811 A modulator and my Hammond 175W

[W9ZSL]

   Been awhile since I've been here. Tricky getting in but Ed, K9FWR said you were still in business. Success! Question: I just scored a cherry Hammond model 2220 175W mod xfmr from eBay, a steal (I hope) for under $75 w/shipping. Pri is 15K, sec is tapped at 3, 5, and 7K, 250 MA. The plan is to use the componants from a HB PS made for a Hotwater 100. 800V, solid. Huge pwr xfmr, 400 MA, easy. Hootch it up to a pair of 811As. Got an NOS UTC S-9 for an interstage and a Triad A-9J for the input to the first stage. I plan on feeding it at broadcast level from my HB limiter, which also has a Triad output xfmr. Perfect match. Final will be a 4-125A w/about 1800V no load from a Gates 250 W BC xmtr plate xfmr. I'm shooting for at least 250 W input to the final.
Do I have enough potential juice here to get 100% modulation? Don't worry about clipping or spatter. I have 40 years in radio as DJ/Producer/Production manager for KSTP-AM,Twin Cities, and I've been a professional tonemeister since 1966. Just wait till I hook this puppy up to my recording studio audio chain!  Look out 3880!
Mike/Passage Recording Studio/ ex K9ZSL
Recently Retired and seeking a Retread

[W3RSW]

Quick answer, first pass, no calc.'s done yet and not familiar with exact pin-out of Gates Plate transformer.

Man, that Gates plate tranny can supply enough whomp for the 811A's too.
I take it you mean 1800 v., no load, does have a choke and bleeder already connected?  If so it won't droop much below 1700 or so at full load. Perfect for a 4-125 mod. by a pair of 811A's.  I haven't worked up the mod tran ratios but with excess audio won't matter too much.  Your approaching the point where the mod tranny won't have enough scrote so I can see why you might want to run the 811's at lesser plate voltage.

Why not build up the 1800v. supply as an economy special similar to the HW101's?
1800v. from bridge, 900+ from ctr.tap.  or does the Gates xfor not have a ctr. tap.?  Save you another supply if you can do that.

[Barrie]

Quick answer, first pass, no calc.'s done yet and not familiar with exact pin-out of Gates Plate transformer.

Man, that Gates plate tranny can supply enough whomp for the 811A's too.
I take it you mean 1800 v., no load, does have a choke and bleeder already connected?  If so it won't droop much below 1700 or so at full load. Perfect for a 4-125 mod. by a pair of 811A's.  I haven't worked up the mod tran ratios but with excess audio won't matter too much.  Your approaching the point where the mod tranny won't have enough scrote so I can see why you might want to run the 811's at lesser plate voltage.

My 250G plate transformer is 1680 volts no load.

Why not build up the 1800v. supply as an economy special similar to the HW101's?
1800v. from bridge, 900+ from ctr.tap.  or does the Gates xfor not have a ctr. tap.?  Save you another supply if you can do that.

Are you saying build two power supplies from one center-tapped transformer?  Two FWB assemblies, one each side of center tap?  Or, am I missing something here?

73, Barrie, W7ALW

[W9ZSL]

  FB! TNX. Plate tranny is from a BC-1T circa 1961 and it is cherry. Very few hours. I worked for the station after it was replaced with an up-grade. WOBT in Rhinelander, WI was running an old 1946 250GY (?) with the 810 tubes. It couldn't be up-graded to a KW and that resulted in the BC-1T 250 watt version with provision for conversion to the KW. The owners were cheapskates and clewless. They didn't believe our chief could get a power raise but the xmtr had to be replaced. Their loss, my gain. Hi!  It came with the 250 watt plate transformer which was pulled and the KW version substituted. This is just broken in. I found it in a cupboard when I salvaged the station about six years ago. I'm sure there is more than enough juice there to power both the mod and the final! After all, damn thing handled four 833's Hi! I wouldn't mind using the same supply for both, but I have to work with my jonque box. I also contemplated using the MV tranny from the Gates for the Mod. The main problem is the whole works is 240VAC on the pri. One option is the Gates xfmr is versitile. It seems to be built to use multi-voltage. If I went with 120VAC and a hefty bridge on the sec, and the HB supply parts, I wouldn't have the problems you might get into if you didn't have a swinging choke with one supply. On the other hand I could wire a new 240V outlet into the power panel. Hassle. Rewiring the studio and control room here was nuts. Besides, I'm newly retired and bored. Too bad I'm getting a cataract in one eye. Makes soldering a B++ch! TNX!  Mike

[W9ZSL]

  Barrie, Good points! I have the Gates MV filter reactor. It's totally over-rated. 10 HY at 5 KV. It's the UTC CG-104. That's my final PS supply filter reactor. I also have a Sangamo 8 MFD at 3KV which was replaced along with that original Gates xfmr. Got the bleeder too. I know that later in it's life, the ENG replaced the old merc vapors with SS equivalents. I still have them, both HV and MV. Built for the 240VAC pri. Center tap in the sec? Sure. With 120V on the pri I measured just over 800V each side of the center tap on the 250 watt version using the 120 primary taps. It has them. That's why I'm looking at a bridge made of four SCH-5000 rectumfryer diodes on there with the Sangamo filter. Cap follower. Single supply for the final. Bleeder is 100K Ohm at 200 watts. Got a rack salvaged from WOBT that used to house the "brain" for FM (how smart can a cassette tape be?). However, if I can operate the system off 120VAC, my life would be far less complicated and I might not electrocute my bod. Hi.  MK

[W3RSW]

Ok here's the description of the 'economy' power supply using one c.t. plate tranny for three different voltages, HV, screen and bias.

Secondary of plate tranny has to have center tap which will not be grounded.  Call the AC voltage clear across both windings, V_rms.
 
Both sides of HV winding go to bridge diodes in usual arrangement but placing the choke in the negative lead coming off the 'bottom' of the bridge; this yields 0.9 VDC of (V_rms. As usual run the HV into a filter capacitor, bleeder and load to ground suitable for such voltage.  This will be your primary plate voltage.

Picking off the center tap, no further diodes required and no additional choke yields 0.45 DC of V_rms. into another capacitor and minimal bleeder to ground. This will be your screen voltage.

You can even get a negative DC voltage, by picking off the lifted ground point between the choke and 'bottom' neg. end of bridge and running the residual AC into a back biased diode followed by another filter cap, bleeder combo to ground yielding minus 0.54 VDC of V_rms.  This is way too high for normal bias but can be lowered by the usual divider string of resistors, VR tube or Zeners, etc.  What you are doing is rectifying the ripple voltage across the choke.

Fun, huh?

All above in usual ARRL manuals, e.g. page 6-7, 1985.

[W9ZSL]

  Eureka! The choke in the negative. I'm so used to conventional Pi nets in the +. And I'm hardly a designer though I've always had a talent picking bits and pieces from a bunch of random curcuits, throwing them together and making them work. I now add: I have the full complement of meters from the BC-1T and then some. Because I'm not planning on a plate voltage much above 1,800 VDC, I scored an exact Simpson Model 29-series matched set including: one  0-2KVDC, one dual-scale MA meter w/ 0-30 and 0-300 MA (Modulator current?), two 0-1 AMP DCV and the original 3.5 KV meter that has gone AWAL. I'm mounting all the meters at the top of the cabinet. There will probably be 4. Rack panels will be in Green Bay Packer Gold and the cabinet in Packer green. Yeah, I'n not only a Hodag but a cheese-head too, fer' sure! Hi!

But, are we thinking 240 or 120 VAC in the Pri? I can go 240 if forced into it and let's face It, we're talking about 300 W input to a 4-125A and another 150 W modulation power. A little much for 120?

At any rate, just zetzed out a payment for the Hammond. A boat ancor is on the way. I think I've convinced the xyl to clear a path to the 6' rack out on the patio. Already she's hoping there is some way to dismantle it! Poor pitiful me!  73 Da Mixer

[W9ZSL]

  PS Rick, TOO MUCH FUN! Da Mix.

[W9ZSL]

  Rick. I'm beginning to get the picture. One valuable iron I have is the UTC from the Gate's MV supply. The choke is a classic CG-104. Is that swinging? Where can I use it? We have to remember, the Hammond mod xfmr is only 175 watts...exactly what I was looking for. I have plenty of rack space and basically want to build an am rig that looks like a classic late 40's broadcast transmitter. Hell, If I am forced to use Merc Vapors, I can and will. I love how they blow glue! I can have a separate supply for every stage of both AF and RF. Got the BC-1T parts. Found a site with 10 1/2' to 12 1/2" rack panels for $20 and under so I can layer the xmtr. The more tubes the less heating expenses in the shack!  Add that to the compfusder in the same room and with a slight adjustment and addition of a humidifier, I might be able to grow schrooms in the shack! HI, HI, HI! X9ZSL

[W3RSW]

I'm not familiar with specs on the CG104 choke.  Also if you plan to build the economy supply it's easier with solid state diode strings.  Otherwise in a bridge circuit you have filament transformers (three) up the waz (npi) and suitable for HV on their windings for mercury vapor tubes. 

The economy supply for two plate voltages was known as the 'duplex' supply in the 1940's where the floating center tap off the plate transformer in a full wave bridge circuit was brought out for approx. half the DC output available from the bridge. In the 'duplex' circuit, two sets of chokes and filter capacitors were in the customary positive voltage side. 

The bridge I referred in previous posts is composed of 4 diode strings of course; "K2AW's famous hi voltage modules" are probably the Cadillac without having to build up your own. With your voltages, four of his model HV 6-1's are probably fine at $5.00 each.

On reflection I'd leave the HV alone if you already have a two diode, full wave, Hg vapor rectifiers, with choke in negative lead lashup.  It's neater watching the purple swing on modulation peaks anyway.  The choke in negative lead supply is easier from HV arc over in the choke purposes but does drop the available HV by the resistance across the choke in addition to the greater drop from it's normal action in either positive or negative lead service compared to a purely capacitive input supply.

So then I'd work the economy supply trick and/or neg. rectification off the choke trick on the medium voltage supply transformer.   That gives you more easily managable  screen voltages and bias voltages rather than such a high 0.54 or 0.45 times (1800) negative voltage to top of resistor, volt. reg. string.   

Anyway, all the tricks aside, you might want a seperate bias supply ahead of time on your class C final to come up live along with filament voltage before screen and plate supply.  Or you want some sort of timer to fire up the bias along with the filaments before screen voltage if you use one transformer for all.

 Always have neg. bias before plate and screen voltage.  Never have screen come up before plate voltage in class C, but if both come up at same time, it's ok as long as protective control grid bias is alreay established.  Lots of circuits to cut off class C amp tube such as clamp tube, VR tube /zener diode establishment of cutoff bias without input carrier, etc. have been published over the years.

Well I've rambled on too long.  If you haven't already (sounds like you have) get Orr's west coast book or any older ARRL manual and read up on the class C amp. and power supply sections.  The older 4 mercury vapor tube bridge rectifer circuits with three filament transformers seem really archaic but they worked hard and well over many years, particulary if the mercury vapor warmed up adequately before hitting with heavy load.   All the boys loved those heavy AC switching circuits, manually operated in sequence, no fancy timers required.  ...but in the heat of battle, lot's of hot switching and too early plate switching happened with loss of tubes and band switches.   These days we're a lot more aware of tube scarcity, have a little more laid back operating habits on AM, etc.   Don't we?