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K1JJ's 813 Maul Questions (was 807 instead of 6146)




 
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Author Topic: K1JJ's 813 Maul Questions (was 807 instead of 6146)  (Read 16288 times)
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K1JJ
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"Let's go sailing, Tommy!" - Yaz


« Reply #25 on: December 03, 2006, 01:28:24 AM »

I see this schematic (http://www.amwindow.org/tech/htm/813/813print.htm) that there are current meters for the screen and the bias power input. If I wanted to round it out and add a plate current meter, where should I put it in this circuit?

Dave,

The plate current meters are already there for both the modulators and finals. They are the 500 ma meters in the fil center tap circuits.

Yes, RG-213 is OK for HV with this rig.

For HV connectors, I use standoff insulators mounted near the rear of the chassis.

73,
T
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« Reply #26 on: December 03, 2006, 10:10:25 AM »

Maybe make provision to put some screen voltage on the 813 modulators if you will have a low B+. You could build a 0-150 volt supply. That would wake them up. 1250 is too low for zero bias. The pair would idle at less than 20 mils. They are cleaner at higher plate voltages probably due to the bias being correct. If your concern is all because of the 110 volt mains supply then spend a little time beefing it up if you can. There should be no problem running a 500 watt input plate modulated rig from a decent 110 volt mains service.
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WA1GFZ
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« Reply #27 on: December 03, 2006, 12:39:18 PM »

PL259s are junk at high voltage. I use HN connectors.
Heck I bet a teflon BNC would be better than any PL259.
I've had no problen putting 6 KV down RG11 through HN connectors since about 1975.
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K1JJ
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« Reply #28 on: December 03, 2006, 01:05:59 PM »

Maybe make provision to put some screen voltage on the 813 modulators if you will have a low B+. You could build a 0-150 volt supply. That would wake them up.

Yes, that will work as long as you regulate the screen supply stiffly. Even a small sag from screen current will negate the benefits. Maybe use a VR-150 tube regulator and then be ready to add some diodes in the CT, if needed, for fine tuning.. ie, idle each tube close to 50 watts each or whatever gives the best waveform by juggling the values.

BTW, I've seen my 813's idle down at 20ma (~1250V) and still produce a reasonable audio signal. Once they idle at or into cutoff the crossover problems really start.

BTW, Dave/APEman, how did that deal finally work out with the red curtain guy?

T
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« Reply #29 on: December 03, 2006, 02:11:14 PM »

BTW, Dave, how did that deal work out with the red curtain guy?

I've never purchased from him.
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« Reply #30 on: December 03, 2006, 10:52:03 PM »



JJ, the usual method of adding bias to the grids of any tube assuming it is xfmr coupled is to simply place it on the CT of the secondary (I'm sure you know)... but one could conceivably keep the grids at ground and put the cathode at some negative supply's potential - but that one is a long way to go to get not very far and probably not worth the effort... Regardless the 813 is a fine tube... many years ago the HR club at the college I was at had a large box full (in the closet) of even larger Navy numbered (not VT) tubes that looked like 813s on steroids... still no idea to this day what they were, looked like a couple of different but similar types. Wonder what they would do...

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K1JJ
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« Reply #31 on: December 03, 2006, 11:51:25 PM »

JJ, the usual method of adding bias to the grids of any tube assuming it is xfmr coupled is to simply place it on the CT of the secondary (I'm sure you know)... Roll Eyes
_-_-WBear2GCR

GCR,
Actually, I'd forgotten that the modulators were driven with a CT p-p audio transformer, thus a positive bias could be placed on the CT if needed to idle the tube up when running too low plate voltage. I've never tried that and wonder if maybe a better method would be to add some screen voltage and then put in fil CTdiodes or some fixed neg bias on the audio xfmr CT. It would then become a standard tetrode modulator.

Or maybe keep the audio CT at ground as is, and add JUST enuff screen voltage to idle the tube at ~50W each.

The reason for the triode connected config was simplicity... no grid or screen voltages needed.

I'm now testing a pair of KLR cathode driven FET audio drivers that could make a beautiful driver for these triode connected 813's. I think with 1800-2KV on the plates, this combo would be ultra hi-fi. I plan to add some neg feedback around the 813 modulators back to this FET driver too.

73,
T
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« Reply #32 on: December 04, 2006, 09:19:04 AM »

Tom's driver should work great!
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« Reply #33 on: December 04, 2006, 12:37:03 PM »


Just keep in mind that Mosfets require a certain amount of power to overcome the inherent gate capacitance... not that much, just more than none... usually a few watts will do the trick, otherwise they do not run so very linearly, afaik...

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WA1GFZ
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« Reply #34 on: December 04, 2006, 12:40:54 PM »

They drive a lot easier in the forward biased mode. Tom simulated the circuit.
I also simulated it as it was the next generation from my simple driver.
It should work fine with a good layout.
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