Ancient Peavey Goodness

The Peavey Artist 240 arrived here Monday and I must say, $200 can take you a long way in this world.

The power tubes were pretty much wasted, and they were never very good in the first place-Mesa branded Chinese coke bottle items and one came apart and left the base behind.

Without further ado, a quartet of JJ 6L6GCs was installed and powering the amp up and letting things settle down a bit got me some nice clean tone from the.....wait for it....fifteen inch JBL that this amp came with.

Unlike the later amps this has an adjustable bias, and I have a set of Rubys I'm going to install mostly because I can sell the JJs to someone and I'm getting out of using Rubys. I've had a couple of them take early retirement.

Going by the serial number this amp was built in 1977, the year after it came out and won the Guitar Player best amp of the year award. No lie.

Looking at the schematic shows an earlier configuration, and this amp does not have the dual pairs of 100uf-350 v capacitors but only one pair which makes me think it was cribbed from the Fender Twin schematic-no bad thing-a single 20uf 600v capacitor with some serious leakage, and a 20-350v number to handle the third node.

For this part of the circuit the later amps use a pair of 100uf 350v numbers back to back with balancing resistors-I guess it was a lot cheaper to build.  There are 20uf 600v Spragues to be had but just one cost me about thirty dollars-ouch!

A good overhaul and some TLC and this will be a peach.

It also predates the use of Basler Electric transformers, as this came with Electrical Windings iron. It's also got the discrete transistors in the preamp which seem to be doing fine.

I had everything in hand except some 25uf 100v  numbers and the aforementioned spendy Sprague Atom. I guess it's there to handle the surge when the standby switch is thrown.

A fair amount of cleaning all the old flux off the board with a cut down acid brush and some 100 per cent isopropyl did the job nicely. Also, some straightening of the chassis was in order as this amp may have fallen over at one time, and the chassis is aluminum. It's not really necessary but I like everything to be at right angles.

Here are some gut shots. The parts were ordered from Mouser and with any luck they'll be here tomorrow or Saturday.

Then, the fun will start.

Vox AC15 Custom Classic spitzen und geschparken und der flammengeschutzen

Ok, enough with the comic German, shall we?

It's been a little slow in the repair business from the shop lately so it has given me a chance to get caught up on a lot of projects that have been hanging fire for one reason or another.

Last week, I managed to get four of them out the door. I got my Fender Bassman knockoff in reasonably serviceable condition, finished up a long term Super Reverb, a Vox AC15CC with a blown screen resistor and something else that I can't remember.

So this AC15 Custom Classic-you know, the first series of Chinese reissues from Vox that were made in the IAG factory under the aegis of Steve Grindrod?

Anyway it had open dropping resistors and a scorched board, so it sat for a while until I was ready to tackle it and fight it to a finish. Because I'd sort of forgotten where I was when I put it down I started by rebuilding the power supply yesterday.

When I threw the switch this morning it was more or less the same thing-a flash of light from under the circuit board and some smells.

This time I jury rigged it so I could observe the underside of the board when powering it up, turned off all the lights, and threw the switch.

And there it was. One of the traces that runs between one of the dropping resistors and the power supply diode was sparking because the substrate of the circuit board became conductive when it got carbonized.

I removed everything that was in the way and commenced drilling out the damaged substrate until I got to solid unburned material. I slotted it out, cleaned it up with a file, painted it with clear nail polish, and laid in a bead of silicone. The dropping resistor was already there with a sleeve of spaghetti tubing around the trailing end.

The silicone will cure overnight and I expect it to be pretty solid by tomorrow, when I will take some images, reassemble everything and road test the amp. If it succeeds it will mean another long term project off the waiting list.

One issue has been the particularly nasty lead free solder that was used in this amp. Cleaning it up required a lot of flux, desoldering braid, and the liberal use of the ZD915 desoldering pump.

In retrospect it was the best $125 purchase I've made in the last year or two and I wouldn't attempt a repair like this without it. Marlin P. Jones has them now and then, but similar devices can be found on the internet. You can read about it here and there is also a great video on youtube on the EEV blog. https://www.youtube.com/watch?v=Ft50m8UU5WQ

UPDATE: The stunt where you turn off the lights and power it up? I used that again on a Crate BX100 solid state bass amp. It's as simple a solid state amp as you can get so I arranged to contribute the labor so as to get some signal tracing in. The first thing I noticed was a crackle from the circuit board and some bad solder joints on one of the power transistors. Turning the lights out and the power on showed me exactly what was happening.

As I was working signal tracing I noticed the briefest wisp of vapor coming off one of the resistors that couples the amp to the drain gates. It was open, and I did the pinky test and found an op amp I think was bad because it got hot. So I shall find out tomorrow if my instincts were correct.

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.