On Wed, Jan 10, 2024 at 06:34 PM, W7WRX wrote:

If your 8k Ultra sags down to 4800 volts you have something wrong.? Possible weak AC line, or failed Oil caps.? I used a Fluke HV probe and mine sits at 5900 volts.? panel meter shows 6000.? ?At rated 5kw output it sags to 5500.?

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The 8K has a very small power supply. Its only a 1 AMP CCA transformer with 2-amp surge.? It was designed for 5kw pep on SSB and no more than 1 amp plate current in Data modes. That is RIGHT IN the manual.? ?Its rated at 8k INPUT to the power supply.? NOT output.?

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This is a good example of my exact point.? The amp will make 7000 plus pep with video all over the net showing this.? It does this on SSB with a 2 amp transformer That I can pick up and walk away with.

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Which of the 7 versions of the 8K ultra are we talking abt ?? ?The 8k manual I have sez...." 5400 dc no load.? Sags to 4800 vdc with a 1.25 amp CCS? cxr".? On the manual I have, they used? a 6000 vac xfmr...and a resonant chole input. No load B+ is .9? X 6000 = 5400 vdc.? The manual I have sez 3.5 kw pep out...on the higher B+ position..... and 2.5 kw out on the lower B+ position ( data modes).

One thing u gotta be careful with on the 8k..... it uses 5 x 20K? @ 100 watt? wire wounds in series for bleeders.? Each wire wound dissipates 60 watts? (300w in total).? If any one of em goes? open, all hell breaks loose.? Then it turns into a capacitor filter, and B+ is now 6000? X 1.414 =? 8484 vdc.? Something to be aware of.? In normal operation, 300 watts of bleeder is used to make the choke look like a choke.?

Are u saying ur 8k sags to 5500 vdc.... while talking on ssb ???

Jim,

Thanks for the explanation.? I understand the principle of what you are describing, eg the energy in the tank circuit causes the voltage to rise on the anode on the positive cycle, before dropping down on the negative cycle of the RF.? With these Russian tubes having such variability as you described, i guess its safer staying with a lower plate voltage and keeping the swing lower.? I don't have the equipment to hi pot test the tubes, but was planning on following the instructions on the internet around "reactivation" of the tube.? Detail copied in below

To avoid flashover in a new or a long time unused tube, it is prudent to prepare ("reactivate") it for his task. There are various opinions and solutions how to do it, but with a GU-43 it can be relatively simple. A DC of 10 to 15 V is sufficient for the tube to draw 900 mA as the grids are connected to the anode. Heat the tube 30 minutes with a filament voltage of 12.6 V. Then supply a "high voltage" of about 15 V and set the voltage to a current of 900 mA. If possible use a current limiter, because during reactivate the current can increase suddenly so that continue monitoring could be necessary to maintain 900 mA. Usually I will not reactivate longer than an hour or so. Soon you will find out at which voltage a good tube drawn 900 mA (or 800 mA/12-12.5 V) so you have an indication if another tube is better or not.

Alex,

Thank you. That makes sense.? His ratings seem way to high to me.? Thanks for the confirmation.

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RG393 will do it easily, it is specified 25kW at 10MHz, 9.5kW at 50MHz.

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73

Peter, DJ7WW

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Betreff: Re: [ham-amplifiers] Building my first amplifier. Where to start?

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On Tue, Jan 9, 2024 at 11:15 PM, W7WRX wrote:

Its not hurting the tube.? You need to remember that this is Single side band with a duty cycle of 15 to 20 percent.? Even with heavy processor is not past 25%.??

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What will the amp be used for???

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This is a very common mistake made all over amplifier groups.?

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Another one is Blower CFM and pressure.? All the time I see these guys saying you need X CFM or you need 300CFM!!!? That blower is to small....? I got advice from a guy that suggested I get a blower TWICE the size of the tube data sheet, Just for good measure.? LOL

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?But those ratings are for 24/7 full power at rated Plate dissipation for 8000 hours.? A ham using SSB will never need anywhere near those ratings.? Even if you ran A digital mode you still would never need broad cast ratings.

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Another one is transformer current.? If the amp is for Ham use it will not need the full rating of the tube spec sheet in transformer.? Ever.?

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?Its comical people buying 175lb transformers only to use the amp on side band and never even pull HALF the current its capable of.?

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It should be built for its intended?purpose.. If it is meant to run Data modes at high power with long operation of a contest, I get it... over build the thing. But most of the time, the amp is just going to be used on SSB with Data or CW less frequently.

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C

The bigger plate xfmr typ results in less B+ sag on ssb / cw.? The 8K ultra sags from 5400 vdc, down to 4800 vdc.? Mine sags just 50 vdc.? ?5500 down to 5450 vdc.? The ebm papst blowers I got a while back (EC type) have variable speed on em....via an external 10 k pot.? I can drop the rpm in 1/2.? You are correct though, on ssb, less air required....and on extended RX, even less air required.?

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On the dc input side, my average plate current is exactly 1/2 of key down cxr.? This is with ssb and a lot of processing.? I call that a 50% duty cycle...on the input side.? On the output side, the average PO is only 29% of the pep out.?

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I have yet to experiment with data /? FT-8 modes.? FT-8 destroys typ amplifiers.? I factored that in, and designed it, so slightly reduced ratings are used on data modes.?

On Tue, Jan 9, 2024 at 07:36 PM, W7WRX wrote:

This guy runs his at 3450volts with 3kw out with 60 in.

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That fellow is well know for his total bs ratings. On that GU43B amp, he uses a pulsed 1.5 khz tone fed into the mic jack, with xcvr on SSB mode.? ?1 on, 4 off, pulsed at 30hz.? ( 20% duty cycle)? ?This is on the DC input side. On all my amps, on SSB mode, with processing, the average plate current is exactly 1/2 of the key down value. Meanwhile on the output side, the average PO is only 29% of the pep value.?

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Note on all his other amps, esp the bigger ones, he uses a 1 on and 9 off (10% duty cycle), again using a pulse generator at 1.5 khz/30 hz, fed into the mic jack of the xcvr...on SSB mode.?

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I borrowed the HP pulse generator from work, and also dragged home the Tek 4 x channel scope.? It's amazing how low a duty cycle you can achieve.? With real low duty cycles on the input side, the plate current is not much higher than normal idle current.? Average PO is through the floor, yet pep out on the scope is sky high.?

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Look at his 4CX-20,000C.? He rates it at 100 kw pep out on ssb.? ?Again using the 1 on, 9 off, 10% duty cycle.? ?There is no way in hell that? amp would put out 100 kw pep on ssb.? His plate choke setup is fubar.? RG-393 coax from load cap to TR relay, wtf ?? ?I don't think so.

On Tue, Jan 9, 2024 at 11:03 PM, desmond auld GI0UTE wrote:

Thanks for the link, im reading that as 3450v UNDER LOAD!? So unloaded he could be at near 4KV dc on a 3 KV rated tube.? Is "over volting" tubes this much typical?? Any rules of thumb to consider here i guess is my question,? Also 3KW PEP from a tube with a 1Kw dissipation must be burning the candle at both ends.? Interesting data point!? thanks,

Listen carefully. ONLY when the tube is driven full bore, the RMS? RF voltage is aprx .6? to .63? X loaded B+ voltage.? Then add 41% for the peak? RF voltage.? ?That peak RF voltage (only when driven full bore)? is in addition to the? loaded? B+ voltage. Do the maths, and the end result is the total voltage on the anode is a bit less than double the loaded B+ value.? This total value is what the tube must stand off.

IE: say 3450 vdc loaded. RMS RF voltage is then .62? X 3450 vdc = 2139 V rms.? Peak RF voltage is 2139 X 1.414 = 3024 V peak.? ? Total voltage on the anode (only when driven) = 3450V + 3024 V peak = 6474 V peak.? ?The tube must be able to stand off at least? 6474 V.?

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This is why you always, always hi-pot test the tubes before using them.? ?An Eimac 3-500Z? will easily hi-pot test to 16 kv.? An Eimac 3CX-3000A7 will hi-pot test to 20 kv.? An Eimac 3CX-6000A7 will hi-pot test to 25 kv.? The 3x10 and 3x15 will hi-pot test to 30 kv.?

Russian tubes are notorious for hi-pot testing all over the map, so beware.? ?I have seen GS35B's that arc at 3.5 kv....and GS35B's that run just fine at 4.5 kv.?

In most cases, we could care less what the UN loaded B+ is on RX.? What counts is what is the loaded B+.? ? I can easily apply 15 kv on an Eimac 3-500Z..and not draw any leakage current between the anode and the control grid.(grid bonded to chassis).??

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Yeah. His output power claims for all of his amps are out with the birds. Pure BS.

73, Alek VK6APK

Thanks for the link, im reading that as 3450v UNDER LOAD!? So unloaded he could be at near 4KV dc on a 3 KV rated tube.? Is "over volting" tubes this much typical?? Any rules of thumb to consider here i guess is my question,? Also 3KW PEP from a tube with a 1Kw dissipation must be burning the candle at both ends.? Interesting data point!? thanks,

The 12v rail is usually around 15 volts so the usually have dropping resistors just outside the meter where the bulb fits in. ?Measure the voltage before the dropping resistor and use a 12v bulb that fits and use whatever drop resistor you need.

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?Hi guys.?I have pulled out my (big) plate transformer and made some measurements.? For voltage options I have 2100 Vac, 2445Vac and 2550Vac measured with 240v ac supply (today over lunch..) as options to pick from.? The transformer is rated at 3.5 amps continuous duty at these voltages.? That is with phase 1 and phase 3 connected out of phase both on primary and secondary.? Idle current with 240v supply was 0.3 amps. The transformer seems?very happy.?

I calculate the DC voltage to be 2970VDC, 3458 VDC and 3605VDC using a bridge rectifier circuit.? The GU43B is rated at 3 KV, so I think the lowest option will be safest, but wonder would the higher options also be “ok”, as the voltage will fall under load?? What are people's thoughts here??

On a separate note I have this 3cx30,000 3cw 30,000 that I don't need and can't really test. Would anyone be interested in it? If you can test it I would be happy to send it to you and make some sort of a deal? It's maybe more appropriate to start a new topic, if it's ok to see things on this group?

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Thanks, Des.