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.

The end of an empire-maybe.

Through the acquaintance of a nice person in Austin I was able to acquire six more or less complete Judybox Revival (or they could be Hooligan) component tag boards, pretty much fully loaded. One's got a little damage but is repairable. Also part of the deal was a new power transformer and a fair amount of bagged components.

These came from when Judybox shut down for good, and the person who originally had these was a worker at Judybox.


There may be more, including some technical information which I would dearly like to get my hands on. It is said that in a garage in Austin awaits more goodies and the file folder with the technical stuff. Although it can be reverse engineered it's much easier to have documentation to work with.

Chances are I will rework these circuit boards to Stage One standard and then see what can be done with them.

Tales From The Technical Fringe: Building A Tube Matcher

This is a little off the subject of Judybox amps but it may prove useful. I'm a semi pro amp repairman in addition to my other hustles, and I've had 4 or 5 Ampeg SVTs here for work mostly from one music shop. Anyone who's spent any time with the newer generation of SVTs knows a couple of things.

First, there's what I call the "Honey, I've got a headache" system. It consists of a couple of circuits that monitor bias current to the power tubes and allow you to adjust it within a certain range. If the system detects a fault in the power tubes, it throws the amp off line and it can't be gotten out of standby until the fault-perceived or otherwise-is rectified.

The general consensus is "Put in a matched set of power tubes-it may fix the problem, it may not." Of course, for a sextet of 6550s or KT88s, you're going to drop a minimum of $160 for the privilege of finding out whether you're right or wrong-maybe.

Why maybe? Because there are four separate integrated circuits in the system, a thermistor and a couple of relays, too-any one of which can produce faults in the system-not to mention the 220 ohm screen resistors on the tube circuit board. In such instances a set of tubes will only do damage to your billfold.

And thereby hangs a tale.

There is no longer such a thing as a good reliable 6550 tube in existence. Of the four makers in existence-Reflektor, Svetlana, JJ, and Shuguang-none are close to the reliability and ruggedness of the old GE or Tung Sol products. Couple that with the SVT's penchant for running the tubes damned hard, and you have a recipe for regular problems. I realized a while ago that if I was going to do any work on SVTs I had to have a good way of evaluating and classing power tubes.

Thus the Jack Price tube matcher, for which a schematic, layout and bill of materials can be found at the old Priceless Amp Restoration website.

A tube matcher's a handy thing to have, but just like Jack I found that the commercial items like the Maxi matcher were awful pricey. The Maxi matcher also tests at only two voltages, 325v and 400v. That's not nearly enough when today's guitar and bass amps are pushing hard-the SVT slams the power tubes with about 660v.

I figured I had the parts to build it, so I started on it a couple months ago.

My tube matcher is based on the Jack Price schematic but with several differences. The chassis was from a Bogen PA amp I had. I sold the transformers out of it and netted $25 so I was ahead before I started.

The filament transformer was donated from an organ that was running about 40 12AU7s so it has ample capacity. It runs at about 6.7 v unloaded and with 3 6550s and an EL34 powered up it only drops to about 6.4v. There's a 330 volt center tapped winding as well but I figure that was a power supply for a field coil speaker.

The power transformer came from an auction I went to in Ames, Iowa some years ago. There was an old split window Dodge van back behind the mobile radio shop that was closing and it was full of Thordarson stuff and rodent droppings. I bought a pickup load of the heavy stuff for a Franklin and this was in it. It's a Thordarson 26R35 television power transformer which is rated at 275 ma plate current and has 400v on either leg. The B+ ended up being 550v at 120v, and with the variac I can up that some.

Because the Thordarson transformer had no bias winding I had to set it up a la Marshall JTM45 off one side of the high voltage winding. It required a little tinkering to get the resistances right but I can now get anywhere from 7 to 100v negative. It's got about half a dozen 5V and 6v windings that I didn't use except for one for the pilot light.

I had most of the other parts in house-tube sockets, hardware, pop rivets, spray paint, and the only thing I bought was a big bag of terminal strips from some guy on ebay.

I had a 3 amp variac in the garage that I used to power the power transformer-it's a good way of adjusting the B+ to where you like it, bringing it up carefully as you go. I start down at about 300v and ramp it up slowly while monitoring the bias current on one tube for excursions and alarms.

I went to test it last night and promptly blew all four fuses. What I'd forgotten was that bias voltage is inversely proportional to bias current, and when I thought I was turning it down it was going up. POW!

After reviewing my buildup and checking everything I tried it again, new .5 a fuses, with the bias control in mid span and it worked fine. I've got four 6V6s on test at about 350v in the picture. I chose not to use the 4 position selector as it was easy enough to bring the test points out to binding posts. I also tested a set of TAD 6L6GCs that were alleged to be matched-and they are, within 2 or 3 ma at 450v.

This is a relatively easy project to build even if you buy the transformers and a chassis enclosure commercially. If you tinker with amps at all it is well worth the effort.

UPDATE: After I completed this project I started evaluating a pair of 6550s and a pair of KT88s that I borrowed from a friend. Things were fine up to a certain point and then my multimeters would all get hysterical and I'd have to dump the voltage. I turned on an AM radio in the shop and it was clear that what was happening was oscillation. Dave Bane of DB Electronics suggested I incorporate some plate load resistors, which I did, and I also added a few snubbers here and there. It's all good and works fine. I've revised the Price schematic which is attached.

Judybox Photos

I've been trolling the web for images and I've come up with some finds. The first series from top to bottom is an unnamed head that's for sale on ebay presently and the second series is some photos of a Judybox Thumper in the build stages.

The lead dress looks a lot different from first to second which leads me to think that the ebay amp has been hacked pretty severely.

Also, there's a photo of Revival serial number 202, which means there were at least 20 or so made as the Redhead is serial number 221 and the sticker for serial number 203 was lying around loose in the back of the Redhead.