Pro Reverb blowing fuses.

A later model 70w CBS Fender Pro Reverb came in that would blow fuses regularly. Initially it had a bad rectifier tube which indicated something was amiss, as the 5U4 GB that was fitted is a very rugged and reliable rectifier tube. They rarely fail.

I replaced the fuse and plugged it into my spiffy new Chinese monster variac, running through a Kill-a-Watt monitor

The Kill-a-watt is sold with the idea that you can police the appliances in your house and see what maybe is drawing a little current when it's supposed to be switched off.

The Kill-a-watt will cost you about twenty bucks and you can read line voltage, amps, watts and frequency. It's a handy piece of kit and it plugs right into the front panel of my variac. I got mine for free from my old pal from the neighborhood Willy Robertson.

Here are some details. http://www.p3international.com/products/p4400.html

So when the amp was powered up, it would run about .4 amps, but when the standby switch was thrown and the power supply was coupled to the load the current went up to over 3 amps-plenty enough to blow fuses.

It had to have new power supply capacitors so they went in, and I went about eliminating sources of excessive current draw one by one by substitution. Output transformer, choke, power transformer. And I still had the problem of excessive current draw.

After thinking about it overnight and considering that this is one of the waxed boards that are known to be conductive at times, I rebuilt the bias system on a new board and made a new insulator board to slide under the main circuit board.

This is a good reason here to buy vulcanized fiberboard material from Mojotone, and the eyelets and staking tools from Mouser Electronics. You can make your own Fender style eyelet boards in any configuration you like, or you can design your own circuits and build them as you wish.

I know, I know, you can buy them off the internet from a couple of vendors who are pretty well known although I am not going to name them, and if that's your thing that's fine.

My experience was not so nice. I got a bias board from one vendor that was not well made and the other big name sent me a 6G3  board that was missing half the eyelets. Even though I sent pictures of it I still don't think they believed me and it cost me a couple of bucks to send it back. All in all the contretemps cost me too much money and a couple of weeks of f**cking around.

That convinced me that the ONLY way to go was making my own, as taught to me by Jeff Gehring. I have detailed the process elsewhere in this blog, it's simple and with a few common tools-a drill press and a Seymour Iron Co. number 1 arbor press-you can make professional grade boards in any way you like.

Don't waste your hard earned money on junky benchtop drill presses from China. They're not worth the bother.

I had one. It was a piece of crap even if it did only cost me five bucks at a yard sale.

One day at an auction I got a nice Walker Turner drill press from 1936 for forty bucks, it needed only a new belt and some grease, and the arbor press cost me five dollars at a yard sale. So don't complain about how much you gotta spend and it's too much trouble...as Stacy David of Gearz says, "Quit talking and get out in your garage and build something."

A smallish Chinese made arbor press from Harbor Fright would be OK if you can mount the staking tool in the press shaft. If you can't, save your money. You can always stake the eyelets with a hammer and an anvil, or some folks just use their drill press as a press of sorts. Whatever. Do something.

So this morning I made a new bias board-which I do frequently-and a new insulator board for the main circuit board. These were installed and the amp powered up nicely and biased properly, current draw about an amp.

Job done, as Edd China says.

Pics to come.

UPDATE: Here's a waxed insulator board which I replaced, the original bias board and the replacement board installed.

The ground connection looks pretty sloppy but that was before I went over to Hobby Lobby for a proper soldering iron for doing chassis grounds.

I'm still not entirely sure whether it was a conductive board going to ground or something dodgy with one or both of the existing bias caps. I did see a bit of red plating so I suspect the bias network was shot. Either way, cutting drilling and sliding an insulator board under the main board was pretty simple and easy.