Chorus II � 2a3 or 300B?

[paully]

My amplifier has the option of being converted into a 2a3 amp. Running 300B now. The 8 watts seems quite ample. I can convert it but before I do I was wondering if people thought that the Chorus II would be fine with 2a3. It is 101db 1watt/1 meter efficient so I seem good there, its impedance curve seems somewhat severe from what I have read. Was wondering if anyone had any experience here or thoughts. I can always go back but if it isn’t going to work I would rather roll some 300B’s than spend the money on new 2a3’s. Thanks!

[Duke Spinner]

chorus 2 want's some watts behind them ..

your missing the deep, tight bottom ...

[Erik Mandaville]

With that kind of efficiency, 8 watts really should be fine -- as you said you found it satisfactory. What matters is what you like. Try with higher power, too, and choose what you prefer. For HT, you might need more....and again you might not.

Have fun,

Erik

[paully]

Well, I picked up a pair of 2a3 anyway. It seems like the only way to find out for sure is to find out for sure.

[Erik Mandaville]

Absolutely. Trying something personally is the only way to really know for sure -- and even then sometimes things change. People have sold their big-size, small-power triode amps in the past and found they missed them later. The opposite of that is true, too. There are some here who can't stand the sound of single-ended triodes, but that's because they just don't know any better. 'Spose I should install a 'wink' here, although it might be kind of interesting not to!

I also have very efficient speakers, and enjoy both low and high power depending on what I'm using them for; and my high power, less than 50 watts, is in the form of both antique and very modern technology.

You may not find a significant difference between 2A3 and 300B output levels, though, but rather some differing characteristics about the tubes, themselves.

What 2A3 amps were you looking at?

Erik

[Cut-Throat]

There are some here who can't stand the sound of single-ended triodes

Especially the ones that have never heard them!

[Erik Mandaville]

"Especially the ones that have never heard them!"

...or heard them under the right circumstances, which with that kind of amp can be important. Even something as seemingly unexpected as a crossover type (as in order of slope) can understandably turn someone off. The DRD 45 or SE OTL (Transcendent Sound) in conjunction with super high order (PASSIVE) networks would not, IMO, be the best combination. That's an extreme example, but I suspect some have listened to these low powered amps with associated equipment that was not able to take advantage of some of the positive aspects of such low power. Then again, I'm sure there are those who simply don't like them, regardless of crossover type or speaker efficiency. Fair enough.

I had a pair of 2A3 amplifiers here to work on, where one of the monoblocks was configured for a different output impedance from it's twin, while the other one on inspection looked as if someone had jammed the 2A3 in backwards (there are four pins, which can be confused). This event sadly turend into another SET mud-slinging contest. These same amps, once repaired and essentially completely rewired (including matching output impedances), with new power supply filter capacitors, became my favorites over not Weborne DRDs, but the Moondogs. That's JUST my personal preference and taste.

But, the Moondogs modified as far as input/driver and hot-rodded for 5 volt filaments (fully regulated/filtered DC) sound actually kind of similar to Parafeed, which if you haven't tried that, you might really, really like. check out Lessards' Horus schematic. It's a straightforward design, and the choke on the grid of the 2A3, which is a cousin to the Heritage crossover autoformer, is a really interesting component. If the Moth amp weren't directly coupled, I would use on in each channel, but you could add them to your Moondogs easily enough. They're installed in place of the grid to ground resistor on the 2A3. The 5687 is an awesome input driver stage tube, too. I once mentioned the paralleling of the 6SN7 to LeoK and the Ultrapth from the DRD, and both of those are very good sounding on the Moondog. He took that a step further to shunt/parallel feed output. Parts in this case are NOT cheap, by the way. 'Course you might like to leave your Moondogs completely stock, and they are good amps that way -- they just don't need two 6SN7s. The grid choke would be an easy retrofit, though. Check the MQ website for the schematic and parts if you get interested sometime. It's a good build from scratch project...........................and I have tests to grade.

Erik

[seti]

There are some here who can't stand the sound of single-ended triodes

Especially the ones that have never heard them!

I would stick with the 300B's.

[paully]

I am running a pair of 300B Paramounts from Bottlehead. The changeover to 2a3 isn't exactly a cake walk but it isn't all that tough either. Move some wires around, take out a capacitor, etc... My gut instinct is that I am better off with the 300B but I just can't help myself wondering what a good 2a3 in this amp will sound like.

I had a Dynaco ST-70 that was heavily, heavily modified. I sold it to pay for some other stuff. Some days I do miss it rather dearly.

[seti]

I'd love to have some paramounts. I've seen the documentation on the switch over. I'd never thought of using the Chorus with SET. I've always wanted a pair.

[paully]

I have been pretty darn happy with my 300B, my curiosity is just getting the best of me and I want to hear what all the raving about 2a3 is.

[paully]

Well, I can always go back if you are right.

[Cut-Throat]

I have been pretty darn happy with my 300B, my curiosity is just getting the best of me and I want to hear what all the raving about 2a3 is.

I was happy with my 300B too, then I upgraded to a 2A3 and I was happier yet, then I upgraded to a 45 and I was even happier!

8 watts > 3.5 watts > 1.8 watts per channel that is! []

[NOSValves]

There are some here who can't stand the sound of single-ended triodes

Especially the ones that have never heard them!

I've heard them countless times and from some very heavy hitting manufacturers included Welbourne labs. Yup can't stand them......just goes to show you different strokes for different folks.

[Erik Mandaville]

Parallel feed outputs can be done with the 45, too.

the MQ grid-choke in the Moondog was really a neat modification. I'll get another pair one of these months. They are not nearly as cheap as the resistors they replace, but are a very nice part of the circuit, IMO. An easy mod, but for the Moondog would need to be installed on the underside of the top plate with two screw holes (plus machine screws/nuts), or against the inner wooden chassis frame.

A very nice little part. No schematic needed, just take out the resistor between grid and ground on the output tube and install the choke in its place. I had one that 'opened' internally (extremely rare), and MQ/Pantheon replaced it at a significantly reduced cost. Good customer service.

Erik

[Marvel]

Erik,

You mean there is room for a grid choke under the hood of the Moondogs? I'll have to look again. Do you remember what the part number was for those. Jeff has a couple listed on the Pantheon site. Priced per pair.

Bruce

[Erik Mandaville]

Bruce:

MQ part# BCP16Ni

The Chokes are much, much smaller than the parafeed plate load choke or the 15H filter choke in the Moondog, and really don't take up very much room. For me, they are easily worthy of a couple of extra carefully drilled holes, or you can also find a place along the inner side of the wooden chassis. Two small wood screws will do the trick there. My Moondogs are so non-stock looking inside right now, that I honestly can't remember what the original looked liked in terms of available space. If you want to post or email an under the hood shot, I can give you an idea of where you could mount them on the chassis. Perhaps better yet (if you decided you wanted to try them), test them to see what you think by attaching to the wooden chassis first, and if you find them unsuitable, I'll by them from you at your cost. *hint: for soldering to the terminal lugs on the choke, you might need to very carefully remove some residual varnish, and then just slip the lead through so it makes contact on the lug at the point where it enters on one side and exits on the other. Don't wrap the wire around a bunch of times. It might even be worth soldering with the choke inverted. If excess solder is applied, it will hit the floor instead falling in on the windings. In other words, the chokes don't have leads already attached to them -- you have to do that. 20AWG is fine.

Erik

[Erik Mandaville]

After that you can try the famous.....drum roll.....'ULTRAPATH CONNECTION" Which is much older than the DRDs. Yank out the Solen across the 2A3 cathode bias resistor, put in another with a higher voltage rating (little more capacitance wouldn't be bad), and connect one end to the top of the bias resistor, and instead of grounding the other side, connect to B+. Ultrapath.

And! you can try removing the cathode resistor bypass cap on the input stage -- the little 100uf job. Won't do any harm in the least, but will possibly drop your gain, particularly at lower frequencies -- but this is in theory. FR can sometimes be a casualty, but there are LOTS of people who do this and find it more musical, smooth, etc.

I'm mentioning these as easy modifications where it really is possible to hear the results of what you do -- I did, anyway, and prefer the bypass cap in the circuit.

[Marvel]

I was thinking it was crowded under there, but finally was able to find the pic I took when I first got these. I think you had them for a while didn't you? Didn't you tell me at one time which OPTs are in my pair? Anyway, there is plenty of room. Thanks for the tips, I am likely to try some or all of these.

[Erik Mandaville]

Yeah. They were here -- I improved a couple of connections, and that was all. We talked about the output iron a few times, actually. You have MQ trannies, which, as far as the secondary output connections are a little more straightforward than those from EP. Electra-Print requires combinations of the leads on the secondary windings for the desired output impedance.

FYI: The Wright amps I liked so much also had MQ output transformers. Not all that large in size, huge in sound.

On removing the bypass capacitor on the front end: It's the small red electrolytic toward the left of the image you posted. If you want to take it out, just two clips with your cutters and it's gone. I would take time to remove it properly, and so another can be put in it's place if you prefer it. I've used very expensive Black Gates, Elna Cerafines, and very cheap counterparts of the same value in that same position and they sound identical to me. The grid chokes I don't mind paying for. Cheaper isn't always better, but sometimes it is..........yawn.

Have fun -- check your filter caps with a voltmeter to confirm that they are discharged before doing anything in there. Be sure the amp is unplugged, etc., etc. (I know you will do that). Just to be safe.

Erik

edit: if you want to try to take out that bypass cap (the red one), be sure you don't cut the leads of the cathode resistor that's part of the same connection. Just the leads for the cap. But: Look carefully: you should be able to just remove the extra leads from the capacitor over to the 6SN7 socket, and leave the capacitor in place in case you want to put it back in the circuit. If you want to do this, I can show you the steps. It might be a mod you like.