Stereomour w/ 2.5V 300B's

[Dr. Toobz]

I've put off building a Stereomour for the better part of a year, but am itching to build again this summer. Plus, my S.E.X. amp may find a new home with my folks in their library/study.

Obviously, there's plenty of reasons why I'm looking into the Stereomour and not a push-pull amp or even a single-ended pentode (SEP), which would certainly be easier, cheaper, and put out more wattage. I do, however, want to build something relatively "future proof" this go around. I like the Stereomour for the reasonable price, the fact that it is integrated and therefore, doesn't really need a separate pre-amp, etc. I don't want monoblocks. Now, the Stereomour configured with 2A3's would put out 3.5W of clean power; I want to aim for 8W, just to have enough headroom for use in a bigger space. Seeing as how a few companies make 2.5V variants of the normally 5V 300B, is there any possibility of modifying the Stereomour to accommodate one of these tubes to full power? What is the B+ in the Stereomour? Lots of people seem to run 300B's at 350V/60mA, though I've seen higher voltages used as well. I'm not sure what the Stereomour power supply provides in its stock form.

Another possible benefit is that the lower filament voltage should mean less hum than a typical 5V 300B heater circuit, and I like the option of sticking with AC heating. Most people seem to think that a typical 300B has too much hum when the filaments are run on AC, so DC heating is used. Yet, AC heating has a huge following amongst people running lower-wattage tubes like 45's or 2A3's, where the lower filament voltage seems to couple less hum into the audio.

I remember Paul Joppa had talked about looking into this last fall. Any thoughts or progress? I would be more inclined to build a Stereomour if I knew there was the option to "beef it up" in case I need more power versus have to build another amp or use monoblocks. Perhaps I'd even just like to hear what 300B's sound like in relation to 2A3's. Though the easy route to 8W would be to build something like a SEP or ultra-linear amp using 6550's or EL34's, I think I would enjoy the DHT SET sound a lot more on my speakers, if the S.E.X. amp is a good indication of that flavor.

[Paul Joppa]

In the stock Stereomour (or any other 2A3 amp for that matter) a 2.5v 300B will perform almost identically to a 2A3. There's a good chance the tube will last longer, but no other difference.

The Stereomour power supply provides about 360 volts at 50mA per tube. Of that, 60v is used for bias, leaving 300v plate to cathode. That's how the 2A3 is operated in this amp. The 45 requires a bit less voltage, and quite a bit less current, so the power supply can be adapted to operate a 45 without strain - so we provided that operation.

All the options for greater power with 2.5v 300Bs require more current, and hence higher temperatures (especially the power transformer). At a rough estimate, the temperature rise would be at least twice that of the Stereomour. Most also require a different power supply design which I have not tried yet and cannot confirm will work.

Bottom line, we'll keep looking but right now I can't say that a Stereomour can ever be modified to give more than the stock 3.5 watts.

[Dr. Toobz]

Thank you for the reply.

I can do without the 300B's, I guess. Since the Paramounts are out of my price range, and would require a pre-amp, I'm just going to stick to the 2A3 Stereomour and worry about finding more headroom if/when the need ever arises. On my 99dBW speakers, I don't foresee there being much problem with 3.5W, especially considering the Stereomour's ability to provide more power at the frequency extremes. I could always build another Stereomour and bridge the channels on each one to mono if more power was needed, correct? (Or, even rig up some sort of concertina splitter with the input tube and modify the two 2A3's to become a push-pull arrangement with a different OPT).

Now the only problem will be how to manage a Seduction, CD player, Crack (to make up for the lack of a headphone output), and Stereomour on the same rack. The Stereomour doesn't look to have pre-outs to go to the Crack, so I would lose the benefit of having three inputs by needing to use a box with two outputs prior to entering the Stereomour. Right now, I don't have to worry so much about this, as the S.E.X. also has a headphone jack.

[Jim R.]

Hi Angelo,

Looks like with two stereomours you are spending the same as a pair of paramounts, no?  Anyway, I can tell you that 3.5 watts on my 100 db Cornwalls is plenty, and as Paul said, it hardly seems worth the extra couple db to go to 8 watts.

As for your input/pre-out situation, you can try this: just connect two of the inputs to the selector switch and/or get another 2 position selector switch, and then run the outputs of the switch both to the volume pot, and to a dpdt switch back to the third rcas.  The third rca will then be a pass-through when the dpdt switch is switched to the correct position.  The switch should be disconnected when the stereomour is being used, and this will keep the load impedance of the cables and crack from loading the inputs to the volume pot on the stereomour.  Of course this assumes that you wouldn't be using the crack and stereomour at the same time.

One of my amps is configured this way and it works very well.

Something to think about anyway.

-- Jim