Thanx. I was just just curious. I built a GM-70 that was in that neighborhood. I used a B+ around 1240V, and got 34W @5% without hitting A2.
I never was a huge fan of the 845, but then I've only had the Shuguang to play with.
Not to barge in – I'll let tubelab respond to this as well, but I might save him some typing. Having a "bigger" (I assume you mean wattage capacity) transformer will not affect your output power – (assuming you already had a transformer that was able to handle the quiescent current of the ouput tube). That's a function of the output tube's power, and the slope of the "loadline" – (the rate of current change in the output tube vs. the change in voltage; a steeper loadline results in more power for the same amount of voltage swing, but more distortion as well).
Remember, I said earlier:
Well, having a larger input impedance will improve your damping factor, but it will also add inductance to the signal path, and reduce your potential output power. What you're essentially doing is creating a "shallower" loadline. More inductance in your output transfomer's primary equals less high-frequency extension, but tighter bass, and lower distortion. It may or may not make a difference to your ear, but there it is. Conversely, having a lower input impedance on your OPT allows greater HF extension and – (given the same voltage swing from the driver stage) – more output power, but at a lower damping factor, and at higher distortion. (Remember how I said everything in audio engineering is a compromise between two evils? This is just another example of it). Also, some (output) tubes are particularly sensitive to load (the input impedance of the transformer it's hooked to), others are less so. But again, the higher your OPT's "reflected impedance" – (I'll explain that in a moment) – the less distortion, the greater the damping, and the lower the output power. There may be some benefit, in your case, to having multiple secondaries – say, a 4Ω and an 8Ω out. Normally, I don't recommend this, as you are adding extra turns (or extra taps) to the secondary coil, and raising the inductance (but not on the input side, where it could have more benefit). But here's the thing: A transformer with an 10K primary and an 8Ω secondary, when hooked to a 4Ω speaker, now has (essentially) a 5K primary. This is what's meant by reflected impedance. Transformers don't have a "set" impedance on any given coil; they only have a primary impedance in relation to the secondary. Drop the load on the secondary in half, the primary's reflected impedance is cut in half. This is why I raise this point. Regardless of what primary impedance you choose (assuming you even choose it – I thought you were originally going to get a kit?) for your output transformer (BTW, 3 to 5 times the output tube's impedance is a good range, but for extreme damping factors, you might go as high 8 to 10 times), having a pair of secondary taps will let you match your speaker's impedance so that the primary impedance is that what it's supposed to be, and lets you "play with" a second (or third) tap that essentially makes the OPT's input impedance half (or double) what it was (in terms of reflected impedance), thus letting you hear the difference. I think that will tell you more about damping and the transformer's affect on it, than any amount of further explanation I (or anyone) can give you
You may find that running an 8Ω speaker on a 4Ω tap (thus doubling the OPT's reflected input impedance) gives you the extra damping you like and not mind the loss of some power. Conversely, you might try a 4Ω speaker on an 8Ω tap and find the damping is still not an issue, and like the increase in power. Either way, I think you should hear it. You might be surprise at how little (or how much) difference damping makes in relation to a given speaker.
Some speakers must have high damping or they sound crappy. Others could care less. But you need to hear for yourself.
To assume you need to implement negative feedback to improve damping, before knowing what effect your OPT's damping has, isn't a good idea. I find (even small amounts) of NFB to be more audible than changes in damping are in most cases. I really think you're starting to become over-concerned with damping factor. It does make a difference, but honestly, using a higher input impedance on your output transformer will probably produce more of an audible effect on distortion than bass – either way, it's a good idea (if you don't mind sacrificing a little power). I think if you listen first to a SET with a 4/8/16Ω-tapped secondary and try the different taps, you'll see what I mean. I don't think you're going to want to implement negative feedback (which kind of defeats the purpose of going SET to begin with). Just my 2¢
I use SE transformers that are rated for more power than I am designing for because it gives me an extension of the low frequency f3 and less distortion at low frequency.
I assume you mean rated for current than you're designing for; "power" rating – (at least in terms of wattage) – won't affect bass headroom in and of itself. But a trafo having an F3 of, say, 30Hz at x milliamps would have an F3 lower than 30Hz at less-than-x milliamps, so....
(Watch out though – go too big and you'll jack up the inductance too).
Personal anecdote: My GM-70 SET is on its third trafo. I started with a 100mA core – the amp sits at about 95mA – and upgraded to a 120mA core, which did help the bass. I noticed a bigger difference, however, going from the 7K to a 10K primary. Of course, there again, it's always a fine line – too high an impedance and you start to lose highs to all those extra mH (and of course you get less output power from the same driver voltage).
Can I say a big thanks to all who have posted on this thread - there is so much useful information here that I think it worthy of being a "sticky" thread for all those who would have similar questions in the future🙂
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