[Milsurplus] Opinion on transformer ?

[Kenneth G. Gordon]

On 6 Feb 2019 at 21:25, Hubert Miller wrote:

> I have a NIB transformer rated  180 V  500 mA and 6 V 5A.  The box says
> "Will deliver 470VDC in doubler circuit." 

Actually, that will deliver +509 VDC output at 250 mA in a solid-state voltage-doubler circuit. 
180 X 2.828 = +509 VDC. That statement on that transformer obviously presumes tube type 
rectifiers which have significant voltage drop. Silicon doesn't.

> I am wondering, would this make a decent transformer for something
> like the Harvey Wells T-90 or Navy TCS transmitter ? 

The voltage sounds a little low to me for maximum output, but not beating the transmitter up 
would make it more reliable. I would use that. 500 VDC at 250 mA is 125 watts input.

What are the final amp tubes in either of those rigs? 807 or 1625? If so, then the maximum 
CCS plate voltage is 600 V. At 500 V in a plate-modulated Class C final amp, your output 
would be around 30 watts. If you had 600 V, output would be 44 watts (at best).

14 more watts isn't going to make enough difference at the receiving end to even be 
noticed.

If that was my transformer, I'd use it for what you mention. It would run cool and would last 
forever.

> Or will more current be required at HV ? ( Ignoring filament requirements. )

I would think not.

> I understand under ICAS  transformer ratings can be "pushed".

Somewhat, yes, but I wouldn't go hog wild either. Again, I don't believe in beating things up 
just to get a few more watts output. It ain't worth it. But, in CW or SSB service, you can 
normally get that full 500 mA from your transformer, depending on how long you hold your 
key down. :-)

> But a doubler is not going to have very good regulation.

That is not necessarily true. In the case of a doubler, regulation pretty much depends on the 
size of the filter capacitors used. According to several handbooks which have mentioned 
this over the years, anything less than 30 MFD total filter capacity makes regulation 
"marginal". This is why those OLDER linear amps which used voltage doubler supplies used 
filter caps which would total as close to 30 MFD as possible.

Higher total capacitance really helps regulation in doubler supplies. In addition, higher total 
filter capacitance also very seriously improves dynamic regulation, which is important for 
SSB and CW rigs, and also, I would suppose for those AM rigs which use the same supply 
for the RF and modulator..

> Is that a real drawback for AM operation?

Well, it would depend, again, on the value of your filter capacitors, but I would think not.

Personally, I always use the highest value filter capacitors I can install in voltage-doubler 
supplies, mainly to help improve the dynamic regulation. For instance, in my SB-200s, which 
originally used 6 ea 125 MFD  450 VDC capacitors for a total capacitance of about 20 MFD, 
when I restored those SB-200s, I replaced those with 560 MFD 450 VDC capacitors, for a 
total capacitance of about 93 MFD.

The unloaded voltage does not change, but the voltage under a steady load is slightly 
higher, and of course, you can't really tell how it improves the dynamic regulation without 
using a storage-oscilloscope.

However, tests by GE back "in the day" showed conclusively that higher values of output 
capacitance in linear amp power supplies makes a significant improvemant in both dynamic 
regulation, and a significant reduction in IMD for linear amps in SSB service. This was all 
published in one of GE's Ham Notes a long time ago.

I really like voltage-multiplying power supplies.

BTW, if you used a voltage-tripler, your output voltage would be 764 volts, but that would be 
too much for an 807 or 1625..

Ken W7EKB