The AM Forum

Are you all sure that a pair of 811A's can't modulate an 813?  I'm looking at a chart that shows a pair operating at 1250 volts on the plates, zero bias, plate current of 50-350 MA with a plate-to-plate load will result in a sine wave power output of 310 watts.  The 813 will have a max of 2KV.  With something like 1350 on the 811A's with zero bias, even with losses in the modulation transformer, (S-22) I don't understand why I can't modulate a probable 300 watt carrier.

811As will modulate a single 813 without any problems.  Although, with 2KV on the 813 and only 1250 volts on the 811As you'll never make 100% modulation.  You will have to use a step-up ratio on the mod xfmr to get close to 100%.

Sure you can.
And what is wrong with a step up ratio mod transformer?

I thought a globe king 500 used a pair of 811's modulating a 4-240 at 1900 volts...1200 on the modulator.

I think you need enough power, Eric (wb3cau) used to modulate a rig with an 8 ohm audio amplifier.

I never heard of needing the modulator voltage to be higher then the rf voltage.
I can run the same voltage and get any modulation level I want, and I am sure I could run less voltage on the modulator and get the same results.

Well now, take a look at the Johnson 500. It runs a pair of 811A's with 2KV on the plates to modulate a 4-400A. That is 250 watts of audio. Johnson spanks those 811A's hard, but I've got 7 years plus on a pair.

Craig,

The S-22 should match anything.  I'm back to considering 2 power supplies again for the big rig.  I would much rather use 811A's for a modulator because after digging into the specs of a pair of 4-65A's, the only advantage would be lower driving power and the disadvantage would be a screen and bias supply. That gets away from my KISS principle.  I'm in an interesting position where I have all the parts to build both a single or P/P 6V6 speech amp driver but my eyesight says do one or the other but not both. A pair of 811A's is the only way to go.

We came to the conclusion long ago that a single 6V6 into the UTC S-8 wouldn't cut it as a driver for a pair of 811A's.  I can always drop the HV to the 813 to something like 1700 because the plate iron is dual-voltage.  Both supplies can be dual-voltage.  I can drive a pair of 811A's to full output just fine.  Decisions, decisions!  Yes, I'm still fishing because I have 95% of parts I need to build something.  Next stop the loony bin.

2 KV on the plates of a pair of 811A's?  The bias must be really high.

At 2000 volts, the 811a should do well over 300 watts for a pair.
I know someone who ran them at 1750 all the time.

You can do 813's as zero bias triode modulators as well.

You can not have your cake and eat it also, class B trides need a stiff low Z driver and even then they do not sound as clean as many AB1 tubes.
AB1 tetrodes need a bias supply (simple) and a regulated screen supply (easy with VR tubes) and NO driving power.

I have both, 4cx250b (AB1, 600 watts at 2000 volts), Push Pull Parallel 100TH (class B with bias) and 811's (class b zero bias).
The 4cx250b's sound the cleanest.

I drive all the mod decks with a 20 watt 8 ohm amp through UTC LS series transformers.

811's are easy, but you need a pair of 6b4's and good iron if you want to do the tube thing, forget about a 6v6.

Everyone uses 811's, almost no one uses 4-65's.

Back to the 6B4's and I think I have the iron to do the driver job!  Need a pair for sure.  N2DTS, if I have to reduce the HV to the 813, to make the thing work, so be it.  I'm less interested in input VS efficiency.  In fact anything at or above 250 watts carrier will make me happy.

Why would you need to reduce the voltage to the 813?
But anything from 1500 to 2000 volts is fine, even on both mod and rf decks.
Cathode bias for the 811's is easy, a zener or a string of diodes will give you bias.
On my 811 mod deck I have a string of diodes with a switch to short some out.
Each diode drops .7 volts and I have a bunch with a 5 position switch to select the bias.
To 1300 or 1400 volts they do not need any bias.
572's are rated for 2000 volts on the plates and do more power output, zero bias triodes......

I can give you 6b4's, 811's, 572b's 813's if you would give them back if nothing gets built.

I've used the S-22 mod transformer as a temporary replacement in my Viking 500 years ago as have others, and it works/worked fine. I later obtained a LS UTC transformer that fit on the 500 chassis. The 500 uses a SE 6B4 as the audio driver. Its on the edge, but it drives the 572Bs in the modulator just fine. That S-22 can be configured in a step up mode for what you might be using there with your HB tx.

I don't hear this distortion or lack of cleanness with my class B triodes used as modulators. You can use negative feedback around the driver and/or around the whole modulator if you want to. AB1 screen grid tubes are OK for use as modulators, but then theres the issue of regulation of the screen voltage etc. That's not always a big deal.

Careful prudent design/personal choice/tubes on hand/availability of good iron, whether its using triodes or screen grid tubes as modulators in a HB tx makes all the difference.

Al VE3AJM

I hear it on my rigs, I could not hear it on other peoples rigs once it goes over the air, every rig/modulator sounds different, even when using the same audio chain, the same mod transformer.
My screen modulator seems to sound best, then the AB1 mod deck, then the others.
Not that any of them sound bad, or you could hear the difference at the far end....

I never heard of a class B tube hifi amplifier.
Class A and class AB1 are what you see mostly.

To get on the air, almost anything will work.
You need to design around the parts you have, voltage, tubes, iron, power levels.

I like the 4-65 idea, glowing plates behind a window, with glowing VR tubes to regulate the screen voltage.
Very cool looking.
A 4-125 modulated by a pair of 4-65's would be a glow a thon, very cool looking (in a hot way) rig.

The amp can be dual-power; high and low since the iron has dual voltages.  I probably won't add that feature.

I have a Mackay Marine that uses (2) 813's modulated by a pair of 811a's with 1600 volts on everything.  Works fb according to my scope.

Your Mackay sounds FB on the air, too, Roy!

Doug