Tube Matcher and Magic Wand Update

Ever since I built the Jack Price Tube Matcher I've been thinking a precision adjustable feature might be very handy to have when calibration time rolls around or there's a need for more precision.

After some thought and research here is what I came up with.

It requires eight 10 ohm precision trim pots, which are the lowest resistance value commonly available. The specifications say that these have no more than 2 ohms residual resistance, so I figured that doubling them up would get me right in the range needed as well as doubling the wattage rating. I mounted the pots on a piece of perf board and I'm going to try it out soon.

I think that setting up the tester with a single tube and then adjusting each socket to read exactly the same on the multimeter will fulfill the calibration function, and this could also allow compensation for any drift in the other parts of the system.

I also added a picture of a neat little signal tracerish idea from the Hoffman Amps website. The idea is to take a small audio amplifier set up for clean sound-in my case a $25 Crate bass amp off of Craigslist. Then taking a multimeter probe and connecting it with shielded wire (old guitar cable) to a project box containing a 1 meg pot for padding and a .1/600v cap for DC blocking completes the gizmo.

Now, you set up the amp under test to your dummy load about which more anon. You have some compliant soul play a guitar through it and using your newly built signal tracer you follow the signal through the amp until it deteriorates, thus locating the source of your unwanted distortion.

Further Tales From The Tonal Fringe: The Ampeg That Nearly Wasn't

A couple of weeks ago I had gotten an Ampeg jones and started trolling Craigslist for likely suspects. I found an ad that had just been posted from the Lincoln, Nebraska area and it was for a 1961 Ampeg M-12-the very amp you see in the pictures, and thereby hangs a tale.

I talked to the owner on the phone and I had his first name, phone number, and a place to meet in his smallish town north of Lincoln. So the Dragon Lady and me climbed into the Honda a/k/a Jackie Chan and headed west.
About 200 miles later we were there. I dialed the phone number I had been given and it was the fellow's work number, an answering service and they were completely unhelpful, didn't know anything and couldn't do anything until Monday. I then went through the Valparaiso phone book and called every Kevin in the book. No joy. I found the local public library and convinced the librarian to post a response to the ad which was still up on Craigslist, and went back to the meeting place.
Just as I was settling in for a good wait, I saw a red Dodge truck hurtling down a side road and it was our guy. His wife had intercepted the message and called him.
The amp was secured, cash was paid out, and I and the Dragon Lady retired to Lincoln for a repast at Five Guys.

How's it sound? It's clean, with a swampy vibrato, and cosmetically it is a peach. It is the early style 'random flair' vinyl covering with silver sparkles that predates the more common herringbone style covering. It sounds so good I'm seriously thinking about selling a couple of other amps I have here.

Vintage Parts Excursion Part IV: Son of Judybox Champion 600?

A friend of mine dropped off an early fifties Fender 5C1 Champ for an overhaul and it sounded so good I decided to make my own. Looking around the shop I found my Champion 600 reissue that has been lounging around with the occasional task of testing preamp tubes for excessive noise. I also gathered up a trio of tube sockets, some capacitors, a bit of perf board that had been used as a bookmark, and the main ingredients-a Stancor new old stock power transformer and a Thordarson output transformer, both of which had come out of the split window Dodge van. It took a few hours to gut the Champion 600 innards and repurpose it.

The first thing I did was ditch the speaker out jack and bore the hole out for a fuse holder. At that time I also installed a SPST power switch and, using a Greenlee type punch from Harbor Freight, installed a trio of tube sockets. Then a trio of 22 uf capacitors and suitable resistors completed the power supply setup. A test confirmed proper voltages at all the important places. Then, I wired up the 6V6 power tube and the 6SJ7 preamp tube.

It sounds remarkably like the old 5C1, but the speaker's a little more hi fi, so I'm looking for a nice period Cletron or similar fine speaker. The combination of the 6SJ7 pentode and grid leak bias is the tonal ticket. It'll slide right into a nice greasy blues tone, and with the right pickups it's a joy.

You could probably re-use the Chinese iron if you wanted to use a 6AX5 rectifier and reutilize the output transformer. But you might miss out on the magic.

To reproduce this project for you, it would cost you about $120 in parts and shipping for Magnetic Components iron, shipping both ways, and about $120 in labor. Tubes would be about $24 extra for a NOS rectifier and preamp tube and a vintage serviceable 6V6. If you like the idea, drop me a line.

UPDATE:

I did have a few issues with this circuit that it is worth knowing about. The lead between the 2-75k ohm resistors and the preamp tube is very sensitive to positioning and has a bad tendency to create oscillation.

After a few fits and starts and some blind alleys I ended up taking some hookup wire and wrapping about thirty turns around a pencil. I then stuck the lead through this coil and grounded one end. The Jensen Mod came in at about the same time so that got installed, offset a bit for clearance. The result's a lot cleaner and the volume can be maxed with no oscillation or interference. I'm still looking for an older 6" speaker-that'll really do the job of recreating a classic.

Brownie Hits The Road

Brownie traveled with me to New Jersey last week and impressed the locals with his verve and can-do attitude.

Vintage Parts Excursion Part III: Building A Usable Capacitor Checker

After a few fits and starts and trips along a few blind alleys I managed to get the capacitor checker project up and running, and it's a pretty nifty piece of gear. This device will test capacitors for leakage in microamps and also can be used to reform electrolytics and check them for leakage. It has three ranges-10 ua, 1 ma and 10 ma. The meter movement is a 199 us Soviet military surplus job, and the cabinet's prior tenant was a Jackson 645 VTVM.

In the picture you can see a new old stock Mallory capacitor leaking about 80 ua of current and an orange drop Sprague showing about 1 ua of leakage, both at 350v. Also there's an electrolytic on the 10 ma range showing about 2 ma of leakage and it settled down after a while.

I've included two different schematics for devices of this kind.

Tube matcher calibration

One of the problems with any kind test equipment is doing some real world calibration and I tried a number of different ways of doing this with my recently constructed somewhat modified Jack Price tube matcher.

Ultimately what I figured out was to take a trio of tubes-6V6, 6L6GC, and 6550. Warm them up until they're stable, say, fifteen or twenty minutes or until the current gets as stable as it's ever going to be, and then get out your leather gloves and switch them to each socket in turn without touching the control settings. I figured that anything under 0.5 ma spread was all good and so it was.

These are now carefully stashed reference tubes, and I'm sitting on my string of 1 ohm resistors from the same batch.

One thing that did occur to me was to figure out a way to incorporate a trimmer into each tube cathode circuit for really tight calibration.

Film at 11.

Vintage Parts Excursion, Part 2: Capacitor leakage tester

Since my last episode with capacitor leakage it seemed necessary to cook up another piece of guitar amp specific test gear to complement the tube matcher I built recently, namely, a direct reading capacitor leakage meter and reforming device.

The original schematic was something I found In Alan Douglas' Classic Tube Testers and Test Equipment, but I've modified it a bit to suit local conditions, as it were.

It started out life as a Jackson model 645 vacuum tube voltmeter that had been lurking in the dark under my workbench. I didn't have a need for it but I couldn't bring myself to part with it-mostly because I like Jackson test equipment.

I'd been thinking about keeping it as is but it needed a special purpose 4.5 volt battery I did not have, so I proceeded to repurpose it. I built the power supply out of a Hammond organ power transformer and more terminal strips. They're great for prototyping things as they can be easily pop riveted in a convenient place.

At present it's been mocked up and I'm trolling through my resistor museum for combinations that meet the schematic value. And that's one good thing you can do with a lot of old carbon composition resistors is find ones that you can use.

In the meantime I ordered up a Russian military surplus analog 100 microamp meter that looks like reverse engineered Weston stuff-a fitting tribute to the men and women of Frelinghuysen Avenue.

I'll probably finish most of the wiring this weekend of the Glorious Fourth, and then station myself by the mailbox for the package from Estonia carrying my spiffy new Soviet microammeter.

Stay tuned.