> 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" <j...@quik.com> >> To: "Discussion of precise time and frequency measurement" >> <time-nuts@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 <paulsw...@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 <b...@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. >>>> b...@att.net >>>> K6OQK >>>> >>>> >>>> _______________________________________________ >>>> time-nuts mailing list -- time-nuts@febo.com >>>> To unsubscribe, go to >>>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>>> and follow the instructions there. >>>> >>>> >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@febo.com >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
