Valiant HV Supply Fix

[stevef]

My Valiant has a non-stock HV transformer.  It is a George Sturley 900-0-900.  I don't know what the previous owner was thinking, but with no other modifications to the supply (other than 3B28's in place of 866a's), the HV B+ was probably around 800v (?).  What the heck should I do to get back down to the stock 675v on the supply side of the choke input?  That's alot of expensive hot power resistors, even more so if I convert it to solid-state.

TNX, Steve KK7UV 

[WA1HZK]

Don't sweat it. Those tubes will work with the higher voltage just fine. Just make sure the bias voltages are set correctly (They will need to be a little higher) and you will not be running as much plate current when you tune it up. Check the screen dropping resistor also. You may need some more ohms here. Too high of a screen voltage blows these tubes kinda quick. I would suggest you beef up the values on the high voltage filter caps also. I think you have something like a pair of 450 volt parts in series now. Your cutting it close. Maybe three in series of a value like 100-200 mf. 450 or 500 volt caps would be safer. You will need one more bleeder resister also.
Keith
WA1HZK

[stevef]

I am going to use 6146B's.  I cannot find any tube spec data that suggests I can put that kind of plate voltage on a 6146B.  It looks like 750v max ICAS for the modulators, and 600v max ICAS for the finals.  Do you have another source?  or are you referring to a different tube than the 6146B?

Steve
KK7UV

[W2VW]

IIRC last time I measured Valiant plate iron it WAS 1800 VCT. Don't worry about it. The toobs will not launch. If it really bothers you then consider a variac on the primary of the plate xfmr or a filament xfmr hooked up to buck primary Voltage.

[W3NP]

My modernized DX-100 with SS rectifiers has an stiff HV of 800+ volts. No problems with 6146B's or plain old 6146's.

[stevef]

Thanks guys.  I shall proceed then.  I will come back later for some help when I get to working on biasing, screen voltages, and plate current ideas.

Steve
KK7UV

[WA2IXP]

I have an Apache that runs 900v on the 6146's with 5881 modulators and works fine. Ajusted bias and increased filter cap rating if I remember correctly. SS rectifiers throughout. The original xformer blew out and the replacement was 1800vct. I was worried about arcing at mod tube sockets but never happened. Good luck jay WA2IXP

[w3jn]

Take toob data max ratings with a grain of salt.  Sure, if you wanna get the absolute max life outta the toob, don't exceed the book.  But ask some around here about how many multiples of 100% they've exceeded the ratings in 50C5s or 813s 

[stevef]

Time to delve into this power supply now.  I am going solid state, and have a 900-0-900 hv tfmr as stated intially...

and you will not be running as much plate current when you tune it up

If I put say 760-800v (just guessing what it will be after the choke-input filter) on the finals plates instead of the stock 620v, and there are no resistance changes in the PA, won't the plate current be higher (according to Ohms Law?).  330mA at 620v = 1879 ohms.  So, 1879 ohms with 800v is 426 mA.  What am I missing?  Or is there some change in the resistance?

Check the screen dropping resistor also.Too high of a screen voltage blows these tubes kinda quick

I suppose I can add/chnage whatever I need to the big droppers R15 and R16.  What about the stock 150v to the clamper and finals screens.  Should they be increased by some amount or stay at 150v?  Same question for bias, how much higher?  Is it a proportional percentage to the plate or screen voltages?

Thanks gang,

Steve KK7UV

[Bacon, WA3WDR]

You might want to load the finals a little lighter, and set it to the original operating plate current.  This will slightly increase the load impedance, but it's not much.  Also you will have a little higher power input.  You could load even lighter to get the original input power, and the mod transformer might like that even more.  The lighter load on the modulator is OK too.  The higher B+ will produce correspondingly higher audio voltage without any problem.

Lighter plate loading results in higher screen current.  If you run it very lightly loaded, you might want to increase the screen dropping resistor in the final amp, but this probably isn't necessary.  I remember DEU-John used 20K in his Valiant.

Probably the clamp tube can be set as before.  When the screen is set to a certain voltage, plate current in a tetrode is not very sensitive to plate voltage.  You can check this and drop the setting slightly if necessary.  Watch out that the clamp tube does not affect positive modulation peaks.