Fun On A Budget

I was listening to some Patsy Cline sides the other day and later on chatting with my pals over at the Basement of Broken Dreams while Bob was plaing some slide on a Telecaster.

That's when it hit me.

A simple lap steel guitar oughtn't cost too much cash and might have some potential for new and interesting sounds and add to my generally skimpy repertoire.

It was the work of a few minutes to find this on the GC page and I immediately placed an order:

It arrived here today, and it was the work of a few minutes to get it unpacked. Shipping was free and even though I had to pay the state of Illinois some four or five dollars in sales tax it was worth the doing.

I have a notion that lap steels are making a modest comeback from their heyday in the thirties, forties and fifties, as people are returning to music that is more authentic and unfiltered and unprocessed.

The Rogue lap steel comes with three furniture style extendable legs that screw into three mating sockets on the underside of the guitar.  One was loose and the guitar was kind of wobbly because of the following.

As the parent wood is kind of soft, a few of the screws were stripped out. However, the cure is found by breaking off a toothpick in each hole that is stripped, putting a dab of Loctite cyanoacrylate glue in, and running the wood screw back down to hold the sockets permanently.

That took all of five minutes. It pays to be handy with tools.

It took a few minutes to find a tuning video on youtube which has come to be everyman's encyclopedia of everything. A C6 tuning gives you a C major on the lower three strings and an A minor on the upper three.

The tone is nice and mellow, and I think it will be just fine as it is. It came with knurled knobs, which could be fun for doing swells if I didn't have the big assed Ernie Ball volume pedal someone gave me.

I will have to obtain a steel bar and some finger picks and learn a few basic riffs. If I'd gone and bought a lathe and milling machine I could make my own steels, but I didn't. Dammit.

Stay tuned as they say.

Why You Need A Tube Tester

I've been around this line of work for a good ten years or so, and if you count the radios, since 1983. Here are images of the three different tube testers I use in amp repair, and each one has a specific purpose.

The Sencore is used only for testing ten pin base tubes such as compactrons like the 6C10 and 6K11 plus the 7868 power tube. It is a basic emission tester.

The Hickok 532 is a mutual conductance tube tester, and the one I use is older than I am, having a build date of 1946. Nonetheless is is a durable piece of kit, and if it ever fails me I have another on standby-even though it was not modified for 9 pin Noval base tubes so I'll have to do that if push comes to shove.

These two testers have one feature which makes them indispensable, and that is a shorts test. They both have neon lights which will illuminate brightly if a shorted tube is encountered.

What does this mean to you? Simple. A shorted tube will render your amp inoperable and if it is a power tube can cause serious damage to associated components.

Both testers will tell you if the tube or grid circuits are open. Either it won't illuminate, or no results will display.

A lot of folks say "Well. The only real test of a tube is how it performs and sounds in your amp." That's true as far as it goes, but the purpose of using a proper tester can save you a lot of work, and it can prevent damage from occurring that can run up the repair bill.

As an example today a Marshall JCM2000 DSL 100w amp came in today, with no output. I knew the amp because it doesn't get used much and I'd marked the tubes with the date when I repaired it two years ago.

If I didn't have a tube tester chances are I'd still be down in the shop pulling my hair out trying to figure out what had failed, and it would be running the customer's repair bill up as I hunted and pecked and hoped to get lucky.

As it turned out it took five minutes to find a preamp tube with an open filament which was all it needed to get it back running again.

Now. The third tester is a Maxi Matcher II made by the fine folks in Seattle at Maxi Matcher which just goes to show there are more things of value that come out of  that fine city besides coffee and big airplanes. It tests a certain range of power tubes for mutual conductance and plate current and it allows the operator to match tubes-which makes for a nice quiet and well balanced performance.

And, if it detects an overcurrent situation-a short-a LED illuminates and you get an overcurrent shutdown, thus preserving your tube tester.

In fact, while matching some brand new tubes the other day, it detected a brand new tube which was shorted, and that prevented an expensive failure and some rework I would not get paid to do.

This was not the first time, either. A customer brought in an amp for service and he provided tubes he'd obtained from an eBay vendor-always a risky proposition. On the Maxi Matcher they went and they were not matched at all-there was a 15 ma split which was unacceptable. So more postage back and forth was required, and a careful reading of the guy's ad on fleabay revealed he did not have any idea what he was talking about when he advertised the tubes as "matched".

Now. You do not have to spend a lot of dough here. A reasonably good quality emission tester like the Sencore, or a Knight KG1, or an Eico 625 and many many others can be had for less than fifty bucks.

If it sorts out one bad tube before it causes damage you're money ahead. And that is something the people who insist "the only true test is in your amp" do not seem to be able to comprehend.

Heating Up A Blackstar

One of the kids at the shop has a Blackstar HT40C which had a persistent "no volume" problem. and because I'm a nice guy and they do provide a lot of work for me I agreed to tackle it.

It seemed that the power tube filaments were intermittently on the fritz, with the result that no joy was to be had. This was the second go around because the last time I'd thought "What's the issue? It works, dunnit?"

Well, it didn't.

Once the problem was localized to the power tube filament circuit, I measured the raw output from SP14 and SP15 and found that unloaded they were quite different-54 and 64 volts respectively.

Pulling the main board for inspection I desoldered pins 2 and 7, redid them with silver solder  and tightened the connectors. On reassembly with a new set of JJ E34Ls I found that the filament voltage was low-about 5.5 and 6 volts. Scrounging through my stash I found some Valve Art EL34s that yielded 6.0 volts on pins 2 so that was good.

The schematic shows the return path as going straight to ground but as I've illustrated it doesn't. It goes through a 100 ohm resistor and a diode in parallel, and then the entire ground goes through a pair of diodes and another 100 ohm resistor and capacitor in parallel before it reaches home base.through SP3. This goes a long way toward explaining why measuring pin 7 to the chassis is not completely grounded. Also, the ground portion of the bias system which contains a diode and a 1 ohm sampling resistor dumps into the above chassis potential "ground" labyrinth described here.

Because of this quirk I will be adjusting these amps to 50mv across the sampling resistor as is noted on the circuit board and the Blackstar HT Club 40 test specifications rather than using a bias probe as has been my habit.

I think the problem is pretty much resolved with the general cleanup

UPDATE 1: I revised the shop sketch here to show the complete return to ground path which includes the paired diodes D42 and D34, the resistor R223 ,and the capacitor C116.

I've no idea what they're supposed to do but they go a long way toward explaining why ground on the power tube pins 7 isn't really ground although the schematic from the factory says it is. Go figger.