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1969 B15-N Repair

W

Wertoc

Feb 2, 2024
8
4
#1
Hey All,

I have a few questions I’m wondering if anyone could help me out with on a ‘69 B15-N. I got it as a standalone head in really bad shape. Someone tried (and failed pretty miserably) to convert it to a b18x. B18x transformers, 7027’s etc. I combed the schematic and board and ordered a bunch of replacement parts and installed all of them. Still waiting on the last tube to arrive before I can test but I’m confident I at least won’t release any smoke when I plug it all in.

Now for questions: whoever “worked” on this thing before massacred the tube sockets. I’m thinking about just replacing all of them. My experience with tube amp repair is limited so I’m not sure what the best route here is. Would it be better to replace the entire sockets? If there are sockets out there that are drop in replacements (no socket/chassie modding for proper fit) then I might just do that. I also might try to just replace each pin individually though. Preserve original sockets and a slightly less involved job. I’ve also heard these amps have some type of shielding around the leads from the transformers. It didn’t really come with anything like that in the first place and I’m wondering if anyone has any experience with how necessary these are. Are they required around all leads of both transformers? Just the primary or secondary of one or the other? I read on fliptops.net that they were included with B15’s that had a PCB, so I want to make sure I’m doing everything to minimize potential hum on this unit.

I’ve included some pictures of the amp as it is right now. I know there’s plenty to clean up in terms of grounding, speaker cable, lead cleanup etc. that I will get to once I can confirm it actually works.
Any info is appreciated!
 

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#6
OW! Hack job, Tears! Was the phase inverter socket changed to a 9-pin for the B18 style phase inverter? If not, tube sockets can be left. Only critical tube socket consideration is around pin 3 of the power tubes - proper clearance to avoid arcing. (btw a modern 7027 is just a 6L6 with pin 1 internally shorted to pin 4) From your pics looks like just the transformers were changed with the original circuitry essentially left intact, check for resistor values which may have been altered especially in the bias circuit.
And make sure the polarity switch is entirely disconnected, the "death cap" removed and thrown away, and a 3-wire cord installed. Personally, I also get rid of the rectifier tube and plug in a diode replacement
 
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#7
Glad to get some feedback on this! Fortunately none of the sockets were changed previously. I had to rebuild the whole power stage of this amp but after a tour through the schematic & PCB discovered that yes, nearly everything else was original. Took care of the death cap, polarity switch, and grounded plug. To be clear I've done a fair amount of amp repair (also work as an EE) so I'm no stranger to this kind of thing. Just less familiar with rules of thumb surrounding vintage hardware like the sockets. I've verified every component in the circuit too so again, with the correct rectifier tube that should be coming this week it should all power up alright *knocks on wood.* I also ordered a new matched set of JJ 6L6GC's.

So this shielding thing... anyone encountered this before? Is there a source people go to to procure replacement shielding if it's gone missing along the way like mine?
 
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#8
Wertoc said:
Glad to get some feedback on this! Fortunately none of the sockets were changed previously. I had to rebuild the whole power stage of this amp but after a tour through the schematic & PCB discovered that yes, nearly everything else was original. Took care of the death cap, polarity switch, and grounded plug. To be clear I've done a fair amount of amp repair (also work as an EE) so I'm no stranger to this kind of thing. Just less familiar with rules of thumb surrounding vintage hardware like the sockets. I've verified every component in the circuit too so again, with the correct rectifier tube that should be coming this week it should all power up alright *knocks on wood.* I also ordered a new matched set of JJ 6L6GC's.

So this shielding thing... anyone encountered this before? Is there a source people go to to procure replacement shielding if it's gone missing along the way like mine?
Click to expand...
Quick check: Once powered up and working, speaker connected, bass flat, treble max, turn the volume all the way up. If it breaks into oscillation you have a shielding problem, if not, it's fine. I have designed and constructed tube guitar amps, always shield the blue - brown output transformer leads. Use pieces of RG58 coax, pull the center conductor out and use the braid / jacket as a shield. Ground the braid to the chassis at the transformer end, a little shrink tubing over the other end leaving about 1 inch of the blue / brown sticking out to connect to pins 3. Dress the shielded leads against the chassis. Saves 1000 Tylenol worth of headaches especially when dealing with an OT near high gain stages. Make sure the main speaker jack shorts the output when no plug inserted, many of these amps originally had an XLR speaker cord with a phase inverter disable loop to prevent being played into an open circuit. Good luck with your B15. It's not a classic (all chrome, 3 jacks, plexiglass lighted logo) but it's close.
 
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#10
Update: 5AR4 tube showed up today so i double checked everything one more time, plugged it all in and crossed my fingers. I knew there would be some noise and there definitely is some hum but it works… After letting it warm up a minute or two all seemed well (no measurements yet). I plugged in, switched from standby and played for a minute or two, then checked the tubes. V5 was glowing cherry red on one side, way brighter than the other power tube! I switched to standby and it went back to normal so I shut it all off, let it cool for a few, swapped power tubes, tried it again and same thing. The tube in slot V5 starts glowing way too bright after a few seconds of switching from standby. I’ve attached the CK I’m working from for reference.

The sound itself was fine - very clean aside from the aforementioned hum. Any thoughts on where this extra glow could be coming from? I would think it’s the result of some runaway B+ or something but I started taking measurements and everything actually seemed far below voltages marked on the CK: 330V where it should read 450V and that kind of thing. I’m going to keep doing some testing and reading but if anyone may have a clue I’m all ears!

CK: [Invalid or Expired Link Removed]
 
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#12
Again these are not my area of expertise but after reviewing some of the theory of operation, yes.

agedhorse said:
Do you (really) know proper safety protocols? If not, you should.
Click to expand...


I would be grateful for any tips you have on safety! You’re making me second guess if I’m doing this properly.. only probe with one hand, isolation from ground (gloves, shoes etc), properly discharge caps, safety glasses. I very much dont want to die for this amp. Again if you have any tips I may have overlooked they’re more than welcome!
 
#13
Study up on the theory, and look up the requirements for safe service procedures. It’s all on you to really understand.

In a certified technician training program, a lot of effort goes into understanding how safe practices apply to different types of equipment.
 
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#14
With both output tubes and the rectifier pulled, other tubes in, amp on, standby on, check the voltages on pins 5 of both outputs. Will vary a small bit depending on the impedance of your multimeter but should be in the range of -45 to -50V but will be equal. The hum is likely due to the imbalance in the output stage. If voltages are not equal 1) Wired incorrectly - most likely 2) C13 is leaky. 3) R30 is open or not connected. With the rectifier pulled the high voltages on pins 3 and 4 will be absent.
When an output tube redplates, it's because it's not biased. And listen to anything @agedhorse says about safety. He knows what he's talking about. The B+ voltage can kill. With the number of times I've been zapped the good Lord must have some reason for keeping me alive! When you take a high voltage measurement, use a clip lead on the ground probe, use one hand and keep the other in your pocket.
 
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#15
There are other causes of red plating as well, including VHF oscillation, leaking coupling cap, failed phase inverter, etc.
 
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#16
Ok here’s an update for anyone interested, along with another question. Red plating has stopped. The scary part is I’m not sure why. I turned it on many times over the last month and have been monitoring voltages carefully every time I power it up. All critical points are reading pretty much exactly what they’re spec’d at on the schematic, and it’s just not red plating anymore. It hasn’t in weeks, no idea why it would’ve just stopped suddenly. I didn’t swap any parts change any connections replaced tubes or anything. I’m happy but confused, would anyone have any idea why this could happen?

another question: the only hum left in the amp is when I turn the volume up. Otherwise it’s completely silent. It’s equally loud in both channels, it’s 120 Hz hum so I’m looking at the power supply. However, all of the components in the power supply are brand new. obviously it’s possible I got a faulty cap or something, but is there somewhere else I should look first before trying to test these caps just since they are all brand new?
 
#17
Look at the supply ripple.

It may also be ripple current injection into the ground system, which it generally a design or layout/fabrication issue
 
#18
Okay sorry my eyes are not what they used to be, on socket V5 you seem to have a very poor if not lifted connection to the PCB I believe pin 8 which is the cathode and if poor will cause red plate. Never assume a thick trace is a ground by the way so make sure everything is clean and well soldered there. By well I mean clean smooth and not overly glopped with solder. It "looks" like a repair was gone in that area, a bridged trace that may have been damaged? clearance between the socket lugs and adjacent pcb traces may require rework.

Edit: I would prefer to see a tighter twist on the red wires to the rectifier and dressed further away from the PC board. If it is 120hz hum that is after rectification, any wire dress away from gain circuits is prudent. I would tuck the filament leads to the back panel as well.
 
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#19
agedhorse said:
It may also be ripple current injection into the ground system, which it generally a design or layout/fabrication issue
Click to expand...

a layout issue in what kind of way? That a part is erroneously connected to ground?

B-string said:
Okay sorry my eyes are not what they used to be, on socket V5 you seem to have a very poor if not lifted connection to the PCB I believe pin 8 which is the cathode and if poor will cause red plate. Never assume a thick trace is a ground by the way so make sure everything is clean and well soldered there. By well I mean clean smooth and not overly glopped with solder. It "looks" like a repair was gone in that area, a bridged trace that may have been damaged? clearance between the socket lugs and adjacent pcb traces may require rework.
Click to expand...

your concerns here are valid, i spent extensive time checking those places on the board you mentioned. There are several places where the previous owner cut traces and, as I mentioned before, broke many of the tube socket pins. Though some of them are not pretty because of that, all of the connections are solid
 
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#20
Layout can influence the noise floor due to electromagnetic or electrostatic coupling of unwanted signals, or the injection of noisy ground currents into sensitive nodes within the amp.
 
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