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GG Amplifiers Bias Cut Off
开云体育Hi Francois. Running the tubes on filament only does not help at all. You must apply drive to activate the gettering process. See That is a good method. Run the amplifier at low anode volts (around 1kV) and apply drive, with plate and load not tuned and and get the anodes red. If you have a 1kV Megger, you can check the tubes initially. It
will show if the tubes need attention. There should be no
breakdown between filament and grid on good tubes. If there is,
then you can do the above process. I have rescued a few tubes in
that way. It works very well. 73, Alek VK6APK On 8/01/2025 4:27 pm, Fran?ois via
groups.io wrote:
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Francois...... Eimac and the other makers of the 3-500Z? used this chemical, called? 'Zirconium'? ?on the anodes.? ?The Zirconium coated anode IS the getter.? It is heat activated when the tube gets red.? IF you want to operate the L7 at only 500-600 watts pep output....... use the LOWER? 1900 no load? B+ (CW)? position.? IE: 500-600 ma.....and aprx 1770 vdc loaded.? Then you will get better efficiency. With 7.2 vdc of bias, and the lower 1900 vdc position used, the tubes will idle at just 40ma (normal).? ?IMD is superb? with 600 watts pep out.? I have measured the IMD on my spectrum analyzer. Yes, you can use a Zener diode...BUT? remember that the zener is in the cathode..... and cathode current is the sum of the DC plate current and DC grid current.? ?IE: 800 ma of dc plate current and 300 ma of DC grid current = 1100? ma of cathode current.? ?1.1 amps? x 7.2 vdc (zener vdc) =? 7.92 watts of zener dissipation.? ?Zener's require a heatsink.? A 10 watt zener will run warm to hot.? ? You could use? 2 x 10 watt zeners in series, like a 4 vdc zener and a 3 vdc zener.? The reason we use 10 x 1N5408 diodes in series for bias is..... they are rated for 3 amps CCS? and have a 200 amp surge rating.? ? The 10 x diodes can be mounted either horizontally or vertically...on some small perf board.? Non critical, since the vdc is so low.? 6A10 / PM600 diodes are rated for 6 amps CCS...and have a 400 amp surge rating.? ?Diodes are cheap to buy,? and do not require a heatsink.? And yes, I use 2 x 100k? @ 3 watt MOF resistors in parallel (=50k)? ?for the cut off bias.? That's called....'redundancy'.? IF one resistor went open, you still have the 2nd resistor in the circuit.? Jim? VE7RF |
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He can also just use the low 1900 vdc CW position, apply a bit of drive, and tune the amp a bit off resonance.? ?That will get the tubes good and red.? A good Eimac 3-500Z? will? hi-pot? 16 kv? (between anode and grid)..... and 5 kv? (between grid and cathode).? Chinese? 3-500Z's? ?will hi pot 9-15 kv? from anode to grid.....and only 1 kv between grid and cathode.? |
开云体育Yes. Very true but I' had it where the tubes will flashover at 1900/2000V, so by changing the HV output wires on amps like the AL-80B and TL-922, you can easily get to around 1000V to start the cooking process as per Tom's article. If the tubes flash at that voltage, they become desk ornaments. Alek. On 9/01/2025 7:20 am, Jim VE7RF via
groups.io wrote:
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开云体育See Thanks for this very interesting post. It should definitely be adapted to a 3-500ZG (graphite anode) because the photograph shows a 3-500Z with metal anode ? So 900 V 250 mA (225 W) and the author states: Watch the color of tube anodes. Anodes should be a very faintly detectable red. Do not run the tubes too hot. ? In the case of the L-7 which uses two tubes in parallel, what would be the best solution to achieve cooking? (remove one tube?) -- F1AMM Fran?ois De la part de Alek Petkovic via groups.io |
开云体育I'm not very familiar with the L7 Francois but if I guess you could do two tubes at once. Even if you just turn on the amplifier and key it, at full plate
volts, if you think the tubes are OK. Run it like that without any
drive and maybe you will get enough anode current for you to see
the anodes glow red after some time. If you get 100mA per tube,
that is 250W. The anodes should glow red after a while. If you know the tubes are not good, you should go with the W8JI
method of lowering the voltage, if possible. On 9/01/2025 2:04 pm, Fran?ois via
groups.io wrote:
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开云体育The old man who sold it to me is reliable. The PA has hardly worked but it has not been turned on for about ten years. ? I will at least regenerate, a little, the filter capacitors, one by one, with an additional voltage so that they reform quietly; that, I know how to do. ? I will try a start-up under reduced voltage by leaving the two tubes in parallel. I will make the cathode and grid modifications afterwards, if the tubes seem ok. ? Thank you for your precious help. -- F1AMM Fran?ois De la part de Alek Petkovic via groups.io |
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On Thu, Jan 9, 2025 at 11:21 AM, Alek Petkovic wrote:
With ZERO bias, and using the SSB position,? you will end up with .220 x 2600 vdc = 572 watts, which will make both tubes go cherry red.? JI's method uses a small lab typ supply, like my 0-30 vdc @ 3 amp? supply? ( I also have a 0-60 vdc @ 10 amps).? ?The advantage is.....? you can then use the CW (1900 vdc position)? and? again, with NO series diode bias / zener bias..... hook up the lab supply with the negative of the lab supply going to the CT...and the positive of the supply going to the chassis.? ?Then key the amp..... and slowly increase the vdc? of the lab supply? till un draw loads of plate current.? ? What u are doing is biasing the grids positive...which will of course cause the ide current to increase by a huge amount.? IE:? with 12-25 vdc applied from the lab supply, you will draw 375 ma of idle current.....and at 1900 vdc? will =? 700 watts on idle....which is enough to make both tubes get good and red.? |