2A3 mode for my 45 SET

This little project got started after the experience of switching from my 45 SET to a 300B amp for an extended period and coming to the realization that the 1.5w of the 45 is just not cutting it on my 100db eff. Altecs. I still LOVE the 45, it’s my favorite directly heated triode so far - within it’s power limits - but I wanted to try the more powerful 2A3 (3.5w! ) in the same amp.

Pretty simple change really but I wanted to easily go back to 45’s for certain types of music or when using more efficient speakers so decided against semi-permanent changes. I added an under chassis switch in each channel that changes cathode resistance to 750R and switches the taps on the output transformer to 2.5K. In went a very fresh pair of OS RCA black-plate 2A3’s and I prepared to be under whelmed (I was not a big 2A3 fan previously) but damn, this is nice!

The additional power makes a world of difference – especially in the bass – and the 2A3 strength of even balance and accuracy is there without the sense of blandness and clinical sound I didn’t care for in the past. I still prefer the midrange of a 300B Western Electric tube but have to think it’s probably bigger and silkier than life, the 2A3 seems to be a cleaner window onto the music without all the syrup (but damn I like that syrup).

The fact that previous 2A3 amps left me cold (literally) may have just been parts mix and execution as opposed to the sound of this tube in general. These RCA’s driven by the Western Electric 417A’s, James iron and the Jupiter coupling caps mixed with the other bits I used on this amp seem to be getting along very well and are better suited to the Altec 604’s in this room than the 45. I’ll be building slightly more efficient Altecs this winter (all horn loaded, maybe 103db eff.) and will certainly go back to the 45 then but for now, the 2A3 is the sweet spot.

Nice switch there. 2A3's are my main drive in my vinyl rig pushing my A7's. I think they are around 104dB efficient.

swapping power tubes.

You guy's really make me jelous, all I have is a 300B ity:

I've never really looked into trying to do anything to mine to try and use a 45
or 2a3, is there a general way of doing so? can any amp do this?

Or do only certain amps have this capabability without doing some drastic mods?

Anything you need to know about your amp to do this?

I know some amps are supposed to be 'convertible' between 300B and 2A3/45 but I believe it would be sort of complicated - the filament and high voltage requirements for the 2A3/45 are the same but not so for the 300B. In addition to this, I don't believe a driver stage like the 417A of my 2A3/45 would work optimally with a 300B. The 300B I mentioned in this comparison was a different amp.

The 45/2A3 conversion is pretty easy if the output trans has separate taps for 2k5 and 5k load and the cathode loading is just a resistor change.

A 2A3 is just 2, 45's in parallel in same tube envelope.

Allright thanks Billfort now I can get that out of my mind, don't need anymore projects!

Bill,

How long does it take for the little one to grow up to be like its daddy in the background?

Cream on the cake

I was sharing my thoughts with a friend on how much I was enjoying the 2A3 conversion and mentioned that I still had a slight reservation with this tube in the midrange – I still preferred the sweetness of the 300B and 45 there.

He suggested I swap out the Jupiter coupling caps (aluminum foil/beeswax) for AudioNote (copper foil/paper-in-oil) caps to see if it changed things for the better.

Well, he nailed it – the copper-in-oil caps added a sweet creaminess to the mids taking the slight edge and ‘hi-fi’ etch off some recordings – especially of the digital nature. Buddy Guy’s slide playing on Folk Singer, Jim Hall’s Gibson ES and tube head tone, Art Pepper’s alto and Allison Krauss’ sweet vocals come across with a flesh and blood presence in the room that can be mesmerizing. The 2A3 in this amp and with these caps had me in that zone where all this audio stuff becomes worth it and the music carries you away.

I’d say the Jupiters had the edge in the neutrality, speed and accuracy areas but the oil caps just pulled the midrange syrup and music out of the 2A3 that I was missing. For me, the Jupiters gel nicely with the 45 and 300B, the AudioNotes best with the 2A3.

I find it pretty cool the way you can play with tone and flavoring in simple circuits like this just by changing key pieces like tubes and coupling caps. Of course this is all in context of a system that suits my taste and has components that get along well with each other (and the room) but this kind of fine tuning and experimenting really drives home for me how a little DIY can help audio become more than just the ‘gear’.

Bill I've been saying this for 2 years now!

"I’d say the Jupiters had the edge in the neutrality, speed and accuracy areas but the oil caps just pulled the midrange syrup and music"

While many others shun the use of PIO caps, I absolutly love them and they are getting
a pretty big resurgance in the mod comunity to where many companies are now making new
ones of all different type materials but still using the oil as the basis but I do prefer paper in oil!

I spent a whole year listening to caps on 1 speaker in mono, I know overkill but I wanted
to hear for myself. Also I agree tubes and caps makes it really cool to get the sound you want.

I use them in speaker xovers to or bypass caps for the electrolytics in the power supllies,
even solid state equipment benefit from there use. GOOD ****!

Gary Kaufman did a convertible 45/46/300B amp a few years back with Electraprint iron. He went a bit over the top with the P/S design but the basic amplifier was pretty straightforward in concept and execution.
http://www.the-planet.org/Ragtop.html





It sounds good too.

FWIW, the 45 and 46 are awfully fine sounding tubes... I like and use 2A3's, but every 300B amp I've heard (ranging over a pretty wide variety of topologies) struck me as sort of 'hi-fi' sounding. Not bad, but with out that lush, liquid MR that the lower-output DHT's seem to provide.

Interesting how personal preference and I suppose experience with different topologies can lead to different conclusions mrh. It seems our assessment of the sound of 2A3 vs. 300B - at least the Western Electric 300B - are polar opposites.

Actually, the reason I posted this thread about the 2A3 was that it completely surprised me when used in a 45 amp I liked so much. I have never cared for the 2A3 and always found IT to be 'hi-fi' sounding and lacking in midrange magic but not so now - if I leave out the 45 due to not enough power, I may just prefer the 2A3 to 300B.

This was a nice slap in the back of the head to remind myself to keep an open mind. I no longer have any qualms about gutting my 300B amps and trying something completely different, it just might surpass this 2A3.

So much more to learn, onwards and upwards. This sure can be a fun hobby when glimmers of success mean bringing the music home a little more each time.

Iron and topology can make a difference. Heck, even passives can make a difference in those ultra simple circuits sans feedback! The neat thing about Gary's amp is that you can compare 45's, 46's, and 300B's under conditions where almost everything else is the same. It's a fine sounding amp, too... but I didn't like the 300B's as much as the low power tubes in it. I don't recall what flavor 300B's he was using, but knowing Gary, they were probably OS WE's.

Tweking does help

I have a diiferent audio tube setup I modified. --- I have an older(1950) zenith tube radio people collect for its style/early FM. I modified tone control to have more bass. I did this to take advantage of full fidelity of FM. I further restored set and connected to modern speaker with nice subwoofer for better frequency response. My wife wont let me part with set as sounds great rivaling many stereos at store yethas warm tube sound. Yeah, when you make something better, it's more than just gear!!!!

:thmbsp: