>GGLL wrote:
>>I've been following this nice discussion, and want to ask something
>>about Ian's post, ¿what's bad with running exciter at a lower level and
>>to have the amplifier "seriously under-driven"? (it seems to be not
>>only a lower power output matter).
>>My modest guess is something related with operating points due to
>>distortion considerations, is it?.
>>
>By "under-driven" I simply mean that you're not utilising the full power
>output capability of the amplifier.
>
>With a good tetrode amplifier - the right tube, the right power supply
>and the right operating conditions - you can obtain low distortion
>right up to the point of grid current, and even beyond that into class
>AB2 if the grid dissipation allows it.
>
>Rich's line of argument is that you under-drive the amplifier in order
>to ensure that it can never be driven into grid current. But he is
>forced into that position because he advocates a high-impedance bias
>supply that cannot handle grid current without changing voltage.
** Not quite. I do not use a regulated bias supply because the tube
manufacturers specify a maximum bias supply impedance for AB1.
Typically, the max Z spec is 100k-ohms. The bias supplies I use have c.
15% of this amount.
http://www.somis.org/D.a.04.GIF
>
>Darn right I'm being stubborn about rejecting that! Not only will such a
>bias supply *cause* IMD if there's any trace of grid current, but also
>it is contrary to tube manufacturers' recommendations. For example,
>Svetlana's recommendations for the 4CX800A are that "for stability, the
>source impedance [of the grid bias supply] should not exceed 1K ohms."
** I read it and they are talking about stability vs, the RF source
impedance of the exciter, not the impedance of the DC grid bias supply.
>
>Simply design the bias supply to have a low impedance, so that it *can*
>handle grid current (in either direction) if necessary. If the bias
>voltage is held constant, there will be no sudden jump in IMD if the
>tube is ever driven into grid current.
** Higher DC grid bias supply impedance means that less grid current is
possible since the more drive applied the greater the negative DC voltage
on the grid. . Does more grid-current equate to less IMD?
>
>Almost all "class AB1" amplifiers actually do nudge into a small amount
>of grid current in order to develop an ALC signal. The tube data sheets
>allow this.
** Eimac/CPI's do not. Svetlana's do not.
>
>
>There is still a need to protect the grid against excessive power
>dissipation due to overdriving. A high-impedance bias supply will
>protect the grid, but it also creates IMD with any trace of grid
>current.
** An unregulated grid bias supply causes a brief reduction in ZSAC
whenever grid-I flows. It would take a steady flow of substantial
overdrive to reduce ZSAC enough to put it into the unlinear region
provided the ZSAC had initally been set correctly with the screen supply.
>We should *all* reject that "solution" because it's unethical -
>you're solving your own problem by dumping IMD onto everyone else.
>
** The amplifier shown at:
http://www.somis.org/pb.top.gif
used a DC grid bias supply with an impedance of c. 15k-ohms. I heard
this amplifier on the air after I sold it and it had minimal IMD when it
was producing 14 out.
>The proper solutions to grid protection are ALC combined with a
>low-impedance bias supply... or else turning the drive power way down
>and under-utilising your amplifier.
>
** Designing the grid-circuit for a specific drive level is one
solution. For example, the amplifier I am currently gathering parts for
needs a grid potential of c. -370V to achieve the needed 0.5A of ZSAC.
The grid will be fed RF through a 9 to 1, trifilar transformer. The grid
will be terminated with 450-ohms. Thus, c. 124V peak from the exciter
will be required to ptoduce 370V at the grid, for a net grid potential of
0V. Thus, according to ohm's-law, the exciter meeds to provide 152W.
Since the trifilar transformer can be expected to have some loss, my
guess is that c. 165W will be need to produce 0V on the grid. OV peak
on the grid may not sound like much until one realizes that the
corresponding anode-I is c. 18A-peak. Of course, even more anode-I will
result if the grid is driven into the +V (Class AB2) region, but the
goals of this project are to hopefully construct an amplifier that has
above average linearity without sacrificing power output, which can be
built with ordinary tools.
cheers
>
>--
>73 from Ian
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