Hi,
Can anyone help tweak a 6SN7 300B SE amp from China?
The rectifier tube will be 5U4G (44 Volt drop)
VCC is 320VAC-0-320VAC (PCB just have 1 unit 47uF 500V to ground) and feed about 406VDC to circuit.
This board needs "matched pair 300B".
Can we modified this board to use with unmatched pair?
Can anyone help tweak a 6SN7 300B SE amp from China?
The rectifier tube will be 5U4G (44 Volt drop)
VCC is 320VAC-0-320VAC (PCB just have 1 unit 47uF 500V to ground) and feed about 406VDC to circuit.
This board needs "matched pair 300B".
Can we modified this board to use with unmatched pair?
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Well--There's Lots of things that could be done, but as its a PCB, there's little re-design options available other than component value changes.
There's no requirement to use matched tubes (300B) as they are Not in parallel or used in push-pull, any tiny differences between a pair of good tubes will never be noticed.
Sadly, I personally think the scheme supplied sucks,--Smacks of Chinese Cheap-n-Cheerful, prolly not a great performer, That gain-stage/SRPP seems to be the Generic one found in 90% of all Chinese amps these days, maybe the valve will be an Octal as in your case or a 9-pin, but the rest is identical to that Yaqin P-P EL84 amp I have--that I severely modded.....
I would dump the boards completely, redesign using maybe gyrator loaded input stage or Mu stage driving MOSFET Follower and Fixed-Bias 300B, none of which you can easily do/change to with a PCB that you have.
Seeing its all rather simple, a P2P would be a reasonable option, then many different things can easily be tried.... If this is a complete amplifier--at least you have all the parts/iron etc, therefore the ingredients are (Mostly) there to come up with something half tidy.
Then again--Thats just my opinion.
There's no requirement to use matched tubes (300B) as they are Not in parallel or used in push-pull, any tiny differences between a pair of good tubes will never be noticed.
Sadly, I personally think the scheme supplied sucks,--Smacks of Chinese Cheap-n-Cheerful, prolly not a great performer, That gain-stage/SRPP seems to be the Generic one found in 90% of all Chinese amps these days, maybe the valve will be an Octal as in your case or a 9-pin, but the rest is identical to that Yaqin P-P EL84 amp I have--that I severely modded.....
I would dump the boards completely, redesign using maybe gyrator loaded input stage or Mu stage driving MOSFET Follower and Fixed-Bias 300B, none of which you can easily do/change to with a PCB that you have.
Seeing its all rather simple, a P2P would be a reasonable option, then many different things can easily be tried.... If this is a complete amplifier--at least you have all the parts/iron etc, therefore the ingredients are (Mostly) there to come up with something half tidy.
Then again--Thats just my opinion.
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22K seems awfully low for input shunt resistors. Unless you're using an SS preamp, I suggest raising them to at least 100K.
Perhaps the strongest reason to use matched tube pairs in single-ended amplifiers without feedback is to equalize left/right channel gain. If your preamp has a balance control, then why worry?
Perhaps the strongest reason to use matched tube pairs in single-ended amplifiers without feedback is to equalize left/right channel gain. If your preamp has a balance control, then why worry?
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Err--No, as I wouldn't use 6SN7 as gain-stage for 300B! The 'SN7 has theoretical max gain of around 35, so this isn't a great choice for the gain stage at all IMHO. Guess you could use it as cathode-follower, but then you still need the gain of 60 odd...
To drive the 300B to maximum, running 300V plate in fixed-bias with zero fb you would need a gain of around 60-70 at least in the preceeding gain stage, for an input sensitivity of 1V.
Add fb into the equation--you'll need more gain The SN7 just wont give it in a single stage. I guess you could use both halves in grounded cathode but then you'll have a three stage amp instead of two, with all the attendant issues that go with that......
Personally I wouldn't use 5U4G as I prefer solid-state rectification,-- Silicon used in moderation--for the jobs its best at is beneficial, only the uninitiated think its not IMHO....
My advice--Look on web for 6SL7 drive to 300B (Classic rather boring and done-to-death scheme, there's been a million done) but add MOSFET Follower driver between SL7 to the 300B to tame the miller, and bring a boring amp to life. Wire 300B for fixed bias 300-350V plate, and 55-60mA Ip, settable via MOSFET follower...
--Again, Use the best choice of Components For What They Are Actually Doing.
(I don't mean here Daft Audiophool Mega-Bux Resistors blessed by virgins at midnight on the winter solstice and Gold Plated Caps wound in unicorn skin and double-overhead-doughnuts!
There's Loads of this crap around that are just cash-cows for their makers--Don't get suckered in, these audiophool things all work on the Placebo Effect, have little if no sonic benefit, and just severely lighten your wallet!...
Good Quality Industrial parts are every bit as good --probably better, Certainly cheaper allowing you to spend the dosh on things that DO make an audible and measurable difference like the Iron.)
MOSFETS make Superb and completely transparent Followers, negating the negative effects of driving the triode O/P tube such as Miller. Silicon Diodes excellent rectifiers, and transistors excellent regulators etc, Even the humble LED is good for low level biassing duties, don't be afraid to use 'em...
Triodes such as the 6SL7 the best voltage amps, Power-Triodes like 2A3, 300B etc the best outputs, so that's where we use 'em...
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