When a tube grid is driven in such a manner that the grid starts to draw current, & if it is using a fixed bias supply, will the tube grid (try to) draw the current from the bias supply?
If so, what effect will this have on the driver?
If the current is available from the bias supply, will the driver tube only need to swing the voltage?
If so, what effect will this have on the driver?
If the current is available from the bias supply, will the driver tube only need to swing the voltage?
Like Tweeker said, it depends on how the driver is coupled. The grid of an A2 output stage will draw current when it goes positive.
If the stage is driven by a follower (tube, mosfet or bipolar) the current will be supplied by the positive supply connected to the plate (drain, collector) of the follower. The negative (bias) supply does not necessarily need to posess a low impedance since no signal current flows through it.
If the stage is coupled via an interstage transformer all of the grid current is supplied by the driver tube. In most cases the driver transformer secondary is connected to the output tube grid. The other lead is connected to the bias supply. The signal current flows through the bias supply (or its bypass capacitors). The bias supply must have a low (and constant) impedance for all audio frequencies.
Since the bias supply is negative, and the grid will only draw current when it is positive the current does not come from the bias supply directly.
If the stage is driven by a follower (tube, mosfet or bipolar) the current will be supplied by the positive supply connected to the plate (drain, collector) of the follower. The negative (bias) supply does not necessarily need to posess a low impedance since no signal current flows through it.
If the stage is coupled via an interstage transformer all of the grid current is supplied by the driver tube. In most cases the driver transformer secondary is connected to the output tube grid. The other lead is connected to the bias supply. The signal current flows through the bias supply (or its bypass capacitors). The bias supply must have a low (and constant) impedance for all audio frequencies.
Since the bias supply is negative, and the grid will only draw current when it is positive the current does not come from the bias supply directly.
If the driver is supplying the voltage, it must supply the current too. That's the definition of any voltage source.
(sorry, just saw George's response)
(sorry, just saw George's response)
How about positive bias supply?
Like Shishido's 801 & 811 designs.
They are transformer coupled, with 317-based voltage regulator to provide the positive bias, about 20-some or 30-some V.
Like Shishido's 801 & 811 designs.
They are transformer coupled, with 317-based voltage regulator to provide the positive bias, about 20-some or 30-some V.
CLS said:
Same deal: the driver supplies the extra grid current, and the positive bias supply is part of the signal's AC return. Therefore, it still needs good regulation and the lowest possible AC impedance. 317-based voltage regulators can manage that.
What operation points does he use?
What driver config does he use?
Is there any special requirements from the OPT?
Will A2 work in Push Pull?
What driver config does he use?
Is there any special requirements from the OPT?
Will A2 work in Push Pull?
A2 could be done in pushpull, but itd be a kinda weird operating point to choose. PP AB2 and B2 is common in modulators.
The 811 was intended for 0 bias class B use, it can be used in class A by using positive bias or other adjustments. Nothing special for the OPT. No idea on the driver.
The 811 was intended for 0 bias class B use, it can be used in class A by using positive bias or other adjustments. Nothing special for the OPT. No idea on the driver.
look at the Altec Lansing PA using a pp of 811 . There was a very interesting study in Sound Pratices . The PP driver is a PP of 6W6 cathode followered with a beefy centertap choke ... 170watts of pure class B but zero biased . Get the B+ low and you have a wonderful A2 PP DHT amp !😉
I've been very curious about the inductance of the OPT of these class A2 SE amps.
They tend to be very low load impedance. For example, as the very high Ra of 811A, if we obey the 3*Ra rule & proper inductance for LF response, turn numbers/impedance of the primary should be very very high.
However, in Shishido's design, the primary of OPT it's only 3.5k. On 801, it's even lower, only 1.5k!! I know it's for positioning the load lines properly among the curves to get linearity & symmetrical swing.
But how do they overcome the dilemma of turns ratio/inductance/impedance ???
How do they perform on the bottom octave, actually?
They tend to be very low load impedance. For example, as the very high Ra of 811A, if we obey the 3*Ra rule & proper inductance for LF response, turn numbers/impedance of the primary should be very very high.
However, in Shishido's design, the primary of OPT it's only 3.5k. On 801, it's even lower, only 1.5k!! I know it's for positioning the load lines properly among the curves to get linearity & symmetrical swing.
But how do they overcome the dilemma of turns ratio/inductance/impedance ???
How do they perform on the bottom octave, actually?
Inductance for LF transformer response vs impedance issues ease as impedance/turns ratio drops. Whether the tubes like such a heavy load is another question.
On the Shishido 801, I saw both global NFB & CFB on output tube...
However, high Ra & not enough inductance (due to low impedance/turns ratio) would not be any good on bass response.
However, high Ra & not enough inductance (due to low impedance/turns ratio) would not be any good on bass response.
Thanks a lot.
So the last sentence answers my question.
I'd better go back to the books & stop posting stupid thing like this😱
Thanks again.🙂
So the last sentence answers my question.
I'd better go back to the books & stop posting stupid thing like this😱
Thanks again.🙂
I think you may have been confusing turns ratio with total number of turns. Both sides of the xfmr need more turns for more inductance. Total turns goes sky high for a high impedance high inductance SE OPT. This leads to a loss in HF performance due to interwinding capacitance and leakage inductance.
CLS said:
I use the Shishido 801A design as my day to day amp. It does indeed have both CFB and global NFB to lower the output impedance. However, I disconnected the global NFB loop because I felt it sounded better. The bass did get somewhat "looser" though.
As far as transformer primary inductance goes, there only needs to be enough to support the desired load (in this case 1.5K). I know it's weird having a load that is less than the Ra of the tube, but if you look at the A2 curves for the 801A, that is what's required. Operating point is Va=220V, Ia=90mA, Vg=+24V.
Cheers,
Pete
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