[Amps] What tube?

[Carl]

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



--------------------------------------------------
From: "Manfred Mornhinweg" <manfred at ludens.cl>
Sent: Thursday, January 29, 2015 11:58 AM
To: <amps at 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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