6146 in the Simple SE
OK, I was sitting in the shack Sunday afternoon, half listening to the Collins net, and the more I stared at a set of 6146A's sitting on top of my Ameritron, the more I thought about putting them in the Simple SE.
I mean, if 6DZ7's can go in the Simple SE, how hard can 6146's be?
In researching this idea, it seems there is a dearth of information on (successfully) using the 6146 in single ended audio, and most of the lack of success falls squarely on the lack of a proper screen supply. My Simple SE has a separate winding that I can use for screen voltage, so I figured it was plenty doable on my particular amp.
Indeed, after studying the pin out of the 6146 types, and the layout of the Simple SE board,
I came to the conclusion that I could drop in 6146's, and drop in a separate screen supply,
without cutting any traces on the board.
The gist of what I have done is this:
1.: I lifted the return line of the choke from the board, and lifted the hot side of R4 , freeing up the tube rectified high voltage, and isolating the remainder of the power supply bus. Having already installed banana jacks for anode caps in a previous overhaul of the Simple SE last year, I removed the OPT connections from the Simple SE board, and wired the B+ directly to the transformers and thence to the banana jacks to complete the plate connection. A 100uf capacitor completes the power supply, so I have CLC, where C is on the Simple SE board and L is on the chassis in the conventional manner, and the final C is installed at the junction of the two leads going to the primary of the OPTS.
I ran a jumper from the new high B+ to the hot side of R4 previously lifted, to restore the original relationship between the B+ and heaters.
This completes the B+ supply.
2.: I built a separate screen supply on a small piece of perf board, using a full wave (2 diode) rectifier in a CRC configuration, using a 22uF first C, a 150 ohm resistor, and then I returned this line to the LI pad I had previously lifted, thus utilizing the 47uf cap on the Simple SE board as the final C in the CRC configuration.
Unloaded, I was seeing 247 volts on the new screen supply.
This completes the screen supply.
3.: Because the 12AT7's will now be running off this lower voltage screen bus, I jumpered R14 and R24 to make sure the 12AT7 does not starve for current.
4.: I figure at some point I'm going to have to build a negative feedback loop, so I removed R10 and R20 and reinstalled them on the solder side of this board to make it easier to implement NFB in the future.
5. Because I had some concern that all of this would not work and I would toast a new set of 6146A's I decided to step back and start the amp with cheap tubes instead of the 6146's.
Once the plate connection is taken care of, the material difference between running 6BQ6's and 6146's, at least as far as pin out is concerned, is that the 6146 has its screen on pin 3, and the 6BQ6 has it's screen at pin 4. Because the Simple SE board brings both pin 3 and 4 to a separate isolated pad, it is a simple matter to wire the new screen supply to pin 3 or 4 as required.
6: I installed one 6BQ6A, put in a 560 ohm bias resistor, crossed my fingers, and powered it up.
The amp worked, surprisingly (!) but hummed intolerably. After a few minutes of colorful language, I stuck some scope probes in the empty 6BQ6 socket, and it looks like the hum is on my screen supply. I added another 100uf across he first C in my CRC screen supply and this did not materially affect the hum.
I reviewed the wiring and don't see any obvious ground loops, but can't rule that out. The input source is a battery powered ipod, so it's hard to see how that could ground loop with the Simple SE.
7.: I lifted my new screen supply line from my new screen supply board, and used an external DC supply to supply screen voltage, using my single point ground in the Simple SE as the ground reference. The hum diminshed dramatically, there is still a bit, but this, in my mind, confirms that my screen supply is the source of the hum.
8.: With 360 volts on the plate, 215 volts on the screen, a 560 ohm bias resistor, in pure pentode mode with no NFB, the 6BQ6 sounds surprisingly good.
9.: It was quite interesting to watch the screen grid action with the variable DC supply. The tube comes alive at about 200 volts on the screen, and from there to about 270 volts, there is about a 20ma swing in the plate current. Because the 12AT7 is still on this bus, it may have some contribution to this.
I'm open to suggestions as to how to tame this hum. I thought about adding some bypass caps
across the diodes on my screen board, but past that, I really don't know where to go.
It looks like if I can tame this hum, 6146 and the like, including 2E26, is plenty doable on
a Simple SE board, easy actually. I could move the jumper over to pin 3, and be running a
6146 right now.
Running 6146A now!
Found it easiest to just move the 100 ohm resistor from pin 4 to pin 3. All other parameters unchanged from the 6BQ6GTB.
Pretty jazzed now!!
Next comes an 813, right? Then on to 4-400A.
I was at a hamfest this weekend and saw a really nice homebrew amp with a pair of Eimac 4-1000A's :eek: in it, but I was in my G8 and needed a truck to move the power supply - didn't feel like making a three hour round trip back to the house to get the truck and return, but I sure would have liked to have had that power supply :) .
What's with the lawyer's magazine?
You could also build a unit with a pair of 4-250A tubes.
I will look to see what transformers I have laying around but the shipping cost would rule out your use of any of them.
The 833A SE Amp Prototype
There are a few 6146's sitting around my lab, I was planning to torture them in a P-P amp, but who knows........
I made substantial progress tonight, including resolving the hum problem. The amp is now as silent as when it was triode wired with conventional audio tubes.
I started by going by my warehouse after work and grabbing up a half dozen or so crusty looking 6146's and 2E26's to use as test mules.
After sleeping on it last night, I decided I could not rule out ground loops and so I redid some of the wiring in and out of the screen supply board. I was also less than satisified with the CRC circuit. I found a small 2H 100ma choke in the parts bin, so I rebuilt the screen supply as a CLC circuit, with a 10uF first C, the 2H choke, and the final C being the 47uF cap on the Simple SE board. I also paralleled each diode in the rectifier with a .1uF cap. I didn't separately test each revision, but somewhere in these changes the hum was banished. Now I have to find room under the chassis for the choke.
There were a few false starts along the way. As luck would have it, the first 6146 and 2E26 I tried, out of the bunch I brought home, was bad and went into runaway after a few minutes. The front panel ammeter is great for catching this. The bad 6146 had gone from 50 mils to 200 mils while I was putting tools away. I would have never caught this by ear; without the panel ammeter, the smoke would have come out of something.
Now that I think I have the hum issue solved, I can start looking forward and one of the things I am considering is whether I want to regulate the screen supply. It seems that having a separate winding on the transformer, its own rectifier and CLC filter, plus being actively loaded by the CCS for the 12AT7, it ought to be pretty stable. I know its stiff. It's as independent of the plate supply as it can get - I can't see plate current variations materially affecting my screen voltage.
On the other hand, I am not entirely certain I want to leave the 12AT7 on the screen supply. Basically, all amplification ceases below 200 volts on the screen line. I am not sure if that is because of inadequate voltage on the screens, or inadequate voltage to the 12AT7 stage. I suspect the latter. Leaving the two tied together is OK if the optimal screen voltage is above 200 volts, but at this point I don't know if that is true or not ( although I think it probably is ), as I haven't begun to tweak anything for optimum performance. I just wanted to stop the hum and make sure everything was stable.
Anyway, substantial progress was made tonight, and I think this will be a worthwhile learning experience. Even if I don't use the 6146's or 2E26's, 6DQ6, 6BQ6, 6889, and 6384 will all benefit and be much easier to use in practice once the screen is freed from the plate supply. There may be other sweep tubes that can be dropped in without a lot of changes. I certainly never expected to drop 6146's in, so there may be others out there to play with with minimal difficulty.
I even found a 4-400A when I was rummaging around for the 6146's.
You wanna go triodes? I have some 3-500Z that I don't use. Actually they are spares for my HF linear amplifier so I should keep at least a few. I even have a spare amplifier in the garage. You never know when you will need a spare amplifier, right? I mean, a pileup on 20 meters needs one!
At Dayton this year I scored a lifetime supply of spares for my homeade HF amp. They don't glow though......MRF150's.
It's been back in service the better part of this evening, the 6146's in it right now are probably pretty crummy; of the bunch of pulls I dragged home the other evening I wound up throwing all but the two that are in the amp now, away, as all of the others wanted to run away.
While it was interesting in a Rube Goldberg sort of way to have half of each power supply on the board and half off the board, once I determined I needed to run the 12AT7 off the full B+, I discarded the split rail idea as being without merit, and reversed those changes I made to the Simple SE board.
The bottom line is that pins 3 and 4 of the octal socket need to be completely isolated, so that the appropriate component / voltage can be connected accordingly depending on the tube in use, and because these pins are brought out to isolated pads on the Simple SE board, they are easily isolated.
For the 6146 / 2E26 family of tubes, pin 4 has to be left externally unconnected as it is internally connected to the cathode of the tube, pin 3 needs correct screen voltage, and plate voltage goes to the plate cap. That's basically it right there, so what looked like an easy project was actually a lot easier once I got a couple of evenings into it.
I wound up with an LC screen supply. CLC (10uf, 2H, 33uF) was settling in at about 245volts loaded, which is too much for 2E26 and 6BQ6, and at the upper limit for 6146 and 6DQ6. LC sets at about 215 volts for 2E26 (above the rating, but they seem to handle it OK) and 205 for 6146. I made the LC from the CLC by moving the hot lead of the 10uF cap to the other side of the choke, so it is easily reversible if I want the higher screen voltage for traditional audio tubes.
Presently the amp is running 400 volts on the anode, 205 volts on the screen, and with a 510 ohm cathode resistor, this gives about 45 mils plate current / tube. Even for old tired tubes, these are loafing. I have the plate load set at 5K, and cathode feedback enabled.
Subjectively, the amp is very dynamic, with loads of power reserve. RCA often described these tubes as compact powerhouses in advertising, and the description fits. I feel it would benefit from a global negative feedback loop as the frequnecy response sounds tilted to the high end of the spectrum to me. On the other hand, my wife thinks it sounds great. Usually, I am the only one to listen to music through the amp, and I do that after everyone has gone to bed, but tonight she spent several hours listening to it, and made several spontaneous commments about liking the sound of it, so go figure. This was after I went into the source setting and set the treble about 6-8 db below flat, however. I expected distortion to be audibly higher than before, and it is, but subtly so and not bad at all. It doesn't really sound like THD to me, more like IMD I would say as there is a subtle loss of clarity in pentode mode, for lack of a better description, as opposed to triode mode. I have 96ish dB speakers and it can flat shake the walls with power to spare now, so I'm Ok with that tradeoff, at least for now. I have quite a bit of pressing business in the forthcoming month, and since my wife seems to like the sound of it now, I'll probably leave it mostly alone for awhile.
I didn't take any pictures tonight because I exploded a cap last night and still have mess on the bench and on the amp to clean up. I'll try to take some this weekend of the temporarily finished project.
If one has to go out and buy 6146's I don't think this is really a justifiable project as there are plenty of traditional audio tubes to cook off, but having them on hand, and all the other components, my only cost is a few evenings of my time. If I decide to keep the 6146's in it, I'll probably pull these pulls, and put in some new 6146B. I need to save my 6146 and 6146A's for my transmitters, as they can't handle the B's and I have no other use for B's.
I've glanced at some pin outs of some of the other popular tubes and it looks like now that I'm this far into this thing that 6BG6 could now be accomodated with a couple of jumpers and a cut trace or maybe two. I'll probably leave that for someone else, although I do have some nice Westinghouse 6BG6GA's that would look good on the amp ....
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