Son of Judybox Mark II

I completed the second Son of Judybox, a/k/a Vamp tonight and at this point it is an unqualified success and ready for prime time.

If you've been following this saga you know that I was able to find a way to reuse the original Chinese made Ningbo Chaobo power and output transformers that the Champion 600 is equipped with to make this project a lot more affordable-at least $68.00 or so, depending on where you shop.

The problem was that the original power transformer had no 5v rectifier tube winding and the secondaries have no center tap. This was solved with a modified Graetz bridge and a 6X5 rectifier tube as indicated in the Yes You Can posting a few weeks ago, and a good solid 330v of DC and 6.4 filament volts was obtained with everything up and running. You could probably use a 6X4 as many Silvertones did, but it is a lot easier to wire up an octal socket-there's more room to work-and you have an octal socket sized hole ready made for the purpose already where you want it.

The schematics for this setup have been posted here, and the only change would be to add a pair of 100 ohm resistors, one on each leg of the filaments for noise reduction as the original power transformer filament winding is without a center tap either. I wired the filaments with two wires, where the original 5C1 uses the chassis for one side of the filament string.

Another useful thing to remember is to use a grounding jack for the input for noise reduction.

To summarize, then. The original speaker jack was removed and a fuse holder was installed after opening up the hole. Two holes were punched in the chassis to accommodate the two additional octal sockets. Care has to be taken to avoid interference between the 6V6 in the center and the speaker basket or magnet.

A piece of perf board did service for mounting the electrolytics and dropping resistors, and the rest of the components were wired point to point. The lead from the jacks to the preamp tube is very sensitive to interference, so I shielded it with about thirty turns of hookup wire wrapped around a pencil and slipped it over the lead and its capacitor.

At the same time I installed a 1 meg audio pot for the volume, a standard Fender pilot light assembly, and reused the original power switch, saving an extra three bucks there.

The last bit of the puzzle was getting the sound right. I first tried a Weber 6x9 inch ceramic speaker but it didn't get the job done.

Luckily as I was up in Minnesota hunting in a dead TV repair shop last week I uncovered a nice 1957 vintage 6" Rola radio speaker and that does the job just right.

Another good idea is to remove some of the aluminum tape shielding in the cabinet above where the fuse holder sits-otherwise you can get a few sparks if the amp is jounced pretty hard.

You can have tube rectification with all the sag and touch sensitivity that conveys and still retain the original iron which seems well up to the task. You save enough dough that you can go out and get yourself a big hefty output transformer and still have thirty bucks to buy some beers.

Fixing a Dead Standby Switch on a YCV40

A Traynor YCV40 arrived recently with a failed standby switch and it presented an interesting problem. Seems the switch actually controls a FET (field effect transistor) that controls the B+ voltage, and a shorted power tube will kill the FET.

The fix according to Yorkville was to replace the FET (Q8) a small 10v zener diode (D13) and an integrated circuit (U3). U3 is the little white square in the picture, Q8 is the transistor on the right and D13 is too small to see easily but if you start at U3 and count to the right it's the sixth device, to the right of five resistors. Make sure you check the part numbers on the FET and the IC because in this case the amp I was working on used a different device-note it has only six legs. Make sure you use some fresh heat sink compound under the FET.

Q8 is an IRF 830 FET and U3 is a 4M35 integrated circuit. Don't quite remember the part number on the zener but it's in Traynor's excellent service manual.

Also these amps were built with lead free solder so be prepared.

Champion 600 modified schematic Mark II

Here's the complete schematic for the low cost 5C1 retrofit using a donor Champion 600 and retaining the original equipment transformers. If you're building this make sure you position the 6V6 so as to have adequate clearance between the tube and the speaker basket.

ERRATA: I got the diode numbers screwed up here so just go ahead and substitute the omnipresent 1N4007 and double them up for safety's sake.

Yes, You Can and the Champion 600

Part of the Mark II Champion 600 to 5C1 modification program was intended to lower the price of the modifications substantially by reusing the original Ningbo Chaobo power and output transformers.
That presented a smallish problem because the power transformer was without a center tap or the more typical 5v rectifier winding.

The first objective was to wire up the filament circuit and see if the 6v filament winding had enough stamina to survive. This was done by comparing the unloaded and loaded voltages. Unloaded the winding produced 6.7 volts ac. Installing three tubes-a 6X5, a 6V6GT, and a 6SJ7-and powering up produced a voltage drop of only .3 volts, and it stabilized at 6.4v which was serviceable.

The second objective was to see if there was a way to use the transformer with the rectifier tube. A bit of looking around and chatting to people led me to a drawing of a hybrid power supply on diyAudio drawn by A. Ciuffoli, which is sometimes called a hybrid Graetz bridge.

It consists of tying the plates of the rectifier tube to ground by means of the diodes shown. I used the 1N5408 which is a bit heavier duty than the omnipresent 1N4007 diode, and then I doubled them up for good measure. Connecting pin 8 of the 6X5 tube to the load produced a good solid 360v of DC right where it is needed.

With all tubes installed and a load connected, I'm getting 330v-316v-273v on the B+ rail which is plenty for this job. By comparison, the original solid state rectifier setup in this amp produced 360v according to my notes, which puts this right on the money. According to the Sylvania technical manual, the voltage drop for this tube should be about 22v, which means I'm close enough for guv'mint work.

So. The proof of concept is good, much money will be saved, and you heard it here first.

Thanks to Mr. Ciuffoli, wherever he is, and the folks over at diyAudio who think hard about such matters.

ERRATA: I got the diode numbers screwed up here and on the schematic so just to be on the safe side use the omnipresent 1N4007 for the legs from the HV to ground and double them up as I did.

Second Son of Judybox Project Underway

As part of the retro mod Son of Judybox project I acquired for a Franklin an allegedly defunct Champion 600 that arrived this afternoon from beautiful downtown Cody, Wyoming. As it happened this was because the preamp tube had come adrift. Once reinstalled it was fine.

After some fits and starts the first Son of Judybox project turned out to be a good exercise in repurposing, but the objective here is a little different.

I want to try and reuse the Ningbo Chaobo (I ain't kiddin') transformers and compare the sound to the fully retrofitted 600, because if I can, I can lower the price of this modification substantially. The original power transformer does not have a 5v rectifier winding, but if it can stand an additional 600ma of heater load I figure a 6X5 rectifier tube will do swimmingly. I could use a 6X4 but with my fingers I need octal territory to work in.

So the objective here will be to measure the original filament voltage with no tubes and then with tubes to establish a baseline voltage and then install the filament string to measure the draw and test the power transformer temperature. The original load, according to the Sylvania manual, should be about .75 amps and the design load adds up to 1.35 amps, the difference being the .6 amps that the 6X5 draws.

The other idea I have is to use one of Ted Weber's 6x9" oval speakers he engineered for the Valco crowd.

Stay tuned.

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.