Using 2 transformers for a HV Power Supply.
I am thinking on building a 6 Meter amp using a pair of 4CX250B's Cathode Driven but I can not locate a suitable transformer for the HV locally I am trying to build it on the very low buget that I have and shipping a transformer here is out because shipping is outrageous. I do have 2 transformers from a Kenwood TS-830 that has 800V @250mA each if I ran them in paralell the into a voltage doubler to obtain the HV needed for the tubes or am I asking for troubles? I have a filament transformer. So before I get started on this I wanted someone that had more experience input on the matter. I was going to use a Microwave transformer that had the voltage I need but it hummed even with no load on it so I nixed that.
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Put the primaries in parallel and the secondaries in series. Then use a full-wave bridge rectifier. That will give you more than 1600-volts DC, probably closer to 2000-volts. Just make sure and get the primaries "phased" correctly! If the output voltage from the series secondaries is near zero volts, the reverse the connections on one of the primaries.
Even if you use a voltage doubler design you might want to ensure you have enough transformer current flowing for the job. You are talking about driving 2 tetrodes though. I think you will need up to 2000 vdc on the plate for AB1 operation although it could probobly play using less. 1500 would be good.
73 de Charles - KC8VWM
North American QRP CW Club #3159, SKCC# 5752
Or you can reverse one of the secondaries in series.
Originally Posted by K9STH
Adrian ... vk4tux
Interesting thread . . . Just got my first tube amp today and interested in learning all I can . . .
What tube did you have in mind? A pair of 811's? Wouldn't running a pair of 800V @250mA xformers in series provide enough current for these two tubes? Perhaps a pair of 572B's? Is my math right; or would the 572B's be a bit starved for current if a voltage doubler was used in a GG config?
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The TS-830 transformers should be worth something on the used market. Sell them and buy an AL-811 transformer for $105.50 or the AL-572 one for $142.00. You can order then from the Ameritron web page. Just search for transformers.
The 830 transformer would be a good starting point for a pair of 6146's for a QRP amp.
A pair of 4cx 250 Bs is a very good choice and putting 2 transformers secondary in series is a very good way to get the required voltage. Do not parallel them and use a doubler. Unless the transformers are perfectly matched one will put out more than the other and be the primary current supplier. The one problem that you might consider is that you only have a total of 400 watts output from the 2 transformers combined. A pair of 4cx 250 Bs can easily draw over 1000 watts. This transformer combination might work in SSB if you were care full but it would be iffy at best and not allow full power out in any mode and defiantly no high duty cycle modes.
I understand building on a budget as building anything today usually cost more than buying but the power supply is not the place to save money. The best suggestion would be to find a proper transformer through swap or buying one designed for a 572 amp. Remember though that a 572 tube is not as stout as a 4cx250b so a 1 to 1 swap is not the best option. If I was building I would look for something with around 1750 VAC at 500 MA. That would at least put you comfortably in the SSB. If you want to run FM or continuous modes this type of transformer will work but don’t think of doing the Alpha thing were you put a brick on the key.
You might put an add here on QRZ explaining that you need a transformer. I tried to find a roller inductor for a Thunderbolt for a year. After 1 add on QRZ I had an inductor in great shape for a great price from a great ham. Needles to say I now have a restored amp and feel great!
Good luck and happy building!
I'm for reversing both the primary and secondary windings.
Originally Posted by VK4TUX
Tubes run cooler that way.
TOM K8ERV Montrose Colo
Remember if U use the 4cx250s, U need a SCREEN SUPPLY and it has to be regulated. There is also a delay time for heating the filaments before you hit them with the High Voltage. You also must assure you NEVER EVER have Screen power without The HV, or the tubes will go PUFF!!!!! So careful consideration has to be given to the power supply. I built just such a device way back when, and had a timing circuit that applied power to the HV circuit AFTER a 2 minute warmup of the Filaments. 572Bs are INSTANT ON, but the 4CX250s are not. I never did get my amp to work quite right but that was mainly because of an incorrect Grid circuit and inadequate cooling. OH, that is another thing to pay DEAR attention to. You are going to need the biggest BLOWER money can buy and an airtight compartment to blow through to get all the air to come up through the tube fins. Make sure you have the correct socket which has the screen bypass built in. SK something. Good luck, this is a WHOPPER of a Project!
I've built many dual 4X150A/4CX250B amps and I doubt the TS-830 transformers in series will really do this job, unless you throttle the amp way back. The idling current alone for that pair of tubes (ZSAC) in Class AB1 is about 180mA. Peak current under full drive/full power will be >500 mA.
I think the pair of transformers would be more appropriate for a single tube amp.
[If you derive screen voltage from the B+, which is a very common (and pretty good) approach, that regulator system will draw about 30mA all the time, also, the whole time the amp is turned on whether you're transmitting or not.]
I like the "sell those two transformers and buy one for an Ameritron AL-572" (or something similar) idea. Much safer.
The transformer in my homebrew 2m dual 4CX250B amp is 1600V @ 750mA CCS, from a surplus Collins military transmitter (probably 50 years old!) and has worked fine since 1979 when I built it. Under "contest" conditions, it gets quite warm! But it's a potted transformer (oil filled) and I don't worry about it.
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