Gu74b amplifier

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These heavy Ameritron Amplifier-Transceiver Keying Interfaces are designed to protect expensive radios from Available in two mo Order this product and get free delivery and handling on your entire order!

GU74B / 4CX800A Ceramic Power Tetrodes DXE-GU74B

Offer excludes truck freight fees. Valid on orders shipped in the contiguous United States. Ameritron AL Amplifiers feature 1, watts of output one hour continuous and operate from through 15 met These h They offer They provide over W PEP output or During the summer I completed my new GUb tetrode power amplifier.

Because there have been a few requests for construction information, I decided to write this article. Please do note that this is not made for first PA project builder and I will not take any responsibility about errors of this document.

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Picture 1 at right : GUb amplifier and power supply Click on bordered pictures for zoom. GUb is made for bigger anode dissipation and current. It also needs more powerful blower than 4CXb picture 2, below.

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Svetlana socket SK3A can also be used but in that case cooling air is feed first to anode cabinet. Socket cooling is important and should not be forgotten. The both sockets have a built-in Screen capacitor and therefore it is possible to connect the cathode to the ground.

GUb socket has also 0.

gu74b amplifier

I would recommend at least 24 hours of heater time before applying other voltages. Plate voltage is V 1. Filament uses 27 V 4A.

Tube parameters

GUb maximum anode dissipation power is 2. With this power the tube needs really big blower that can give 75 mm H2O pressure and 2. With 1. The blower is in separate box and cooling air is feed in the 90 mm aluminium tube to PA. On TX the pressure is 32 mm and on RX 20 mm in the input cabinet. Smaller fan could also be used as can be seen from air temperature. This feature is good for a soft start.

I use a L regulator and a pair of 2N transistor for this circuit. The filament resistance is 2. The tube manufacturer recommends that cooling should remain up to 5 minutes after filament is turned off. Please be very careful that anode voltage is not disconnected before G2. The input is directly from there and the Q-value is decreased with the resistor. But still the gain is about 16 dB. There is no need of neutralizing the PA with this construction.

Aluminium is not so expensive, I thought. At first I had difficulties to find the right length of the anode resonator. Because of this the cabinet is about mm longer than needed and there are extra holes for tune and load capacitors.

However now it is possible to change the tube to 4CXb. It needs 75 mm longer resonator. The efficiency is now a little bit worse with this construction.

Amplifier cabinet is 1 mm aluminium. The mm diameter anode resonator tube is made from this same Al-plate and connected with Al-rivets.

Of course copper or aluminium tube would have been much better choice.It may NOT work for you. It is not published anywhere else or recommended by ACOM.

How to exchange the GU74b / 4CX800 tubes on your Alpha91b (Alpha99) Amplifier

There were lots of conflicting opinions on the use of NOS tubes. One was manufactured in June,the other in February Both tubes appeared to be in like new shape, and they were in their original sealed packages with an insert containing instructions in the Cyrillic alphabet that I could not understand. This is what I did:. Do not touch anything yet. Then, holding the shorting wire by the insulation, back about 4 inches from the bare part, touch the stripped end for seconds on all parts on the HV rectifier PCB, especially: - the HV output J3; - the wire-wound resistor R12 on its both sides; - the four HV diodes on their both sides.

Lift and pull the cover to remove it, exposing the RF deck and allowing access to the tube. Use the shorting wire described above and touch the copper strip of the plate anti-parasitic suppressors and the tube anode. First carefully push the temperature sensor that looks like a small transistor out through the side, being careful not to bend the leads.

Unclip the anode spring clip from tube cap, and while pressing down on the tube cap, work the the rubber chimney up off of the tube, together with the anti-parasitic suppressor and copper strip. Make sure that all 16 of the socket's spring contacts are outside the tube, and clipped tightly against the tube screen-grid ring.

gu74b amplifier

You may have to adjust the idling zero-signal plate current of the tubes for low- and high- RF-drive levels. In order to determine if this is necessary, the amplifier must be powered on. As mentioned in the disclaimer at the top of the page, these are lethal voltages and it is better to send the amp back to the factory than to risk your life.

First, you have to defeat the crowbar and "cover presence" interlock. I used a plastic plate and masking tape. This is so you can work on the bias voltage adjustments for the idling currents with the power on. If this is not the case, you probably should adjust RP1 and RP2 as described in the aforementioned document. I will not go into it here because it falls outside the scope of these instructions; is available elsewhere, and since I did not have to do it, I am unable to confirm that the documentation I found is correct.

Now the interesting part! Assuming everything has been reassembled correctly, it is time to power on the amplifier with the NOS tube. Since there is no way to know what the idling plate currents will be without power, this step has to be done with the main cover off and the interlocks defeated as shown above. Plug the amp into the wall socket and turn on the main power rocker switch at the rear. If it doesn't, something is wrong, and I can't help you. Call your ACOM dealer! The amp should power up as usual and begin the second warm-up countdown.

If it doesn't, I can't help you. Once it goes through the warm-up, you will get the usual LCD display. Toggle through the screens and ensure that things look normal. In particular the plate voltage should be and the idling plate currents should be within mA and mA. If so, you can either power it down, remove the interlock defeating tape or however you did it and put the main cover back on. This is the safest way, but since I had two tubes to condition plus putting the original tube back in, I decided to do everything with the cover off.

Either way, now is when you begin "conditioning" the tubes. That's easy. You wait. Let the amp sit powered on in idle mode for hours. I left it 5 hours per tube. Maybe 3 is enough, and maybe would be better.N4JA my response to a nasty email from n4ja.


The GI-7B t is a very rugged metal ceramic tube built during the cold war for use on Russian tanks and other military vehicles. The GI-7BT is a more ruggedized version of the same tube featuring a fatter ceramic element but otherwise the same as the thinner GI-7B. You will find these tubes that have white ceramic and some that have pink or purple ceramic and we have found no difference in the performance of these tubes.

Always be aware of what you are buying. They are easier to retrofit than the MLA and will give nearly identical performance.

Please read all the notes below on the MLA before you decide to start this project.

gu74b amplifier

It is important to note that, as with any retrofit, you should NOT expect more out of this amplifier than it was designed to provide. You can not get more power output from this amp than the power supply is capable of providing. Also, the retrofit of the GLA this applies to most of the other sweep tube amp retrofits too requires changing the power supply to a voltage doubler. When you do this, the plate voltage will be higher.

Obviously if you can not get more power from the power supply, then the plate current will be lower. What this means is that the plate load impedance is going to change.

What that means is that the Pi-network will no longer provide a proper match to the antenna jack. You can get the amp to work but it will not come up to full output. There are other opinions on this subject so please understand this is just one of them. I am not in a position to hold your hand through a project like this so be well aware of what you are getting yourself into before taking this on.

If you do not have experience doing metal work, schematic reading, figuring things out from pictures, troubleshooting and intuitive layout design based on available components, you may not want to take this project on. You are not going to find a list of detailed instructions here for doing this retrofit.

You are not going to find a detailed list of parts for doing this retrofit. You will find a detailed schematic that reflects most of the changes that can be made but not necessarily all of them. If you do not find the information you are looking for here, I am afraid that I do not have it. Carefully study the pictures in these retrofits. You should also study other MLA retrofits done using other tubes. All of this study will give you better ideas for doing your own work. If you still want to do the conduction cooled retrofit, we can supply the new cooler as well as the socket.

Even if you do the retrofit as a conduction cooled amp, it is wise to blow a small amount of air around the tube to keep the base of the tube properly cooled.My son Robert Stone has taken over the part of our business supplying GU74b tubes.

Email Rob at info vinecom. Customers already on the Vine Antennas waiting list have been moved to Rob, who will supply tubes when they become available. We've sent the following tubes back for replacement as promised by Ru-Tubes, as they are either low-gain or dead with internal shorts. After one year of waiting, it is certain that they have no intention of replacing them. Doubtless Ru-Tubes will be seeking another victim to send those non-working tubes to.

If you get one of these, you have been suckered, as "Nikolay" there has the business ethics of a skunk. Vine is the world's premier supplier of fully tested guaranteed GU74B vacuum tubes. These tubes are used in many commercial amplifiers such as all the ACOM amplifiers, and amplifiers from Alpha Power, Command Technologies, and several other manufacturers.

Our service engineer has built a special tube tester for the GU74B, which tests it by applying different values of grid bias at a fixed screen and anode voltage. Please see the pictures below. As well as performing the dynamic performance tests we also apply a "gas" test and reject any tube that does not pass it. Please see the pictures of our tester, as below. Our tests give anode current plotted against different values of grid bias and recorded.

Every tube we sell comes with a certificate of conformity showing the tester results. As far as we know we are the only tube supplier to make tubes available that have been tested in this way, and with documented evidence of performance. From a large quantity of tube tests, we are then able to select individual tubes with close results and set them aside as matched pairs.

Check out this image. It shows that the probability of any random tubes being matched is near-zero. Because prices we have to pay vary, and have been increasing recently, we cannot publish a price for our tubes on this web page.

Please contact us for prices and availability.It finally happend… After a decade of reliable service, my Alpha91b amplifier broke. Most likely on it's way to Spain one of the tubes broke. The first shock was big, but a few measurements confirmed that just the tubes were broken.

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In this blog post I will share the knowledge I gained during during debugging, comissioning and breathing new life into the amplifier. Do not perform ANY work on your amplifier if you are not exactly sure what you are doing. Again if you are not absolutely sure, contact RF-concepts or your local dealer. It has worked for me, it might work for you, but it might also not work for you.

This article claims no professional advice. Look for professional help from your next dealer. While I couldn't figure out the root cause of the tube's failure, the symptoms where quite obvious. The most likely theory is that one of the tubes was damaged during the shipment. The tube in question got so hot that it ignited the red, high-temperature silicon-rubber chimney. Even 30 minutes after removing the power, the tube was still not touchable by hand.

The Alpha91b is an old school amplifier with just a few Integrated Circuits and no Microcontrollers. The Schematic is quite simple and easy to understand. Everything else including the input network, which is accessible through a cover plate on the bottom of the amp had no visible damage. Measurements proved that the supply voltages were working as expected and located within the operational boundaries. Unfortunately, this test didn't work at all and I couldn't get out any power of the amplifier.

It is absolutely important to replace the tubes with a matched pair. Matched pair means that they have been selected as two tubes with equal parameters. Usually a matched pair is a bit more expensive because the selection has to be done manually. Googling for these tube brings up several webshops. Almost all are selling NOS new old stock.

These tubes have been manufactures decades ago, have been stored in russian military depots without being used a single time. I think both provide a good service.

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The owner, Ralf DL3JJ responded very fast and pre-heated both tubes the remaining 9 hours before they left his shop through express shipment the next day. Before using transmission tubes after a longer storage period in my case almost 20 yearsthe tubes must be conditioned. This process is called Gettering. Andrey, AE1S explains on his blog:. There is no such thing as a perfect seal!Your JavaScript appears to be disabled. JavaScript is necessary for various functions, such as order checkout, to operate on this web site.

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Order this item and get free delivery and handling on your entire order! Offer excludes truck freight and oversize fees. Valid on orders shipped in the contiguous United States. The performance characteristics of the 4CXA allow its use as a high gain grid-driven RF amplifier or as used in grounded grid service. These tubes may also be used to make great homebrew amplifiers.

When used in a modern amplifier with auto-tuning and vacuum tube protection circuits, they may produce much more power and exhibit very long life. We prove that, contrary to some reports, these tubes are still available. Get your amplifier back on the QRO bandwagon! DX Engineering Verified Purchase. Was this review helpful?