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In Reply to: RE: Need some opinions and maybe a lot of help with 300B design. posted by hifipaul on March 14, 2016 at 19:35:11
What a dumb mistake on my part. What makes it even worse is that Edcor does not make a transformer that goes above 900v ct. HHHHMMMMMM.
Follow Ups:
It's not so bad, just increase that 10uf input capacitor to say 40 or 50uF. Download PSUD and play around to get the right voltage level, you're on the right track. If you wanted a choke input power supply you would need a higher voltage transforner but considering this is a class "a" amp, which has little variation in current demand from zero to full load, the better voltage regulation of a choke input supply doesn't offer any real advantage and then you may have to deal with a buzzing choke.Also, I have a couple other comments on the power supply
It's kind of arbitrary to go dual mono at the first choke - why not either go full mono with a separate power supply for each channel,l (considering the amount of iron in this amp, there's little reason not to) or just use a single power supply for both sides.
One more comment: layout is going to be really important here, there is tons of opportunity to pick up hum with all those transformers, so to be fool proof go for a big chassis with lots of space between the power supply and the amplifier transformers, and be careful not to make any ground loops.
Another thing I have to ask: what's the intention of the interstate transformer between the first two stages? It's a 1:1 transforner and isn't isolating DC due to the 1uf coupling capacitor.. I think it's a redundant component, couldn't you just use a grid reaistor on the second tube and cut that transformer out of the circuit ? Speaking of which, why use the choke on the first tube instead of a resistor for the plate load? It's not as though this stage needs to produce the extra output voltage a choke will permit, and it will introduce a low frequency pole which won't help the overall performance of the amplifier. Also, those coupling capacitors are way bigger than they need to be.. I didn't calculate values but I would expect 0,1uf - 0,47 to be absolutely fine, and still allow the transformers to dominate the low frequency roll off.
One final comment: a single ended 6L6 will make about the same amount of power as this amplifier, if what you're after is more "grunt" why not try say a 15 watt push-pull amplifier, I don't think 20% more or less power will make a material difference, you may want substantially more for the "grunt" you're after.
Edits: 03/14/16 03/14/16 03/14/16 03/14/16
Thank you for your reply. I am beginning to rethink my plan of attack here. My funding is limited and this amp, even with the parts I already have on hand is going to be a killer on the wallet. I am considering a different design using different tubes. I really don't like the way PP amps sound so I may build another SE EL34 amplifier with just the 396A. I know what you mean about layout. I was going through what I would have to do in my head and it was giving me a headache.
Another thing to consider is using a well known design as a starting point, for example the JE labs circuit, it's more or less the most ubiquitous entry level 300B amplifier on the 'net, and won't cause any major headaches during construction. If you decide it's enough power you could later modify it with fancy stuff like plate load chokes and interstage transformers. But this will be a quick path to a proven amp design and will also let you know if it's enough power without breaking the bank.
Edits: 03/16/16 03/16/16
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