[Amps] Novel Screen Grid Regulator

[Carl]

Its IXCP10M90S Jim, sorry about that!
http://ixdev.ixys.com/DataSheet/DS98729A(IXCP-CY10M90S).pdf

Carl


----- Original Message ----- 
From: "Jim Garland" <4cx250b at muohio.edu>
To: <amps at contesting.com>
Sent: Saturday, September 29, 2012 1:25 PM
Subject: Re: [Amps] Novel Screen Grid Regulator


> Thanks for your comments, Paul, Carel, and Carl. I think a zener
> across the + input is a good suggestion, Paul. I hadn't worried about
> protecting the op amp because the 50:1 voltage divider would reduce the
> voltage spike at the input to 8V, assuming the TVS did its job. According 
> to
> the data sheet, the TVS acts in a nS or so and fails in a shorted mode, so
> there shouldn't be a problem with it. Still..... one can't be too 
> cautious.
> And Carl, I can't find any info on the IXCP0M90S. You sure you typed
> the part numbeer correctly? And Carel, that's an interesting circuit, 
> which
> is similar to the Alpha circuit (except for the polarity and use of 
> bipolar
> devices). Thanks for flagging it.
> Now I'm weighing a design decision for the amp as to the best way to
> key it up. Normally, in standby one leaves on the plate voltage and screen
> voltage and applies cutoff bias (for the GU-74B about -150V). When the amp
> is keyed, the bias is reduced to about -50V,to get the proper resting
> current.
> An alternate way is to keep -50V operating bias on the tube at all
> times, and key up the amplifier by keying the screen voltage. I see two
> benefits to the second approach. The first is that the bias circuit is
> simplified, since one only needs a single -50V bias supply. The second
> benefit is that the approach eliminates possible turn-off problems for the
> amp. In my design, when the amp goes off-line a vacuum relay instantly
> interrupts the HV. If I use the first approach, I'd need another very fast
> way to kill the screen voltage at turnoff, because it would otherwise 
> decay
> slowly compared to the HV, thus potentially damaging the tubes.
> The main problem with the second approach (keying the screen
> voltage) is maintaining flashover protection. I'd like to use a fast solid
> state relay to key the screen voltage. Previously I've used the Clare
> PLA-140,  which handles +/- 400V at 250mA and comes in a 6pin DIP, and I
> like it a lot, but it would never stand up to a flashover current pulse.
> There are similar issues with implementing a screen current trip circuit. 
> So
> anyway, I'm still mulling over possibilities....
> 73,
> Jim W8ZR
>
>>
>> Very interesting circuit since the MOSFETs function like a high-power,
>> active voltage divider.  I did note that the non-inverting input to U1a
> may
>> be susceptible to the effects of both the normal screen voltage and
> perhaps
>> a flash from the screen.  However, provided that Z1 acts quickly enough
> and
>> R10 remains intact, then concern is probably minimized.  Still, it may
> make
>> sense to Zener-clamp the non-inverting op-amp input to a level just above
>> the value found in normal operation.  Then, if adding Zener-clamping, I
>> think it would be a good practice to add series R at the non-inverting
> input
>> of 1K.  The input Z to the non-inverting input of the LM358 is quite 
>> high,
>> allowing for some additional series R protection.
>>
>> Paul, W9AC
>>
>> ***********************
>>
>> Jim,
>>
>> I made (and used for my current 160mtrs tetrode th289 amp) almost the 
>> same
>> for grid supply. But will work also good for positive screen supply (use
> NPN
>> and reverse diodes).
>>
>> http://www.pc5m.net/projects/160mtrs-th289
>>
>> 73's Carel.
>>
> ****************************************>
>> Ive been using the IXCP0M90S, good to 900V at 1-100ma. Keeps those
> sensitive
>> screen grids safe from over current or collateral damage when there is an
>> internal failure.
>>
>> It does keep 8122's happy in my case.
>>
>> Carl
>
>
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