Ampeg B15-NF restoration project

[Dave Moberg]

Hi all, looking for some help with an old Ampeg B15-NF rescue/restoration project.  This amp has been modified and hacked on a little....well, a lot really.  I replaced all the tube sockets, one was smoked and the rest had missing/broken tabs so I just swapped them all.  Starting putting all the wiring back in place and I've found a couple connections that were terminated in the wrong place.  I'm going to have to trace every component in the schematic and try to verify everything's wired according to the original diagram.  It's time consuming and I've got lots of questions.  I'm an old Air Force radio troop, started working on tube radios 40 years ago....I'm a bit rusty.  First question....the attached schematic is the same one glued to the bottom of the amp.  There are two circled F symbols, one on the OT-214 secondary winding (black wire) and the other is at the 1K resistor tied to pins 3&6 of V3.  I noticed these points are grounded on an older 1961 schematic, but I'd rather verify my information before I take a chance on smoking this thing.  Can anyone educate me on the circled F symbol?  Thanks in advance.

[shooter]

look at the schematic, now plug the speaker in(mate the plugs), you'll see how ground is "made"

[Dave Moberg]

Thanks for the info.  I can see the path to ground thru the 4-pin connector.  This thing was modified with a standard 1/4" tip/sleeve connector.  I'm going to wire in the 4 pin connector to match the schematic.  What about the 1K resistor?  I just can't decipher what the circled F is supposed to designate.  If the resistor is simply grounded.....why the F?  There's a ground tie point right there, has me wondering if I'm missing something.

[66Strat]

The circled F at the PI cathode connects at the circled F on the speaker plug.

[vampwizzard]

There appears to be several iterations of their output section on those amps. Heres an interesting article on restoring that output socket/plug with an XLR with instructions on how to ground it: https://www.premierguitar.com/articles/Ampeg_B_15_N

[66Strat]

It's a roundabout way of grounding. INDOT must have designed it.

[shooter]

INDOT

they're not that competent, probably Army Core of Engineers 

[Dave Moberg]

Thanks vampwizzard, the article answered the question perfectly.  The circled F must designate the optional grounding method.  Simply ground the cathode resistor and the phase inverter is on when power is applied.  Ground the cathode resistor through the speaker connector and the phase inverter is disabled until the load is connected.  Still don't know what the cirlced F stands for but I'm good.  Back to the soldering iron, thanks for help.

[DummyLoad]

safety for amp - if no speaker load is connected, no signal is passed to output tubes since phase inverter is non-operational if 1K is not grounded.

if the original johnson plug was replaced with a 1/4" jack, you'd use a jack with a NO sleeve switch, so when you plug the speaker in the 1K for the phase inverter is grounded by closing the sleeve switch now 1K is grounded and phase inverter passes signal to the output tubes.

i'm sure most of you figured that out - just being capt. obvious. 

--pete

[DummyLoad]

Still don't know what the cirlced F stands for but I'm good.

Fault protection? does it matter? F is wired to F.

--pete

[shooter]

most of you

I got as far as "that's odd", since it's a proven design, I accepted the schematic as drawn and went back to garage painting 

[SoundmasterG]

Most schematics will show a symbol such as A, B, or F in this case to indicate that a point connects to something else labelled the same way. This way you can just write F, but you don't have to show the actual connection, especially when drawing it in might make the schematic messy. It is sort of the same idea as how they don't often show the heater connections on schematics. They could but everyone who works with tubes know  you have to wire up the heaters so its assumed. Take a look at a Sunn 200S schematic for another example of the practice.

Greg

[NewYorker]

"if the original johnson plug was replaced with a 1/4" jack, you'd use a jack with a NO sleeve switch, so when you plug the speaker in the 1K for the phase inverter is grounded by closing the sleeve switch now 1K is grounded and phase inverter passes signal to the output tubes."

I think a Switchcraft Type 13 jack would work? - https://www.tubesandmore.com/products/14-jack-switchcraft-mono-isolated-make-circuit (Doug doesn't have this one listed.)

Supply Sargent Obvious 

[DummyLoad]

"if the original johnson plug was replaced with a 1/4" jack, you'd use a jack with a NO sleeve switch, so when you plug the speaker in the 1K for the phase inverter is grounded by closing the sleeve switch now 1K is grounded and phase inverter passes signal to the output tubes."

I think a Switchcraft Type 13 jack would work? - https://www.tubesandmore.com/products/14-jack-switchcraft-mono-isolated-make-circuit (Doug doesn't have this one listed.)

Supply Sargent Obvious 

that jack would work perfectly.


--pete

[NewYorker]

Caveat - It would work perfectly and serve the intended purpose so long as the speaker cable is in fact properly terminated.  I say this because my head / cab setups typically involve speaker cables with 1/4" plugs on both ends.  If the head-end of the cable is plugged in and the cab-end of the cable is left dangling, the OT would be at risk.

Supply Sargent Obvious, OCD 

[Dave Moberg]

Thanks for the info, I'll order a switch tomorrow and see if I can get it wired in.  I've managed to figure a few things out but this forum has been very helpful.  It's been many many years since I've attempted this level of bench work and my theory is very sketchy.  I worked on some antique systems back in the day, I'm a retired Air Force radio man.  Mid 80s was the last time I did component level repair on tube amps....my powers of recall are being tested.
I'm trying to restore this old amp for a bass player friend of mine, he's had it since 1966.  It looks rough, and the amp was in pretty bad shape.  I'm gonna give it my best shot and see if I can save it for him.  The obvious symptoms when I got it was an arced up tube socket (pins 2&3 of the PI), blowing fuses, broken tube socket pins, lots of sketchy connections.  Replaced all the sockets and started verifying wiring and connections.  This amp has a 1/4 mono plug instead of the 4-ping speaker connector on the schematic which shows the ground path.  I was tracing my connections, that's when I got confused by the circled F symbol.  The grid resistor was grounded directly at the external amp jack, not wired through the speaker connection (amp protection circuit).  Considering this is the grid resistor circuit it may have contributed to the arc damage....my theory is weak.  Also found some creative wiring at the standby and ground switches.   Should I leave the ground switch circuit in place, or just remove the cap?  I put a 3-wire AC cord on the amp.  Thanks again.
 

[sluckey]

Should I leave the ground switch circuit in place, or just remove the cap?

I would replace the cap with a "Class Y" type cap and leave the switch functional.

[Dave Moberg]

I've been working on this B-15 and although I've solved a couple problems I still haven't fixed it.  I've got a problem with the power supply voltages.  I checked the AC at the outlet, 118 VAC.  Schematic calls for 430 volts to the power tube plates....I'm getting about 500.  Kept blowing fuses, so I hooked up the dim bulb tester and am able to at least take some voltage readings.  Found the -50 vdc bias voltage was low, so I replaced the bias cap and now I'm getting -49 vdc....acceptable.  I can now power up the amp without blowing fuses as long as the power tubes are not installed, put in V4 or V5 and the dim bulb tester starts to glow.
 Problem has to be in the power tube circuit or power supply?  To verify power supply voltages, I started at the plates of V1 and V2, supposed to get 165 vdc....I'm getting just over 200 vdc.  Not perfect, but with the primary B+ being high it doesn't seem too odd.  Main problem is with V3 circuit voltages, they are way too high.  Plate voltage is supposed to be 235 vdc, but I'm getting the full B+, a little over 500 vdc.  I'm reading full B+ on both sides of the pair of 120K resistors, and the 470K and 510K resistors.  There are two .022uf feedthru caps isolating the grid bias circuit, I read -49 vdc on the 1K resistor side of the caps and the full B+ on the V3 side.  Seems the caps are fine, and I've ohmed out the resistors and all are fairly close to the expected value.  Schematic shows 2.2 and 2.4 vdc respectively at pins 3&6 of V3, I'm getting close to 5.  This explanation may be confusing but I'd appreciate any help or recommendations.  I've not been able to find a bad component or solder joint or anything.  I'm not sure what to check next, just awfully frustrated, I've spent too many hours trying to figure this one out.

[sluckey]

I would use a light bulb in the range of 60 to 100W for this amp. Put the power tubes in. It's OK if the dim bulb tester starts to glow. Check some voltages Should be low. If things look OK then plug the amp straight into the wall and recheck voltages.

[Dave Moberg]

Thanks.  I was wondering if the voltages were high because I had no load.  I put the power tubes in, using a 150 watt bulb in the tester.  Powered up, flipped the standby switch and started to measure the plate voltage at V3.  After a few seconds the bulb started to glow brightly so I shut it down.  Chop sticked around a bit checking for loose connections...nothing obvious.  I understand it's okay if the the dim bulb tester glows a bit but this was pretty bright.  When I power up, the bulb starts glowing after a few seconds and then ramps up to bright quickly, and I can hear random popping noise in the speaker.  Not being sure how much glow is too much, I plugged that baby straight into the wall outlet and powered up.  It took a few seconds before the fuse popped.  I double checked each pin at the power tubes, traced each connection back on the schematic and everything is connected correctly as far as I can tell.  I have yet to find a component shorted/open or way out of spec. 
I hate to even think the OT is bad, it was replaced once already.  I ohmed out the transformer when I was reinstalling everything after replacing the tube sockets and everything looked good.  I may have missed something.  Would a bad OT give these symptoms?  Any advice on how to isolate and check it?  Can I disconnect the blue and brown wires and power it up safely?   

[shooter]

pull ALL the tubes
put it back on the limiter
measure the PS tranny secondary side volts
measure the filaments volts
if they seem good, hook up rectifier
measure volts out of rectifier volts
150W bulb glowing bright means you're amp is sucking big current

[Dave Moberg]

I checked my voltages from the PT earlier.  I have about 500v with everything but the power tubes installed.  I had my 6.3vac and 5.0vac. I'll certainly double check everything.  The B+ is high, but that's with no power tubes installed.  Soon as I put the 6L6s in, it starts drawing current and pops the fuse.  I just can't find any bad components, or figure out how to isolate the problem from here.  The bias circuit had a bad cap, but after replacing it, the bias voltage is fine but it still blows fuses.  I've replaced most of the caps, the filter caps and feedthrough caps.  I've checked every resistor, wire connection. 
Little history.  This amp was blowing fuses, the original problem.  Told my friend, he bought this thing in '67-68, that I'd try to salvage it for him.  Half the tube socket pins were cracked or missing pins.  Installed all new tube sockets, isolation washer for the speaker connection, routed wires, put everything back together except the speaker cable.  Someone had modified the amp using a phone jack connector and a 4ohm/8ohm switch??  I removed all the mods and wired it per the schematic, it's a 4pin connection that provides the ground to the phase inverter, amp protection circuit.  Far as I can tell, they had directly grounded the V3 circuit, bypassing the amp protection.  I wired the amp exactly per the schematic....as best I can tell, and I've double and triple checked. 
I'm back to the original problem, blowing fuses.  Could the bypassed amp protection have caused the problem?  What normally fails first when the amp is operating with no speaker connected?  Am I off on a wild goose chase?   

[AmberB]

Just curious, have you checked your power tubes for shorts?  I toasted a power tube once, that caused lots of fun problems...

[Dave Moberg]

I tried to checked all the tubes best I can.  I have an old EICO 635, seems to work just fine.  I checked each tube and they all read "Good" or "?"....for whatever that's worth.  The rectifier tube reads about center of the "diodes okay" scale....  Side note, it's impossible to find a tube tester these days.  I called around, I wanted to compare readings with my old tester but couldn't locate one to use.  One tech offered to test some tubes for $5 a pop.  It could very well be a bad tube...but if I trust my test equipment?? 

[SoundmasterG]

The power transformers seem to blow a lot on these B15's. I suspect the transformer was barely meeting the specs from day 1 when these were designed since so many fail. I have a 1963 B15N with a replacement transformer in it because the original blew. A customer has a 67 B15N that he had me look at because it was blowing fuses. It had blown a fuse at a gig and PT and OT were dead in this one. The PT got so hot it leaked all of it's goo inside the amp and I had to take the PCB board and everything out of the amp to clean it up and fix it. Someone had done a cap job on the amp previously but neglected to replace the first cap and only replaced the can cap. There was likely a bad power tube also. After replacing the PT the amp is stable but there is very low volume. Customer still hasn't sprung for a new OT yet.


I always keep a set of burner 6L6 types around that function so I can use them to test. The Sovtek 5881 is a good choice because they are cheap and near indestructible. Where are you located? Maybe someone else has a tube tester they will volunteer to double check your tubes?


Greg

[shooter]

did you measure bias without tubes AT the tube sockets?

[PRR]

> Soon as I put the 6L6s in, it starts

Put them in one at a time.

[Dave Moberg]

Thanks for the info and recommendations.  I tried to follow through starting with the power supply. Followed your suggestions and checked voltages with no rectifier, then with rectifier, comparing readings to spec tables, (attached). Nothing was way off.  All the voltage readings are without the 6L6s installed, it's the only was I can keep it powered up.  With that in mind, I went through the voltage table (attached) and noted all readings (readings taken at the tube sockets).   Note V4&V5 voltages are fairly close.
 Nothing seems way off except V3, plate voltage at pins 2 & 5 is high, no load with power tubes not installed?  V3 pins 3&6 are much higher than the schematic, should be 2.2 and I'm getting 40.  I checked all the OT leads for voltage, the readings are penciled in at the top of the attachment. 
I'll complete the resistance table readings tomorrow, but at a glance V3 readings are off. 

I verified my speaker is working, checked it with a different amp.  Reading about 6.8 ohms between pins 1&4, jumper installed between pins 2&3, checked with ohmmeter.
I double checked the tubes in my tube tester, check okay.  I checked the pins with an ohmmeter and the only shorts I found were the filament pins...2&7.  Unfortunately I don't have a spare set available.  Figured I'd post what I have documented so far, hoping someone spots something obvious.  I'm an antique..working on an antique...theory is weak.  Next step?  Should I go ahead and install the 6L6s one at a time? 
Trying to isolate the problem, can I eliminate the power transformer?
output transformer?  Voltage readings normal-ish? 
can I eliminate V1 & V2, everything before V3? 
Thanks for all the help.

[Dave Moberg]

The other attachment...

[sluckey]

Measure the 1K resistor that's connected to V3 pin 6.

[Dave Moberg]

Checked the 1K and the 220 ohm resistors, well within spec.  The 25uf-25V cap is new, just replaced.  Replaced the 10K resistor between the OT green wire and V3 pin 3, double checked it as well.   

[sluckey]

Measure resistance from V3 pin 6 to ground.

[Dave Moberg]

Initially I get about 20K then it starts building quickly till I read an open, I believe I'm seeing the 25/25 cap charging?

[sluckey]

Initially I get about 20K then it starts building quickly till I read an open, I believe I'm seeing the 25/25 cap charging?

That 1K resistor has no path to ground. It gets it's ground through the speaker plug. If the speaker plug is connected, then the interlock is miswired. Do this test... Use a gator clip test lead to connect the end of that 1K resistor (point F on the schematic) to chassis. Do the voltages on V3 look better?

[Dave Moberg]

I was wondering if the interlock circuit is the issue.  I took out the mods on this amp and wired in a new 4 pin speaker cable.  As best I can tell, I've wired it exactly like the schematic.  The black and red wires of the speaker cable are tied to the black wire from the OT secondary and terminated at the 1K resistor.  The white wire from the speaker cable is connected directly to ground.  I've verified the jumper is in place at the female 4 pin receptacle on the speaker end, verified with ohmmeter.  The speaker is connected.  I verified my connections, I read an open with the speaker disconnected, and I have ground at the 1K resistor when I reconnect.  I've double and triple checked the green and yellow wires are terminated correctly at the speaker jack.  Speaker jack is isolated from ground, new isolation washer verified with the ohmmeter.

[sluckey]

Do this test... Use a gator clip test lead to connect the end of that 1K resistor (point F on the schematic) to chassis. Do the voltages on V3 look better?

[Dave Moberg]

Outstanding.  Things look very much normal around the old V3 now....  I am thoroughly confused.  I have ground terminated at the pad right there at the 1K, verified with the meter, plug/unplug speaker, ground/open...  So, then I clip a jumper from the resistor lead adjacent to the pad (same spot) and ground the other end, everything seems to be within specs now.  I'm just an old retired master sergeant, ain't no engineer, but I thought ground was ground, the world around, paint it green it won't rust....  I've been chasing this issue for a while, couldn't find any failed components, had ground there.....never gave it a second thought.  Maybe a poor solder joint at that junction?  Maybe I just don't get it, but at this point I'm just grateful for the help and glad I can move on.  I'll pick up in the morning and see what happens with ALL the tubes installed.  Thanks again for the assistance, this one was starting to keep me up at night....got personal.

[sluckey]

I have ground terminated at the pad right there at the 1K, verified with the meter, plug/unplug speaker, ground/open...  So, then I clip a jumper from the resistor lead adjacent to the pad (same spot) and ground the other end, everything seems to be within specs now.

Something not right there. Investigate further. You want the 1K to be grounded when the speaker is plugged in. Can we see a hi-rez pic?

Unfortunately, this has nothing to do with the fuse blowing issue. Put a new set of 6L6s in the amp. If fuse still blows you got some expensive problems. 

[Dave Moberg]

I'm not sure what was going on with the grounding at the 1K resistor, interlock circuit.  This morning I fired the amp up with the jumper removed and the voltages are normal.  I'll take another look at it, maybe reflow the solder joint, definitely have to make sure the connections are solid.  Took a quick look at the resistance table and compared my readings, all were relatively close...nothing crazy.  With voltage and resistance tables fairly close, I pressed on to checking and installing the 6L6s. 
I backed up and tested each tube in the tester, and once again they both "passed" as good according to the old tube tester.  So I marked them, and then tried them one at a time in the amp. 
One of them was okay, I can install it in either V4 or V5 socket and it heats up and no current draw....dim bulb stays dim. 
The other one lights up the dim bulb.  I inspected the pins with a magnifying glass and found evidence of overheating between pins 2&3.  Using my trusty ohmmeter, I find pins 2&3 of the tube are shorted.  Hopefully that's gonna be the problem, heater voltage and plate are shorted.  I've ordered a set of tubes, sure hope that gets me to the next step.  Thanks again PRR, sluckey, shooter, and soundmasterG for all the info and assistance.  Wish me luck.

[Dave Moberg]

Meanwhile.... I just removed the death cap from the circuit for the time being but I intend to put the ground switch back in at some point.  Sluckey, your previous message prompted me to do some reading on the cap classifications and from what I can gather I should use either Class X2 or Y2.  Searched for one online and I think maybe I'm chasing the wrong rabbit.  The few parts I found with Y2 in the specs all have estimated delivery dates as far out as Feb next year.  Anyone have a part number/source for common safety caps?