Single Ended EL34 Amp

[dsavitsk]

I was sitting in my quiet office the other day, when it dawned on me that a) it was very quiet because I don't have a stereo in there, and b) I have almost all of the parts to build a stereo which would rectify the situation.

So, I drew up a schematic using stuff I have around, and am wondering if there are any comments on changes I might consider. Basically, it is a single ended EL34 amp (though a pair of EL34's is the one thing I don't have, so that could change) with a 6SN7 as the driver. The 6SN7 has a CCS plate load, and I just put both sides in parallel for simplicity. The schematic is quite close to Tubelab's SimpleSE, but at a lower voltage and current. This is really just because the transformer and chokes I have are all rated at 150mA, and the caps are all 500V, so I didn't want to push it too far.

The tubes are biased via LEDs. I just drew one on the schematic for simplicity, but I would expect 2 or 3 for the 6SN7, and perhaps a 3x9 grid for the EL34.

I figure about 270V on the plate of the EL34, and the grid biased up about 18V. This gives 60mA of current. The OPTs are spec'd at 70mA, so this seemed a decent conservative operating point.

I also have other things around I could use as driver tubes (6J5, 6n1p, 6dj8, 6n6p, 6h30, 5687) but the 6SN7 seemed promising and aren't being used anywhere else.

For speakers, I am not sure what I'll use yet, but probably I'll just pick up some small boxes at parts express and a reasonably efficient full range driver from madisound. I have not thought that far in advance though.

Comments?

[tubelab.com]

Well, you already know that I have built a few of these. I couldn't get enough gain from a 6SN7 to get to full output with a CD player input. A 6SL7 worked fine, but I wanted to use a tube that I had lots of, so I settled on the 12AT7. I have since discovered a few hundred 6SL7's in a big box.

I would loose the cap from the B+ side of the CCS to ground. The CCS provides plenty of isolation for B+ noise. For a detailed explanation read this thread:

(Newbie) CSS Question

In fact with your B+ voltage you may not even need the resistor. I added it when I started approaching 500 volts. The CCS got too HOT!

[Original Burnedfingers]

I have since discovered a few hundred 6SL7's in a big box.

I would be very very happy to help you use those pesky 6SL7's before they go to waste in the salt air.

[kevinkr]

Small error with the cathode connection to the second half of the 6SN7 since they are supposed to be in parallel.

[dsavitsk]

Oops. I'd like to think I wouldn't have actually done that.

[arnoldc]

I think you'll like the results of your project.

I whipped up a simple EL34 last week, but I used a single 5687 section with diyAudio CCS on the plate. I was surprised it sounded good.

[Geek]

Originally posted by burnedfingers


I would be very very happy to help you use those pesky 6SL7's before they go to waste in the salt air.

Yeah, I'll rescue a few too

[tubelab.com]

I would be very very happy to help you use those pesky 6SL7's before they go to waste in the salt air.

Before everyone gets excited, all of the tubes that I got a few years ago were very poorly treated over the years. They are all loose unboxed tubes, and were stored for at least 10 years in a warehouse that had holes in the roof, in Central Florida. They were exposed to bird, and rat "substance" and a lot of rain. I have been going through boxes one at a time, over the past 4 years. After cleaning the "stuff" off, and throwing away the broken, or severely corroded tubes. I am yielding about 20 to 40% testable tubes. Most of the miniature tubes work. I have found issues with octals because the "stuff" has worked its way in between the base and the glass. This makes small signal tubes noisy, but can make flames on an output tube! Many have been painted over with some type of military clear lacquer. I have seen the same stuff on vintage military PC boards. It glows green under UV light.

Back on thread, I have experimented with LED bias on my SimpleSE. The input tube can be LED biased fairly easilly. I put Some pin sockets in place of the cathode cap (and left the resistor out). I then tested a bunch of LED's. I look for the right plate voltage, and minimum signal across the LED with a scope. The best ones came out of an old bag of LED's.

I tried a series - parallel string of LED's for cathode bias in the KT88 output tube (like the Red Light District amp). I had over 100 LEDs wired together and connected into one channel (I didn't have enough LED's) When I cranked the amp into clipping a few LED's got dim, then the output tube started glowing, followed by a few more fried LED's. I turned it off before things got ugly. I don't think that I had enough paralleled strings, and these were LED's that I have had for at least 15 years. They were not all glowing at the same brightness when I started the test. I think that you are going to need a lot of LED's, and they need to be somewhat matched. At least each string should drop the same amount of voltage.

I tried a 39 volt 5 watt zener diode for bias. It was a waste of $2. Sounded terrible.

[kevinkr]

I'd recommend fixed bias in this application. Take a look at my pages for the 45 SE amplifier - driver circuit would suit the EL34 perfectly.

[dsavitsk]

Originally posted by tubelab.com
I tried a series - parallel string of LED's for cathode bias in the KT88 output tube (like the Red Light District amp). I had over 100 LEDs wired together and connected into one channel (I didn't have enough LED's) When I cranked the amp into clipping a few LED's got dim, then the output tube started glowing, followed by a few more fried LED's. I turned it off before things got ugly. I don't think that I had enough paralleled strings, and these were LED's that I have had for at least 15 years.

I have done 2 parallel strings of 3 before without issue. I figured here I'd need 9 in series to get the bias to 18V, and probably 4 in parallel for the 60mA. A few less than you used, but still a lot of little parts to solder together.

The chance of this amp ever going really loud, or into hard clipping, is pretty low, though. I tend to run anything I build pretty conservatively.

Originally posted by kevinkr
I'd recommend fixed bias in this application. Take a look at my pages for the 45 SE amplifier - driver circuit would suit the EL34 perfectly.

Interesting. Is C3 in the signal path? If it is, what is the advantage over cathode bias, and if not, why a Blackgate?

Part of the reason for the 6SN7 as opposed to a 6SL7 is that with both sides in parallel I can run the current high enough for LED bias. My understanding is that it needs something over 10mA for this to sound good. Otherwise, I'd use the 6J5's I have.