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Re: [Amps] What tube?

To: "Manfred Mornhinweg" <manfred@ludens.cl>, <amps@contesting.com>
Subject: Re: [Amps] What tube?
From: "Carl" <km1h@jeremy.qozzy.com>
Date: Thu, 29 Jan 2015 18:52:39 -0500
List-post: <amps@contesting.com">mailto:amps@contesting.com>
The 4-400 and GU-81M can both be run in GG. In a SB-220 with100W of drive you can expect 1000W due to a bit lower gain than the 3-500Z. Crank up the voltage at the same 100W and 1500W is likely attainable even on 10M with full emission tubes, those are often $20-30 NIB military surplus into the 70's. Sockets are the same as the 3-500 and the flat glazed ceramic Johnson style are $15-30 at US hamfests and Fleabay. The Eimac SK-410 runs about twice that from the same sources.

Im using GU-81M's as AM modulators and the sockets are often more expensive than the tubes from most European sources. A reflector member picked up tubes and sockets for me at Friedrichshafen a few years ago.

A tube that Im surprised you and others overlooked is the QB5/1750, popular in BCB and SW broadcasting. Expensive at retail but often bargains where commercial transmitters are being scrapped for SS. I bought a pair off Fleabay a few years ago for $20 NIB...only one was listed and he tossed in the other as I was the only bidder. They were the spares for a BCB TX. The socket is the same as the 4-1000A but the European version is more common and cheaper as several RF and and industrial tubes, especially in EU, used them. Mine are the RF end that the GU-81M's are modulating.

Carl
KM1H



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From: "Manfred Mornhinweg" <manfred@ludens.cl>
Sent: Thursday, January 29, 2015 11:58 AM
To: <amps@contesting.com>
Subject: Re: [Amps] What tube?

Well, I got suggestions for three different tubes. All of them being directly heated glass bottles, which makes a lot of sense given my requirements.

I don't think I will ever actually build that amp, but I wanted to see what could be done, in the line of a reasonably cheap and efficient amplifier using a conventional tube-type RF section, combined with a modern switching power supply.

In calculating the efficiency of amplifiers, I think we need to consider _all_ power taken from the supply line. That includes filament power, and in teh case of tetrodes, screen power.

Instead I think we do not need to include the drive power in the formula, despite the fact that GG amps feed most of it to the output, while grid-driven amps either need very little drive power, or burnit up in a dummy load. In fact most ham amplifiers are built to be driven by standard 100W radios, set up to 100W or near that, so this area will be similar for all amps.

There are three power drains to consider: At full output, in TX at no output, and during RX. In order to get lowest possible overall power drain from a simple, conventional tube amplifier, I set the requirements for class AB2, with low idling current, and instant-on filaments that can be shut down during RX, at least for slow ragchew-type communication. Shutting down filaments means that also the fans can be shut down, putting the amp into near-zero power drain mode.

So, let's see:

A pair of 3-500Z bottles indeed seems like a pretty optimal choice, being able to run in a simple, zero-bias GG circuit. Although it looks like to really operate at legal limit and low idling current, it would need some bias - but that's easy enough to do. Running at a tad above 3000V, 120mA bias, 800mA max at PEP, it delivers 1500W PEP out at 61.7% plate efficiency. Considering 146W filament power, the overall efficiency is 58.2%. Total power input of 2578W, plate dissipation of 932W. Cooling requires a good air stream from fans, but no noisy blower. The output matching is reasonably easy, and the drive requirements are 92W over 57 ohm, allowing a radio with higher Q output to drive the cathodes directly, while a typical radio would have an easier job driving it if simple resonant circuits are used at the cathode.

At idle during TX it consumes 511W, and during RX it's down to 146W.

To the above figures we have to add the fan power (maybe 20 watts), and power supply losses. On the other hand, if we shut down the filaments during RX, and after a minute shut down the fans, power drain is nearly at zero.

Cost for those tubes ranges from about $340 for the cheapest ones, to $570 for ones with better reputation. I don't know what the sockets cost.


Now let's see what happens with a pair of 4-400: Judging from the data sheet, class AB2 operation at 1500W would require roughly 3230V, 700mA at the plates, 500V 39mA at the screens, and 146W for the filaments. That means 61.8% overall efficiency, slightly better than the 3-500Z. The price for that is the screen supply. Plate dissipation is 761W. Grid bias would be -83V, which means that a 100W radio can drive the grids directly, no impedance transformation needed, just a dummy load at the grids, which also has a stabilizing effect. So we have simpler drive than with the 3-500Z.

During TX idling, plate current is 150mA and screen current is zero. Including the filaments, that is 630W idling power. Worse than with the 3-500Z, but this can be tweaked, probably sacrificing some IMD performance.

Power drain during RX is the same as with the 3-500Z.

Cost for these tubes, Taylor brand, is $478. No idea about socket prices.


The third submission I got (well, actually it was the first!) was a pair of the GU81 pentodes. These are really huge bottles, gorgeously beautiful, and very inexpensive compared to the other two! From an emotional point of view, I would say, go with them and build a window into the front of the amp, so that the tubes can be seen all the time!

Performance data for class AB2 operation is not as easily available for these tubes. I worked it out from the data given in the sheets, but I'm not very sure of it all. These huge tubes are frequency-challenged by their high capacitances, so that at 30MHz they need to work at reduced voltage, not much more than 2kV.

It seems that to get 1500W output, the parameters would be roughly 2200V, 1.1A at the plates, 600V at a whopping 400mA at the screens (can that be possible?), and 277W for the filaments! That would mean an overall efficiency of only 51%. On the 10 meter band, the plate tank Q would need to be higher than 20, because of the huge capacitance! And the grid requires a rather high drive voltage, so that bandswitched PI tanks would be needed at the grids, or maybe a very well made broadband transformer, followed by a dummy load.

It seems to be a far less than optimal choice, from the technical point of view. But the sheer beauty of these tubes, and their rock bottom price, around 60 dollars for a pair, are attractive... It would be a fun project, but not really a technically competent amplifier, it seems.


Comparing these three tube options to my cheap MOSFET amplifier project, which also seems to be pretty much shelved: I'm getting an efficiency of roughly 55%. Of course there is no filament power, and the low efficiency is due mostly to the poor saturation characteristics of the MOSFETs, given by the voltage-dependent internal capacitances. So, even with the power lost in the filaments, good tubes are more efficient. If we shut off the filaments during RX, tubes win this efficiency contest, although only by a small margin. On the other hand, my MOSFETs cost only about $70 for the full set, and allow making a no-tune amplifier. Then again, they are easier to kill than tubes, if an antenna connector comes loose or such.

It's somewhat of a tie.

One more question, maybe a bit stupid: How fast is the heating of 3-500Z and similar directly heated tubes? Are there specs available? Is it reasonable to switch off the filaments during RX, or would that mean cutting off the first few words of every transmission? Would it damage the tubes to switch the filaments on and off very often?

Manfred


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