For TigerTrading2021 -- EQ tube section Noise

[sluckey]

I'm gonna try to start this thread since Tiger is having difficulties with it. I don't recall his exact words but basically the original thread stated he has noise in this Altec amp and the noise goes away when he pulls the EQ tube (V3).

I replied...

     Ground V3 pin 7. Does the noise go away?
     Ground V3 pin 2. Does the noise go away?

Here's his schematic...

[pdf64]

It looks like V3 pin2 is fed from P7 wiper (master vol?).
So turning that down doesn’t kill the hiss?

[TigerTrading2021]

Thank You so much Sluckey. In response the master and all pots are verified good and are turned down. And also my noise is WAY beyond a hiss or hum. The 1st time I powered this on and made the noise I thought I would have a frkn heart attack it was extremely loud. I have just not encountered anything like this. It is a multi spectrum noise with both lows and highs and very loud. This is not normal type 60hz hum getting through a filter cap. I have replaced a lot of components on this amp in both an effort to bring it to a new level and also solve this noise. I did all the bias circuit stuff and replaced the selenium rectifiers with diodes. I have almost every coupling cap upgraded. I have looked at and gone over to ensure the wiring is correct. I also swapped tubes for know good tubes. With V3 pulled and the amp powered up I can set bias and it is very quiet. Add that tube or another known good one and Chaos ensues.

[TigerTrading2021]

Tomorrow I am going to ground one at a time those pins and see if the noise happens. I even though about the way I have it on my bench that pressure on a pot could be causing this.

[pdf64]

Try replacing everything connected to pins 1&3 of V3, eg R23,24….

[HotBluePlates]

... my noise is WAY beyond a hiss or hum. The 1st time I powered this on and made the noise I thought I would have a frkn heart attack it was extremely loud. ... It is a multi spectrum noise with both lows and highs and very loud. ... With V3 pulled and the amp powered up ... it is very quiet. ...

There is a feedback loop from the speaker to V3 Pin 3.  Disconnect that and see if the "noise" goes away.

It sounds like your negative feedback is positive feedback, and making a loud howl.

[TigerTrading2021]

Thank you guys for all the replies>
So I have grounded the 2 & 7 on V3 and still get the noise. I need to look on replacing those components because I think those are new and verified. I will re visit that and also disconnect the feed back loop. If the disconnected feedback loop solves it does that mean I have some kind of transformer issue?

[pdf64]

... If the disconnected feedback loop solves it does that mean I have some kind of transformer issue?

It means that the polarity of the balanced signal section of the power amp may need flipping.
ie swap the wires at V5 pin5 and V6 pin5, or alternatively, swap the wires at V5 pin3 and V6 pin3, whichever is most expedient.

[TigerTrading2021]

... my noise is WAY beyond a hiss or hum. The 1st time I powered this on and made the noise I thought I would have a frkn heart attack it was extremely loud. ... It is a multi spectrum noise with both lows and highs and very loud. ... With V3 pulled and the amp powered up ... it is very quiet. ...

There is a feedback loop from the speaker to V3 Pin 3.  Disconnect that and see if the "noise" goes away.

It sounds like your negative feedback is positive feedback, and making a loud howl.

Hot Diggity progress No Sound with negative feedback disconnected. So am I facing on of the resistors there being bad or is this a filter supply cap type issue? This unit has the original Caps and it was made in what like 195X something. I know you guys are going to flame about the caps already. But can I tell you to power this up and not have it howl at me was like getting somewhere LOL

[sluckey]

Did you build this amp? Or is this a factory-built amp that you are restoring? If from factory, I doubt that the NFB is wrong phase unless the OT has been replaced.

[TigerTrading2021]

It was given to me after someone else tried to repair it and lost interest. I did not build it per se. I found that the interface from the splitter to the PP pair grids was wired backwards and restored that to the original schematic. I just checked the resistor values around the NFB wiring and they are spot on. I do have a spare OT if that is the issue.

[TigerTrading2021]

Do you think I caused this putting the inputs of the PP pair as they are in the schematic. I have it exactly like it is in the schematic.

[sluckey]

Do you think I caused this putting the inputs of the PP pair as they are in the schematic. I have it exactly like it is in the schematic.

Yes. You have two choices to fix the NFB issue and get rid of the noise. You can switch the grids back like they were, or you can swap the OT blue and brown plate leads.

Since you now have the grid inputs wired IAW the schematic, I vote to swap the OT plate leads.

[TigerTrading2021]

Ok and just to be sure Im on the same page. When I swapped the inputs to the PP pair I did not move the OT leads as those are correct to the schematic. But yes swapping those is lots less trouble. And just for understanding how am I in this situation because it is all right with regards to the schematic? I do not get that at all.

[sluckey]

Well, there was a reason why the grid inputs were initially swapped. The question is, did the factory do it? Or is there simply an error on the schematic? It happens frequently.

Doesn't really matter though because you will have to either swap the inputs or swap the OT plate leads to make it work right. I still vote to swap the OT leads and use this schematic so you will feel good about it.   

[TigerTrading2021]

Guys thank you so much for the advice walking me through this. That was the charm and with all the new coupling caps and other items...well even with 195Xs something filter caps this baby has dam close to zero residual noise and sounds fantastic just like my other one. I will invest in filter caps now for both units as a precaution to protect these very old and rare transformers. Just put a big smile on face to have this working. These 342b's are getting harder to find and are really a gem from the past. With 2 I can have some tube stereo WooHoo Thanks Again! 

[TigerTrading2021]

I went ahead and pulled all the pots out while I had this on the bench and rebuilt them and that improved it even more plus with the correct grease now the pots feel high quality. Now I am doing that to my 2nd one. It has seen service before as someone replaced the nice carbon comp 470K resistors (R14 R15 R17 R18) coming off of each pre amp wiper to eq stack with some horrid looking giant 2 watt jobbies. This particular 342b is near showroom new, like the inside looks brand new. Can I get away with 330k in place of the 470k? What would the result be? These are mostly to isolate the channels is that right? I have some nice 330K and maybe some 680K carbons on hand. Otherwise it would be order and wait.

[sluckey]

330K will be fine. I'm surprised that they have been replaced. I've never seen any mixing resistors fail.

[TigerTrading2021]

Thank you Sluckey. I think those were just robbed to put in something else. The majority of this amp is like brand new but the pots were slightly different and the mixing resistors (now that I know the proper terminology) are big funky looking 2watt jobbies.

[TigerTrading2021]

Put it all back together and have very low out put. Like I can't even hear a guitar into it. Mic level is way down too. I think after I look and make sure for the 20th time that I put it all back correctly I will put some 470K mixing resistors in but I have no carbons oh well.

[sluckey]

Might be a good idea to find out what you did to break it before you improve it further.   

What kind of grease did you put in the pots?

[TigerTrading2021]

Silicone heavy grease, I also did the other amp the same and it works fantastically. But I guess I could have gotten to much in a pot or 2 I can flush em out some. Your thoughts.

[sluckey]

Silicone heavy grease, I also did the other amp the same and it works fantastically. But I guess I could have gotten to much in a pot or 2 I can flush em out some. Your thoughts.

I would have never done that! Silicone is an insulator. We used to use the silicon grease with a mica insulator on power transistor heat sinks. High voltage transmitters use silicone oil as an insulator, similar to the way the power company big transformers are filled with mineral oil. And I keep a tube of silicone grease with my spare O-ring collection for pool maintenance. Keeps the o-rings from drying out and also helps hold the o-ring in place while rebuilding a pump.

I think the dielectric properties of the silicone grease will cause problems with the pot wiper making reliable contact with the carbon track.

I use CAIG DeoxIT D5 and/or CAIG DeoxIT Fader F5 to clean pots. I'm not likely to ever take another pot apart except as a last ditch effort to repair an obsolete pot.

[PRR]

The Ford DuraSpark II ignition DEMANDED a magic Silicone grease or it would short-out in a slight dew. Get the grease right, no electricity leaked away. (So it can't be good where you WANT electricity to go through.)

> I keep a tube of silicone grease with my spare O-ring collection

I'm sure you know there are different kinds of "O-ring". Some swell in some kinds of grease. I had a 6" ring swolled so big it could not go back in the filter-canister. I bought several kinds and have a stock of compatible rings and grease at my filter.

[sluckey]

Try some Buna-N o-rings. Hold up very well in pool chemicals, gasoline, most any petroleum products, etc.

[TigerTrading2021]

  This thread has now gone to pot. Ok Im finding the correct O-Rings to secure the condition of my potentiometers and will in the future be certain to connect them firstly to the Flux Capacitor to save the world from nefarious overflow of electrons.....

[sluckey]

Don't forget to grease the tube sockets. 

[shooter]

 
The X-Ray tube sockets on Cat-Scanners got greased, it was a PITA, clean out old, then grease up the candle-sticks, then shove n shake till the pins seated, lock down the nut.  You had Popeye fore-arms after a couple years!
the plus side, 30 years later I still got a couple tubes of high-voltage grease for plug wires 

[66Strat]

The X-Ray tube sockets on Cat-Scanners got greased, it was a PITA, clean out old, then grease up the candle-sticks, then shove n shake till the pins seated, lock down the nut.  You had Popeye fore-arms after a couple years!
the plus side, 30 years later I still got a couple tubes of high-voltage grease for plug wires 

Do/did you work in clinical engineering?

[shooter]

Ya, field engineer, did 32 years in CT & MRI.  I glow in the dark and have paperclips all over me 

[66Strat]

I had a Clinical Engineering client for five years. It was amazing the level of service documentation required by CMS for medical equipment. Every device was required to be covered under a Medical Equipment Management Plan, and documentation maintained that evidence that service was performed for each device in accordance with the MEMP. CMS would audit the effectiveness of Clinical Engineering without ever looking at or examining a single device or piece of equipment.

[TigerTrading2021]

Don't forget to grease the tube sockets. 

Oh I've got some very nice Rod & Reel Lube for the Tube Sockets I like it cause it is waterproof. 

[TigerTrading2021]

Im going to buy some dog gone Nyogel or my name aint Albert Fienkelstienman