Revised 5E1 Schematic

This schematic reflects the changes which I made as I built my project amp. Notice the hybrid rectification method using SS diodes and the 5Y3. Fender didn't see fit to show the 6.4vac feeds to the audio tubes in their schematic, so I added them. My 60hz hum is still present. I tied a 10mfd electrolytic cap in series with the second 8mfd cap in the power supply, just after the choke, and this reduced the hum by 50%. I am now testing an old 20mfd can cap removed from one of the console TVs I scrapped. If this thing is good, I will swap it for the 8mfd that is in place now. The voltage values in red are what I actually am reading with the amp powered on. I will adjust this with resistors later. Don't forget to click on the image to see a larger version, or click on the link below to go to the actual blog.

Fender "Champ" 5E1 is finished!

Well...mostly. Earlier this month I completed the assembly and began testing. Since then I added a switch for the feedback loop which runs from the output transformer back to the cathode of the 6V6GT. Since the power transformer does not have a center tap for the HV secondary winding, I wired a diode to each leg of HV windings and then tied the other end of the diodes to ground. The HV windings were then wired to the 5Y3 as per the schematic. It is my understanding that these diodes working with the 5Y3 provide full-wave rectification. I will post a revised schematic soon, but for now will include these photos. Don't forget to click on the photo to see a larger version. The first photo above shows the external components and where they came from.The Second photo shows the inside of the amp from the bottom. The wiring looks cluttered, and is, but I do have star or central ground and all grounded components are tied to this via the dark green wires. The other noise reduction effort involved using a cordless drill to wind the 6.3VAC heater wires which are light green. Since all of the components are pulled from scrap (except the output transformer, filter caps and cathode bias caps), I had to approximate values by combining parts. There are several resisters tied in series to achieve the proper resistance, and you can see two caps tied in parallel (yellow at bottom left) for the coupling from the 12AX7 plate to the 6V6GT grid.

The third picture shows the cabinet that I am using for now. This cabinet was given to me about 25 years ago by Frank Hilvert. THANK YOU FRANK! I believe it was built by a friend of his family. This unit was built with an outer screen which the amp is sitting on, while the actual speaker cabinet is lying on it's back in this photo.


It turns out that this was a folded horn design by Klipsch, which I located in a loudspeaker desing book from the '50s. This is visible in the forth picture.

3 for 3

My parents were in town this past weekend for another visit. This time my father and I had more time to spend working with three amp projects. First we looked at the Champ 5E1 project that started all of this. We confirmed that I do not have a center tap on my RCA power transformer so my options are to either create an artificial tap and use the 5Y3 for rectification, or create a bridge rectifier using 4 diodes, donated by my neighbor (thanks Bryan!). Next we spent time slowly powering up the Western Electric 124c Dad pulled out of the town dump! This thing was THE workhorse of the early '50s. As you can see in the photo above, this amp has 19" rack mount ears on either end of ths chassis and a full-size face plate that mounts on the bottom which is actually the front. Click on the photos to see larger versions.

The third amp was the Hammond AO-35 which was donated by Ron Ashcroft last year. I installed new tubes which I bought at the Dayton Hamfest in June 2008. These were two Sovtek EL-84, two Svetlana 12AX7, and a NOS Sylvania 5Y3 made in 1975. We slowly powered this up as well, using the variac I bought at the '08 Hamfest. This amp has no gain control pot, so after a little squealling with a microphone, we plugged my Samick Telecaster in using the Switchcraft adapter I bought from Antique Electronic Supply (visible on the right of the chassis). The amp sounds good, but a thorough test of all the parts will likely turn up some leaky caps. A gain pot would also be helpful.

Pulling Parts from the RCA HG-765L

My folks were in for a visit, and after my dad looked at the layout of the TV chassis, he wondered if their wasn't a rectifier tube somewhere in the circuit. If so, this may indicated the needed 5 volts to operate the 5Y3 rectifier tube in the Champ plan. We'll look closer at the tube layout for this console to learn more.

Tektronix Oscilloscope for Parts & Variac Made Functional

Another Craigslist freebie! The Tektronix Oscilloscope seen here is a vacum-powered tube unit from probably the mid-60s. (click the photo to enlarge) I hope to pull various components off of this. I had the opportunity to spend a couple of hours rewiring the Variac I bought at the Dayton Hamvention last year. My Dad told me I may be able to rewire the coaxially mounted M5 variable transformers, from 3-phase 240vac to 120vac. I did this and the test I performed afterwards is in the video below.

Console #4: RCA HG-765L

This unit was likely made in the mid to late '60s. Paula is overjoyed that yet another old console will sit in the garage while I strip it. This is another Craigslist freebie. What I am likely to do with this is pull the TV chassis out, along with some other very small pieces, and dump the rest. Though the TV chassis is tube, the audio component is solid state. This is the same configuration that I found in the Magnavox that I got at the very beginning of this project. Yes the Pavarotti LPs came with it. THANK YOU BRIAN FOR YOUR HELP GETTING HOME!

Inside the Variac

I haven't touched the amp project in nearly a year. I have acquired additional test equipment though, to include a an a capacitance decade box, capacitance analyzer, a bridging transformer for impedence mathing in audio inputs, an RF Signal Generator, a DC ampmeter, and an older multimeter. But of course the hard part has been finding time to put this stuff to good use. I did pull out the Variac that I bought last year, with the goal of getting it into action. I will need it to safely test amps and individual components like transformers. For starters I cut the wires off, and removed the cover. I have obtained enough technical information about this that I hope to get it wired for 120vac-to-120vac use soon. But not without a call my pop first. Please click on the photos here to see enlarged versions.