A very good friend and bandmate of mine scored yet another insane Kijiji deal. This time it was a 1976-79 Traynor YRM-1 Reverb Master 45 Watt head and a closed back Yorkville horizontal 2x12 for around $300. Sickening. Anyway, the head was bone stock, and needed the obvious work done to it, so he sent it to me. We plugged it in at our practice space, and hearing the brutal 60hz hum pump through with the volume all the way off, I recommended it not be powered until the filter caps were replaced. Good thing too.

The tubes were old, but not original. The two power tubes were mismatched 6CA7's. One a "Made in Japan" Sylvania labeled 6CA7 EL34, and the other a "Made in USA" Westinghouse 6CA7.

As you can see, all original 1970's Made in Canada Mallory can caps. The schematic is included on the lid. It has blotchy stains on it from when the filter caps spewed their guts all over the place. Shame, too, as this particular schematic (while barely representing the actual circuit in the amp), is not found online. I might yet try to digitize it and clean it up in Photoshop. This schematic is labeled "YRM-1 YRM-1SC + Eng", meaning the YRM-1 head, the YRM-1SC (4x10 Combo version) and the English export version, with 220V mains.

The guts feature the typical Traynor hodgepodge of cloth wire and various plastic covered wire, "mustard" caps, and those yellow brick things. Not a single carbon comp resistor in this guy though! Nice.

Oh, that cap sure doesn't look good...

Hmmmm, wonder where all that 60hz hum was coming from...

So a full cap job was done, naturally. All discreet capacitors replacing the cans. It really, really isn't worth shelling out the Big Bucks for can replacements, especially when they're hidden away inside anyhow.

Then I had to rewire the mains supply, removing the evil ground switch on the back. Unlike a typical Fender amp, this ground switch had 3 positions: Ground, Ground and Lift. One was normal chassis ground, the other was ground through a 12 ohm resistor and an .047 death cap, and the lift was literally a lift of the chassis ground! Yikes! Also, it was wired so the fuse was on the hot side and the switch was on neutral. Both are now on the hot side.

After the mains were sorted, I modified the bias supply. I put smaller filter caps in. The amp had 68u @ 68V (labeled on the schematic as something different naturally...). Since the bias voltage is supplied right off the mains rectifier bridge, this means if the standby is off, so is the bias voltage. If the bias supply filter is larger than the mains supply, (which it was by 28u as the main filtering was 40u), the plate voltage could come up faster than the bias, and that would not be at all okay for the tubes.

I upped the screen supply filter to 50u, and knocked the rest down to 22, and left the 10s as they were. Then, the bias supply was changed to 2x 10u, so as to allow the bias to come up first. I then converted to a variable bias, by changing the main 27k resistor to a 20K, and adding a 10K pot. This gave enough wiggle room. Interestingly, I originally thought the circuit already had a bias trimmer, as there was a trimmer right in the bias supply area, however, what it is is a 2M trimmer to set the maximum intensity for the trem! Neat!

Unfortunately, after biasing up, and running the thing for a few minutes, one of the mismatched power tubes arced over. I saw and heard the flash, but the damn thing kept operating! I waited to see what would happen next. Another arc about 30 seconds later, and the signal died. I hit the power. The massive 10 watt screen resistors saved the day. They weren't even warm. Overdesign at it's best. If you've ever had a tube arc over in your Fender amp, and had the 1W screen resistor burn through your tube socket, you'd be well grateful for this big cememt guy.