Wideband tube amp (or, trying to be) - Page 1
Forum section: electronics / projects

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 06, 2014, 10:09:23 am
content: Partly for curiosity and partly for lab use (something to bolster the function generator when it's not powerful enough on its own), I want to build a wideband power amp.Problem: I have kW worth of semiconductors on hand. They're all useless above a MHz or so.2N4401 will go into the 10s of MHz easily, but, watts? Come on.I have more watts worth of tubes on hand, but with such high impedances, surely I'll never get useful bandwidth out of them? Can't seem to find any examples -- every single project in the ARRL Handbook, for example, is tuned like a piano, with that classic high-Q Pi filter output section!Ah well, back to intuition. Well, I'm limited by plate capacitance and whatnot. So, impedance better be low. Low low low, what do I have, well, sweep tubes, regulator triodes, and a few odds and ends. With peak currents over 1A, sweeps are alluring, but quite large physically; the inductance might become a problem! The capacitances are also quite high (though quite favorable given the possible load lines). 6AS7/6080 are monsters in current (and low plate resistance), but the low gain is probably prohibitive. A smaller version, like 12B4, might be something to try. 12BY7 also looks good, a little high on the load line, but probably the smallest plate capacitance per cathode current I have. And then there's 5687, the illegitimate bull-dyke cousin of the 12AU7: a load line around delta 80V, 0.28A (~300 ohms) looks quite plausible.So, I set it up:The matching transformers are 50 ohms to 1632 ohm CT on the grid side, and 1088 ohm CT to 50 ohms on the plate side. Cathodes (and the 'hot' end of the heater) bypassed to ground; grids get grid stopper action, plates don't seem to need anything.Results: 0.3 to 8MHz (-3dB) bandwidth, 11dB gain. I was hoping for more like 30 or 40MHz, but it seems Miller is a strict mistress (erm?!) in this case. Tried neutralization (~10p from each plate to opposite grid), no effect.Question: any of you HAMs know about using tubes for linears or wideband amps? Seems to be very little out there besides narrow band, tightly tuned rigs. Only solutions to save bandwidth that come to mind are, maybe something with feedback windings (in series or parallel, between the two transformers or winding ends?!).Or grounded grid, of course. Which should at least keep Miller at bay, but comes at a hefty 3dB penalty, and that means to get the theoretical 20W output from this poor sucker I need more than 3W in, which I also don't have. Turtles, I say!Tim

author: Precipice
author_rank: Frequent Contributor
timestamp: November 06, 2014, 11:22:58 am
content: Isn't this the holy grail? A robust, cheapish, wideband power amp?I caved in and bought a cheap Amplifier Research 60LA from ebay (4-150MHz, 100W, mismatch-proof).It's basically a simple transistor preamp, then a bunch of 8 blown-air-cooled ceramic 8121s).I can send you the scanned manual if you want to see how AR did it in the 70s. Depressingly, I think things haven't moved on that far yet, although surely solid state must eventually catch up?

author: KJDS
author_rank: Super Contributor
timestamp: November 06, 2014, 02:05:58 pm
content: High power semis are available, it's fairly straightforward to do what you're after, however it takes a lot of silicon and a lot of specialized packaging to get what you're after. A pushpull class AB should be fairly straightforwards, just a lot of care needed for the baluns, and the heat dissipation, and the device matching, and biasing and power supply, but apart from that it's easy.

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2014, 10:39:14 pm
content: I would have at least expected to use a tetrode or pentode or a cascode configuration to avoid Miller capacitance. Tektronix used lots of triodes or pentodes in distributed amplifiers for this sort of thing although the 80 MHz 585 oscilloscope used a 6939 or 7699 dual power pentode (more like a beam power tube?) for its output stage (driven by a distributed amplifier) which suggests the type of tube to look for.As far as 2N4401s and 2N4403s, you can get a surprising amount of power if you use lots of them in parallel which also conveniently adds to the surface area available for heat dissipation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2014, 01:05:07 am
content: Quote from: Precipice on November 06, 2014, 11:22:58 amIf you would, please!Looks like a distributed amp (which is another thing I never quite understood..).Probably, tubes remain oddly relevant for this sort of thing (robust RF amps, anyway) because their efficiency is poor to begin with (so the added dissipation of crappy mismatch doesn't cause much trouble) and their thermal time constants are measured in the seconds to minutes (as opposed to microseconds!).Tim

author: Marco
author_rank: Super Contributor
timestamp: November 07, 2014, 02:03:42 am
content: Quote from: Precipice on November 06, 2014, 11:22:58 amIf it's still going to be a couple times more expensive than the narrow band amps is it really that interesting? It seems to be a bit niche to me ... at 10$ for a 50W LDMOS RF FET is it really the silicon holding this back?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2014, 05:14:27 am
content: It does work better with grounded grid (using a somewhat lower turns ratio on both transformers): now I'm getting 10ns rise time, but only 7dB gain too. So, that at least is in line with my expectations, but man, 4W drive still...Tim

author: Precipice
author_rank: Frequent Contributor
timestamp: November 07, 2014, 08:58:33 am
content: Quote from: Marco on November 07, 2014, 02:03:42 amSwapping amps (and checking to make sure you've done it right) in the EMC chamber just gets unbelievably irritating. Also, the price of a dozen narrowband amps starts to mount up, although a common power supply would help.The unblowupability of that AR amp is also very, very nice. It's impossible to guarantee anything like a well behaved antenna VSWR when I'm testing stuff. The antennas are wideband (hence a bit random), and even in an anechoic chamber, there are reflections and standing waves at times.So yeah, there's probably always going to be a need for a robust wideband amp.(I've got refrerence designs knocking around for LDMOS FETs, that I've picked off the web from time to time, but never built. If, in some parallel universe, they all worked and were robust (the datasheets all extol the virtues, but real life and datasheets...), these four might make a bearable stack.MRFE6VS25NR1 Broadband Reference Circuit Schematic ? 1.8--30 MHz 25W.PDFMRFE6VP100HR5(HSR5) Broadband Test Circuit Schematic 30--512 MHz 135WMRF6VP3091N_Broadband_470-860MHz_18W.pdfCLF1G0035-50_1G0035S-50 broadband amplifier (500 MHz to 2500 MHz) demo circuit 50W .pdfMHV5IC2215N_1-2GHz_20W.pdf

author: Precipice
author_rank: Frequent Contributor
timestamp: November 07, 2014, 09:22:32 am
content: Quote from: KJDS on November 06, 2014, 02:05:58 pmIndeedAnd the fact that AR can still charge over $0.5M for their bigger amps pre-owned shows there's a market - best get started!I don't know how possible it is to build a bulletproof output stage - those LDMOS devices have so little thermal mass in the packaging that maybe you could watch them thermally, but it'd need to be on a hair-trigger.

author: Marco
author_rank: Super Contributor
timestamp: November 07, 2014, 09:35:11 am
content: Quote from: Precipice on November 07, 2014, 08:58:33 amI'm not disputing the need. I'm asking if whether with relatively cheap LDMOS devices it's really the silicon holding back their availability ... instead of the size of the market and the old equipment which is still around.PS. there must be some HAMs which have build some wideband amplifiers with LDMOS (here's one at least).

author: Richard Head
author_rank: Frequent Contributor
timestamp: November 07, 2014, 09:49:53 am
content: T3sl4co1lYou can use a tube in grounded cathode configuration without a high Q parallel resonant tank network. The only reason hams use the resonant tank is that it provides attenuation against harmonics and provides a convenient impedance matching function for the antenna. If you prefer a wideband amp then just use a decent series choke (high self resonant freq) without the tank network in the anode and capacitively couple the output.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2014, 08:32:28 pm
content: Quote from: Richard Head on November 07, 2014, 09:49:53 amHow much bandwidth can I expect from this configuration?Tim

author: Zero999
author_rank: Super Contributor
timestamp: November 09, 2014, 10:48:02 am
content: Perhaps you need more stages?How many devices do you have?What about using a cascode configuration to give more power gain, possibly with a follower on the output to reduce the impedance and provide more current gain?

author: Richard Head
author_rank: Frequent Contributor
timestamp: November 09, 2014, 01:23:44 pm
content: T3sl4co1lI'm not sure what bandwidth you can get from the tubes you are using but with the correct tube a bandwidth of 50Mhz should be no problem.Your decision to use a tube does have some merits in my opinion. Yes, I know it's antiquated technology and a high voltage PSU is a pain, as is the heat generated but the IMD performance compared to transistors and ruggedness shouldn't be underestimated.I would be inclined to use a common cathode configuration (for gain) with neutralization to combat the Miller capacitance. A suitable gain sloping network should also be used to REDUCE the gain at lower frequencies.If there are a few lumps and bumps in the response you can always sample the output level, compare it to a reference and adjust the gain (PIN diode) to yield a super flat response. I've done this before and it works really well. Perhaps overkill!If the amp is going to get abused then a circulator in the output and a termination resistor is an option. This should make it really rugged.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2014, 10:06:32 pm
content: Quote from: Hero999 on November 09, 2014, 10:48:02 amMore stages, certainly... with only 7dB per stage, I need two or three to get full (saturated / clipping) output using my function generator (~0.5W) as source.Cascode would work out disturbingly well (the cathode and anode load impedances are pretty close), but this would cost serious supply voltage. I could do folded cascode, with transformer coupling; but I have to worry about the frequency response of all those transformers, which means more turns, and more opportunity for oscillation if they decide to be unstable.A follower might improve the dynamic output impedance, but doesn't change the load line: a 50 ohm output impedance doesn't help me any if it supplies less power than the input (gain < 0db)!Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2014, 10:15:25 pm
content: Quote from: Richard Head on November 09, 2014, 01:23:44 pmWell, I don't know about IMD... I've seen plenty of THD under various conditions (such as reduced supply voltage -- it runs into class C -- under condition of fixed grid bias, adjusting plate voltage within limits is an easy way to adjust bias), and the transition from high impedance grids to grid current is definitely reflecting power back to the generator, and introducing distortion.I'm not concerned with heat -- for test equipment, I'd build a class A amplifier without a second thought. Although as long as I have a pair of active devices, I might as well go class AB and kick up the power level while I'm at it.As for ruggedness, the other day I build a sort of amplified Tesla Coil, using a matching network to drive the helical resonator and a feedback winding to drive the amplifier. With only 7dB gain, understandably it took quite a lot of feedback to achieve, but I was seeing plate supply current over 100mA, enough power output to light a neon bulb (not really saying much..), and enough plate dissipation to make the plates glow dull red.5687 is brick-shithouse territory, so I'm not afraid of breaking anything, no.QuoteIs this not what I tried in the OP? Grid drive, plate output, attempted neutralization (10pF from each plate to opposite grid)?The results were pretty shitty (11dB gain, 8MHz BW)...QuoteMmm, how so?I would rather have more gain at high frequencies, than attenuate at lower, to maintain flatness...QuoteNot sure what you mean. I have no PIN diode here, and this is not a radio transmitter application, this is wideband. In general, I may be pushing many frequencies through at once, not just a single modulated carrier. I can't tweak gain to fake response in a case-dependent manner.QuoteEspecially so for solid state amps!Do you have a wideband circulator circuit handy? It's my recollection they're not easy to build from lumped components, and that's narrowband besides...Tim

author: Richard Head
author_rank: Frequent Contributor
timestamp: November 10, 2014, 08:04:38 am
content: T3sl4co1lFor wideband applications the IMD performance is very important. If you have multiple input frequencies with poor odd order IMD response then the input signals will mix together to produce inband distortion. THD products are normally out of band so don't create a problem.To reduce the IMD a simple solution is to under run the amp. This is a brute force approach but always works well.I wonder if your input matching was poor and that's why you got such a low gain. At such low frequencies I would expect a lot more gain.A trick to select the correct amount of neutralisation is to power down the circuit and feed the signal from the output backwards through the amp. Measure the feedthrough power and adjust the neutralisation for minimum through power.Yes, you are quite correct about the AGC I suggested. It is only applicable for narrowband applications. Sorry, my mistake.For output protection instead of a circulator (expensive, relatively narrowband etc.) you could use a Bruene type VSWR sensor. You can make them quite easily but you do need some RF equipment to set them up properly.Keep plugging away. I'm sure you will find a solution.

author: GK
author_rank: Super Contributor
timestamp: November 10, 2014, 08:37:11 am
content: Not sure why you're surprised about the low gain with the 5687 with a plate load of ~1k. Just giving the characteristic curves for that device a quick glance, at 200V on the plate and 10mA bias current, the gm is only about 4.5mA/V. That works out to about 13dB voltage gain with grid drive. At a low voltage operating point the tube isn't going to perform better so your measured 11dB seems about right.If you're dead-set on twin triodes, find a 6DJ8 and wire both halves as a single cascode amplifier (with 180V on the upper plate and 90 on the grid of the cascode, as the tube was specifically designed to be operated, for television IF stages). That tube has higher gm and the cascode configuration should keep the Miller effect further at bay. Fat chance of getting decent voltage gain at 50MHz without a narrow-band, tuned, high impedance (relative to ~1k) plate circuit though, IMO.Even if you combat Miller c, stray c from plate to ground will corner with the effective plate resistance and limit your bandwidth. 1k corners with 3.2pF at 50 MHz. There is probably that much capacitance just in the tube socket. With a resonant tank plate circuit the stray c from plate to ground is just lumped in parallel with the tank circuit capacitance, so it's much less bother to design a high impedance narrow-band plate circuit to achieve high gain in a single stage at high frequencies.

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2014, 01:00:02 pm
content: I would be complex to calculate and perhaps make but what about using a bridged T-coil network to handle the output capacitance?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 10, 2014, 01:34:13 pm
content: Quote from: GK on November 10, 2014, 08:37:11 amYes, the gain is what I expect, but it's still disappointing that it's so low. Alas, beefy triodes tend to have that effect, so...Then take off another 4dB for the cathode drive. Feels like the damn IRS...QuoteSee, that's the tricky part, isn't it? I could throw around cascodes or pentodes or whatever I want, but they're all small signal, meaning (output) capacitances of a few pF, which when paired with a load line of say 10kohms, gives SFA bandwidth!(FWIW, note that, although I'm concerned with bandwidth in the baseband, it remains roughly constant for any tuned bandwidth. A device that rolls off at 10MHz baseband might be usable at 5-15MHz with light tuning, or 90-100MHz with sharp tuning. In all cases, the band *width* is ultimately limited by output capacitance.)I'm not at all set on triodes, but I don't have any pentodes that look promising. 12BY7 is the top contender, but it's kind of oddball in its own way; plate saturation is terrible ("Rds(on)" is a whopping 1kohm) and operation is almost always screen current limited. It's intended for class A, RC coupled applications -- video amps and oscilloscope deflection. Tektronix used more than a few, but for interstage, not outputs I think.The only other way to get more bandwidth seems to be distributed amplifiers, which if I'm not mistaken, work by connecting everything in parallel, but canceling the capacitance through inductors. So, normally, you can cancel capacitance to ground with inductance to ground, but this creates a narrow (tuned) bandwidth. If, instead, the inductance bridges one capacitance to another, and so on, you get a lumped transmission line. And therefore, operation will necessarily be a traveling wave sort of phenomenon (which is how they're arranged: input on one end, output from the other, with source and load terminators on the opposite ends of each respective transmission line). It should also have an even sharper cutoff than the narrowband case, because an N-element lumped transmission line is also known as an N pole lowpass filter; if the transmission line is nondispersive (N/2 repeated real poles, I think), it will be very sharp indeed just beyond the characteristic frequency.QuoteIn the 5687 case, it's the Thevenin impedance of the source and load. The load impedance is in parallel with the plate resistance, but since this is a pretty steep load line, the load dominates over plate resistance. So really, it's even better, more like 300 ohms to 4pF for each plate, and a bit more for the cathode circuit.For example, connecting a "swamping" resistor in parallel with either circuit visibly improves the bandwidth, at direct expense to gain and efficiency of course.The killer in the grounded-cathode case seems to be Miller effect, which would've demanded about 4 times lower grid drive impedance, which is absurd (they're only grids drawing a few watts), and would result in even lower gain than the grounded grid case.Tim

author: GK
author_rank: Super Contributor
timestamp: November 10, 2014, 10:40:20 pm
content: In a cascode circuit the internal plate impedance (in parallel with the external load impedance) goes up dramatically. You are after all emulating apentodetetrode. That gives you a little extra gain. To combat the Miller effect you could even wire a pair of 6DJ8's (four triodes) as a cascoded push-pull amplifier. But you still have shunt c (not just that inside the tube) from plate to ground as the ultimate bandwidth killer, and especially so if you try to raise the plate load resistance for more gain.

author: G0HZU
author_rank: Super Contributor
timestamp: November 10, 2014, 11:03:19 pm
content: QuoteI don't claim to be a wideband PA design 'guru' but I've designed several solid state amplifiers that cover a decent bandwidth. The latest wideband designs use GaN technology and there are reference designs out there that cover a few MHz through to 6GHz at several watts. I've got an old lashup PA design of mine here using GaN parts and it covers about 6MHz to 1GHz at about 15W with pretty good flatness across this range. The harmonic performance is pretty poor though.It has been possible to cover 20-520MHz at 100W using LDMOS or VDMOS for quite a few years now and GaN pushes this further. At work some of our 50W PA designs cover up to 6GHz and use GaN.Here's a fully packaged GaN device that does 25W over a wide bandwidth.It's from RFMD and is pretty much state of the art. I've got experience of using this part at work. It is effectively a drop in amplifier. However, I don't think it's available to everyone to buy but it is an interesting part anyway...http://www.rfmw.com/datasheets/rfmd/RF3833_rev.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 11, 2014, 03:43:57 am
content: "Confidential: NDA required"Wow, it's like they actually want people to buy their products or something!Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 11, 2014, 03:45:55 am
content: Quote from: GK on November 10, 2014, 10:40:20 pmYes, but I'm still limited by the fact that 1. a 6DJ8 is only good for a couple watts into a fairly high impedance (1-2k),which fundamentally limits my bandwidth regardless of how much gain is stacked in front of the finals, and 2. cascoding drops half my supply voltage, so I either need twice the supply, or accept that my power capacity goes down by a factor of four!Tim

author: GK
author_rank: Super Contributor
timestamp: November 11, 2014, 04:24:28 am
content: You didn't actually state how much output power output you actually need. I suggested the 6DJ8 for consideration for experimentation because it was specifically designed to operate on a low plate voltage (which makes cascoding easier) and it also has ~3 times the gm of a 5687 (just from looking at the curves), so you could still keep the "effective" (notice that I said that word in my 1st post - as in Rp || external plate load) plate impedance low (or even lower) without getting less gain.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 11, 2014, 04:58:40 am
content: As much as I can, I suppose, and much more than 1/2W which is what the sig gen does. 20W sounds like a good number; nice thing about the 5687 is, it's right about there, at least the load line says so. Similar results from sweep tubes (or more), but I'm not sure if they're really optimal (high Cin, Cout; large inductance); I still need to try though.Or to put it another way: I have 2N4401 and 2N3866, either of which I can rub a few together to get a few watts SS. Which I'll still want to do anyway, so it would be nice to have something even higher power, without having to go out and buy LDMOS parts (which I'll probably do anyway, but in the mean time, hey, tubes!).Tim

author: coppice
author_rank: Super Contributor
timestamp: November 11, 2014, 05:10:49 am
content: Quote from: T3sl4co1l on November 11, 2014, 03:43:57 amGaN is a hot topic right now. Everyone is trying hard to see what their competitors are achieving, and how. Serious customers aren't put off by NDAs. They may be a pain, but they are now the norm.

author: David Hess
author_rank: Super Contributor
timestamp: November 11, 2014, 06:11:40 am
content: Quote from: coppice on November 11, 2014, 05:10:49 amI am sure their competitors are put off even less by NDAs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2014, 12:47:40 pm
content: Try two...12BY7A, two in parallel, class A. "Parafeed", the plates are supplied from a large choke and coupled to the OPT.Grid circuit impedance: 160 ohms. With 20pF input capacitance, these things need a lot of drive, despite the low power requirement. I don't think there's much I can do about this. A higher impedance would give considerably higher gain, but would drop the cutoff frequency by the same amount. The coupling transformer should have no effect on high frequency cutoff. To absorb power, the grid circuit is terminated (180 ohms 1/2W, close enough).Plate circuit impedance: 450 ohms. With 3.5pF per plate, this should give up to 100MHz bandwidth, but probably a lot less. The OPT for example is wound with about a meter of wire, which will look like about 50 ohms as a transmission line, but it needs to be nine times higher than that, so it will have the capacitance of a matched line nine times longer, or a 1/4 wave cutoff of 133MHz.With such a high impedance, I need to pay attention to the plate choke. I decided on a T50-52B core with 100 turns of #28. The winding is one layer progressive, but turns are piled up as they go. This minimizes shunt capacitance -- if it were wound in two or three layers, the overlapping wire would again form ~50 ohm transmission lines between layers, which have much too low impedance for the circuit (= effective shunt capacitance), and being unterminated (and dispersive), would ring like a bell and ruin the frequency response.The remaining downside is, #52 powdered iron is pretty conductive stuff, so there's still the capacitance of winding over core. This could be avoided with a gapped NiZn ferrite core (low conductivity, moderate dielectric constant), but I don't happen to have any toroids with notches in them.To counter the remaining capacitance, a small ferrite bead (Fair-Rite 2673000101) was wound with 4 turns, giving a pretty reasonable impedance at the highest frequencies, plus some damping.Now, since there's two tubes in parallel, I decided to try to peak them a little. There's a bit over 1uH from plate to plate, and about 0.3uH from grid to grid. Input goes to the first grid, terminator on the second; plate output connects to the second. This makes a bit of a distributed amp, which seems to help.So, I'm getting 13.7dB gain and 7.6ns risetime (46MHz BW?). Should be about 4W output from 0.2W input.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2014, 07:58:23 pm
content: Circuit:

author: Richard Head
author_rank: Frequent Contributor
timestamp: November 13, 2014, 08:15:34 am
content: I wonder how well those tubes are sharing the power. I also wonder if the quiesent current through both are equal, or nearly equal.Next step is a two-tone test. That should be interesting.