811A x 811A. AM Rig suggestions

[T23]

I am gathering parts for an 811A x 811A AM rig.

Schematic here ----> http://schmidling.com/811_am.png

Just had a few questions.

1. In the schematic. It uses a 4uf 1kv cap in the power supply. I would be using a cap with a higher voltage rating ofcourse.
But the other 811A amplifier schematics that I have seen have 4 270uf 450v caps in series. Which should I go with?

If I go with either setup what value would the bleeder resister be 10 watt for the 4 uf and and maby 4 100k 2 watt resistors for the series caps?

I will be running the plate at 1500v instead of 1000v volts.

2. Any suggestions on the Mod iron here?

3. Any other comments?



Thanks,

T23

 

[T23]

Bump.

 

[Captain Kilowatt]

I am gathering parts for an 811A x 811A AM rig.

Schematic here ----> http://schmidling.com/811_am.png

Just had a few questions.

1. In the schematic. It uses a 4uf 1kv cap in the power supply. I would be using a cap with a higher voltage rating ofcourse.
But the other 811A amplifier schematics that I have seen have 4 270uf 450v caps in series. Which should I go with?

You want good dynamic regulation of the plate supply to help prevent flattopping on voice peaks and for good linearity. Go with whatever you can get to achieve as much capacitance as you can get. A single large is always preferred but nothing wrong with a series string either.

If I go with either setup what value would the bleeder resister be 10 watt for the 4 uf and and maby 4 100k 2 watt resistors for the series caps?

Depends on the value of resistance used and how much plate bleeder current you can afford to waste. Bleeders draw current from the transformer and power supply and thus rob it from the tube as far as transformer capacity is concerned. If yo have a beefy plate transformer then no worries. Heavy bleeder current helps regulate the voltage but at the expense of extra heat.

I will be running the plate at 1500v instead of 1000v volts.

Just watch those 811A's don't arc over running the voltage at their maximum limit in plate modulated service. That's a lot of voltage swinging back and forth on those plates.

2. Any suggestions on the Mod iron here?

The mod xmfr from an old Heath DX-100 should be perfect as well as some other old rigs. I have heard of people using a high power (100 watt) 70 volt PA line distribution transformer as a mod tranny but that would be my last resort.

3. Any other comments?

Just one, GO FOR IT! Nice to see some home brew AM stuff. Some day i hope to do the same thing but I have far too many things I want to build first and far too little money to make them happen right now.

Thanks,

T23

 

[T23]

Ok, thanks for the response. I really appreciate it.

So the 4 uf would work in that setup I assume from your comments?

And how might I lower the plate voltage?

I was either going to stick with the 4 uf or have 4 270mfd caps setup like an ameritron 811h.

The tubes I am using are g-811s, russian 811a equivelents.

Also, what ouput impedance for the audio drive transformer would be best for the modulator grids?

T23

 

[T23]

This is the transformer I had in mind -----> MFJ Enterprises Inc.

It should be able to power the modulator aswell, but the HV tap is 1500 volts?



T23

 

[Captain Kilowatt]

I had a brain fart. Forget about what I said about the bleeders helping regulate the plate voltage. This is a class C plate modulated rig. The RF amp runs all the time and maintains a minimum constant load on the plate supply. this will drop the voltage a bit to begin with. As for bleeder current, just use a 100K resistor rated for 25 watts or more. It will need to handle 22.5 watts. Maybe three 33K 10 watt instead. All it needs to do is bleed the supply on shut down and not help regulate the supply because as I said the RF amp will do that. The 4 uF is pretty light without using a filter choke as well. I have used broadcast transmitters that used 4 uF filter caps however they used choke input power supplies so a small capacitor is fine. Personally i would use more than 4 uF.