Question about RD06HVF1 amp

[OgreVorbis]

What is the purpose of the big resistor leading into the drain on this amp. Wouldn't that limit the power output? Could it be removed for more power?

[teckniqs]

It looks like it limits the voltage for the bias circuit.

[OgreVorbis]

But doesn't it lead to the drain part of the amp (it's not going to ground).

[thewisepranker]

I believe it's to slightly damp the drain inductor (under the watermark "B") to hopefully stop oscillations.

[Gigahertz]

Could it be removed for more power?

If you want more power, be better to buy a bigger amp!

[rettimsnarT]

Very simple to change for RD15HVF1.

[Maximus]

Not quite as simple as that, unless you drive it hard lol


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[Albert H]

The output impedance for a 6 Watt amplifier at 12V supply is 12Ω. The output impedance for a 15 Watt amplifier at 12V supply is 7.5Ω. The output match will therefore be quite different if you want to match into the usual 50Ω.

The big resistor is there to quench parasitics. I have a similar board here (Chinese made) and disconnected the resistor. Spurs appeared each side of the carrier because the FET self-oscillated (slightly) at around 200kHz. Putting the resistor back gave a nice clean output.

It used to be quite normal to have something like a 100n capacitor in series with 22Ω as part of the decoupling in a power amplifier.

[sinus trouble]

My question is who built this amp? NASA? The housing is machined from a solid block of ALU with gold connectors and feedthru DC cap! Not no cheap sh*te im guessing!

[3M amateur]

Hi,

where have you purchased? have searched on eBay and Ali express even can not find a similar one.

[thewisepranker]

http://www.ebay.co.uk/itm/87-5-108MHz-F ... 2058586361

[3M amateur]

http://www.ebay.co.uk/itm/87-5-108MHz-F ... 2058586361

Thanks

[thewisepranker]

My question is who built this amp? NASA? The housing is machined from a solid block of ALU with gold connectors and feedthru DC cap! Not no cheap sh*te im guessing!

The feed-through capacitor is good engineering, however gold plated connectors being "the best", especially for use in AC applications below about 10 GHz is a common misconception. Nickel would be far, far better.

Gold, 100 MHz: 7.47 µm, 10 GHz: 0.747 µm
Nickel, 100 MHz: 0.93 µm, 10 GHz: 0.093 µm
Look at that, the skin depth of gold at 10 GHz is the same as that for nickel at 100 MHz. Source here.

Nickel is normally plated (using either the electroless method or the conventional electroplating method) at around 5 µm in average thickness, less for cheap stuff but not by much.
Gold is normally plated at around 0.5 µm if you're lucky, so won't even support the skin effect at 10 GHz.

You could argue that the gold is better at resisting corrosion than nickel but I'm not really convinced as the gold has to be pure in order for that to be true, and it never is.

[sinus trouble]

That's a good point Mr P! I wish I paid more attention in chemistry classes (serious its really interesting)
From what I understand? certain metals are used as an 'alloy' (mixing elements) to improve structural integrity and durability? for example, Pure silver jewellery would be useless in its own state without adding a touch of copper to strengthen it up

[thewisepranker]

True, but not always - they are added for all sorts of reasons. For example, nickel, chromium and molybdenum are added to iron to make (in this example 316L) stainless steel, not for mechanical strength but for corrosion resistance.

[Radio Garfield]

Hi everyone.
Since this is my first post and public appearance here I would like to say hello to all of You out-there.

Does someone knows what input power for this device RD06HVF is enough to deliver about 5W on 88-108 at about 13V?

A friend of mine sent me his fm transmitter with blown output fet, which has deleted markings on it, so I don't know what type it is.
Input power from buffer is 50mW so I wonder what output device could I use instead of that blown FET. This RD06HVF seems to be option!?

Thanks in advance.

[Albert H]

The RD06HVF is a good choice for 50mW of drive - you should be able to get close to full power with that much assuming the PA circuit's right. Remember - these FETs are static sensitive, so be damn careful when you handle them. I work on an earthed metal workbench, wear an anti-static wristband and keep the terminals of these FETs shorted together until they're safely installed into their board.

You want to start with the bias turned right down. Check that the supply actually is 13V, then apply the 13V supply to the PA. You shouldn't see anything on your power meter - the FET shouldn't be self-oscillating. If it is, turn off the power and carefully examine the board, looking for open or short circuits and make particularly certain that all the decoupling capacitors are OK.

Assuming you've got this far OK, connect the 50mW drive up. You might see a bit under a Watt coming out of the FET, Slowly and gently advance the bias. The power will rise quite smoothly. When you get to 6W, don't go any further. Over-bias can kill these FETs (that's probably what your friend did in an effort to get more power out), If you don't achieve the full 6 Watts, don't despair - 4 Watts will have pretty much the same coverage as 6 Watts!

[Radio Garfield]

Good day, thanks for reply.

RD06HVF is my first choice if I get to modify board for it, cause in original there was FET in case SOT 123A, like BLF245. I dont know what exact device there was, which was good for 50mW input and few W output. I cant remember any device for such a low input.

Anyway, thanks for static tip. I was lucky enough not to destroy any.

BIASing is done with variable resistor, so it will be possible to do it correctly on the way you described.
4-5 W of output power is my goal. 6 W will not show anything noticeable in the air, just like you said.
I think it would be fine to start new thread with this transmitter. It was destroyed partially so it needs to be repaired in the output matching network. I just want to show you guys interesting project. Some factory ( or someone ) did an interesting job with it.

hear you again. Bye.

[RF-Head]

I think you have a old board that i made many years ago
Using alot of SMT parts and a motorola PLL chip and run on 24V
The transistor is a Philips device and i may have a few left
Please post a picture of it and we can see if it's this one

[Radio Garfield]

I think you have a old board that i made many years ago
Using alot of SMT parts and a motorola PLL chip and run on 24V
The transistor is a Philips device and i may have a few left
Please post a picture of it and we can see if it's this one

Hello. Well, You never know! What a small world.

I am at work right now and I will do it tonight probably. Yes, it is smd work ( a nice work by the way) It has MC145151P2 for PLL
with u664 prescalerc. VCO is factory made. Few OP-amps 723 used. I remember that buffer stage have SMD device with S5 mark on it. I could not find anything on the net what device it is. Maybe something like BFR96 or so in SMD case.

Transmitter is in the metal case, with feed-trough capacitor for power supply connection. LPF is made with inductors in straight line.
It ended up in Germany, then now in Bosnia and Hercegovina at my place. What a ride!

I did not saw product like that on the internet before.