B-15 Clone build

Discussion in 'Amps and Cabs [BG]' started by Wal Pawlik, Nov 9, 2015.

  1. For some time now I’ve been contemplating building a tube bass amp, but could never decide on what vintage amp to base it on. Would it be Fender or Ampeg? I have previously built some Fender clone guitar amps successfully so I was confident that I could tackle a bass amp, but which one?

    In any case which ever I chose, it would be costly, and it would probably be cheaper to buy a used amp, but in my part of the country old tube amps are rare, let alone old bass amps. Anyway there’s more fun doing it yourself.

    Recently Ampeg have release the B-15 Heritage amp combining the best of their 1964 and 1966 Bass Combo models at a recommended retail price of AU$8600. Whoa! That’s only available to well heeled players, one of which I’m not. The B-15 is of course an iconic amp, one we’ve heard countless times in our youth, even though we didn’t know it at the time, played by the likes of Jamerson and Babbitt.

    My mind was made up. I can build a clone for a fraction of that price. It may not look like a B-15 Flip top, but it can sound pretty close. So of to Schematic Heaven http://schematicheaven.net for schematic of the B-15 amps and a general search on all things B-15 via Google.

    This paid off because I found the CtG web site CtG electronics - Home which has published a complete build of what they call the “CtG FCN-51-B custom amplifier (based on the Ampeg B-15-N series)”. The web site has PDFs of the schematics and layout and description of the amp.

    “The FCN-51-B combines the original circuits of both the B-15NC (1964) as well as the later B-15NF (1965-67). Maintaining these circuits keeps the same gain structure and biasing found in the original B-15N amp designs, helping to preserve their true heritage.”

    This appears to be close to what Ampeg is offering with the release of their new B-15 Heritage amp. My mind made up, it was time to gather the parts. Parts were sourced from Amplified Parts Amplified Parts in the US, Evatco Electronic Valve & Tube Co. Evatco Pty Ltd | Electronic Valve & Tube Co. and WES Electronics WES AUSTRALIA here in Australia.

    After an anxious wait, parts began to arrive, and work could begin. First, was deciding a layout for the chassis. I followed the layout suggested in the CtG build, however this proved to be problematical. (If I were to build another I would allow a little more space between the rear of the chassis and the tube sockets).

    The chassis is a Hammond 1441-26BK3 16 x 8 x 4 inches box from Evatco. The power transformer is also a Hammond 290DEX and output transformer is a Hammond 1650HA Ultra linear also from Evatco. At this stage I won’t be using the ultra linear windings, but may try later, once things are running. The reason for this transformer, is because it was the only one I could find with the correct plate impedance.

    While waiting on hardware I began to assemble the circuit on a tag board I already had, and from my stock of resistors. When the hardware arrived, this was put aside to work on the chassis.

    The chassis was duly bashed, and filled with appropriate holes to mount the hardware and wiring began. This is when I found that a little more room at the back would have made for an easier access to components for wiring, but I think it will be ok.

    Laying out the chassis
    Chassis with holes

    Transformers, capacitor and tube sockets mounted

    Wiring begun

    Wiring is now to the stage of mounting the volume and tone pots.

    Stand by for more.
    Last edited: Nov 10, 2015
  2. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Yes, will be staying tuned to this one! Looks like a fun project. Ampeg did just release new low wattage tube amps that sound quite awesome and don't cost near what a Heritage B-15 does, but it's got to be pretty satisfying that you can undertake a project like this.
  3. Jimmy, I already have a couple of Fender clones under my belt and various others. Thing is, I have more amps than I need, but it's the process I enjoy the most.
    In this part of the world even low powered tube amps of any brand, especially Fender and Ampeg are at premium prices. This has already been an expensive exercise, but still cheaper than the real thing. And, as I said, I enjoy the journey.
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  4. This morning I went to my local Men’s Shed to use their guillotine to cut the brass ground plate that the pots and input jacks will be located on. You can see the brass plate in the picture. I had hopes of completing the wiring of the pots and tone circuits today, but the temperature was 36 deg C here today, and my shed was like a sauna, so all I managed to do was mount the pots and a couple of resistors and get out of there.

    The forecast is for 28 deg C tomorrow so I don’t think I’ll get too much done, unless I get a really early start before the heat sets in. I think I started this build at the wrong time of year, as the summers here can be brutal, especially in my shed. I was hoping for a test run by the end of the week, but if this heat continues, that wont happen.

    Well, I guess, that’s it for now, we’ll see what happens tomorrow.

  5. The weather man was wrong. Didn’t get to 28 deg C today, but was overcast and muggy.

    Got an early start this morning while it was still cool. I managed to finish all the wiring and begin testing. All good till I plugged in the power tubes and discovered I had an oscillator. Reversing the output transformer plate wires cured the oscillation. However all was not well as there was no output when a signal was applied. I retraced the circuit to find that I had missed the connection of channel 2’s coupling capacitor to the volume pot. Channel two is now working, but for some reason channel 1 is mute. Again, retraced the wiring of that part of the circuit, but I can’t see anything obvious. Tomorrow is another day, so I’ll get back in then.

    First impression is that the amp is reasonably quiet, hum and noise wise, but at this stage I want a signal flow. I will look at hum and noise levels later after I have a complete circuit.

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  6. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    You're a million miles more ahead than I would ever be. But you'll get it figured out. If not, there's a resident B-15 expert on here by the name of beans-on-toast who will no doubt have some good advice for you.
  7. I'm sure it's something simple I've overlooked as there are no rude noises, heat or fireworks and voltages are all ballpark. Hopefully tomorrow it will reveal itself.
  8. C:cool::cool:Lio project.

    I've visited the CtG site before and dreamed of the day...

    Gonna see if I can build a few pedalia first.
  9. cfsporn


    Aug 20, 2011
    New York City
    Two B15 builds in one day! Awesome!
    pie_man_25 likes this.
  10. Well this morning I began retracing the connections on the mute No 1 channel but nothing was obvious so I applied a signal and began following it. There was plenty of signal until it reach the cathode resistor on pin 6 of V1. This resistor is located on the tag board and connect by a wire to pin 6. Closer inspection revealed a poor solder joint. I resoldered and the signal came up.

    Fix one problem and another shows itself. When channel 1 was muted I couldn’t hear any hum, but now that it was working a hum presented itself. This hum is volume dependant. Volume down, none, and as the volume is increased , so is the hum. Channel 2 is quiet, so the hum is confined to channel 1 somewhere. All my low level wiring is screened at one end so I can’t see any ground loops. Tomorrow I will have a closer look and see if I can cure this.
  11. Got no technical advice but I'm rooting for you, this is an awesome project.
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  12. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    Is there a chance that one of your heater wires on channel 1 is touching something it shouldn't be?

    Awesome project by the way.
    pie_man_25 likes this.
  13. Not happy! Though I swapped V1 and V2 around and got rid of some noise I’m still not happy. I’ve decide I will strip the amp down and revise the layout. I think I will move V1 and V2 to the centre of the chassis. This will remove them from the proximity of the output jacks and probably remove a source of noise. I think the proximity of V1 and V2 being so close to the output jacks doesn’t help with the noise floor.

    Placing V1 and V2 in the centre of the chassis between the transformers will mean that the terminal board will have to go. I have ordered a heap of tag strips so I will be able to solder the components directly to the tube sockets. This should open up the wiring somewhat and hopefully reduce potential sources of noise.

    Another thing, all the voltages seem to be rather high, about 20 volts above the shown circuit voltages, although the cathode voltages look good, and the thing gets hot. Almost too hot to touch around the tube bases. I had a look at the output on the ‘scope but couldn’t see any supersonic oscillation. Anyone care to comment?

    Stay tuned.
  14. Voltages will be higher if you are viewing old schematics. They usually assumed 110 to 115 VAC, not the 120 average of current times. 20 volts is of no concern.

    Tubes do get hot especially octal tubes 150-350c average so I'm not why you would be trying to touch them? If you see plates getting orange or red, that is a worry.
    Last edited: Nov 12, 2015
  15. Just to clarify. I'm in Australia where the domestic voltage is 240 volts and I'm using a 240 volt primary transformer.

    No, I'm not touching the tubes :) It's just that the amp seems to be radiating lots of heat, more than I expected.
  16. I'm on a phone but a quick look showed a redrawn schematic? The man that was doing these provided a nice clean document but not always without errors. I still wouldn't be concerned with 20 volts over.

    Octal tubes run much hotter than say a 12AX7 9 pin. What is the filiment supply voltage? That is the voltage you might be concerned with being a little high and causing more heat than "normal".
  17. The Hammond 290DEX transformer does supply the correct filament voltages so I'm not concerned about those, but I guess you're right about the octals being normally hotter than 9 pins. Just have to provide plenty of ventilation when I get round to building a case.:)
  18. beans-on-toast


    Aug 7, 2008
    The original B-15's were ultra quiet. I hear no hum, no noise out of mine. Layout and grounding are very important. I can't stress that enough.

    If you want a quiet amp, study and copy the vintage Ampeg layout.

    Rather than start again as you mentioned, I'd try to figure out what is wrong and change the layout a bit.

    One thing that Ampeg did was to use a power supply bus. The ground line starts at the node A cap and runs to the input jack. The various stages connect to the bus. It is important that there is only one ground point or there will be a ground loop and hum. They grounded the bus to the chassis at the input jacks. The OPT primary red/yellow center tap grounds where the node A capacitor grounds onto the bus.

    The output jack should be isolated from the chassis. The speaker return is quieter if it is grounded where the phase inverter is grounded. It is also good to run the OPT secondary ground to this point as well.

    It is also important to isolate the ext speaker jack from the chassis. Without this, there is a ground loop. The ext amp jack does not need to be isolated but it doesn't hurt to do it.

    Wires carrying an AC signal such as the PT and OPT primaries and secondaries should be tightly twisted. These wires should run along the chassis. The heater lines should be twisted. If you have a hum balance pot, check that it is working. If there is a heater center tap on the power transformer either ground it or use the hum balance pot. Connecting both will cause a ground loop.

    If there' s hum it could be the tubes. I once had a bad batch, they all hummed so swapping them didn't help.

    It is important to differentiate between 50Hz hum and noise. Two different possibilities with different solutions.

    There are good and bad shielded wires. Some offer more shielding than others. The shielded wires should be grounded at one end only. The routing of these wires is important in terms of noise. From the pots and input jacks, I tuck them along the chassis and run them to the point where they turn 90 degrees to go to the tube sockets or the eyelet board. I've attached some images that should how the shielded wires are run. Perhaps this will give you the general idea as to what to try.

    The OPT primary yellow and green wires are twisted. You can see how they run under the board to the jacks. Close proximity to the tube sockets is not an issue if they are run close to the chassis.

    speaker jacks.JPG speaker wiring B15.JPG speaker wiring ext jack copy.JPG shielded wires 1.JPG shielded wires 2.JPG

    shielded wires 2.JPG original B15 wire layout .JPG sheilded wires II.JPG
    Last edited: Nov 12, 2015
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  19. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Told you he's got good advice :D
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  20. beans-on-toast


    Aug 7, 2008

    Thanks Jimmy. An issue like this is complicated to resolve on the internet. There are so many possibilities and chances are, it's something simple.
    BassGreaser, pie_man_25 and JimmyM like this.