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

[Jack_Hester]

I chanced upon a good deal on a non-working B-12-N.  I set it aside until this past Saturday.  I wanted a bit of a break from the Gibson Falcon Reverb hum issues, so I pulled the Ampeg out and opened it up.  Here's the schematic that I'm using, though I have the one that is glued to the chassis board:

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

Before doing any troubleshooting, I fixed quite a bit of obvious issues.  The speaker had been disconnected and connected to a jack mounted in the hole that at one point was the tilt-back leg bracket.  The wires were clipped to the origninal octal speaker plug.  I now have new cable connected between the speaker and the octal plug.  The jumper is still intact on the plug, that is needed to complete the ground path back to the PT CT.  Fortunately, the speaker cable and octal socket is intact, and still wired in to the amp.  I cut the socket off and installed a new hood with clamp, and stripped back fresh leads to solder in.  So, that's in good shape.  Now that I think about it, I did not check the tie points for the other end of this cable, in the chassis.  I will when I open it back up again.

I had to correct the power cord wiring on the chassis inside.  I won't try to remember what all I found, but it now matches the schematic.  I lifted the PT primary leads and ohmed everything out.  Then, I applied power to be sure that I had 120vAC to the proper places (power switch, ground switch, fuse).  I noticed that a component was missing.  It appears to be something like a piece of ni-chrome wire, though I have been unable to find a name for it.  It is 100 ohms, when cold.  And, .49 ohms, when hot.  It goes between the grounding switch and the fuse.  Once I cleaned up the power wiring, it left an empty spot on the eyelet board.  I put a temporary jumper there, until I can identify the component, and whether it needs to go back.  I would like to put a direct replacement for it, if possible.  Can anyone tell me what this component is?  And, a replacement.

Anyway, I replaced the blown 10 amp fuse with a proper 3 amp fuse, by specs.  I fired it up on the lamp limiter, and got a very bright and hot 25 watt bulb.  Disconnected everything and lifted the secondary leads (no tubes installed during this testing).  Fired it back up on the limiter.  Dead short, again.  Disconnected and measured the primary resistance.  Approximately 2.5 ohms.  I don't have any specs on what this reading should be on a good primary.  

So, I wired it all back up to the correct tie points and put it back together.  I went to FlipTops.com and ordered a Power Supply rebuild kit (some pricey, I might add).  As the folks around here used to say, "I'll have to sell a load of chickens, to pay for that one".  I did not see any listing for the missing component.  I'm sure that I overlooked it.  

Anyway, that was my brain-break from troubleshooting the Falcon.  The Ampeg is tucked away, until the PS kit comes.  And, the Falcon is back on the bench.  Maybe I will see something obvious, with a fresh mind to troubleshoot.  I'll also recap the troubleshooting that I've done, just as a reminder.  

Please comment on what I've done, so far.  Looks like the PT is breaking down, under power (and no load).  Are there some things to look out for, in these amps, that will help me to avoid the same happening with my new PT.  It surely won't have a hard life, in my possesion.  Maybe a trip to Church, when they have a song service, and we have lots of musicians.  Thanks for any advice.

Jack

[xm52]

The device that you are asking about is an NTC thermistor. It is a current inrush limiting device. It is rare to find these still in place in these amps. If you want a replacement, you can use a CL-60 which is 10 ohms when operating, 5A. Fender uses these in their amps, check out the circuit for the reissue 57 Deluxe on their web site.

The replacement transformer that fliptops sells is a good quality product, made by Heyboer. The metal can and potting drives up the price a bit. Original PT spec: 750 VCT (375-0-375) secondary, 150mA, 5 VAC @ 3A, 6.3V @ 5A with 117 VAC primary. The primary DCR is 2.5 ohms, the DCR across one half of the HV secondary is around 55 ohms.

Remove all the tubes when testing the transformer. Ensure that there are no shorts wired in. Check the hum pot in the heater circuit. Make sure that the standby switch is wired properly. It is originally wired so that the center tap of the HV is not connected to ground unless the speaker is connected. It is a feature intended to protect the amp in case it is turned on without the speaker connected. It is quite dangerous. They feed the B+ out through the four conductor speaker cable. You can undo this, hardwire the HV center tap to ground and move the standby switch after the first power supply capacitor. To all protection for the output transformer, you can add a  250 ohm 10W resistor (Ohmite makes a nice one) from the 16 ohm tap on the OPT to ground.

[Jack_Hester]

This is really good.  Thanks for the reply.  Even though I had no tubes in, at anytime while testing, I did not lift anything but the HV secondaries.  The 5v and 6.3v wiring remained connected to their tie points.  Did not check the hum pot.  That will go on the list of things to check next. 

I will digest the rest of you suggestions and have this thread pulled up on the shop computer, when I am once again back inside the amp.  Thanks, again.

Jack

[Jack_Hester]

The device that you are asking about is an NTC thermistor. It is a current inrush limiting device. It is rare to find these still in place in these amps.

Ok.  Did a quick search at FlipTops.  My Power Supply rebuild kit did not come with the thermistor.  However, they had some NOS, so I placed an order for a couple.  I will solder this in, when I get back to the amp.  

I will comment on the rest of your post, later this evening.  Thanks for the tips.

Jack

[Jack_Hester]

The replacement transformer that fliptops sells is a good quality product, made by Heyboer. The metal can and potting drives up the price a bit. Original PT spec: 750 VCT (375-0-375) secondary, 150mA, 5 VAC @ 3A, 6.3V @ 5A with 117 VAC primary. The primary DCR is 2.5 ohms, the DCR across one half of the HV secondary is around 55 ohms.

Remove all the tubes when testing the transformer. Ensure that there are no shorts wired in. Check the hum pot in the heater circuit. Make sure that the standby switch is wired properly. It is originally wired so that the center tap of the HV is not connected to ground unless the speaker is connected. It is a feature intended to protect the amp in case it is turned on without the speaker connected. It is quite dangerous. They feed the B+ out through the four conductor speaker cable. You can undo this, hardwire the HV center tap to ground and move the standby switch after the first power supply capacitor. To all protection for the output transformer, you can add a  250 ohm 10W resistor (Ohmite makes a nice one) from the 16 ohm tap on the OPT to ground.

I measured the Primary DCR, when I was troubleshooting on Saturday.  It measured approximately 2.5 ohms.  I removed the tubes before putting the chassis on the stand.  I had the cage removed, and didn't want to risk breaking them.  They won't go back in until the new PT, and other components are installed.

I will need to confirm the wiring of the chassis end of the speaker cord, but for now, I will put everything as shown in the schematic, just for troubleshooting sake.  I'll come back to your last suggestions, when I have the amp up and running.  Your comments and suggestions are greatly appreciated. 

Jack

[birt]

the primary of most pt's is around 30 ohms as far as i know. but this is based on my limited experience.

[Jack_Hester]

the primary of most pt's is around 30 ohms as far as i know. but this is based on my limited experience.

When I disconnect all PT secondary leads, I plan to put it back on the lamp limiter to see how it fares with the filament leads disconnected this time.  I fairly confident that the PT is bad, but I want to make this one last test, to see.  It would please me greatly, if something downstream in the filament wiring is at fault.  But, with the tubes out, I'm not counting on it.  I'll save this PT, as a candidate for repotting.  If the new one is like the old, in appearance, I'm not sure it's worth the effort.  FlipTops repotting cost is very close to the new cost.  I still tend to save stuff like this, for a very long while.  

Jack

[smackoj]

hi; I am not familiar with "Fliptops" ?  could you post a link so I can check 'em out?  thx, smackoj

[PRR]

> I am not familiar with "Fliptops" ?

Google is your friend.

[Jack_Hester]

hi; I am not familiar with "Fliptops" ?  could you post a link so I can check 'em out?  thx, smackoj

http://www.fliptops.net/

[Jack_Hester]

The device that you are asking about is an NTC thermistor. It is a current inrush limiting device. It is rare to find these still in place in these amps.

To all protection for the output transformer, you can add a  250 ohm 10W resistor (Ohmite makes a nice one) from the 16 ohm tap on the OPT to ground.

Got the NOS thermister(s) in from FlipTops.  I will put one with the amp, the next time I'm out in the shop. 

I completely overlooked getting a resistor for the OT.  I may already have one of this value, but if not,  I'll order one of those this week.  More to come.  Thanks for the help and suggestions.

Jack

[xm52]

The resistor is not necessary as it is a safety measure. Most amps don't have this sort of output transformer protection, although it isn't a bad idea. I mentioned because later revisions of the B12/B15 (they were the same amp) had this resistor. It seems that Ampeg got some grief from the regulators at the time because of their original protection scheme. Adding the resistor was their answer. They also removed the standby switch on the center tap of the high voltage PS secondary. Instead they went with disabling the phase inverter in later revisions. By doing this they were disabling the power amp and avoided any potential spikes through the power tubes and output transformer.

You might find it interesting to check out the schematic for the Heritage B-15, Ampeg's latest offering of the B-15. It is available in the user's here: http://www.ampeg.com/pdf/Heritage_B-15_OM.PDF


If you ever decide to repot your original transformer can, the original design used a 8411 Stancor transformer. Here is the 61/62 catalog: http://www.clarisonus.com/Archives/Trans/Stancor61.pdf. In later revisions, when they went from a 5U4GB to a 5AR4 tube rectifier, they switched to a transformer with a little less heater current. I like the specs of the Stancor for this amp. It is nice to be able to run the 5U4GB if you want a looser power supply.

It is very common to find these amps with replaced power transformers. They run a bit hot and I've always suspected that the way the transformers were potted contributed to the problem. The potting is supposed to wick away heat more efficiently than one would attain with an open air transformer. If it isn't done properly, it can do the opposite. You normally pot a transformer under negative pressure to ensure that the compound doesn't have any air bubbles and that penetration is optimized. This can cause local hot spots on the transformer windings which could eventually lead to a breakdown of the coating of the winding wires and a short.

They used a gooey black tar for potting at the time, the modern epoxy products are a lot better. If you stick the can upside down in an oven, the tar will melt. You can collect the tar in a pie pan and reuse it. A word of advice, don't do this with your wife at home, you will get into a lot of trouble. Find a toaster oven that is big enough at a yard sale.

[Jack_Hester]

They used a gooey black tar for potting at the time, the modern epoxy products are a lot better. If you stick the can upside down in an oven, the tar will melt. You can collect the tar in a pie pan and reuse it. A word of advice, don't do this with your wife at home, you will get into a lot of trouble. Find a toaster oven that is big enough at a yard sale.

Just so happens, I have an old oven in my shop.  Whenever I wanted heat heads for new valve guides, or crankcases for new bearing races, I'd warm them up to about 250°F.  I also used it for baking anything that I coated with Glyptal, such as the inside of cases or up in the topside of heads.  Anywhere oil flowed or splashed.

I'm going to save the Stancor catalog.  That will make for good reference material.

Anyway, I've got a place for perform the melting, but I'd better not get into potting/repotting.  Sounds like you know, first-hand.  Thanks for the good info.

Jack

[Jack_Hester]

Got some parts on hand.  Got a NOS thermistor, a power supply rebuild kit, and some more nails for the corner protectors.  Some of the nails are missing.  The power supply kit consists of the potted PT, canned caps, and two other electrolytic caps.  And, a 1K/10W resistor. 

The picture for the kit only showed a replacement rectifier tube, but I received a whole tube set (5U4GT, 3 x 6SL7GT, and 2 x 6L6GC).  So, I need to contact FlipTops and see if this is a mistake.  I've got several old stock of each, so I need to get these on the way back to them.  I haven't tested the ones that came in the amp, but I really hadn't planned on new tubes. 

Maybe the weekend, after this upcoming one, I can put this one back on the bench.  I need to finish troubleshooting the Falcon, first.  Work hours are too long to borrow a megger and test the tube socket on the Falcon.  That will have to happen next week.  More to come.  Have a good one.

Jack

[Jack_Hester]

I was able to get a day off, yesterday (Wed, 07Nov12).  Those haven't been as frequent lately.  Anyway, while I'm pondering my next move with the Gibson Falcon, I put this chassis up on the bench, for some initial troubleshooting.  Being as I had some repair parts on hand, it was a good time to see why this amp was in a bad way.

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

Replaced the PT and put a temporary jumper from ground to the CT of the PT secondary.  Replaced the decoupling capacitor can.  It was not correct for this amp.  Mine is supposed to have two 40uf and two 20uf.  The kit had one with three 40uf.  So, I connected a 40uf on either side of R38, and connected R36 and R37 to the third single 40uf.  I have the correct one coming from AES, and will replace next week.  Fired it up on the lamp limiter, and things looked much better this time.  All tubes were in, and amp was connected to the cab.  Each lamp, from 25 watts to 100 watts, showed that the amp no longer had the dead short.  I shut it down, at this point.  With other errands thrown in, this was a full day.

This afternoon (Thurs), I fired it up again with limiter, to make some voltage checks.  Dead quiet.  Not even a hum.  After a good warmup, I flipped the chassis over to be able to take readings.  That's when things got exciting.  The speaker started sputtering loudly, and the lamp limiter (100 watt bulb) went to full bright.  Shut it all down, disconnected from the speaker cab, and put the chassis on the bench.  I unsoldered the speaker cord and removed the strain relief.  There was a very bad break in the cable, inside of the strain relief.  I cut it off and skinned the outer jacket back about 2".  Reinstalled with strain relief, and resoldered the wires to their proper termination points. 

Found another problem, when I started to connect the octal cord socket to the cabinet plug.  The bakelite center pin on the plug was missing it's key.  The socket appeared to have been forced onto the plug, breaking the key.  I was careful to match the socket pin 1 to pin 1 on the plug.  Fired it up again, on the 100 watt bulb.  Now, I have a very subtle background hum, and the amp works.  Removed the 100 watt bulb and installed a circuit breaker that fits the lamp socket. 

Power on and guitar connected.  This is one loud amp, with a good mid to low range.  This may be due to the large voice coil speaker.  Tomorrow, I will connect a speaker with a smaller, standard voice coil.  The external speaker jack parallels both speakers to a 16 ohm tap.  My shop speaker cab does not have a good quality speaker, but it is good enough to see if the amp brightens up a bit.  I don't really care, as it works so well. 

I'll replace the cabinet octal plug with a good one.  I'm also going to put a grounded power cord on.  The existing ungrounded cord is cracked all the way through, at the plug.  I'll save it and store in the speaker cab.  The amp has a ground switch, and a hum pot.  The ground switch makes no change in the amp sound.  Hum is so low.  I haven't touched the hum pot. 

I have some replacement nails for the cabinet corner protectors, as a few are missing.  I also have a tilt-back leg assembly to install, as the original is missing.  This one isn't like an original.  It appears to be heavier duty.  I plan to get a dolly for it, to round out the repair/restoration.  I don't really need it, but will pick one up later.

Anyway, this amp should be finished as far as I can go, tomorrow evening.  When the correct caps get here, and installed, that will complete a rather enjoyable repair.  I needed one of those.  The Falcon has me stumped.  But, I'll clear the bench, and put the chassis back on the cradle, and iron that one out.

Hope I didn't ramble too much.  This amp really didn't need a great deal.  It was over and fixed rather quickly.  I'll post a picture, before I carry it to the house.  Have a good one.

Jack

[smackoj]

can you post a picture of the Ampeg? 

[xm52]

It is amazing how quiet these amps can be. It helps when recording.

If you want to keep the four-conductor speaker cable, I use Belden 8454. It is a rubber jacket cable, 4-conductor, 18ga. The outer dimension is the same as the original. They sell it by the foot here http://www.surplussales.com/Wire-Cable/Wire3.html.

[Jack_Hester]

can you post a picture of the Ampeg? 

I'll see if I can accomadate you with one, tomorrow (Sat), when it's buttoned back up.  I really wish that I knew some history of this amp.  The speaker cord was probably the original problem. 

It puzzles me that someone kept blowing fuses, I guess hoping to clear the problem.  And finally, putting a 10 amp fuse in and frying the PT.  But, judging from the amount of lint and little spider webs inside the chassis, this was an active amp for the better part of its past.  And, very well taken care of, as it's in a good, clean condition. 

Anyway, I'll snap a couple pics, tomorrow.

Jack

[Jack_Hester]

It is amazing how quiet these amps can be. It helps when recording.

If you want to keep the four-conductor speaker cable, I use Belden 8454. It is a rubber jacket cable, 4-conductor, 18ga. The outer dimension is the same as the original. They sell it by the foot here http://www.surplussales.com/Wire-Cable/Wire3.html.

That's good info, for the cable.  I will order some, as I plan to keep it as close to original as possible.  Super quiet, regardless of the power cord plug orientation.  It's getting a grounded one, this evening.

Jack

[xm52]

I should have mentioned, cables come with a white lettering printed on them identifying the product. To remove it, I apply some acetone to a paper towel, wrap it around the cable and pull the cable through. It removes it fairly easily.

I buy power cable by the yard at Home Depot. I use four yards of Carol (General Cable), SJ0 #18/3, rubber jacket. It has a .299" outer diameter and again fits the original chassis hole and works with the original strain relief bushing, a Heyco 6W-1 (part #1184). Heyco still makes this part and will send you a free sample if you need a couple. If you want to get fancy, you can get a Heyco strain relief bushing pliers, No. R-29 (part #0022). They are expensive, but I've seen them on sale at Amazon.com.

With this cable you will need to add a plug. Home Depot sells a Leviton 5-15 15A 125V. They come in black to match the cable.

[Jack_Hester]

I should have mentioned, cables come with a white lettering printed on them identifying the product. To remove it, I apply some acetone to a paper towel, wrap it around the cable and pull the cable through. It removes it fairly easily.

I buy power cable by the yard at Home Depot. I use four yards of Carol (General Cable), SJ0 #18/3, rubber jacket. It has a .299" outer diameter and again fits the original chassis hole and works with the original strain relief bushing, a Heyco 6W-1 (part #1184). Heyco still makes this part and will send you a free sample if you need a couple. If you want to get fancy, you can get a Heyco strain relief bushing pliers, No. R-29 (part #0022). They are expensive, but I've seen them on sale at Amazon.com.

With this cable you will need to add a plug. Home Depot sells a Leviton 5-15 15A 125V. They come in black to match the cable.

More good info.  I have some strain relief pliers, but don't remember the make.  Never thought that strain reliefs were a problem.  Then, I got the pliers.  What a difference using the right tool. 

We are always short handed on Fridays.  I'm always the 'old reliable' Tech that works Fridays.  If we had more people here today, I would have taken a half day and spent it in the shop. 

The rest of the cleanup on the amp won't take long.  Changing the decoupling caps out next week, will be just as quick.  I just can't get over how such a good amp can go so bad.  The PT and parts were some pricey.  But, that's really it.  Everything else was mostly correcting wiring errors, on the power end of the amp.  Repairing the speaker/ground cord.  Not difficult at all.  I did put a new OEM thermister in.  I wonder if this is a weak link.  When I do get it to the point where I can call it done, I plan to hook up a DC milliammeter on the front end of the B+ line, and see what it pulls at an idle. 

I also need to date this amp.  I haven't done my homework on it, yet.  I'd like to put an accurate date on it, as I've been doing with the others that I've resurrected.  I hang a tag in each, with working conditions, correct date, and  needs (if any).  Needs being whether a speaker is incorrect, or power cord is not original.  Or, footswitch belongs on a different model.  Those kind of things.  Anyway, I'll try to do some of that tomorrow, before I post pictures.  Otherwise, I'll post my findings when I have them.  Thanks to everyone who commented and shared info.  Hopefully, posting my ramblings will do someone some good.

Jack

[Jack_Hester]

can you post a picture of the Ampeg? 

Note: By posting these pictures, you are entering my 'new' shop ('78), and a world of clutter and collectables.  I can't even walk in the old one.  But, it's still fun to try.

Here are some quick snaps of the amp:









The cab has been cleaned.  The bottom front corner protectors have new nails in place of the missing ones.  The new tilt-back leg socket has been installed, in place of the missing one.  The speaker/ground loop plug has been replaced in the cab.  Ungrounded cord has been replaced with a grounded cord. 

The amp picked up a hum when I installed the new grounded cord.  I attached the ground to a post on the PT.  It's not bad, but there.  I'll need to study the schematic and look the circuit ground over a bit more, in the chassis. 

Also, the mail carrier just delivered the correct decoupling can capacitor(s), from AES.  I might slip that in after lunch.  Anyway, more to come.  Have a good one.

Jack

[sluckey]

I love the look and feel of those portaflex amps!  I fell in love with the B12X back in '68. That amp just has such a great sound and REAL EYE APPEAL for anyone with electrons in their blood. The only time I ever saw one in person was the day I walked into Capital Music in Montgomery with $375 in my pocket. I'm sure I drooled when I saw it but I knew I didn't have quite enough cash. I did walk out with it's smaller, more affordable sibling, the Gemini II (G-15) combo.

Is that a JBL or EV in yours?

[Jack_Hester]

Is that a JBL or EV in yours?

I'll let you know, this evening.  I'll have it open again in a little while, to put the can cap in. 

Jack

[Jack_Hester]

Is that a JBL or EV in yours?

I was pretty sure that you were asking about the speaker.  Now that I've looked, I'd better be sure.  Forgive my ignorance, if that was not your question.

Just in case it was, it's a Utah, but I can't see the number on the rim.

Decided to record some voltage readings, as I have no reference to go by:

V6 (5U4):  Pin 4 - 355vAC
                Pin 6 - 355vAC
                Pin 8 - 344vDC
V5 (6L6):   Pin 3 - 323.5vDC
                Pin 4 - 212vDC
                Pin 5 - .5vDC
V4 (6L6):   Pin 3 - 322vDC
                Pin 4 - 212.8vDC
                Pin 5 - .2vDC
V3 (6SL7): Pin 2 - 220vDC
                Pin 3 - 2.4vDC
                Pin 5 - 212.8vDC
                Pin 6 - 2.2vDC
V2 (6SL7): Pin 2 - 88.4vDC
                Pin 3 - .9vDC
                Pin 5 - 138.8vDC
                Pin 6 - 1.8vDC
V1 (6SL7): Pin 2 - 94.9vDC
                Pin 3 - 1.0vDC
                Pin 5 - 128.5vDC
                Pin 6 - 1.8vDC

The filaments are 5vAC and 6.3vAC.

Here's what's bothering me.  I connected my Bias tester to the two 6L6's, and measured the Bias voltages as follows:

160mA and 154mA (read as mVDC on my Fluke 8062A DMM).  

I would think that these values should be down in the 55 to 60mA range.  Maybe this is correct for this configuration.  These tubes get real hot, after about 15 to 20 minutes. It's obvious that they've been hot in a past life, as the clear Ampeg plate has a slight warp in one side, where it's mounted to the cage.  I've only had the amp on for short spells.  

Please advise as to how I should proceed with further tests.  Thanks.

Jack

[xm52]

Here is a schematic with voltages that may help you. It is a slightly older revision of the amp. A B-12 is the same amp as the B-15, the speaker cabinets are different. The B-15N had the 5U4GB initially, then went to a diode rectifier with the B-15NA (and NB which was the same amp), then went with 5AR4 rectifier in the B-15NC. The voltages are higher in the B-15N, the 5U4GB drops a lot more voltage. You can see if your voltages are proportional to the ones indicated on the schematic. Most B-15N's have a 5AR4 tube rectifier and the voltages are higher.

For reference, here is the schematic with voltages for the short lived 1963 B-15NA. It has a solid state rectifier.

[Jack_Hester]

Thanks alot, for those.  Something with voltage readings is a big help, for comparison.  Mine is running quite a bit lower than either of these.  But now, to figure out why my bias current is high.  I'm basing my concerns from the results of an online calculator for my tubes:

http://www.webervst.com/tubes1/calcbias.htm

Until this is resolved, I'll not run it longer than it takes to warm up and take readings. 

Thanks again, for the schematics.

Jack

[sluckey]

Please advise as to how I should proceed with further tests.

Measure the voltage on pin 8 of the 6L6s. Then divide that voltage by 250Ω. That's the real current flowing thru both 6L6s. Divide that by 2 for an idea how much current is flowing thru each tube.

[xm52]

yes...and use an ohm meter to verify that the resistor is the correct value. You never know what's really in there unless you check.

These cathode resistors get hot and when they are old, you often find that their value has drifted out of spec. When the power supply is re-caped, it is a good time to change the cathode resistor and capacitor if they are out of spec. Because the resistor runs hot, it helps to use a higher wattage flameproof part and mount it away from the eyelet board. Just raise it up a millimeter or so it is not in contact with the board. If I use a cement block resistor, I mount it on its side, so the bottom with the glue is not facing the board. I find that when they fail, they often burn on the bottom. That can scorch or burn through the board. Turning it away prevents this.

[Jack_Hester]

Measure the voltage on pin 8 of the 6L6s. Then divide that voltage by 250Ω. That's the real current flowing thru both 6L6s. Divide that by 2 for an idea how much current is flowing thru each tube.

I have some adapters (now where did I put them?) that I can put between the tube(s) and the socket, and measure each pin from the topside of the chassis.  Even so, that's what I've come to like about this amp configuration.  Everything is so easy to get to.  At any time.  I usually don't work on any projects on Sunday.  Unless I'm at the plant.  This is not work, when you're enjoying it, is it? 

yes...and use an ohm meter to verify that the resistor is the correct value. You never know what's really in there unless you check.

These cathode resistors get hot and when they are old, you often find that their value has drifted out of spec. When the power supply is re-caped, it is a good time to change the cathode resistor and capacitor if they are out of spec. Because the resistor runs hot, it helps to use a higher wattage flameproof part and mount it away from the eyelet board. Just raise it up a millimeter or so it is not in contact with the board. If I use a cement block resistor, I mount it on its side, so the bottom with the glue is not facing the board. I find that when they fail, they often burn on the bottom. That can scorch or burn through the board. Turning it away prevents this.

I replaced the cathode cap, when I changes the other electrolytics.  I did not measure the value of the 250 ohm resistor.  I don't believe that I have a replacement, but I have a 100 ohm and a 150 ohm, both 10 watt.  They are rather big, but I can get creative and work them in, if need be.

Good idea about the orientation of the resistor(s). 

I have the amp on the back seat of my truck (crew cab), to carry to Church.  I'll make my checks this afternoon, and post my findings.  Thanks, muchly. 

Jack

[Jack_Hester]

Measured the cathode resistor, for the 6L6's.  In circuit, it measures 248 ohms.  Out of circuit, 249 ohms.  

I took a voltage reading from Pin 8 to ground and got 42mVDC.  I did this more than once, to be sure of my reading.  Got 38mVDC, one time, right after turning the Standby switch on.  The other reading was after a minute or so of turning it on.  

I took a reading across Pin 3 to Pin 8 of each 6L6 tube and got 330vDC.  It's as if I have an open connection from the plate to ground.  Of course, I know that's not true, as it works. 

Just for grins, I swapped the 6L6GC's out with a couple USN metal can 6L6's, and took the same readings.  Got 54mvDC at Pin 8 of each.  Basically, the same.  I'm going to shut down and head for the house, and ponder it there for awhile.  Thanks for the tips to get me to this point.  Have a good one.

Jack

[birt]

I took a reading across Pin 3 to Pin 8 of each 6L6 tube and got 330vDC.  It's as if I have an open connection from the plate to ground.  Of course, I know that's not true, as it works. 

an open connection from plate to ground would result in 0vDC. what you have measured is plate to cathode. add to that the cathode to ground voltage and you have the full plate to ground voltage. i see no problems there.
reread your measurements and logics. it seems you are more confused than you should be.

[Jack_Hester]

an open connection from plate to ground would result in 0vDC. what you have measured is plate to cathode. add to that the cathode to ground voltage and you have the full plate to ground voltage. i see no problems there.
reread your measurements and logics. it seems you are more confused than you should be.

I suppose I stated that wrong.  My meaning was that it was as if I were reading B+ to ground. 

You're right about the confusion.  The only time I can get a current reading is with my adapter to go between the tubes and sockets.  And then, I'm reading millivolts across a built-in 1 ohm resistor.  That's what started my chasing this rabbit.  Way high current for what I was expecting from each tube. 

Jack

[sluckey]

I took a voltage reading from Pin 8 to ground and got 42mVDC.  I did this more than once, to be sure of my reading.  Got 38mVDC, one time, right after turning the Standby switch on.  The other reading was after a minute or so of turning it on. 

That can't be right. Not if the amp makes a sound. 42mV/250Ω = 0.168ma current. That's pratically cut off.

[Jack_Hester]

That can't be right. Not if the amp makes a sound. 42mV/250Ω = 0.168ma current. That's pratically cut off.

I agree.  I connected my meter to the ground point for the cathode resistor/capacitor, as my ground reference. 

That's one thing I like about this amp design.  No dependance on the chassis for signal/supply ground.  All grounds are hard wired to each other, and tied to the decoupling can capacitor case.  This is above ground, as it is insulated from the chassis.  This ground point goes down the cable to the speaker cabinet plug, through a jumper in the plug, and back up the cable to the PT center tap.  I have followed all the grounds on the eyelet board, to be sure that they are tied back to the cap case.  Confirming the grounding was my first concern when I started troubleshooting.

That being said, I confirmed that all the connections on the power tubes were as in the schematic.  Of course, I can say that and immediately have doubts.  But, that's what troubleshooting is all about.  I even tried a second meter, just like the Fluke that I've been taking these readings with.  My next step is to pull out my VTVM and take measurements with it, just to back up the DMM's.  That will have to be tomorrow afternoon. 

See, this is what creates my confusion.  My bias test device shows high current on each tube.  But, I can't duplicate it with a voltage reading on Pin 8, because from there to ground, I may as well have no reading, being as it's so low. 

Also, the tubes get super hot in just a minute or so.  When I swapped them with the canned tubes, the cans got so hot that I had to remove them with my leather gloves.  And, with only minutes of operation (standby switch closed).  I'm satisfied that I'm chasing an existing problem (if it is indeed a problem) that was with the amp before it died with some previous owner.  The clear plastic Ampeg plate that lights up, is mounted right in front of these two tubes.  It is warped on one side to the point that it has almost pulled loose from one of the screws holding it to the cage.  I will replace it when I can cool things down. 

And, the amp makes much good sound.  There is a hum that is present on both channels, more so on Channel 1 than 2, that increases with volume and goes away with the volume at minimum.  I'll chase that later. 

Tell you what.  I'll take all readings tomorrow afternoon, VTVM only, and post them tomorrow evening.  My DMM's are supposed to be high impedence.  I haven't looked at the manual in a long while.  'Back in the day', we used a VTVM exclusively for signal measurements, because of it's high impedance probe.  I'll plug it up in the morning, before going to work, and know that it will be good and warmed up, by tomorrow evening.  I ran a cal on it about a year or so ago.  Should be good enough for troubleshooting. 

More to come.  Thanks for the help and the additional head scratching.  Have a good one.

Jack

[sluckey]

Measure the resistance from pin 8 of the 6L6s to ground. What do you get?

It's puzzling that the tubes get so hot but yet they don't appear to be conducting??? Your DMM is fine for any measurements in that amp. I would not bother with a VTVM.

[Jack_Hester]

Measure the resistance from pin 8 of the 6L6s to ground. What do you get?

It's puzzling that the tubes get so hot but yet they don't appear to be conducting??? Your DMM is fine for any measurements in that amp. I would not bother with a VTVM.

Will take that reading, tomorrow afternoon.  Have a good one, and thanks.

Jack

[xm52]

Let's step back a bit.

First of all, did you install the replacement power transformer from fliptops? When you take your readings tomorrow, check what the AC mains voltage is.

When you take you amp readings, measure DC volts to the chassis with the amp off standby (in operating mode), nothing connected to the input, volumes down all the way, tones at noon. There are different types of 5U4 rectifier tubes and they are not the same. Check that the rectifier tube in the amp is a 5U4GB. The power tubes in the earlier B-12 amps were 6L6GT's (like a 6L6), the later B-15N amps used a 6L6GC which can take a higher plate voltage. Please verify which you have installed.

On some of these amps, the wiper (center terminal) of the hum pot is connected to ground. In others, which are cathode biased, the wiper is connected to the power tube cathodes. The DC offset voltage is an attempt to lower hum in the heater circuit. In other versions of these amps, the hum pot wiper is connected to ground. If the hum pot is connected to the cathodes, until you sort out this problem, it might help to disconnect this. This might not do anything but it eliminates a factor associated with the cathodes for testing purposes.

It would be interesting to redo your readings with a 5AR4 installed instead of the 5U4GB and see what is happening. A higher plate voltage will lower the current through the power tube. The power tube cathode voltage, pin-8, is typically around 30VDC with 117 VAC at the wall; 36VDC with a 5AR4 installed.

[Jack_Hester]

It would be interesting to redo your readings with a 5AR4 installed instead of the 5U4GB and see what is happening. A higher plate voltage will lower the current through the power tube. The power tube cathode voltage, pin-8, is typically around 30VDC with 117 VAC at the wall; 36VDC with a 5AR4 installed.

I'm going to dig through my sorted tubes and see if I have an OEM 5AR4.  I doubt that I have one.  I'll put my hands on the JJ equivalent that I do have.  I'll also dig out a 5T4, as well.  I promise not to do any amp 'work' today.  But, I will make comparisons in voltage readings and bias current for each change in rectifier tube.  I'll also see how the amp sounds with the guitar.  A record of the mains and B+ differences, along with the bias current readings, will be put in the cabinet to stay with the amp.  Even though I haven't documented my line voltage feeding this amp, I'll do so this time.  Should have done it from the beginning.  'Better late than never'.  More to come.  Have a good one.

Jack

[xm52]

The tones at noon and volumes off, is more of a consistent way of taking the readings. That's why I suggested it. I don't think that it will make any difference in this case since you are looking at the power amp.

Some of Ampeg's, and other company's service manuals, tell you how to set up the amp when running tests and setting the bias. They provide reference voltage readings and these require certain settings and perhaps a signal generator connected to the input. The tone setting would be necessary of taking readings in those stages. In Ampeg's case, they often specify tone settings at noon as part of the initial setup for testing.

The Ampeg labelled tubes were often supplied by Sylvania, rectifiers were often Mullard. There are a lot of interesting photo's at tubemongerlib.com and tubeworld.com that can be used to identify who made any tube that you may have in hand. Many companies, including RCA for example, often seemed to get their 5AR4 rectifier tubes from Mullard. I'm not sure why.

[Jack_Hester]

Walked out to the shop, this morning, to take a quick peek at the Hum pot and how it is connected.  The outside terminals of the pot are attached across the filament of V1.  The wiper terminal is routed back to Pin 8 of V4.  Interesting discovery made while tracing this conductor.  Both this wire and the wire going to the cathode resistor/capacitor are looped tightly through the terminal on Pin 8.  But, no solder has ever been on this terminal. 

Makes it easy to disconnect the Hum pot.  Ampeg used solid conductor wire throughout this amp.  The only stranded wire are the leads from the PT, OT, and power cable. 

Anyway, this afternoon, I will disconnect the Hum pot lead from V4, Pin 8, and use an alligator jumper to attach it to chassis ground.  Then, I will warm up the amp and take readings on Pin 8 of both 6L6's.  If I get voltage readings, I'll calculate the current flow and compare it with an actual reading, using my adapter. 

More to come.

Jack

[sluckey]

The outside terminals of the pot are attached across the filament of V1.  The wiper terminal is routed back to Pin 8 of V4.

So, the filaments are floated to the positive cathode voltage.

By chance, does your new PT have a centertap for the filament winding (usually a green/yellow wire)? And if so, did you connect it to chassis ground? If so, thats why you have no voltage on the cathode (pin 8). Measuring resistance from pin 8 to ground will show very low ohms (reading thru filament winding) rather than the expected 250Ω.The cure is to disconnect that center tap wire.

[xm52]

By chance, does your new PT have a centertap for the filament winding (usually a green/yellow wire)? And if so, did you connect it to chassis ground? If so, thats why you have no voltage on the cathode (pin 8). Measuring resistance from pin 8 to ground will show very low ohms (reading thru filament winding) rather than the expected 250Ω.The cure is to disconnect that center tap wire.

That sounds like it is a good possibility here.

If the center tap wasn't connected to the chassis, is there a chance that the end is shorting against the chassis? It is a good idea to put some shrink wrap tubing over the end of the wire, roll it up and tuck it somewhere safe.

[Jack_Hester]

So, the filaments are floated to the positive cathode voltage.

By chance, does your new PT have a centertap for the filament winding (usually a green/yellow wire)? And if so, did you connect it to chassis ground? If so, thats why you have no voltage on the cathode (pin 8). Measuring resistance from pin 8 to ground will show very low ohms (reading thru filament winding) rather than the expected 250Ω.The cure is to disconnect that center tap wire.

No filament CT.  The only CT is on the mains, which is a red/yellow.  This lands on the eyelet board with a lead from the Standby switch.  The old PT did not have a filament CT, either.  That being said, the old one had a white wire that was landed on an eyelet, and nothing attached to it.  The new one does  not have a white wire. 

My plan is to lift the pot wiper and land it temporarily to ground, leaving only the cathode resistor/capacitor attached to Pin 8 of each tube.

Jack

[sluckey]

My plan is to lift the pot wiper and land it temporarily to ground, leaving only the cathode resistor/capacitor attached to Pin 8 of each tube.

How about making the resistance check I asked for before and after you do this?

[Jack_Hester]

How about making the resistance check I asked for before and after you do this?

Will do.  I'll give you a before lift resistance, and an after lift resistance.

The wife reminded me that we are going to the Golden Corral for a Veterans Day meal.  She's not a veteran, but she insists that I get a freebie.  Man, I dislike crowds.  And, it will be.  But, I fully intend to spend at least an hour or so in the shop, when we get home.  Resistance check, first.  Then, I'll work my way through the other checks.  

Jack

[Jack_Hester]

How about making the resistance check I asked for before and after you do this?

Resistance readings from Pin 8 to signal ground:

.24 ohms - Hum pot wiper connected
249 ohms - wiper disconnected

28.8vDC on Pin 8

28.8 / 249 = 115.7mA

115.7 / 2 = 58mA per tube

The tubes never got really hot.  At least, no warmer than the other tubes.  But definitely, not as hot as with the wiper connected as it was.  Also, the amp hum is completely gone, with both volumes at minimum.  But, increases back to fairly loud, as volumes are turned to full.  Looks like I'll be chasing that one over in the pre-amp section.

Didn't have much time, this evening.  I ate too much at the Veteran's Day meal.  Will be able to get an earlier start, tomorrow afternoon, and take a whole set of new readings.  Though I didn't write them down, my B+ jumped to well over 400+vDC, at Pin 8 on the rectifier tube. 

More to come.  Have a good one.

Jack

[sluckey]

.24 ohms - Hum pot wiper connected
249 ohms - wiper disconnected

28.8vDC on Pin 8

28.8 / 249 = 115.7mA

115.7 / 2 = 58mA per tube

That's a lot of progress! The pot may be defective or there is a short between the filament string and chassis. Should be fairly quick and easy to determine exactly which it is.

[Jack_Hester]

Tomorrow, I'll measure the pot resistance, with all leads disconnected.  Also to ground, one terminal at a time.  

At this point, I'm suspecting the pot.  I'll also see how the hum is affected with the pot out of the picture.  

Poor memory tells me that my B+ jumped to around 425vDC.  I lifted the wire at the wiper, as the other end was wrapped so tightly around the terminal, that I would have had to cut it.  So, I left it alone.  The wiper end was easy.  By lifting it there, I proved that the wire was good.  Wonder why the Hum pot wasn't a higher value?  Like maybe something in the 200 - 400 ohm total.  As it is, the wiper being tied to either chassis ground or signal ground, the pot does not reduce the hum to null (either volume up full).  However, moved away from center, the hum gets extremely loud.  

Something else about the wiring in this amp.  The wires are wrapped together with waxed string, like we used to make our bundles.  Nowadays, tie-wraps, cable ties, etc. are the norm.  It sure is good to see something done like this.  Of course, the bundles were done outside the chassis, and installed afterwards.  Too tight a fit to do so in the chassis, unless they were a surgeon.  

And, I'll prove my math by inserting my bias tester in between the tubes and sockets, and taking a direct reading.

Anyway, one step at a time.  I'll pick up on it again, later.  Have a good one.

Jack

[sluckey]

Wonder why the Hum pot wasn't a higher value?  Like maybe something in the 200 - 400 ohm total.

100Ω is a good value.

As it is, the wiper being tied to either chassis ground or signal ground, the pot does not reduce the hum to null (either volume up full).  However, moved away from center, the hum gets extremely loud.

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. 

Something else about the wiring in this amp.  The wires are wrapped together with waxed string, like we used to make our bundles.

That was typical for the time period. I still have a white spool and a black spool of waxed lacing cord and even remember how to lace bundles or make spot ties. Haven't used it in years though.