[time-nuts] Nuvistors and FETs...
lists at rtty.us
Mon May 3 12:01:07 UTC 2010
Last time I went down that road, finding a FET with *low* enough Gm to match the tube was the issue. Some circuits are fine with 10X the gain others not so much so. The one thing that helps you is that the output conductance of the tube is generally higher than the FET's so you get a bit of a swamping effect. It's a bit easier if you are looking at swapping triodes than swapping pentodes.
Capacitance wise, you can find things going either way. The higher gain will boost your miller capacitance, Things like sockets and wire likely contribute as much as some of the tube capacitances. Fiddling is going to be the order of the day if you want to get a broad band circuit to track.
You see a lot of circuits based on now extinct mosfets for doing swap outs. If if you have a stash of those parts, then it's going to be much simpler.
On May 3, 2010, at 1:28 AM, Don Latham wrote:
> You can also look up the gm of the nuvistors and get a reasonable match to a fet as well.
> ----- Original Message ----- From: "J. Forster" <jfor at quik.com>
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Sunday, May 02, 2010 9:23 PM
> Subject: Re: [time-nuts] Nuvistors and FETs...
>> Interesting idea. In fact, the later versions of the loop used FETs in
>> place of Nuvistors. I've not compared the circuits.
>> Because the thing is only 60 KHz, the change in grid/gate capacitance may
>> not matter much. There may be some mounting and stability issues though
>> because the plate caps were on the opposite side of a metal shield from
>> the grids.
>>> For what it may be worth, I've swapped Nuvistors out with FETS. I
>>> see from the schematic that the Nuvistors are running on 35
>>> Volts. It would be simple to drop the 35 Volts to 12 or 8 Volts with
>>> a 3 terminal regulator in the receiver. If desperation sets in, try
>>> stuffing N-Channel FETs in place of the Nuvistors. The gate is the
>>> grid, the drain is the plate, and the source is the cathouse. Since
>>> the FETs are usually pretty low in capacitance you may need to shunt
>>> the tuned circuit/s with a small piston type capacitor. In one low
>>> Voltage preamp circuit I used a National U-310. In another one, a
>>> Nems Clark receiver, I replaced the two RF front end ceramic triodes
>>> stages that ran on 150 Volts with two U-310's and rewired the plate
>>> supply to run off of the 12 Volts already in the receiver. This made
>>> for a low noise, very sensitive front end. All I needed to do was
>>> add a small quartz piston capacitor to bring the tracking in at the
>>> high end and the bottom end fell right end. If you can make that
>>> work, you'll never have to replace those pesky wittle Nuvistors again.
>>> Burt, K6OQK
>>>> From: paul swed <paulswedb at gmail.com>
>>>> Subject: Re: [time-nuts] 6CW4 Nuvistors on eBay...
>>>> Indeed there are.
>>>> And even 7587s thats the TRF frontend. In one of the units V3 appears bad
>>>> all Rs are good voltage high on plate, 0 cathode current. But oddly when
>>>> swap v1 into position same effect so somethings odd. Or my methods not
>>>> The other units working. Had to replace a shorted 6.2 V zener.
>>>> The 100 kc vco /lock osc is awful touchy.
>>>> Pretty amazing the units pretty hot sensitivity wise -80 dbm.
>>>> Interesting afternoon. Way in the deep corner of the basement I think I
>>>> have a nuvistor or two. Time to see what they are.
>>>> On Sun, May 2, 2010 at 4:14 PM, Burt I. Weiner <biwa at att.net> wrote:
>>>> > For what it's worth, I just took a look and found a bunch of 6CW4's on
>>>> > eBay. See:
>>> Burt I. Weiner Associates
>>> Broadcast Technical Services
>>> Glendale, California U.S.A.
>>> biwa at att.net
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