This 60’s YBA-3 was completely submerged under water during a basement flood. 4 days later you’d never know it had been wet.
It’s in for the full meal deal. New tubes, caps, 3 prong power cable, a new power tube socket that got burned up before the flood, and any water damaged components. Though I have a feeling all is well otherwise. Traynor: built to outlast the cockroach after the apocalypse.
This is the ORIGINAL model of the YBA-3. There are at least 4 different variations on this amp, but this one was the first, originally set up with 7027A power tubes, no master volume, and no bias pot. This model also uses one less gain stage than later models, meaning there's an unused half of a 12AX7 just sitting there waiting to be used.
This YBA-3 is being set up to be a guitar amp, so I decided to leave the amp setup for 6L6s, and assume I'm going to have to make some additional changes to the preamps and/or PI (but we'll get into that later). This was a good decision, since the existing tube sockets had their pin 1 removed, meaning to switch to 6CA7/EL34's, I'd have to replace all the tube sockets. If I REALLY wanted it to be a Traynor, I'd also have to add the bias supply to to pin 1 rather than ground. Ohhhh Traynor...
After allowing the amp about 2 weeks to FULLY dry out (mostly worried about moisture inside the transformer windings), I got down and dirty in this old gal.
First thing I noticed was that you'd barely be able to tell this thing had ever been wet. The old paper caps looked discoloured, and the tagboard was quite warped, but otherwise, you'd never know.
Some of the pots were ceased right up, but a few squirts of Deoxit F5 (the fader stuff) had them moving freely. While I was at it, I filed all the jacks, and cleaned with more Deoxit F5.
After inspecting all the wire connections and solder joints I only found one resistor leg that had pulled loose due to the tagboard warpage. Not bad!
Not sure how this would have happened, but the cathode of the 4th power tube wasn't connected to anything, so that tube would have been effectively out of circuit. Hmmmm...
A mod had been done to the phase inverter, made obvious by the two big Orange Drop decoupling caps. Someone made a boo-boo there, and only had 1K5 grid stopper resistors going to one tube per pair. The other tubes were getting a direct feed from the audio/bias supply node. Opps! Also, one existing 1K5 was carbon comp, so I ditched that and replaced with metal film.
Then, I replaced all the electrolytic caps. I managed to reuse the side terminals used to series connect the big 80/450V caps and 100K balancing resistors. As usual, instead of removing the old can caps, I just unhooked them and left the cans and bleeder resistors in place. Why bother with that business?
The new caps replacing the dual 40@450V cans were twisted together, strapped with a 100K 1W resistor and soldered to chassis. Nice and strong.
The tube socket of V3 had burned up, melting off it's screen and plate pins, so I replaced it with a nice NOS phenolic socket (as the ceramic socket I had was too skinny for the chassis! It fell right through the hole).
I touched up the solders to chassis where the cathodes were connected, as someone had blobbed up on top of the existing solder with an under-powered iron. Seriously kids, if you want to work on these amps, you HAVE to have a high-power iron with a big flat tip.
Before powering up, I replaced the original 2 wire power cord with a nice new molded end 3 prong, and soldered the ground to chassis, of course. While I was at it, I reversed the power leads, as the "hot" side of the line was connected directly to the trafo primary, and the "neutral" side to fuse/switch/trafo. Now, "hot" goes to the fuse/switch.
The mains power switch was bad, so it was replaced with the ground switch, which I also had removed. It's nice to recycle when you can.
The mains fuse was - unsurprisingly - blown, so it was replaced.
Now ready to power up, I plugged the amp into my special limiter: a normal duplex 120V outlet wired in series with a 60 watt incandescent light bulb. This is an important device for testing an amp that may have a bad power tube, or a short somewhere in the power supply. Here's how it works:
Since the device under test (the amp) is connected in series THROUGH the light bulb, then to the wall, when you power up the amp, it PULLS its power through the light bulb. When you power up the amp, the heater lines start drawing current to power the heaters (if any tubes are installed). Also, the HV circuit energizes and attempts to fill the supply caps. When you flip the switch to "ON", the amp starts pulling current, and the light bulb will light up BRIGHT. The bulb lighting raises it's internal resistance, dropping the voltage available to the amp, thus limiting the amount of current the amp can draw. As the amp draws less current after the caps charge up, and the heaters warm, the bulb dims to almost off.
If there is a short in the power supply, the light will stay lit bright, as the amp attempts to dump all that current right to ground. If there is no short, and the amp is operating normally, the bulb will dim, and the available voltage to the amp will go up.
So, if there's a short, it will light right up, and be very visible, it will save you popping a fuse, and it will prevent the amp from doing any damage to itself. Cool eh? I also have a lower tech version which is just a 60 watt bulb with alligator clips so I can clip it right onto the fuse holder. This is only useful on the old amps where the fuse holder has exposed pins.
One downside is that there is always some inherent loss while plugged in to the bulb, thus, it's not possible to get accurate voltage readings inside the amp. Several times I have forgotten to unplug the amp from the bulb and was flabbergasted when I measured only 4V on the heaters. Opps! Use it to check for shorts, then assume everything is ok enough to go full power.
Luckily, this old beauty powered up fine, not showing any signs of shorts or issues.
With power but without the power tubes, I measured the bias supply, and found it to be off of where I figured a good starting point would be. The voltages in the amp were about 20% higher than indicated on the schematic, due to the mains voltage slowly creeping up since the 60's (originally 115V, now around 121V at my shop).
I changed the bias voltage set resistor to a 47K, and added a 25K pot. Now I could get the voltage into a very safe range. A must since the poor 6L6's are taking a very high plate voltage. The modern 7027A tubes are identical in construction to a modern 6L6GC, and thus are not technically rated to take this high voltage, but I know from experience that these tubes will stand up quite nicely to these high voltages.
The screen voltage was only measuring about 0.5V less than the plate voltage, and each tube only had 470R on each screen, so I decided to add a 1K 5W resistor to feed the 4x470R's. This dropped a little voltage, and also added some more protection to the screens. It paid off, because I didn't see ANY glow in the screens. Surprised was I!
The power tubes being installed were Ruby 6L6GCMSTR - a robust and quality tube made in the Shuguang factory (nothing but good reports from these guys!). They are used a lot in Mesa amps, and are supposed to be a drop-in replacement for 6550's, so they gottsta take a whoopin'!
Biased up real "cold" according to the original schematic, and the waveform looked great, lacking crossover distortion or hum. Just fine! Don't listen to the papers (or internet forums), listen to your test equipment and ears! Like a snowflake, every set of tubes is different.
All powered up and running, I go to measure the total power, and am shocked to see a max of around 60 watts RMS clean output.
This amp had been designed with VERY low gain gain stages. Two of three stages are unbypassed and use 22K plate resistors! Woah... not to mention the phase inverter/negative feedback were tuned for absolute minimum distortion.
I fixed up all the gain stages and PI/NFB to be more in line with any Traynor guitar amp (read: any Blackface Fender amp), and voila: tons of available gain to drive those output tubes.
Now doing around 98 watts clean, and over 160 at heavy distortion! Wow! Even pushing that hard, those 6L6's screens weren't lighting up at all. Primo!
And that was it! Very little work considering this thing was completely submerged in water just a few short weeks ago.
After this blog post went up, the owner wrote back to me to relay a story passed to him via his father - the original owner of the amp, and I thought I'd share it here:
The blog is great. My Dad got a kick out of reading it, got more out of it than me trying to relay what was done. He told me a story that was quite interesting. After you told me about the grid stopper resistors [missing from two tubes] and stated that I won’t be able to hear CFBC through the cab. He stated one night when he was practising, depending on how he turned his bass he could hear “Moscow Radio”. I thought was pretty neat, but now no more Moscow Radio.
