OD wire wheel in my battery operated Dremal tool and then treating the pins to a wipe down with Deoxit red, in fact it got to the point where now whenever I would even be placing a new tube in a old good working socket I would give that tubes pins a wipe down with the Deoxit and then install and remove the tube a couple of times and I would see old tarnish and crap come up on the tube pins even though the socket worked fine!
Also note that if the plate voltage on that PI section is close to, or over 300 and the power suppy node side then you should step. up to 1 watt resistors for a 50% or so voltage safty factor!