Donna left for Florida on Thursday morning. She’s visiting her sister in Sarasota and they spent the weekend in Miami for her niece’s wedding celebration. I’ve stayed busy building my guitar amplifier and playing some pickleball. In my last post, I wrote about my amp project and the complicated design of Dumble-type amps. It’s been a tough build.
Thursday I completed the eyelet board to vacuum tube socket wiring.
The 12AX7 preamp tube and the phase inverter tube have nine pins and are called noval tubes. The 12AX7 tube is really two tubes in one – it has an A and a B side with separate triodes. Sometimes it’s called a duo-triode. The Dumble circuit uses both sides of each preamp tube, making four gain stages in the circuit.
The power tubes have eight pins and therefore are called octal tubes. Here are a couple of pictures from different sides of the chassis to illustrate the complexity of this circuit.
Vacuum tubes – called valves in England – aren’t too plentiful nowadays. There was a time when you could get them at the supermarket or drug store. Now we have to rely on a few online suppliers. The main sources are factories in Russia, China and Slovakia.
There was a Russian factory in Saratov that made vacuum tubes for the Russian military called Reflektor. That factory today is operated by New Sensor Corporation headquartered in Long Island, New York. New Sensor bought the rights to many old tube manufacturer names and they offer reissues of old favorites like Tung-Sol and Mullard. The big tube factory in China is Shuguang, but they closed the factory to relocate and it’s put a real strain on the tube market.
For this amp project, I bought Mullard CV4004 12AX7 tubes for the preamp and a Sovtek 5751 for the phase inverter – these are New Sensor tubes from Saratov, Russia. For the power tubes, this amp calls for a pair of 6V6 GT tubes or 6L6 if you want more power. Remember I said that the Reflektor factory made tubes for the Russian military? Well, in the 1970s and 1980s, the Russian military apparently stockpiled a boat load of tubes. After the fall of the Soviet Union, these tubes flooded the market as surplus – there are still lots of them available.
A few enterprising guys bought large numbers of Reflektor tubes made in the ’70s and ’80s such as the Reflektor 6n6c. This is the Russian military equivalent of the 6V6 GT and they are supposed to be a good sounding, robust tube for guitar amplifiers. I bought four of these at a very reasonable price from NessTone in Los Angeles.
I had reached the moment of truth. It was time to power up the amp and see if it worked. First, I built a bulb limiter. This simple device provides two very important functions for firing up an amplifier for the first time. I bought a short, three-conductor extension cord. The three-conductor cord is essential – it has a black wire for the hot lead, a white wire for the neutral or common lead and a green wire for ground. On modern household outlets, the white neutral or common wire is always on a wider blade of the plug so it’s always oriented in proper polarity when you plug it in. I split it open and cut the black wire. I connected a light bulb socket to each end of the black wire I had cut in half.
I plugged the extension cord into a wall socket, then plugged the amplifier into the other end of the extension cord. Now, the current on the hot lead coming into the amp had to go through the 150 watt bulb first. This made two important things happen. If the amplifier had a short circuit to ground or an incorrectly wired lead that drew high current, the bulb would light up brightly. If that happened, I would shut it down quickly before anything started to smoke. We all know that once you let the smoke out from an electrical device, it won’t work anymore! The second benefit is the fact that the bulb burning brightly would consume power and the lack of full power to the amp would give me time to shut the amp down, hopefully, before anything bad happened.
With the amp in standby, I flipped the switch. The light stayed dark. After a minute to allow the tubes to warm up, I flipped the standby switch to “On”. The light bulb glowed dimly for a few seconds then became progressively dimmer as the capacitors in the circuits charged and the current flow diminished. Perfect. I could breathe again.
Next I had the scary task of checking voltages in a live amplifier. This is best done with one hand in your pocket. That way you’re unlikely to have one hand on the amp chassis while the other hand accidently touches a live connection. If you did that, the current would flow up one arm, across your chest and out the other arm – likely to stop your heart along the way. With only one hand in the amp, you’d get a jolt but would survive.
My voltages checked out. When I say it was a little scary it’s because at the first check point, B+1 the voltage was 441 volts DC. At the power tubes it was 439 volts. The other tubes were “only” 200 to 280 volts.
After the voltage checks, I shut the amp down. I clipped a lead with a 100 ohm 10-watt resistor spliced in series to the last capacitor in the power section with one end of the lead grounded. This drained the capacitors, otherwise dangerous voltages would remain in the capacitors even though the amp was turned off. The resistor in the lead prevents it from arcing or sparking when you clip onto a live capacitor. That lead is pictured with the bulb limiter above.
I made one mistake. I didn’t unplug the amp. I accidently touched the jack inside the chassis where the power comes into the amp before the power switch and got a jolt of 120 volts. I won’t make the mistake again.
Everything seemed to work. It was time to plug in a guitar and see how it sounds. I had a couple of noise issues and wierd bass response. I found a problem with the tube filament (heater) wires being too close to the negative feedback wire – that was causing a 60-cycle hum. I’m still working on getting the bass response right, but otherwise it sounds good and doesn’t hiss or hum at all now.
I took our Nissan Frontier truck back to Sullivan Motor Company on Monday morning. We’re still having airbag issues and they are finally going to replace the wiring harness. Meanwhile they put me in a loan car – a beat up Kia Forte. Oh well, it beats walking. Hopefully they get it done in another day or two.
Tomorrow afternoon, I’ll pick Donna up at the airport. She’ll come home to nice weather – daily highs in the mid to upper 80s with overnight lows in the 50s.
*Just so you know, if you use this link to shop on Amazon and decide to purchase anything, you pay the same price as usual and I’ll earn a few pennies for the referral. It’ll go into the beer fund. Thanks!
