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.

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.