B-15 Clone build

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.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.