The Edcor meets the 6AV5

[tubelab.com]

That's no surprise- with screen drive near class B, I have no problem hitting 150W with 6LF6.

OK, SY put me up to this, so now its time to abuse the poor old 6AV5 in screen driven P-P. I thought I would mention my initial experiments here, and then I will start a new thread later when I have time to take this further.

I got this wild idea for a new Tubelab prototyping system, so I built it first. This one really does make it possible to prototype a tube amplifier without turning on the soldering iron, and all of the parts come from Home Depot and Radio Shack. But this is another topic that I will cover fully later.

Now that I had a new prototyping lab, I just had to try it out, so I built this amp. The first stage is a LTP phase splitter using a 6BQ7, CCS in the cathode, 10 mA total. Next is the driver. This one took 4 tries to achieve success. 4 tries, 4 different tubes, one hour total time, never touched a soldering iron. The final circuit uses a 5965, 550 volts of B+. It can pump out almost 400 volts P-P into a 100K ohm load (plus the gate capacitance of a fet) at 80 KHz. A mosfet follower drives the screen grids of the 6AV5's. At full power the screen grid swings from -60 volts to over 300 volts. The driver is still the limiting factor. I can't get any more swing because there isn't any more knob left on my power supply. It is maxed at 550 volts. The output stage runs at 550 volts on the plate, and adjustable negative bias on G1. G2 is also adjustable with a second power supply. A third power supply provides -25 volts for the CCS in the first stage. I will draw up a schematic next week.

Does it work? Well, I put in a pair of the wimpy slotted RCA 6AV5's, and tried to make them glow! Operating right on the edge of class B, I can get over 80 WATTS at soft clip! There is NO GLOW visible in the tubes with the room lights off! At 50 watts the distortion is under 3% from 100 Hz to 25 KHz. There is NO FEEDBACK local or global in this amp. Operation below 100Hz is limited by saturation in the OPT. I ran this thing at clipping for an hour, the hottest thing on the bench was the load resistor.

I have not listened to this amp, or made detailed measurements yet. I need to find a better OPT. I am using one from a guitar amp of unknown impedance. I get best results with the 8 ohm load on the 4 ohm tap, indicating the need for a pretty high load impedance.

[tubelab.com]

One more picture.

[tubelab.com]

I am going to need some bigger P-P OPT's if I am going to keep this up. I will need a really large one for the 6LW6's.

[valveitude]

I got this wild idea for a new Tubelab prototyping system, so I built it first. This one really does make it possible to prototype a tube amplifier without turning on the soldering iron, and all of the parts come from Home Depot and Radio Shack. But this is another topic that I will cover fully later.

I hope its not much later

The pictures pretty much tell the story on construction, but I'm real interested in hearing the details (cost, which peg board, ect.).

Very cool!!

Casey

[Tom Bavis]

If that output transformer is the same Schumacher part I bought on eBay, I have a data sheet - 4KCT to 4/8/16 as I recall. Which reminds me - I have the Harris data sheet for the monster power transformer you got from me - email if you want copies - tbavis(at)rochester(dot)rr(dot)com

[tubelab.com]

I am in the process of documenting the next generation Tubelab prototyping system. I will put it on my web site when I am done.

I bought some (about 200) Schumaker transformers from a guy who was selling them on Ebay about 5 years ago. They were surplus from the closing of the ADA guitar amp company. They are exactly the same physical size as the ones you mention, but they are 6600 ohms. I have been using them for amps ever since.

The transformer that I used here was not one of these. The same guy gave me this one, and said he had more if I wanted them. I have not tested it out. It was the first transformer I found when I was building this amp, so I used it. It is not a Schumaker.

I have found a known transformer, and I am planning to try it later.

Tom, I would like to see the spec for the Harris monster, so I will send you an e-mail. That project (833A-SE) is on hold for lack of 2 equally monster OPT's.

[Brian Donaldson]

"Tom, I would like to see the spec for the Harris monster, so I will send you an e-mail. That project (833A-SE) is on hold for lack of 2 equally monster OPT's."


I'm waiting on wire. I hope to do the first prototype this week.

[tubelab.com]

Now back to the 6AV5 in SE mode. I decided to perform some basic measurements on the Edcor - 6AV5 combination. I discovered (while playing around with the 6EM7) that the Edcor transformer really likes cathode feedback, so I tested that too. I set an arbitrary 5% test limit for distortion.

In triode strapped mode I got 4.75 watts at 5% with 300 volts and 60 mA. Adding cathode feedback allows 8 watts at 5% using the same voltage and current. Distortion at all power levels is reduced considerably.


In UL mode I got 6.3 watts at 5% with 300 volts and 60 mA. Cathode feedback allows 9.0 watts without adjusting the power supply! This is like adding a second tube.

I have not listened to this yet, but I will soon. It has been my experience that when cathode feedback works, It sounds good. It tightens up the bass without killing the sound.

[tubelab.com]

Every once in a great while you stumble across a combination that works way too good considering what it is made of. I knew that I was on to something last night when I tried the 6AV5's through the Edcors in UL mode with cathode feedback. The measured performance was good.

Well when I got home from work today Sherri was not home, so I connected up the speakers and cranked it up. This thing positively ROCKS, I played a lot of different music, the CD's that I have previously identified for their abilities in finding amplifier flaws. Complex music pushed well into clipping still sounds good, there is NO intermodulation from heavy bass guitar over a nylon stringed acoustical guitar. Voices and saxes do not get affected at all by drums or bass even with the volume knob all the way up. I can tell by the bouncing current meter that I am not in class A any more, but distortion can only be heard on a few heavy bass notes, and it doesn't sound gross.

There is no bleepin way that I am listening to 3 dollar tubes through 18 dollar transformers. This thing has bass. Crystal clear highs too. I got to run a frequency response test.

Surprise! 3db points are 24Hz to 45KHz AT FULL POWER! 8 WATTS.

Yes I am still using a regulated power supply, and a 5842 driver with PowerDrive. The next step will be to try the real power supply, and try to find a 3 dollar driver tube. When I started using 5842's they were $3, I bought 75 of them. They are not cheap any more.

If I am succesful this could be the ultimate "El Cheapo SE".

Frequency response plot at 8 watts is included. The vertical scale is 1db per division.

[Kyle K]

That's great news! Do you know what the damping factor is with that configuration?


Kyle