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Substitution of 3-500Z with 4-250 or 4-400?

mfle2001 All Messages By This Member #39270 It has been suggested to me that a 4-250 or 4-400 could be used in a grounded grid amp that is designed to use a 3-500Z. I wonder if anyone here has experience of doing this as I am in the process of acquiring a home brew amp that originally used 2 x 3-500Z. I see the base and filament requirements are the same but can the grids all be strapped together and directly grounded? What would be the bias requirements for ssb operation and cutoff in Rx to achieve by taking the filament CT above ground? Mike. Mike Fleetwood Canberra Australia and Sidcup UK. Worldwide email address is: mfleetwood@...
Jim VE7RF All Messages By This Member #39271 It's done all the time.? DON'T strap the grids together.? That's? BBI video fubar.? ?Run a separate strap from each grid pin to chassis.? 6 x straps in total.? ?Drill 6 x holes in chassis, if needed. Don't bond the grid pins to the aluminum standoffs that are used to sub mount the white Johnson style sockets either.? ?You will end up with an int every time.?? ? Cut off bias is the same, just use a 3 watt mof resistor in the CT, like anything from 10k to 100k.? ?Shunted out on TX via relay contacts.? ?Diss in that resistor is almost zero watts on RX..and is zero watts on? TX.? ? Depending on B+ used, operating bias is way less vs a 3-500Z.? ?Either no bias, or just 2-3 volts.? Use a few 1N5408 diodes in series for bias.? ?100 ma of idle current is ample for any 3-500Z / 3-400Z / 4-400 A / B / C? You will get a little less PO with the 4-400's.? They need a bit more drive vs the triodes.? Jim? ?VE7RF
mfle2001 All Messages By This Member #39273 Thanks Jim that all makes sense to me, will give it a go as I have several 4-400s. Mike VK2AMF. At 02:18 PM 8/04/2023, you wrote: toggle quoted message Show quoted text It's done all the time.? DON'T strap the grids together.? That's? BBI video fubar.?? Run a separate strap from each grid pin to chassis.? 6 x straps in total.?? Drill 6 x holes in chassis, if needed. Don't bond the grid pins to the aluminum standoffs that are used to sub mount the white Johnson style sockets either.?? You will end up with an int every time.? ? Cut off bias is the same, just use a 3 watt mof resistor in the CT, like anything from 10k to 100k.?? Shunted out on TX via relay contacts.?? Diss in that resistor is almost zero watts on RX..and is zero watts on? TX.?? Depending on B+ used, operating bias is way less vs a 3-500Z.?? Either no bias, or just 2-3 volts.? Use a few 1N5408 diodes in series for bias.?? 100 ma of idle current is ample for any 3-500Z / 3-400Z / 4-400 A / B / C You will get a little less PO with the 4-400's.? They need a bit more drive vs the triodes. Jim?? VE7RF Mike Fleetwood Canberra Australia and Sidcup UK. Worldwide email address is:????? mfleetwood@...
Louis Parascondola All Messages By This Member #39275 The TL922 should be changed from forced bias to self bias if it hasent already been done.. This is easy to do. ?If using 4-400’s make sure to ground all the grids direct to chassis. ?Since 4-400’s don’t need any operating bias at less than 4000v in GG you should run a jumper across the zener to short it out. ?The resulting ZSAC with no bias should be fine. toggle quoted message Show quoted text On Friday, April 7, 2023, 10:50 PM, mfle2001 <mfleetwood@...> wrote: It has been suggested to me that a 4-250 or 4-400 could be used in a grounded grid amp that is designed to use a 3-500Z. I wonder if anyone here has experience of doing this as I am in the process of acquiring a home brew amp that originally used 2 x 3-500Z. I see the base and filament requirements are the same but can the grids all be strapped together and directly grounded? What would be the bias requirements for ssb operation and cutoff in Rx to achieve by taking the filament CT above ground? Mike. Mike Fleetwood Canberra Australia and Sidcup UK. Worldwide email address is:? ? mfleetwood@...?
mfle2001 All Messages By This Member #39276 Many thanks for the kind response to my queries, much appreciated! 73 and Happy Easter to all. Mike VK2AMF. At 08:47 PM 8/04/2023, you wrote: toggle quoted message Show quoted text The TL922 should be changed from forced bias to self bias if it hasent already been done.. This is easy to do.? If using 4-400?€?s make sure to ground all the grids direct to chassis.? Since 4-400?€?s don?€?t need any operating bias at less than 4000v in GG you should run a jumper across the zener to short it out.? The resulting ZSAC with no bias should be fine. On Friday, April 7, 2023, 10:50 PM, mfle2001 <mfleetwood@...> wrote: It has been suggested to me that a 4-250 or 4-400 could be used in a grounded grid amp that is designed to use a 3-500Z. I wonder if anyone here has experience of doing this as I am in the process of acquiring a home brew amp that originally used 2 x 3-500Z. I see the base and filament requirements are the same but can the grids all be strapped together and directly grounded? What would be the bias requirements for ssb operation and cutoff in Rx to achieve by taking the filament CT above ground? Mike. Mike Fleetwood Canberra Australia and Sidcup UK. Worldwide email address is:??? mfleetwood@...? Mike Fleetwood Canberra Australia and Sidcup UK. Worldwide email address is:????? mfleetwood@...

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