Ampeg B-12-N........

[xm52]

Great news. So the hum pot was shorting the cathodes to ground. A blown pot can cause a lot of hum. So can a bad tube. Before there were hum pots in the heater circuit, there were hum or balance pots between the two cathodes. This would allow you to compensate for tubes that were not matched. These pots were often 100 ohms. They didn't want too much resistance added so maybe 100 ohms was a good value to choose. Then they started using the same valued pot in the heater circuit. Probably because they had them.

I forgot to mention, the white wire on the old power transformer that you mentioned is a ground to the frame of the transformer.

Sounds like the problems with the amp are almost sorted. Famous last words...

[Jack_Hester]

100Ω is a good value.

I expect that is because there is still a short to ground somewhere in the filament circuit. When you fix that, the hum balance will work properly and you will be able to minimize the filament hum by ear. 

The pot values that came to mind were because of the typical values that I see others using for their virtual grounds.  With the pot centered, each leg would be 100Ω, using a 200Ω pot.  The same for a 400Ω pot, 200Ω per leg.  I'm sure more values have been used than these.  These are just the ones that came to mind.  So, 50Ω per leg is a good value.  I'll jot that down in my notebook in the shop.  Paper and pen are my short-term memory these days.

I'll confirm the integrity of the Hum pot this evening.  I did not measure the filament voltage, with the pot wiper lifted.  I don't expect it to be much different from my original reading.  That one was normal. 

More to come.  Have a good one.

Jack

[Jack_Hester]

Before there were hum pots in the heater circuit, there were hum or balance pots between the two cathodes. This would allow you to compensate for tubes that were not matched. These pots were often 100 ohms. They didn't want too much resistance added so maybe 100 ohms was a good value to choose. Then they started using the same valued pot in the heater circuit. Probably because they had them.

I forgot to mention, the white wire on the old power transformer that you mentioned is a ground to the frame of the transformer.

This is a 'beefy' pot.  It's size reminds me of a rheostat, though it's enclosed.  I expect it is capable of some serious watts.  I'll check FlipTops and other sources for the same. 

As stated before, the white wire was unused.  Just soldered to an eyelet.  I guess the PT being hard mounted to the chassis made it unnecessary.

Anyway, I'll have the pot lifted this evening, and I'll temporary in a couple resistors to signal ground, until I have a replacement in hand.  Assuming a bad pot. 

I was having doubts about my new PT being of the same voltage as the original, from the voltages on the schematics that you provided.  My doubts were proven unfounded, after taking a quick measurement of the mains.  It had a serious load on it.  Fortunately, my power tubes still check very good. 

I really hope that this resolves the hum over in the pre-amp.  Even if it doesn't, that will be just another avenue to troubleshoot.  This has been an enjoyable 'fix', up to this point.  Pretty much, everything tried has yielded results. 

A shame these amps are so pricey.  Any found that are considered projects are tagged with a price that is not reasonable for most people.  I love the history behind anything, and this one just doesn't have it.  The cab is in excellent condition, with a couple tears in the fabric.  They just give it character.  This is an easy amp to work on.  Comes apart very easy.  Minutes, to have the chassis free and flipped over.  The transformers and the cage studs hold it very stable, upside down.  Of course, I lay it on towels to protect the surface of the transformers.  If they weren't so pricey, I would look forward to putting my hands into them more.   

Anyway, I'll be back with more, this evening.  Have a good one.

Jack

[Jack_Hester]

I'm in the shop a 16:00 hrs..  Removed the Hum pot and was prepared to megger it, casing to each terminal.  Was not necessary.  DMM measured hard ground from wiper to casing.  I'll be ordering another pot.  I'm going to hook a couple 100Ω resistors to the leads and signal ground, after I power it up without them.  I want to hear the difference with the pot removed from the filament circuit.  Then, with the virtual ground.  Sure was a good find.  Thank y'all so much.  I'll be back shortly with the results of this test.

Jack

[Jack_Hester]

Ok.  New readings with Hum pot completely removed, and a 100 Ohm resistor connected to each lead of the filament power on V1, and the other ends connected the same wire that connects to the cathode of V4: 

V6 (5U4):  Pin 4 - 365.7vAC
                Pin 6 - 365.4vAC
                Pin 8 - 420.6vDC
V5 (6L6):   Pin 3 - 414.2vDC
                Pin 4 - 388vDC
                Pin 5 - .03vDC
V4 (6L6):   Pin 3 - 412.5vDC
                Pin 4 - 388vDC
                Pin 5 - .03vDC
V3 (6SL7): Pin 2 - 280.2vDC
                Pin 3 - 3vDC
                Pin 5 - 270vDC
                Pin 6 - 2.7vDC
V2 (6SL7): Pin 2 - 113.1vDC
                Pin 3 - 1.1vDC
                Pin 5 - 172vDC
                Pin 6 - 2.3vDC
V1 (6SL7): Pin 2 - 119.7vDC
                Pin 3 - 1.2vDC
                Pin 5 - 165.7vDC
                Pin 6 - 2.1vDC

The filaments are 5.08vAC and 6.49vAC.

I connected my Bias tester to the two 6L6's, and measured the Bias voltages as follows:

V4 - 58.4mA and V5 - 55.26mA (read as mVDC on my Fluke 8062A DMM). 

For all practical purposes, Channel 2 has no hum at any volume setting.  However, Channel 1 has a rather loud hum that changes from totally quiet to loud with volume from minimum to full.

I love this amp.  New Hum pot should be an easy fix.  Then, on to troubleshooting Channel 1.  Time to go to the house.  It's been a good night, and I'm hungry.  Closing up at 17:35 hrs..  More later.  Have a good one.

Jack

[xm52]

The numbers are looking more respectable now. So far so good. They will be higher with a 5AR4 in place. That rectifier will allow more headroom and later onset of distortion than with the 5U4GB.

CTS makes a nice 100R, 5W, wire wound pot. I get them from Digikey.

You can try swapping the V1 and V2 6SL7 tubes and see if the hum moves to channel 2. That will tell you if the tube is bad. Some Deoxit on the socket terminals and tube pins and re-tensioning the tube socket connections if required would be my next step.

[Jack_Hester]

I have NOS 5T4's, which appear to be very much like the 5U4 in comparison.  The only 5AR4's that I have are a couple of JJ's. 

100Ω, wire wound pot on the way.  I ordered one from FlipTops, a I needed to replace the plexiglass logo on the cage.  Ordered one of those, too.  These power tubes have been getting hot (for someone) a long time.  The logo is right in front of them, and had just about melted loose from one of the mounting screws. 

I'll try the tube swap, when I go out to the shop, this morning.  Hope that it's that simple.  Re-tensioning the socket connectors is not something I've tried. 

The amp is louder than before.  And, I thought it was plenty loud.  It's cleaner, brighter, and the tone pots make a really big difference in tone.  Before, they still made a difference, but I was not overly impressed.  I had no other amp like this one to compare, so I figured it was typical for a Bass amp, and was supposed to sound like such.  That ground was a problem in every imaginable way.  Maybe there are some here that can testify to the fact, but my thinking is that this can be a really good guitar amp.  With another higher frequency ranged external speaker attached.

I'll also need to pull the speaker.  With the volume (either one) up to half or better, there is a very distinct and loud buzz, when plucking the 'D' string (guitar).  I'm hoping something (trash) has dropped down into the back.  But, I'm not ruling out a split or tear.  With volume at full, it will do it on the low 'E'.  Back off the volume, and the low 'E' is very clear.  I consider this minor, compared to the rest of the troubleshooting.  I'm just happy to get it to this point. 

Again, this has been a very painless troubleshooting, with all the feedback from you guys.  Each time I went into the amp, something was fixed.  The amp got remarkably better.  The schematic on the mounting board has some ever so faint pencil markings.  The ones that I can read are wire colors.  The others, I just can't make out.  I mark up a copy of this amp with my final readings, and print out one to put in the cab. 

At some point, I need to date it.  I'm satisfied that the speaker is original to the amp.  I will make a search for Utah speaker codes, and start there.  Are there any other obvious markings to back up what I find for the speaker?  The Hum pot had no obvious markings, but then, I didn't look real close.  It has all the appearances of being original to the amp. 

It's ain't over, but I can feel the finish coming on.  Wish that I had another just like it to troubleshoot.  I never expect to find the deal that I did, on another as nice as this one.  Man, I wish that I knew the history on this amp.  And why it was allowed to go down like it did.  I had the same desire to know the history of the old motorcycles that I restored.  Just to many hands, from beginning to the point of restoration, to know it all. 

I'll get off topic just a bit, and then come back.  If you ever want the most fun old motorcycle in the world to ride, get an old H-D 45 WL (or military WLA).  The most under-powered V-Twin H-D ever made.  Suicide shifter/clutch, and 3-speed.  They will run 60-65 mph (downhill slightly), but mostly 55 mph.  And, will never be in front.  But, they are workhorses and will always get you to where you are going.  I've witnessed them being cranked by hand.  I prefer to 'step' crank them, as no kicking is necessary like on the big twins.

Now, I'm back. 

I'm looking forward to an accomplished guitarist giving it a workout, as I'm nowhere close to being qualified to say more than 'it works' and 'sounds really good'.  Anyway, I'll make the tube swap and maybe start looking at the speaker.  Thanks again, for all the really fine help.

Jack

[Willabe]

Maybe there are some here that can testify to the fact, but my thinking is that this can be a really good guitar amp.  With another higher frequency ranged external speaker attached.

A friend of mine has a recording studio with 16 track 2" tape and all the gear to go with it. He's a very good blues/R&B drummer. He has an old B15 and uses it for guitar often. I was suprised but he said it sounds great. You might not need the high frequency speaker.


                 Brad    

[Jack_Hester]

Swapped the tubes in V1 and V2.  Hum stayed on Channel 1.  I did make a couple discoveries. 

The probably half of the loud hum is because J1 of Channel 1 is an open circuit jack.  They took a closed circuit jack and removed the inside contact, and used that terminal as a tie/junction point.  I grounded this jack, and a large portion of the hum went away.  Still fairly loud at full volume.  But, a goodly amount disappears when grounded.  I have a cord that has a shorting switch on one end.  This will work nicely for this jack, using the switch-end at the guitar.

The guitar that I used last night, that caused a buzz on the 'D' and low 'E' strings was a different one from what I normally use.  I just got that one back from Don DeJong, our music leader at Church.  It's a Harmony H-19 that he installed a new pick guard on, and did a bit of setup. 

I removed the ground and connected my usual shop guitar.  No reason for changing.  It's just the one I pick up to check my amps with.  It's an early 80's Japanese-made Harmony, and sounds very good.  I don't remember the model, as it's not displayed.  The buzz on 'D' and low 'E' strings is gone.  Can't make it happen with this guitar.  I'll still take the speaker out, as it warrants inspection, being as I'm doing so for the rest of the amp.  Now, I've got to troubleshoot the H-19.  Won't do that in the shop.  Will take it to the house. 

So, a bit more progress and a bit more to come.  Have a good one.

Jack

[Jack_Hester]

A friend of mine has a recording studio with 16 track 2" tape and all the gear to go with it. He's a very good blues/R&B drummer. He has an old B15 and uses it for guitar often. I was surprised but he said it sounds great. You might not need the high frequency speaker.

Glad you responded, saying so.  I think that it sounds quite well.  But, I just can't leave it with my judgement.  

There's a fellow over in Durham (30 or so miles South of me), Wade Baynham, that has a couple of older amps that he borrowed from me.  Both needed only decoupling caps, as they were shot.  One is a '53 Epiphone Pathfinder, and the other, a '55 Gibson GA-40.  I intended to replace the other electrolytics in them, but they haven't come home yet.  He's recording with them.  My friend Don is good friends with him.  They both live in Durham, and Don has him over from time to time, for music and such.  I usually take my 'fixes' to Don for trials and critiques.  Don's son is a fairly active musician, and comes over to give them a try.  When Don thinks one sounds especially different, he has Wade over.  Wade just sent me a track yesterday, from an album that he has in the works.  I know that it has one or both of the amps in it.  But, unless he tells me about the track, I won't know anything other than it sounds really nice.  

Anyway, Don will eventually get the Ampeg, and I'm betting that Wade will have it shortly thereafter.  That's why I'm hanging on to it for awhile.  I can't let everything go.  

I really have a passion for the smaller 'Art Deco' amps, like this one:





They don't have near the great sound as something like B-12-N, or the other two that I mentioned.  But, this one is a quality build.  Nothing short about it.

Jack

[Willabe]

I love the 2 tone tolex on that amp.   

I bought the iron for a GA-40 several years ago from Merc. Mag. I also bought some 5879's and a couple of 6SQ7's (metal tube) and 1 of these days I'll build it.

How good does it sound? And how about the trem? Deep? Smooth?

Tone Quest Report Mag. did a review of 1 years ago and they loved it which is why I'm going to build one.


               Brad     

[xm52]

The B-12N/B-15N is probably one of the most recorded bass amps there are. They have a deep harmonically rich tone that is hard to beat. You get inspired when you play through these amps. It is amazing how many song's from the 60's and 70's were recorded using them. They are still a favorite in many studios today.

Based on the pics of your amp, I would estimate that it is a 62 to early 63. The earliest ones didn't have codes on the pots, the later ones did. The pots were made by CTS, so look for a stamp on the pot starting with 137. In that era, the next to digits were the year, followed by two digits signifying the week of the year. If you remove the output transformer, there is sometimes a paper label on top of the tar. There are four shock mounts that are between the cabinet lid and the amp tray. You need what they call a clutch head screwdriver (fliptops sells them) for those bow tie type screws. The mounts were manufactured by a company called Lord, and each has a date stamp on them. The cap can usually has a date stamp on it as well.

The warped Lucite logo on your amp is not uncommon to see. They look neat but the power tubes and the power transformer can get quite hot. Ampeg used to include a card with these amps that you filled in and returned with your lucite logo. They would engrave up to six letters. People often put their name on them.

The price of the Ampeg reissue is up to $5K if you can get your hands on one. They are planning on releasing a lower cost circuit board based version within the next year or so. Unfortunately, the value of these amps just keeps going up since the re-issue came out a couple of years ago and they are getting to the point where they are unaffordable for most regular folks.

As for using it guitar, the B-12 was and is a favorite from day one of east coast jazz guitarists. Accordion players also used them. There is a famous clip on Youtube of Jimi Hendrix using one on the Dick Cavett show. Although I figure that he could make any amp sound good.

They really went to town when some of those older amps were designed. They are works of art. The simpler electronics with the octal pre-amp tubes are very musical.

[Jack_Hester]

I love the 2 tone tolex on that amp.   

I bought the iron for a GA-40 several years ago from Merc. Mag. I also bought some 5879's and a couple of 6SQ7's (metal tube) and 1 of these days I'll build it.

How good does it sound? And how about the trem? Deep? Smooth?

Tone Quest Report Mag. did a review of 1 years ago and they loved it which is why I'm going to build one.

Don and Wade had one of my '54 GA-40's and both fell in love with the sound and Tremolo.  Then, they got hold of the Pathfinder and the '55 GA-40.  The early 40 came home.  Wade says that each amp is very different, in tone.  And, both are excellent for recording.  The Pathfinder has Reverb and Tremolo, and is the Epiphone version of the Gibson Falcon.  The GA-40 only has the Tremolo.  They each are capable of being very deep.  I honestly don't know how to describe how they sound.  I could say smooth, but that just isn't enough. 

I will ask Wade if I can share the two tracks that he sent.  The first is a track he covered by Emmy Lou Harris, Red Dirt Girl.  I like it better than her version.  Maybe a bit biased, here.  The other is a cover of Shenandoah.  On the first, he has other artists, but he plays guitars and Bass.  On the second, he plays all instruments.  Both of my amps are on the first.  I know at least one is on the second.  If he is ok with it, and I hope he is, I'll email them to you if I can.  They are both large. 

Jack

[Jack_Hester]

How good does it sound? And how about the trem? Deep? Smooth?

               Brad     

Brad -

Here's a note that Wade sent Don and I, about the Shenandoah recording.  I missed it, as it was on the website that he uses to send these large files through:

hey folks,

here's a rough mix of an instrumental version of 'shenandoah' that i've been working on for the past little while. it still has some rough places, but i think it's worth a listen... jack and don, the electric is my Gretsch through the Epiphone, and the pedal steel is through the Gibson with tremolo turned on- and both are recorded with cascade ribbon mics. thank you, thank you!

Jack

[Tom_Hull]

The B-12N/B-15N is probably one of the most recorded bass amps there are. They have a deep harmonically rich tone that is hard to beat. You get inspired when you play through these amps. It is amazing how many song's from the 60's and 70's were recorded using them. They are still a favorite in many studios today.

hi
Thats nice to know my daughter plays the bass and maybe this is what she needs ..
She has free recording time ,but you feed the bass directly and the computer has what amp and  and all ..lots of software goes with it

 so maybe she will record  this amp on the computer , and she will need one .

i did not know that recording an album is $100,000,,is that about the right cost ??

tom

[xm52]

i did not know that recording an album is $100,000,is that about the right cost ??

Hope that she has the time of her life making the album.

There are many ways to derive a good tone. Recording bass directly is done quite often, some like to mic an amp, some use a combination of the two. Sometimes the music will dictate the best approach to use. What is important is that you are inspired by what you hear when you are playing.

Costs can vary depending on the facility. It is an involved process that includes recording, mixing, and mastering. There are costs for a producer, an engineer, and all the little things that add up. A few days in a world class studio can cost as much as a nice new car. There are many affordable studios that will do a great job. Spending as much time as possible in pre-production before going into the studio can help minimize costs. No matter what the budget, you can burn through it pretty quickly.

[Willabe]

Jack I'd love to hear them!

I'd bet others would too. If you could why not post them in the sound clips section?


                Brad    

[Jack_Hester]

If you could why not post them in the sound clips section?

Shenandoah is around 14Meg in size.  Red Dirt Girl is right at 10Meg.  These may be way to big to load up the server, I'm thinking.  If he answers back with permission, I'll see if there is a freebie site to put them, and link them to here.

Jack

[Willabe]

Sounds good Jack. There's a number of free sites for that. IIRC some of the guys here have mentioned several different ones?


            Brad      

[xm52]

Soundcloud is very popular for hosting music files.

[xm52]

Swapped the tubes in V1 and V2.  Hum stayed on Channel 1.  I did make a couple discoveries. 

The probably half of the loud hum is because J1 of Channel 1 is an open circuit jack.  They took a closed circuit jack and removed the inside contact, and used that terminal as a tie/junction point.  I grounded this jack, and a large portion of the hum went away.  Still fairly loud at full volume.  But, a goodly amount disappears when grounded.  I have a cord that has a shorting switch on one end.  This will work nicely for this jack, using the switch-end at the guitar.

These amps are normally very quiet. Plug in an instrument, stand a few feet away and point the instrument in a direction to minimize hum, turn the amp on full and you will barely hear any hum. I think that there is a problem with channel one.

Channel 1 of your amp is wired correctly, one jack is shorting, the other isn't. With nothing plugged in, the shorting jack shunts the input to ground. I would check that the shorting jack has a clean contact on the switch. The main ground terminates at the chassis at the channel 1 input jack. Check for corrosion or anything that would make a bad contact here. I would clean it well and put a good star washer behind the jack inside the chassis. Ensure that the nut is tight. The volume and tone pots also need to have a clean and tight connection with star washers. They cheat and ground a terminal of one of the tone pots to the case, which in-turn is grounded to the chassis. Sometimes there is just a blob of solder between the pot terminal and the pot case. Solder doesn't make a good conductor. I like to run a wire from the terminal to the case. Tight nuts and clean contacts can make a big difference with hum. If that doesn't resolve the issue, at least things will be in good shape.

Another possibility is a problem with the gray shielded wire that runs from the jacks and pots. Sometimes the outer braided shield gets badly corroded and can cause problems if it breaks within the cable. The solution is to change the shielded wires. To avoid a ground loop, they should be grounded at only one end. Note how these wires are routed before you start as lead dress is important to prevent oscillation. Be careful with the connections at the eyelet board. One strand of wire out of place can cause a lot of grief.

If channel 2 picks up radio stations, one thing that you can do is add a grid stopper resistor (channel 1 uses 120K but a smaller value such as 32K works) at pin-1 on the V2 tube socket. This won't affect hum but will greatly reduce RF.

[Jack_Hester]

Sounds good Jack. There's a number of free sites for that. IIRC some of the guys here have mentioned several different ones?

Sorry.  The tracks are proprietary, for now.  They are the first mix, before cleaning up to put on an album.  I kinda suspected they would be proprietary.  And, I can understand.  This is his livelyhood.  As soon as I can get my hands on the first release, I'll make sure that I have one to send to you. 

Jack

[Jack_Hester]

These amps are normally very quiet. Plug in an instrument, stand a few feet away and point the instrument in a direction to minimize hum, turn the amp on full and you will barely hear any hum. I think that there is a problem with channel one.

Channel 1 of your amp is wired correctly, one jack is shorting, the other isn't. With nothing plugged in, the shorting jack shunts the input to ground. I would check that the shorting jack has a clean contact on the switch. The main ground terminates at the chassis at the channel 1 input jack. Check for corrosion or anything that would make a bad contact here. I would clean it well and put a good star washer behind the jack inside the chassis. Ensure that the nut is tight. The volume and tone pots also need to have a clean and tight connection with star washers. They cheat and ground a terminal of one of the tone pots to the case, which in-turn is grounded to the chassis. Sometimes there is just a blob of solder between the pot terminal and the pot case. Solder doesn't make a good conductor. I like to run a wire from the terminal to the case. Tight nuts and clean contacts can make a big difference with hum. If that doesn't resolve the issue, at least things will be in good shape.

Another possibility is a problem with the gray shielded wire that runs from the jacks and pots. Sometimes the outer braided shield gets badly corroded and can cause problems if it breaks within the cable. The solution is to change the shielded wires. To avoid a ground loop, they should be grounded at only one end. Note how these wires are routed before you start as lead dress is important to prevent oscillation. Be careful with the connections at the eyelet board. One strand of wire out of place can cause a lot of grief.

If channel 2 picks up radio stations, one thing that you can do is add a grid stopper resistor (channel 1 uses 120K but a smaller value such as 32K works) at pin-1 on the V2 tube socket. This won't affect hum but will greatly reduce RF.

I'll check J2 on Channel 1.  I have a burnishing tool that I can dress up the contact.  And, I'll put a bit of No-Ox on it.  Saying this out loud, I'll burnish all surfaces that will be in contact with instrument cables, etc..  And, a light coat of No-Ox on each. 

I did find J1 loose, during my checks.  I've begun a habit of loosening and retightening all jacks, pots, switches, that depend on the chassis for a ground connection.  Fortunately, this amp was built with no dependancy on a chassis ground (from what I've seen).  Yet, it has lockwashers behind each, like it might. 

Speaking of stray wire strands.  I've found a few, but not in contact with something else.  I have not looked the whole board over.  Channel 1 will get a close inspection for such, now that you've mentioned it.  If Channel 1 cleans up like Channel 2 is now, this will be one killer amp. 

Channel 2 is showing no signs of picking up stray RF, other than when one of my shop lights goes on the blink.  That light needs a little TLC.  I'll leave that channel as-is, for now. 

Learned about shielding, back in the 70's, when I first went to work out at the power plant.  Everything is an antenna, with 240KvAC leaving the main bank transformers.  The old analog control systems had nothing to clean signals up, so shields and corrosion were a real problem.  Anyway, I know where you're coming from on that.  It's a good reminder, though. 

I'm really hoping that the hum is something simple.  But then, it wouldn't be any fun to troubleshoot, would it.  More to come.  Have a good one.

Jack

[Willabe]

Thanks for trying Jack, I understand.


              Brad     

[Jack_Hester]

Two things for sure.  The man's got talent.  And, he's got a studio that stays busy. 

Jack

[Willabe]

Well that's 2 good things. I look forward to hearing the CD.


            Brad       

[Jack_Hester]

I'm planning to do a bit of troubleshooting and signal tracing on Channel 1, while I'm waiting on the new Hum pot.  I would like to say that I'm starting this afternoon, but the weather is starting to fall a bit, and the temps are dropping, also.  No heat in this part of the shop where I'm working.  The sun makes it nice and warm.  No sun, no heat, today.  I'll have Saturday to do it, otherwise.  But, I'm planning to dig out my little 'listener' that I fabbed up from one that I thought I got from something Doug had on this site.  I can't seem to find it where I saw it. 

Anyway, I'll set up another amp to connect the output of the listener to, and start walking through Channel 1, until  I find where I first hear the hum.  Or, where I lose it.  I did not touch the coupling caps on either channel, and will probably change them, as they are easy to get to.  More and more, I'm liking eyelet boards.  The schematic shows four of them (.02uf) for both pre-amp sections.  I've got quite a few .022uf on hand.  I can't rule out the tone stack, I suppose.  But, I'll start with the obvious.  The tone stack components appear to be mounted entirely on the pots.  I know that I don't have those caps on hand, and would have to look for them.  They are about the size of postage stamps, thin, and with multi-leads. 

Given that the bias condition probably existed for a long while, I can only imagine what other problems have been there with it.  Like this hum.  In a club atmosphere, this hum would not be an issue.  That's just how well it sounds, with it.  But, it's cleaning right up, and won't be any fun to work on, anymore.  I'll post the afternoon's findings in another few hours.  Have a good one.

Jack

[Jack_Hester]

16:30 hrs.  Started out by pulling V1.  Silent amp.  Expected as much.  Channel 2 is really quiet at full volume, as before.  Put V1 back in and turned the volume down.  
Made a couple DC voltage checks:  

116vDC on Pin 2.  
11mVDC on the other side of the .02uf coupling capacitor.  
166vDC on Pin 5.
8mvDC on the other side of the .02uf coupling capacitor.

Put my listener on Pin 1.  Can't really tell a change as I'm working with a handicap.  Rain on this metal roof makes listening difficult.  I think there is a hum on Pin 1.  I have the listener turned all the way up and the listening amp likewise.  I may as well button the shop up.  It's pouring down rain.  I will lift the wire off of Pin 1 tomorrow, and hopefully have good results.

Jack

[sluckey]

Don't pull the wire off pin 1. The tube will lose bias if you do. Instead, put a gator clip jumper lead between ground and pin 1. If the hum dissappears replace that 5.6M resistor with a 1M resistor.

[Jack_Hester]

Don't pull the wire off pin 1. The tube will lose bias if you do. Instead, put a gator clip jumper lead between ground and pin 1. If the hum dissappears replace that 5.6M resistor with a 1M resistor.

I did put a jumper on J1, to the terminal where the shielded conductor was landed.  Sorry I didn't mention it.  I had the volume on the Ampeg at minimum and the volume on the listener amp at full.  The hum going to Pin 1 I believe went away, but was replaced by a different hum, almost as loud.  Again, noise from the rain hindered any listening.  If I'm not mistaken, at some other point, when I was troubleshooting the filament ground, I plugged a guitar cord into J1 and shorted the tip and barrel.  I believe that I heard the same change in the Ampeg speaker. 

But, not depending on memory, I will make this same test tomorrow.  With the cord, and with the jumper on the inside.  Also, I'll check the 5.6M resistor for value and verify that is properly connected.  I neglected to check the shorting contact on J2 to see if the 120K and 1M resistors are being paralleled with the 5.6M.  xm52 made mention that I should check that contact.  We'll see what happens. 

Thanks to everyone following this thread.  I'm making it rather long by writing as I think and troubleshoot.  But, it keeps me from spinning my wheels, when I get suggestions to get me back in line. 

Except for callouts, we work in pairs at the plant, when troubleshooting problems.  Just for this very reason.  One man follows the drawings and directs the troubleshooter.  The troubleshooter repeats back what he just checked.  I'll repeat back what I check, tomorrow.  Have a good one.

Jack

[Jack_Hester]

http://www.el34world.com/charts/Schematics/files/ampeg/b15n_portaflex.pdf

Pondering my next moves in troubleshooting, the burnishing of the contact surfaces will come first.  The tool is ever so fine, and will only remove the slightest amount.  Oxidation is all that I'm after.  And, a micro-thin coat of No-Ox to protect it.  I put it on with a cloth, and wipe it back off with a dry one.  I'll dress up the contact points that mate to the plug tip.  I don't really have a good way of cleaning the inside of each jack, other than a piece of crocus cloth rolled up and make one pass through them.  I can q-tip a bit of No-Ox in there, afterwards, and wipe it out with a clean one.  That should give the plug barrel a better contact point, though I'm really not chasing a hum, by doing this.  Just some preventative maintenance.  

This will get me to the point of good mechanical connections.  Wish that I had pinched the J2 contact slightly, last night, to see if that completed the path to ground (through the 120K/1M resistors) for J1.  That may have taken care of part of the hum.  That will be confirmed and out of the way, when I get back out to the shop this afternoon.  More to come.  

Jack

[Jack_Hester]

17:00 hrs..  First thing was to burnish contact surfaces and apply No-Ox.  

Then, I put a jumper from V1-Pin1 to signal ground.  Worst of the hum went away, but a different hum remained.  And, almost as loud, changing from nothing at minimum volume, to loud at full volume.  When I say loud, either hum would not be so noticeable in a club environment.  You would not record from this channel.  Channel 2 has the same hint of a hum, at full volume, as Channel 1 when it's quiet.  

Looking at the older hand-drawn schematic, the downstream coupling cap from Pin2 of V1 is labeled C1.  I moved my jumper to the downstream side of it, where it heads into the Tone Stack, and the amp went dead quiet.  At full volume, just a hint of a hum, and that may have been the lights overhead.  I didn't cut them off, as I do sometimes when I testing.  No point in it.  This is one really quiet amp.  Lifted the jumper (loud hum, again) and moved it to between C2 and the volume pot.  Dead quiet, again.  

What I neglected to do was put it on Pin4, the grid of the second triode.  I'll just have to do that in the morning.  

Even though I had low DC voltage downstream of C1, yesterday, I'm wondering if my next move would be to swap that one out.  The first triode appears to be the source of all hum, from this channel.  I'll think a bit more on it, in the morning over some French Roast.

Update - 17Nov12 - 04:45 hrs.: I'll leave the coupling cap alone, for now.  It's doing it's job.  It's only passing what the triode is sending.  To completely eliminate the input jacks, I'm going to lift the frail little shielded wire from V1-Pin1, and put a 1M resistor between it and signal ground.  I'll warm the amp and see what I hear.  I'm expecting hum/noise from an open input like this.  Then, I'm going to ground it, and hopefully it all goes quiet.  

If this does clear the hum(s), I'll put everything back as it was on P1, and head over to the J1 and J2.  As these are some crowded with resistors and other connections, I'll have to carefully slip them out of the chassis for inspection.  

More to come.  Have a good one.

Jack

[Jack_Hester]

10:00 hrs..  Retraced my steps from the past several days, to do some of the same checks.  Just getting it back in my head.  Plus, I really wanted to isolate the first triode, just to see what would be the result.  I knew that grounding downstream of C1 would quieten Channel 1.  

I lifted the B+ to this triode and fired it up.  Dead silence.  Soldered it back and began a general inspection of signal grounds.  I found J1 loose, the other day, and re-tightened it.  I removed both J1 and J2 from the chassis, this morning, to inspect solder joints.  That's when I realized that J3 had a signal ground attached to it, but J1 and J2 did not.  Unless I'm missing something, this ground on J3 is the absolute only hardwired signal ground to the chassis.  J1 and J2 are dependant on this for signal ground.  Not good enough.  There are now jumpers soldered between J3 and J2.  Another from J2 to J1.  

When I fire the amp up like it is, nothing plugged into any of the input jacks, I don't get the loud hum at full volume.  There's some still there, but when I connect my alligator jumper from signal ground to V1-P1, quiet amp.  Lift the jumper (J1 being an open circuit jack), hum.  I move the jumper to the other end of the shielded input wire and amp goes quiet.  Next, I move the jumper to the part of the jack that contacts the tip of the 1/4" plug.  I get hum, but not bad hum.  It's gone.  I plugged in a short guitar cable and grounded the tip.  Same hum as the one when I ground the jack at the tip contact.  

Grabbed the guitar and plugged it up to J1.  Volume on guitar and Channel 1 at full.  Sounds like the same intensity of hum, as with cord tip grounded.  Just for grins, I connected the guitar to Channel 2.  Full volume on both, and only about half the intensity hum of Channel 1 (with guitar).  And, I can hear this one change as I move around.  

So, here's where it stands.  Bad hum gone.  Was it re-flowing the solder on the B+ connection at the eyelet board, where I disconnected to isolate the first triode?  Was it adding hard-wired signal grounds to J2/J1?  I'm curious as to how much effect the Hum Control pot would have on my present hum.  The Ground Switch has no noticeable affect.  

Now, I only have the difference of hum between the two channels.  Channel 2 is very quiet, no connections.  Closed circuit to ground.  Channel 1 noisy, no connections.  Now, I'm really only chasing what may be an issue with any other working amp, of this configuration.  I know that my closed circuit contact on J2 is in good shape.  Again, if I ground (signal ground) either end of the shielded conductor that connects between V1-P1 and the junction of the two 120K's and the 5.6M resistors, quiet amp.  

I feel good about the morning's work, not completely understanding what the fix was.  I did multiple tasks, without checking in between.  But, I'm just refining a good working amp, to make it better.  Maybe make it like it was, as new.  I will note that this Channel is probably quiet enough at half volume, for recording.  

11:00 hrs..  I'm taking a break from standing on this cold concrete floor, and sample a bit of the French Roast I left in the Thermos.  Please comment on the fix, and a path the head down, chasing the noise on Channel 1 (full volume), so that I can make it like Channel 2.  Have a good one.

Jack

[sluckey]

replace that 5.6M resistor with a 1M resistor.

You can just temporarily tack a 1M across the 5.6M for a test.

[Jack_Hester]

You can just temporarily tack a 1M across the 5.6M for a test.

Consider it done, when I get back out to the shop.  Will post the results.

Update: 12:30 hrs. - Connected a 1M resistor across the 5.6M.  No noticeable difference in the noise.  I'm calling this noise, to differentiate from the original problem, which no longer exists.  The lack of noticeable change tells me that the 120k in series with the existing 1M, being paralleled to the 5.6M, is working.  If I remember from the other day when I did the math, the total is just shy of 1M.  

13:00 hrs. - I pulled V1 and took an ohms reading from J1 tip contact to ground.  Got just a little over 1M ohms.  Just confirming that the J1/J2 configuration is correct.  I pulled the tube, just to be sure that it didn't affect the reading.  Put the tube back in, and the reading was basically the same.  Heated up the filaments, and the reading went down to 80K or so.  I'm satisfied with J1/J2 wiring and component values.  

http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_B12N_B15N.pdf

http://www.el34world.com/charts/Schematics/files/ampeg/b15n_portaflex.pdf

Jack

[Jack_Hester]

Based on the pics of your amp, I would estimate that it is a 62 to early 63. The earliest ones didn't have codes on the pots, the later ones did. The pots were made by CTS, so look for a stamp on the pot starting with 137. In that era, the next to digits were the year, followed by two digits signifying the week of the year.

Just occurred to me to look at the volume pots, as they are easy to see.  The numbering on both are: 1376311.

So, judging by what you've given, I have a 1963 model made in the 11th week.  I'll hang a tag on it now, with the date and model.  Thanks for that good bit of information.

Jack

[Jack_Hester]

So, here's where I am.  

http://www.el34world.com/charts/Schematics/files/ampeg/b15n_portaflex.pdf

Channel 1 has an open circuit jack (J1) that I know will be a noise issue, left as such.  It has a grid stopper (120K), and it has something of a series/parallel combination of resistors (combined with those of J2, as long as nothing is plugged into that jack) to make up a grid leak resistance of just shy of 1M.  In this amp, it measures just over 1M with my DMM.

Channel 1 has a closed circuit jack (J2) that has a grid stopper (120K), and a grid leak resistor of 5.6M.  It is unaffected by J1, as long as nothing is plugged into it.  

Channel 2 has a closed circuit jack (J3), and only one grid leak resistor (2.2M) in its configuration.  

Channel 1 is noisy (not comparing it to the open circuit noise), no matter whether something is plugged into J1 or J2 (guitar), or even if the tip contact of J1 is shorted to signal ground.  Different sound to the noise, from that of J1 being open.  Let me clarify the noise level.  It changes with the position of the volume pot.  At minimum, no noise.  Full volume, noise.  However, ground either end of the shielded conductor connected to V1-Pin1, and the channel goes quiet, no matter the volume pot position.  

Channel 2 is quiet with nothing plugged in.  It's the same as Channel 1 with V1-Pin1 being grounded, which is exactly what happens when J3 is unused.  

However, there is noise from interference that I'm not chasing which is that caused by the instrument and instrument cord acting like an antenna.  This applies to both channels.  

I'm open for suggestions.  Am I dealing with a problem being caused by the grid stopper and grid leak resistors?  I burnished all contact surfaces, to assure good contact closure and connections of plugs.

None of these resistors are mounted to the eyelet board.  Only to the jacks.  They are not CC in appearance, but I'm not familiar with all types to recognize just from looking at them.  They appear the same as ones that are mounted on the eyelet board, and those don't seem to be causing a problem.  

Please comment.  Thanks.

JacK

[birt]

locate the source of the noise in channel 1.
i understand it didn't change as you used a different tube?
does the noise change as you use the tone stack? if so, the source is at the first gain stage circuitry. if not, chances are its in the second.
have you cleaned and tightened the tube socket?
if you narrow it down to a small part of the circuit, chopstick it and see if you can hear more or less noise by pushing and/or tapping components and wires.
have you checked for leaky capacitors in channel 1?
do you have an 'audio probe' and listening amp to search more precise?
carbon comp or not, in a noisy gain stage i always mildly suspect the anode resistor.

[xm52]

Let's summarize. The tip of J1 runs through a 120K resistor and connects to the bottom let of a voltage divider (120K+1M || 5.6M ~= .83M) that feeds to the input of the tube. No issues with noise as far as the design is concerned. When you plug into J2, the signal runs through a 120K resistor into a 5.6M bottom leg of a voltage divider then into the tube. There is a small stub with a 120K resistor hanging on it connected from the tip of J1. This doesn't prove to be an issue in terms of noise.

You have to sweat the simple things first by breaking down the problem. I would check the circuit in detail against the schematic just to make sure that it is wired correctly and the correct components and pots are being used. Check your preamp ground connections. Redo the solder joints if in doubt. Look under the board eyelet board with a dental mirror and look for any stray strands. Pay particular attention to the gray shielded wires, their layout and their connections. Bad solder joints and strands shorting to other eyelets can be a source of all sorts of problems. Is the tube socket making good contact with the tube. Is the socket damaged?
Once all these basics are taken care of, then start looking for issues such as a bad component as birt was saying above.

Maybe it would help to have a reference showing how the wires are routed on channel 1 from a 63.

[xm52]

Here is the shot from another angle showing other details.

[Jack_Hester]

i understand it didn't change as you used a different tube?

Correct.

does the noise change as you use the tone stack? if so, the source is at the first gain stage circuitry. if not, chances are its in the second.

No change.

have you cleaned and tightened the tube socket?

Can't say that I've ever cleaned or tightened a tube socket.  I will have to learn this.  Is there some kind of burnishing tool to get down into the individual pin sockets? 

if you narrow it down to a small part of the circuit, chopstick it and see if you can hear more or less noise by pushing and/or tapping components and wires.

I've pushed wiring and components around with chopsticks.  No change from that.

have you checked for leaky capacitors in channel 1?

Only by measuring DC voltage after the coupling cap.  Small millivolt reading. 

do you have an 'audio probe' and listening amp to search more precise?

Yes, I do.  That's how I did my first troubleshooting to get to this point. 

carbon comp or not, in a noisy gain stage i always mildly suspect the anode resistor.

You've got me curious on this one.  I believe that I've got some 470K-1/2W on hand.  Maybe some 1W of the same value.  I'll replace the plate resistor tomorrow, and hang on to the original, just in case there's no change.  I like to keep original components, if I can't prove them to be bad.

Thanks for the comments, and I'll start on it in the morning.  Got some days off this week.  I want to do shop cleanup, but I'm afraid that I'll devote whatever time it takes to resolve this, and clean the shop later.  It's all inside, so nobody can see it, unless I open the front door.  Have a good one.

Jack

[Jack_Hester]

Here is the shot from another angle showing other details.

Looks just like a cleaner version of mine.  Mine was full of lint and spider webs.  I blew it out with air, before I started repairs, and it looks nice now.  I have a dental mirror, so I'll take a peek under the board.  I re-flowed all the solder on the jacks, and added ground jumpers from J3 to J2, to J1.  J3 was the only jack with a hard-wired signal ground.  As a matter of fact, if these jacks had been isolated from the chassis, the Hum Control pot probably would have not been a problem.  I can see no other ground connections to the chassis, other than these jacks.  And, it may still have not been much of a problem, if I had not added a grounded cord.  The cable was cracked at the original plug, and I could see bare copper on one lead.  So, I added the grounded cord. 

Anyway, like I stated in my reply to birt, I'll replace the plate resistor on the first triode of V1.  I'll make some inspections with the mirror, before I fire it up.   I took resistance measurements on all Channel 1 resistors, to confirm their true values.  Then, readings through the signal path to ground.  Tomorrow, I'll take an open cord or plug and take the same signal path resistance readings.  J1 gave me just over 1M to ground.  I'll confirm the readings for J2.  Though J3 is not an issue, at this point, I'll take a reading there. 

I've confirmed the Channel 1 wiring with the schematic, but I don't mind doing it again.  All part of the troubleshooting.  Just making sure. 

I certainly can't say that this has been a difficult repair.  Up to this point, something was fixed almost every time that I went into the amp.  That's real progress.  I'm proud of how the amp turned out.  This is just making it better.  I will feel foolish if the hum pot resolves this. But, I don't care.  A fix of any kind is a fix.  I'll have more to report, tomorrow.  Thanks to all who have commented and helped.  Have a good one.

Jack

[xm52]

J3 was the only jack with a hard-wired signal ground.  As a matter of fact, if these jacks had been isolated from the chassis, the Hum Control pot probably would have not been a problem.  I can see no other ground connections to the chassis, other than these jacks. 

That black wire (on the left hand side) going from the eyelet board to the jack is where the amp's ground bus connects to the chassis. Other grounds are tied to this bus from the cap can (it should have an isolation washer), all the way down to the first pre-amp stage. On the back of the amp is a jack, ext spkr, which should be isolated with a fiber washer. The other jack, ext amp, is usually not isolated so you have another ground connection there. There is also a ground connection to the chassis through the tone pot case. They tie one of the pot's terminals to the case which in-turn connects to the chassis. Except for these two exceptions, the main ground is the one terminating at the jack. This makes the amp very quiet.

[Jack_Hester]

That black wire (on the left hand side) going from the eyelet board to the jack is where the amp's ground bus connects to the chassis. Other grounds are tied to this bus from the cap can (it should have an isolation washer), all the way down to the first pre-amp stage.

Mine now has all three jacks bonded together, to this black wire.  

I replaced the cap can, making sure that the new isolation washer was installed properly.  I saw where the can was being used as a tie point for the ground loop coming from speaker cord to the eyelet board ground buss.  

On the back of the amp is a jack, ext spkr, which should be isolated with a fiber washer.

I remember seeing this, but didn't give it much thought.  Just out of curiousity, I'll inspect this fiber washer.  Not as a source of my problem.  Just curious.

The other jack, ext amp, is usually not isolated so you have another ground connection there.

I did notice this, and forgot about it being a ground to the chasis.

There is also a ground connection to the chassis through the tone pot case. They tie one of the pot's terminals to the case which in-turn connects to the chassis. Except for these two exceptions, the main ground is the one terminating at the jack. This makes the amp very quiet.

I'll take your word on the tone pot cases.  The way they are configured, I would not attempt to remove either channel, unless one or both were giving me trouble.  They both appear to be doing a fine job.  

J3 grounds the input of Channel 2, and as long as this circuit remains closed, that channel is quiet, no matter where the volume pot is positioned.  That is where I'm trying to get with J1.  Of course, knowing that J1 is open, I'm trying to get there by grounding the tip contact and take the whole signal path to ground.  Also knowing that it has a 120K grid stopper that J3 doesn't, along with the fact that J1 has approximately 1M for a grid leak resistor and J3 has 2.2M  Going straight to the shielded conductor (just the other side of the J1 grid stopper) with a ground gets me there.  Just like J3.  

I'm assuming that the one in your pictures is yours, or one that you have access to.  How does it sound with nothing connected to the inputs, and Channel 1 volume turned up to some point greater than minimum?  Also, with something to short J1 tip contact to signal ground?  This is really what I need, a comparison to a known trouble free amp.  If this is not an amp that you have access to, that's ok.  

Forgive me if I repeat myself.  Just my way of thinking out loud, and hopefully, I repeat with accuracy.  

Anyway, if you do have access to the amp in the picture, please give me some comparisons if possible.  Thanks for your help.

Jack  

[xm52]

On the back of the amp is a jack, ext spkr, which should be isolated with a fiber washer.

I remember seeing this, but didn't give it much thought.  Just out of curiousity, I'll inspect this fiber washer.  Not as a source of my problem.  Just curious.

How does it sound with nothing connected to the inputs, and Channel 1 volume turned up to some point greater than minimum?  Also, with something to short J1 tip contact to signal ground?  This is really what I need, a comparison to a known trouble free amp.

If the washers were not in place on that ext spkr jack, there would be a ground loop hum.

With nothing connected to the inputs, irrespective of volume, the amp is dead quiet. With the volume on full, you have to put your ear to the speaker to hear any hum. Seriously, these are about the quietest amps I have ever heard.  The pics were in my computer library. One of them was mine. I don't have the amp with me at this time to test a shorted jack in J1. It couldn't get any quieter though.

For the purpose of your tests, since you are working on channel 1, it might be a good idea to pull V2 so channel 2 isn't on to take it out of the equation.

Other general things that help in keeping the amp quiet: I connect the speaker return as close as possible to the ground of the phase inverter. I fly it over the circuit eyelet board. I don't run it in the 4-pin speaker connector. The high voltage CT ground goes to the return of the first power supply capacitor. Anything carrying AC is twisted. This includes the OPT paired wires. Those wires go along the chassis. Keep wires with low level signals away from AC signals. Fly them above if necessary. If they must cross, do so at 90 degrees to each other. I just thought that I would mention these things for completeness.

[Jack_Hester]

If the washers were not in place on that ext spkr jack, there would be a ground loop hum.

With nothing connected to the inputs, irrespective of volume, the amp is dead quiet. With the volume on full, you have to put your ear to the speaker to hear any hum. Seriously, these are about the quietest amps I have ever heard.  The pics were in my computer library. One of them was mine. I don't have the amp with me at this time to test a shorted jack in J1. It couldn't get any quieter though.

For the purpose of your tests, since you are working on channel 1, it might be a good idea to pull V2 so channel 2 isn't on to take it out of the equation.

Other general things that help in keeping the amp quiet: I connect the speaker return as close as possible to the ground of the phase inverter. I fly it over the circuit eyelet board. I don't run it in the 4-pin speaker connector. The high voltage CT ground goes to the return of the first power supply capacitor. Anything carrying AC is twisted. This includes the OPT paired wires. Those wires go along the chassis. Keep wires with low level signals away from AC signals. Fly them above if necessary. If they must cross, do so at 90 degrees to each other. I just thought that I would mention these things for completeness.

I've already come to the conclusion that you've had your hands and eyes inside more than one Ampeg.  I'm coming in, a bit later in life.  Glad I'm experiencing one.  It's worth the effort.

Good idea about pulling V2, to get it out of the equation.  That'll happen first thing, before firing it up again. 

The amp is basically stock.  There are a few signs of someone having hands-on, but most of that is gone, because of my installing a new PT and decoupling caps.  The power cord had been landed contrary to the schematic, and of course, the thermistor was missing.  That was my first correction before troubleshooting the original PT.  I made my PT leads follow the same path as the originals.  Tightly twisted and laying close to the chassis.  And, when I discovered the break in the speaker cord, at the strain relief, I was careful to put them back as I found them, when I cut and stripped back the old cable.  Which, by the way, I plan to replace with a new one.  Once all else is resolved. 

I'm liking the thermistor, as I watch the lamps (backlights for the plexglas logo) brighten slowly, when I power up the amp.  Someone cared enough about the circuit(s) downstream, to include such in the build design. 

You've given me the standard I'm looking for.  Quiet amp with all jacks as they are, no input connections.  I'll swap out R5 (470K plate resistor) and proceed from there.  We'll see what tomorrow holds.  Have a good evening.

Jack

[Tom_Hull]

On the back of the amp is a jack, ext spkr, which should be isolated with a fiber washer.

I remember seeing this, but didn't give it much thought.  Just out of curiousity, I'll inspect this fiber washer.  Not as a source of my problem.  Just curious.

How does it sound with nothing connected to the inputs, and Channel 1 volume turned up to some point greater than minimum?  Also, with something to short J1 tip contact to signal ground?  This is really what I need, a comparison to a known trouble free amp.

If the washers were not in place on that ext spkr jack, there would be a ground loop hum.

With nothing connected to the inputs, irrespective of volume, the amp is dead quiet. With the volume on full, you have to put your ear to the speaker to hear any hum. Seriously, these are about the quietest amps I have ever heard.  The pics were in my computer library. One of them was mine. I don't have the amp with me at this time to test a shorted jack in J1. It couldn't get any quieter though.

For the purpose of your tests, since you are working on channel 1, it might be a good idea to pull V2 so channel 2 isn't on to take it out of the equation.

Other general things that help in keeping the amp quiet: I connect the speaker return as close as possible to the ground of the phase inverter. I fly it over the circuit eyelet board. I don't run it in the 4-pin speaker connector. The high voltage CT ground goes to the return of the first power supply capacitor. Anything carrying AC is twisted. This includes the OPT paired wires. Those wires go along the chassis. Keep wires with low level signals away from AC signals. Fly them above if necessary. If they must cross, do so at 90 degrees to each other. I just thought that I would mention these things for completeness.

Hi mx52

thank you so much for your posting..

tom

[xm52]

If it helps, it's my pleasure. If it doesn't, never mind.

[Jack_Hester]

http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_B12N_B15N.pdf

09:30 hrs. - Replaced R5 (470K) plate resistor on V1.  No change to noise.  Popped it back out and reinstalled the original.  Made an interesting discovery, putting this one back in.  My original bad hum reappeared (to my dismay).  Turned the standby off and re-flowed the solder on the end of the resistor where the plate lead connects.  Standby back on and bad hum is gone.  Confirmed the original fix was here. 

Taking a break to do some chores.  Do a little more pondering.  Be back after a snack.

12:15 hrs. - Mail carrier stopped by, just as I was walking out to the mailbox.  Small package from FlipTops.  Got my new Hum pot.  It's nowhere near the quality in appearance, compared to the old.  Don't care, for now.  I need to get it installed.  Lifted my temporary virtual ground and wired in the new pot, just like the old one.  Fired the amp up, and let it warm a bit.  I centered the pot before starting, so when the standby switch is turned on, I will have less adjustment.  I have way lots less hum than before.  I have installed a plastic knob on the Hum Control, so that my fingers don't cause interference.  I tweak the pot for even less hum. 

I start out with the guitar plugged into J3, as this is my standard for quiet.  It's even quieter.  Full volume, and there's only some hiss.  I move to J2, full volume.  I know that I can hear some hum in there, somewhere.  Mostly the subtle hiss, like J3.  Now the test.  Guitar connected to J1, full volume.  Keep in mind that the guitar is laying down, away from the amp.  I hear the same as J2.  A little hum, maybe.  Same hiss.  A slight decrease in volume and I'm not sure there is any hum at all.  Note: Hiss disappears at around 3/4 volume. 

With nothing plugged into the amp, I detect some hum at about 1/2 volume.  I take a 1/4 phone jack and short it with my jumper, and plug it in.  Channel goes quiet, with subtle hiss a full volume.  I am happy.  Sorry I had everyone on a rabbit chase, with the virtual ground.  It just made me inspect the setup much closer, and reconfirm the amp against the schematic. 

This amp can be considered good enough for studio work, as quiet as it now is.  Do you suppose the Hum Control was connected over at V1 for a reason?  It is all the way on the other end of the chassis from the PT and first filament connection at V6.  Thanks ever so much for your patience and putting up with my lengthy essay.  I'll now break it back down and make the final voltage readings, along with new bias readings.  Hope they are in good shape.  Otherwise, here we go again.

I'll be back and post the readings in a short while.  Have a good one.  I am!

Jack

[Jack_Hester]

New readings with new Hum Control pot installed:

V6 (5U4):  Pin 4 - 368.0vAC
                Pin 6 - 367.9vAC
                Pin 8 - 425.5vDC
V5 (6L6):   Pin 3 - 418.5vDC
                Pin 4 - 379.1vDC
                Pin 5 - .0203vDC
V4 (6L6):   Pin 3 - 417.2vDC
                Pin 4 - 379.1vC
                Pin 5 - .0222vDC
V3 (6SL7): Pin 2 - 283.5vDC
                Pin 3 - 3.05vDC
                Pin 5 - 273.5vDC
                Pin 6 - 2.8vDC
V2 (6SL7): Pin 2 - 114.2vDC
                Pin 3 - 1.18vDC
                Pin 5 - 174.1vDC
                Pin 6 - 2.27vDC
V1 (6SL7): Pin 2 - 119.6vDC
                Pin 3 - 1.23vDC
                Pin 5 - 166.4vDC
                Pin 6 - 2.28vDC

The filaments are 5.12vAC and 6.60vAC.

I connected my Bias tester to the two 6L6's, and measured the Bias voltages as follows:

V4 - 58.97mA and V5 - 54.04mA (read as mVDC on my Fluke 8062A DMM). 

I am extremely pleased! 

And now, that the amp is as it should be, I still have a question about the Bias current.  Is this an acceptable range?  I can run the online calculators and come of with values from the mid 30's to mid 40's in milliamperes.  But, there's nothing like real life values to reference. 

Anyway, I'm going to enjoy it a bit, before farming it out to do service.  I can't say thanks enough to all who have contributed to this fix.  I've got good notes.  Just got to clean them up for better reference.  Have a good one, all.

Jack