[time-nuts] Nuvistors and FETs...

J. Forster jfor at quik.com
Mon May 3 15:43:21 UTC 2010


> Hi
>
> 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.

In the 117A the first three stages are in individual shielded compartments
of a box. The cathode and grids are on one side of the shield, the plate
is on the other side...  sort of a "feed through" tube. I suspect the
three stage 60 KHz amp has a lot of gain and likel stability issues.



> Fiddling is going to be the order of the day if you want to get a broad
> band circuit to track.

Fiddling, for sure! But the circuit is not broad band. I doubt it's more
than a KHz wide, although I've not seen it for years. It is only
interested is essentially a 60 KHz CW signal.

-John

====================

> 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.
>
> Bob
>
>
> 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.
>> Don
>>
>> ----- 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.
>>>
>>> -John
>>>
>>> ================
>>>
>>>> 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
>>>>> I
>>>>> swap v1 into position same effect so somethings odd. Or my methods
>>>>> not
>>>>> correct.
>>>>> 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
>>>>> may
>>>>> 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
>>>> K6OQK
>>>>
>>>>
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>>>
>>>
>>>
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