The AM Forum

Here is a rough sketch of what I am planning for my dual 807 modulator (Class AB1). The plate supply will be roughly 700 volts and the screens will be 300 V regulated.

Looking at a power out of approximately 60 watts  The plate transformer is a Hammond 275X and I will use the 6.3 v winding (5A) for the tube filaments.

Bias voltage for the 807 grids is variable from -27 to -35 volts.

I will steal 10 ma at 200 volt or so for the 6C4 driving a 3:1 interstage transformer (Hammond 124B)

Anything major out of place? The 807 plates pull 139mA not amps at noted.

Also is a photo of the chassis and front panel.

Thanks

Pat
N4LTA

Pat,

   With all that Hammond Iron, you must be working for them?  :P

The 400-0-400 tranny secondary needs a CT to be shown, and from there the LV filter attaches. The current attachment point on the FW bridge (diode anodes) should go to ground instead.

The OA3 is a 90v tube (VR90). I think you meant to put OD3 (VR150).

Looks like a fun project.

Jim

Thanks -I know better. That is one of my favorite power supply circuits also thanks for the tubes number correction. I mean't to put 0A2 as it is the miniature 7 pin 150 volt version. I am short of space.

My rememberer doesn't work as well as it used to.

I have a Hammond distributor acount still active from a while back. I can save you some money on Hammond stuff.

Pat
N4LTA

Pat,

   As a reference, I attach a schematic I used in a Central Electronics 20A. It is similar to your supply, but adds a series regulator instead of a shunt regulator, and includes an adjustable zener diode in the bias supply portion. Since I drew that circuit, I changed the LV series pass tube to a 6W6, and the error amplifier tube to a 6CB6 (the 6AH6 got a heater cathode short).

Jim

When using gas regulator tubes, some designers make use of the 'jumper' inside the VR tube to interrupt the power supply via a primary relay in case the VR tube(s) are not in the socket(s) for some reason.

Keeps unregulated voltage from getting to the screens. Unfortunately does not help if the VR tube is just worn out.

Glad to see VR tubes being used though. It's the RIGHT thing to do :) .

An electronically regulated supply is the ne plus ultra.

Pat,
The voltage to the 0d3 regulator tubes is 5 volts below the minimum recommended for reliable starting.  It will probably work anyway, but if not, I would use 720 volts instead of the center tap of the transformer.

I have a similar modulator running the same plate and screen voltages as you are.  The only difference is I am driving it in class AB2 using NPN transistor followers.  It gives more oomph than AB1.  If interested, I can provide a schematic.

73,
Fox

Not sure what your design is supposed to be doing,  because it seems the screen voltage will be a large negative voltage with respect to ground. Did you mean to use separate rectifiers for it instead of a bridge?  Is this supposed to be an “economy” doubler arrangement that’s drawn wrong?

Ed

Yep, As stated above the schematic  is drawn wrong.

I initially was going to run class AB2 but couldn't find a good source for a driver. I decided to go with higher voltage on the plates and use a low cost 3:1 interstage transformer. I used one with a better core for better frequency response. I would be interested in seeing your transistor driver circuit as I have a 120 watt modulator in my future plans.

Pat
N4LTA

Pat,

Sorry for the rough drawing, it was my working copy.  The driver transformer is from an ART-13 and I believe for class B modulators.  I think 1:1 primary to total secondary.  I used bipolar transistors for the drivers, but I would use MOSFETs if I did it over. 

Thanks for the information. It will be helpful on the next project.

Best 73,

Pat
N4LTA

Got the power supply PC Board and the caps in and built the power supply board for the dual 807 modulator. The two VR tubes mount off board. The rectifier bridge is a 1600 v PIV .unit

Used to solder wires directly to the board but it makes trouble shooting very difficult. Hopefully the screw type connector for the bleeder resistors and chokes do better.

Pat
N4LTA

My wife is at the coast so I am drilling and punching on the apartment kitchen table. Got to get it cleaned up by Wednesday.

Made some progress this afternoon. Have the punches for the 12AX7 and the 807 and have the driver transformer. Hope to get all that mounted by bedtime.

I need some spacers to mount the PC board. I have some somewhere.  If I can get the board mounted, I can power up the power supply and get it tested.

Going to have to find a way to cut the large  hole for the modulation transformer and a way to mount the power inlet on the kitchen table.

Pat
N4LTA

I'm right behind you on this, N4LTA!

I got out the sockets for the 807s this morning and have been playing parts Tetris to get them onto the chassis. Mine will be a hybrid signal path-the mic amp and audio shaping will be silicon.

Pat,

It's starting to take shape already.

I'm curious - where did you get the front panel? I've seen them before and not known where to source them. Also, how did you cut the "viewing window" in it?

Thanks and 73,

Richard
WR6J

The front panel is a great buy and I am surprised that more people are not using this service.

It is made by a company  SendCutSend located in Las Vegas

I took a CAD copy of a Hammond standard 19" Rack panel and added the cutouts in CAD. They laser cut all the openings including toggle switch cutouts with the tab so they don't rotate, Meters and the window and finished the panel for cheaper than you can buy a 19" rack panel.

The panel shipped free was around $28.  They will do most any material and any size of item. Tube sockets and any type cut. If you can draw it , they can cut it

I have used them for cutting SS chassis tops to fit the Hammond walnut chassis for a very nice looking audio amplifier. The amps unfortunately were destroyed in the shop fire.

Pat
N4LTA

about SendCutSend...do they need a dxf or a pdf will do ?

I don't have a cad program but I do have the FrontDesign software that can export dxf (see for example the attachment to which I added a .txt to upload).

also, do they provide the material, say a black 4mm thick 5U rack panel ?

thanks,
Peter

I use sendcutsend all the time for my business. Excellent service and product.

The panels come bare metal, no coating or color. The need a scalable vector graphic, to do the online quotations. .DXF, .ai, .EPS they will all do instant quotes on. Anything else costs a little extra as they have to redraw it.

I cheat a little. I own an engineering company and one of my designers can whip out what I need in a minute or so.

I am not CAD proficient except for a few PCB programs.

Pat

N4LTA

Little more progress today. All tube sockets and transformers and the main power supply board are mounted. Not as good as I like but I guess ok for drilling and punching on the apartment kitchen table. Still have to cut the opening for the modulation transformer and mount the AC inlet. I have a new nibbling tool coming tomorrow and I hope it makes the job easier.

Question - I am running a balanced line input, and I am not real familiar with the proper input/output standards. Should I be using a male or female connector for the input. Is there a standard?

Microphones generally have a male connector? I think.

Pat
N4LTA

Female XLR for the chassis. The mic has a male XLR. Pin 2 is hot/positive/in phase, pin 3 is cold/negative/out of phase, pin 1 is shield.  They are not in order -- Pin 1 and 2 are opposite/further apart. See image.

Ed

Thanks Ed. That is how I installed it, but I had both connectors.

I'll start wiring it now that I know for sure.

Pat
N4LTA

Thanks for the information.

Richard

If you have a sheet metal brake available, you can lay out the chassis in CAD too and have them laser it out, then fold and rivit or fold and weld. They won't do already-folded boxes, though. I already asked.

Me to. It would be great to send them a chassis and have it all cut.

The Hammond Walnut chassis have a top and I have had then do all the tube sockets etc. Sure does make building easy by taking away the hard stuff and errors.

Pat
N4LTA

Yeah. My fledgeling metal shop business is just getting started, or I'd offer a chassis service. I just have them cut out the flat pattern and I fold it all up when it gets here for mine, if I don't have a box the right size.

Having them laser out the tube socket holes makes it easy to use the S-type sockets that fit a 1 11/64 hole with a key in it, the ones with the wavy spring washer.

Maybe in the future once I get established!

I'll be your first customer.

Pat
N4LTA

Got a good bit done this weekend on the modulator. Got most of the punching complete , all components mounted , the heater wiring done and the VR tube screen supply done.

The VR tubes are running at 25 mA and are regulating at 307 volts which is probably close enough to the 807 designed screen voltage of 300. The 100uA meter on the front panel is calibrated in volts x 10 and it is working well as shown in the photo. (the supply is not connected to the screen) There is a front panel switch to switch it from screens to plates.

The plate supply is now 784 volts, which is too high. I have a 47K bleeder but is does not pull sufficient current to get the 10H choke to drop the voltage to the desihn value of 680-700 volts.
Th ezero signal plate current will add another 35-40 mA and that should be enough current to get the choke to reduce the voltage to get me close. I am assuming this is OK and that I don't need to install a smaller resistance bleeder. The voltage should drop when the modulator is keyed.

The two other meters read 0-30 mA for the screens and 0-300 mA for the plates. Expected plate current is approximately 150 mA.

I need to wire the two 807s and the 12AU7 and build the bias supply.

Pat
N4LTA