Yes, More PEPCO

Well, the Paul amp seems to be behaving so far: no blown tubes or smouldering resistors. It does have a strange habit of going dead silent (instead of the usual faint hiss of a live amp) after about 10 seconds if there's no signal. As soon as a note is played or the patch cord touched, it springs back to life. This isn't normal behaviour, so I'll have to do some more probing.

I also plan to tune up the Pine amp this weekend with new parts, as well as start work on my "amp from parts" project. Here's a few pics of the Pine:


This is the cabinet, with the groovy mac-tack covering and "white noise" grill cloth. Not sure if the handle is original. Looks a little odd.

The chassis, showing (from left to right) sockets for the 5Y3GT rectifier, 6V6 power tube, 12AX7 tremolo circuit, and 2 6AV6 pre-amp tubes. Note the empty socket hole, as the same chassis was used by PEPCO for a number of different configurations.

And finally, the control panel for the amp. Four inputs: enough for the whole band!

PEPCO Care and Maintenance

It's been such a long time since I used this space to write about the goings on in my studio. My deepest apologies to all you spambots and diehards.

I did finally finish off a four song EP in March, called A Small Collection of Songs for a Small Selection of Friends and released by my "band" Yes With a Capital S. Ask me and I'll give you a copy.

But I'm here today to talk about amplifiers, in particular 1960's Canadian made tube amps. About 12 years ago I bought a small tube amp made by Pine Electronics of Montreal at a flea market in Kingston for $40. It has two 8" speakers, a nasty tremolo feature, suave cabinet styling, and dates from 1965/66. The tube setup is a little weird: 2 6AV6s for the pre-amp, 1 12AX7 for the tremolo, 1 6V6 power tube, and a 5Y3GT rectifier. Class A operation, maybe 7 watts of power. In recent months I've been preparing to build my own tube amp from parts, but I thought it'd be good practice to work on an existing amp and bring it back to life. So I opened up the Pine (aka PEPCO - Pine Electronic Products Company), made a note of parts needing replacement, and placed a big order with a tube specialty place in the States.

In the meantime, there was a listing on Craig's List of a dead PEPCO tube amp called The Paul, which I bought last weekend for $100. Here it is:



Now, this is the same circuit and tubes as the little Pine, but with two 12" speakers in a much bigger cabinet. The previous owner had bought it from Capsule Music on Queen St., and many of the parts had been updated, and new tubes added. The new tubes surprised me, as dead or dying tubes are often the cause of poorly functioning amps. The 6V6 wasn't lighting up, so I replaced it with a new 6V6 I had, and ta-da! it worked. At least for a little while. After about 90 minutes of playing, the tube went south. Obviously there was a bigger problem with the amp. So I opened it up again:



I started checking the resistors and capacitors on the 6V6 and found that the screen resistor on the tube looked toasted and in fact read "open" (i.e. busted) instead of the 470K value it should have been, when I checked it with a multimeter. Without getting too technical, the screen resistor affects how the tube operates: the higher the value of the resistor, the more restricted the tube is, preventing runaway current draw and failure. There's a lot of audio mojo and voodoo around the ideal value for this resistor. Fender amps with a similar circuit, like the Champ, use a low value like 470 ohm, while Marshalls use a higher value 2.2K ohm resistor. In this amp's case, the failed resistor was a very high value and was too undersized for the wattage and heat in this usage (perhaps incorrectly replaced, but hard to say - I've never found a PEPCO schematic). When the resistor failed, the tube ran flat out at max power with no resistance on the screen, and failed after a few minutes. A properly running tube should be good for years. So, I replaced the resistor with a 1.5K, 2 watt resistor, put in a new 6V6 and so far, so good. My parts for the Pine arrived this week, so I also have a range of low value, 5 watt power resistors to try out as well.

So how does it sound? Pretty good, me thinks. I tried it with guitar, and it gets quite loud and distorted when cranked, but it also dawned on me that this amp was designed for bass use, perhaps as a cheap Canadian tribute to the Vox amps that Paul McCartney was using in The Beatles at the time. Plugging in my P-bass got a nice solid tone. 7 watts is not a lot of power, but two efficient 12" speakers can move some decent air, and it sounds loud enough for my studio use. Rock on.