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1963 B-15N question regarding schematic...

Discussion in 'Amps and Cabs [BG]' started by Frederick34, Nov 10, 2019.


  1. Frederick34

    Frederick34

    May 12, 2017
    Texas
    So I’m working on a very early 1963 B-15N (everything appears to be January or earlier). The schematic shows the 5U4 rectifier and 40/40/20/20 cap can (although the actual multicap installed at the factory was a 40/40/40/40, not unusual for Ampeg, from what I’ve heard). When I got it, someone had been trying to diagnose it and apparently gave up.

    They had installed what appeared to be a makeshift SS rectifier. Here is a photo right after I removed the cluster of diodes and resistors which were soldered to the rectifier tube socket:


    2F480E11-F419-47E4-B2C3-649B707F554D.

    What I determined was definitely wrong was the PT, so I sent the old one to Fliptops for a correct new PT and potting service. I’ve installed the new PT and a new Multicap can (4 x 40, replacing what was there originally). Before powering it up, I am looking over the wiring, to correct any issues. The person that was trying to diagnose had disconnected certain wires to test the PT, which became obvious when I went to test it. I assumed the funky SS rectifier might have been a temporary thing, just so they could check out the PT.

    As I was’ looking over the circuit, I see the odd 1600 ohm resistor you’ll notice in the upper right corner of my photo there. Just looking for a second opinion before I remove that resistor and replace it with a 7500 ohm, which is what the original schematic shows (attached image). I believe that resistor may have been swapped for the SS rectifier circuit mod. If I’m reverting back to the 5U4 tube, I’m thinking I should follow original schematic. BUT, might that resistor be a 1600 because of the 40/40/40/40 cap, instead of the 40/40/20/20 shown on the schematic? That resistor is the screen resistor for the 6L6GC tubes.

    Thoughts?
     
    Last edited: Nov 10, 2019
  2. Frederick34

    Frederick34

    May 12, 2017
    Texas
    Here is the schematic from the actual amp (I’ve marked the suspect resistor)...

    EDIT: I misread the color bands (a bit color blind... had my wife verify). The installed resistor is a 5600 ohm, not 1600 (green-blue-red, not the brown-blue-red I saw). So that may be a factory installed part... solder looks pretty original to me. So now the question is... should I leave the 5600, or install a 7500 as per the schematic?

    Also, I will install 5U4 tube, leave all others out and check voltages before powering up fully assembled... any other tips before I turn this thing on? Sure don’t want to fry my brand new PT.
    6A758E54-5526-4A3F-9344-30831EB812D3.
     

    Attached Files:

    Last edited: Nov 10, 2019
  3. byacey

    byacey

    May 16, 2008
    Alberta, Canada
    That 5600 looks original; I would leave it alone.
    Why does that green heater wire by the 6l6 socket appear to be disconnected?

    All the tan colored resistors are to be newer vintage than 1963; make sure they are all the correct value before powering up.
     
    Bill Whitehurst likes this.
  4. BogeyBass

    BogeyBass

    Sep 14, 2010
    Solid state rec will give the amp slightly more power.
    otherwise expect a 30 volt drop with a tube rec.

    depends on what you want the screen voltage to be. a higher resistor will drop the screen voltage.
    probably a change you could make later on if you feel screen voltage is to high.

    as for why someone gave up on it. they probably tested the transformer, found out it was bad and sold it
    and passed it on to whoever wanted to replace the transformer
     
  5. Frederick34

    Frederick34

    May 12, 2017
    Texas
    This picture was taken not long after I recieved it. The green wires were disconnected, I believe, when the last person was testing the PT. A few other points were also disconnected (like the white going to the 1000 ohm 10w ceramic, also seen in the photo).

    The makeshift SS rectifier sure did seem like someone added it later, but because it has an original 40/40/40/40 multicap, it sure makes me wonder if Ampeg was playing with the idea of a SS rectifier on this one... I know some later 1963 versions did have a SS rectifier. All the SS rectifier Fliptops seem to use that 4x 40, not the 40/40/20/20, as on earlier models.

    I original assumed someone botched a SS rectifier mod and burned up the PT, but now I’m not so certain. I’m sure there is a story behind this amp.
     
  6. byacey

    byacey

    May 16, 2008
    Alberta, Canada
    The 20ufd sections are for the preamp and phase inverter B+ supplies. I don't really see why it would be any benefit to increase this to 40 ufd sections, silicon rectifier or 5u4. More likely, Ampeg bought these caps in quantity for other amp models, and simply continued using them for this particular unit.

    I doubt Ampeg had anything to do with installing silicon rectifiers instead of the 5U4. Had this been a manufacturer experimental unit, they would have known enough not to connect the rectifier heater windings, or even bother installing the 5U4 socket, for that matter.
     
    Last edited: Nov 10, 2019
  7. Frederick34

    Frederick34

    May 12, 2017
    Texas
    Good point, that makes sense. I guess bumping that up from 20 to 40 in those sections really makes no difference one way or the other. I almost put a 40/40/20/20 in there, to follow the schematic, but decided to just go with what was in there from the factory.

    My other theory is that the 5U4 went bad, burned up the PT and then repair guy said “all the newer models have SS rectifier, let’s just eliminate the rectifier tube”. Agree 100%, the 5v filament would not have been needed, Ampeg would not have wired that up.
     
  8. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    beans-on-toast likes this.
  9. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    Check out some of the schematics either at vintage blue or in the wiki above, ampeg liked to change things up fairly frequently. I've had about 10 amps from this era and the schematics in the amp don't always match what appear to be original parts in the circuit.
    Schematics
    Ampeg Portaflex Wiki | TalkBass.com

    For what it's worth the amp would also be earlier than 63 with a 5U4 and the 8 pin connector. Actually as I'm looking at them now, both the 6/60 and 3/61 schematics from vintage blue/in the wiki are different from what you have here. The early amps don't have the 7.5k screen resistor, the later amps have a 1.5k, yours could have come from the factory with a 5.6k.

    The first B15NA schematic show has all 40uf values for the caps.
     
    JimmyM likes this.
  10. Frederick34

    Frederick34

    May 12, 2017
    Texas
    All the date codes (caps, pots, even a ceramic resistor have 1st thru 6th week of 1963. I initially thought for sure it was a 1962 also.

    One reason I’m tempted to rebuild it exactly as per schematic is that my absolute favorite Fliptop is a 1961 that sits in my bedroom as a practice amp. I’m not looking for huge volume, just nice thump and that ultra sweet tone that the ‘61 has. It is one of the early ones with no externally visible multicap... it has those red cardboard tube caps inside it. Love that amp, and not bothered at all by the lower value filter caps.

    Since I don’t really play live anymore, I don’t need massive headroom, otherwise I might consider the SS rectifier. But I am a but of a traditionalist, no doubt.
     
    Last edited: Nov 10, 2019
  11. Frederick34

    Frederick34

    May 12, 2017
    Texas
    Definitely! Beans has helped me with a few other Ampegs... always spot on.
     
    JimmyM likes this.
  12. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    It's true that Ampeg changed the values around. The higher the capacitance, the more headroom. Personally, I prefer 40/40/40/40.

    An amp with a lower value such as 30uF that Ampeg settled on for node A (the first node in the amp's power supply) will perform a bit differently. Less headroom.

    First of all, the A node capacitor should be rated for 600V. That first cap is strained due to spikes for a short time when the amp is powered up and the cap is discharged. A 40uF cap value also strains the 5AR4 rectifier tube because it takes longer to charge and there can be arc'ing (called flashover). It could lead to a shorter 5AR4 service life. Service calls are expensive. Ampeg settled on a lower 30uF cap that is rated for 600V. There is a solution if you want to use a 40uF cap in the first node. Add two diodes inline with the plates of the 5AR4. It increases the total peak inverse voltage (the tube plus the diode rating is additive) and prevents flashover. How to do this is described in the TB portaflex wiki. This is what I do and it works very well.

    They also changed the plate voltage on the phase inverter. The early amps had a higher plate voltage than the data sheet maximum specification for the 6SL7 tube, it worked fine the tubes could take it. They eventually lowered the V3 plate voltage. This was accomplished with a change in the dropping resistor.
     
  13. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    They went from a 5U4GB rectifier to a solid state one towards the end of the year with the NA/NB. The NC revision came out in early '64 and had the 5AR4. The 5U4GB rectifier has more sag, the voltages are therefore lower in the amp. They call it softer sounding with that rectifier when the amp is pushed.
     
  14. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    interesting I had thought the 5u4 was gone by the end of 62 but I guess that date is off a bit.
     
  15. When I got my B15, I experienced flashover in the GZ34 ( OK I’m British! ) It had me flummoxed. At David’s suggestion I added said SS rectifiers to the plates which solved the problem. It’s a mod that I recommend be done to any of the B15 variants. It makes no difference to how the amplifier operates.

    I installed one rectifier on each plate. With the devices I chose it was enough. I still have some in the shop. If you want a pair I’ll send you two. :)
     
    Last edited: Nov 10, 2019
    beans-on-toast likes this.
  16. Frederick34

    Frederick34

    May 12, 2017
    Texas
    Thanks much for the offer! I actually bought a few of those diodes about a year ago, on Beans’ advice. I’ve been meaning to install them in all my B-15 tube rectifiers but have not gotten around to it. Will do it on this one for sure.

    Thanks for all the advice. Going to check this over again and be sure it’s all proper before I power it up. The 40/40/40/40 can I got was 525v... hopefully that will suffice until I can build the mod that Beans has refered to.
     
    Last edited: Nov 10, 2019
  17. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    You weren’t off by much. NAMM was July 21, 1963. Figure that they would have announced the new revision then.

    NAMM Show Location & Date History 1901-2019
     
    coreyfyfe likes this.
  18. Frederick34

    Frederick34

    May 12, 2017
    Texas
    Kinda like Fender’s model year actually running from June to July... so a Feb 1970 guitar/bass could still be considered a 1969.
     
    beans-on-toast likes this.
  19. TemplesOfSyrinx

    TemplesOfSyrinx Supporting Member

    Sep 8, 2013
    New Jersey
    Beans, I'm not sure what language you are speaking. It reminds me of when I was a kid and the TV stopped working. The TV repairman would come by, take the back off that behemoth console and all I could see was tubes and wires everywhere. He spoke a similar language....
     
    beans-on-toast likes this.
  20. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I wish that google translate could handle the language “amp geek” to “plain english”. :laugh:

    https://translate.google.com
     

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