HV Supply Loading Question

[N8UH]

Hey all! I have been making progress on my Viking II. I Re-capped the LV supply, plugged in a 5V4 and 6AL5 and brought it up on the variac. Looks good, but I may end up either adjusting the bias supply to compensate for higher voltages, or going with a bucking transformer. Line voltages here hover around 120V RMS at times.

As for the HV supply, R13 was open, so I pulled it. I'm planning on the following mod to have regulated screen voltage for the 807s:

http://members.cox.net/n4jk/viking_ii.htm

Basically, I'm replacing R13 with the 2 0A2s and taking the original tap wires and feeding those from the regulated output. So I'll be sending regulated voltage to the 807 screens and  the clamper circuit.

Soooo, I wanted to see how much voltage I'm working with in order to calculate the series resistance for the regulator circuit. I put a 1M safety bleeder in, plugged in some 5R4s and slowly brought up the HV section. I think I see why they put that monster 20K bleeder in there... I chickened out after 850V. I'm pretty sure it was on it's way to 1KV

So it's obvious that the HV section needs some loading to bring the voltage down a bit. But how much? The original bleeder was pulling around 30+mA; 0A2s are only good for a maximum of 30mA, and I don't want push them too far.

Do I need to still run a big-ish resistor in there to present a constant load and regulate it from there, or will 15-20mA or so being passed by the 0A2s be enough to load it down? Is there a way to calculate all this?

Thanks!!

[KB2WIG]

It's too late at night  to make a comment on the oa2's and voltage regulation.

 I think you are using one, 1 Meg resistor fer a safety bleeder while you ramp up the voltage. A 1/2 watt carbon resistors' max voltage rating is around 350 volts.  A tad bit small.

FWIW, on top of the usual bleeders, I'll use a 2nd safety string to back up the bleeder.    10, 100k 1/2 resistors X 350 will give ya  1 meg, 5 watts and 3500 v max protection....

gud luck with the vk

klc

[KA2DZT]

Bleeder current should be about 20-25ma.  The problem I see with using the regulators to provide the current is what happens if either OA2 goes south, then you have no bleeder current.  Another thing to consider is to wire the OA2's so that if any one of them is pulled from their socket the screen voltage is disconnected.  There is a way to do this, look at the tube's pinout.  Also remember that if either OA2 fails to conduct you'll end up with HV B+ on the screens (not good).  These tubes are a little unstable when first turned on and also if they are overloaded.  But, for modulator screen regulation it shouldn't be a problem.

I use two 20K 50W resistors in series for HV bleeders in my HB xmtr.  Under full load the HV B+ is about 650V.  The HV supply is left on at all times and there is a HV PTT relay.  The xmtr is a single 6146 modulated with 6550's.  I also have four 180K 2W resistors as backup bleeders,  2 sets of 2 in parallel in series.  Another feature is each 20K 50W has a 180K 2W across it.  The idea here is if either 20K opens, it will cause the 180K 2W carbon resistor to go up in smoke.  This lets you know that your main bleeders are open.  You don't really need this, just something I did.  I have about 200,000 resistors on hand, thought I would use a few.

I have a separate regulated 300V supply for the screens of the 6550's.  It uses the larger old style regulator tubes which are good for about 40ma.  The output from this supply is relayed on and off.

Not really sure how the Viking II power supplies are set up to operate.  So, you would need to determine what your HV B+ is under full load to be able to calculate the value of the series dropping resistor for the regulators.  Probably not a good idea to exceed the 30ma max current rating of those regulating tubes.

My HB xmtr has separate power supplies for everything, LV, HV, bias, PTT relays, VFO (regulated 6VDC filaments and regulated B+), 6550 screens and the microphone amps (12AY7s) filaments run on DC to prevent hum.  The 12AY7s filaments run from the 21VDC relay supply (21V down to 15V).  Also, there is a 80VDC negative peak limiter supply on the secondary of the mod xfmr.

Hope some of this helps,

Fred, KA2DZT

[K9PNP]

I installed the 807 mod SG regulator system using the 2 0A2's with a 10K 20W dropping resistor and it works well.  My R13 resistor was still good overall, the adjustment tap was well worn for some reason and was intermittant.  Personally, I would want at least a rough equivalence of the original bleeder resistor in the circuit.  My line voltage here is likewise around 118-120 V normally and have not noticed the great increase in HV B+ with the 20K WW bleeder in the circuit.  Only instability I have seen is arcing due to dirt (and 1 small flying critter) in the plate caps.

[WD5JKO]

Tim,

  There are many options, and most have been discussed. Remember that with choke input filter system where the minimum load current is met along with the critical inductance, the DC voltage will be ~ 0.9 * the RMS AC voltage either side of the transformer CT. If the minimum load current drops, or the choke inductance drops, then the voltage rises, and taken to an extreme (no load, or no choke), then the DC voltage rises to ~ 1.414 times the RMS AC voltage either side of the transformer CT. So with very little load, the DC voltage could approach 1 KV.

  I went through a few Viking 1's back in the day.

  In one case I solid stated the HV, and LV B+. That freed up 3 octal sockets. I used the LV B+ rectifier socket for PTT relay, and the two HV B+ rectifier sockets for a pair of OD3's (VR150). These tubes can take 40ma which is 10ma higher than an OA2. If you do something like this make sure there is still a bleeder current once the VR tubes go off, so you will need at least two resistors.

  In another case I did most of the above, but instead of using VR tubes for the 807 screen voltage, I used an extra contact on the PTT relay to key the LV B+ for the 807 screen voltage. Following this idea, you have to make sure the 807 screen voltage is NEVER on when the plate voltage is OFF.

Jim
WD5JKO

[N8UH]

Excellent info guys, just what I was looking for. Thanks!   

I just ordered a couple of big assorted resistors to try out. I may even end up doing a bleeder/fixed divider feeding the 0A2s.

Fred, thanks for that info. When (not if...) I end up making an HB transmitter, I think I'll be taking a similar approach to the power supplies.

K9PNP (cool call!) I'm glad to hear that it works well for you. I was a bit worried about the stability of the 0A2s when they get switched on/off so much, but I guess if the current surge is not too big, they'll be fine.

Jim, thanks for the theory! If I ever end up SSing the power supply, I like the idea of using 0D3s in place of the rectifiers. On a side note, what type of relay are you referring to for PTT, considering you would be switching 300+ volts from the LV B+?

Thanks again guys!

[WA1GFZ]

my V2 is loaded down to about 760 volts and is quite stable under load with modulation. I mounted a second filter cap from a dead V2 on a bracket between the chassis and cabinet.