ACOM HF Amplifier - which one to choose? - Page 1
Forum section: electronics / rf-microwave

author: noreply
author_rank: Frequent Contributor
timestamp: December 08, 2020, 05:00:27 pm
content: Just wondering if there is someone who could make some informed comments on which ACOM HF Amplifier is best in terms of reliability and cost of operation ...The choices to choose from are the ACOM 1000 and the ACOM 1500The 1000 uses a single Svetlana 4CX800A (GU74B) high-performance ceramic-metal tetrode with plate dissipation of 800WThe 1500 uses a single 4CX1000A tube - a ceramic and metal radial beam tetrode with plate dissipation of 1000 WThe amplifier will most likely be never 'tuned' above 400W for the immediate future - but might go to full power when and if QTH changed to country which has higher legal limit.Other than the associated circuitry to cater for the different 'tubes' - the design of the amplifiers is identical.Here is some rationale for why these two models from the ACOM range are chosen ...ACOM seems to have the best reputation for reliability and value for money with their 'tube' amplifiers.The 1010 model which uses a single Svetlana 4CX800A (GU74B) - does not support 6M band - other than this its most likely best option as the 4CX800A will be running at a level which will prolong its life.The 1000 has a cheaper 4CX800A compared to the 1500's 4CX1000A which costs considerably more.The 1500 with its 4CX1000A if operated at the 1000 level will prolong the life of the amplifier and 'tube' much longer than simply running the 1000 with its 4CX800A at its rated limit.The 1500 has 3 switched antenna inputsThe 1500 power level covers most countries which support the 1.5KW legal power level - so if QTH changes to appropriate country - you could use that countries max legal power.The 1500 might have a better resale value as it is more desirable (meets legal power level) to those in countries which support higher power levels.Anyway ... thought this might be an interesting topic and perhaps some useful information could be exchanged for those looking to acquire a 'tube' based HF linear amplifier (yes the 4CX's are very linear devices when operated correctly)Take Care

author: bob91343
author_rank: Super Contributor
timestamp: December 08, 2020, 06:46:04 pm
content: You are overthinking this. Assuming the design is well thought out, either one will do.However in my opinion I don't like to use ceramic tubes. Aside from their higher cost, they require warmup before use. I much prefer amplifiers using 811A or 572B or 3-500Z tubes, as they can be used within one second of being switched on. Waiting the necessary time before use means ceramic tubes have to be in standby for upwards of a minute or alternatively left with filaments lit ready for use, which shortens life.I have been using 3-500Z tubes for years and appreciate both the nearly instant warmup and the reduced energy by not having them lit at all times. If that doesn't bother you, go for either ACOM unit.The bottom line is that you pick and enjoy; any amplifier will do the job.

author: noreply
author_rank: Frequent Contributor
timestamp: December 08, 2020, 08:08:30 pm
content: Thanks for the feedback - much appreciated.I hear what you say about the 'tubes' - yes totally see your point - I guess after a period of time it all adds-up in power usage. I think the filament on the 4CX is 6A @ 10V?? about 60W - not exactly low power stand-by.You are correct BOTH amps are nice - but the price differential between the 1000 and 1500 is significant - about GBP800 - about US$1000 - so need to justify this - but I guess you get what you pay for.The interesting thing for me is the reason for getting an amplifier in the first place.I love working on LOW power and having a brilliant antenna with perfect matching to frequency (yes this IS possible without a 'tuner')The problem is when you are trying to 'work' a DX station - there are always 'other' stations with KW power levels in essence killing any chance of your signal being spotted and more important getting a reply.If the playing field was even - whereas you need to uses good propagation conditions AND clever antenna design / implementation - this would be great - but it does not happen.Today most 'hams' have the KW's and use them inappropriately - killing any opportunities for people like me.There ARE true - more justifiable reasons for using a KW amplifier.I founds that under 'closed-band' conditions - a KW will still get 'heard' a significant distance - something you cannot do with a QRP set-up.I have managed to 'work' a station over 3000KM on 40M when the band was TOTALLY closed - I was using 90W and a well tuned antenna - he was using 1KW and a much higher orientation dipole antenna. His signal was 5,9 at least 4 's points' above the noise at my end - my signal was 5,7 - 1 to 2 s-point above 'his noise' at his end.If I had a 1KW amp - he would have 'heard me at 5,9 at least 4 's-points' above his noise.For now - it does not matterIf you are a serious 'contester' - then it does matter I guessSo getting an amplifier is in some way 'forced' on you to level the playing field - which should already be level if everyone played by the rulesCheers

author: Theboel
author_rank: Frequent Contributor
timestamp: December 08, 2020, 09:13:17 pm
content: is there any specific reason You want a tube amp ?If Your target is 1kw I think solid state amp from acom is very capable, no need to worry about tube wear out. no warm up

author: bob91343
author_rank: Super Contributor
timestamp: December 08, 2020, 11:34:40 pm
content: I am not trying to justify using an amplifier. I use one some of the time, depending on several factors. If a station is popular I will use it to save my personal energy by not having to call many times. If propagation is poor, it helps. I run 'barefoot' much of the time.Be all that as it may, a directly heated tube amplifier is convenient and does the job. A modern solid state amplifier overcomes the heating time and usually costs significantly more than a tube amplifier. And if it fails it's generally a bigger deal to repair.Since it's a hobby, money needs to be factored in. If you have to spend an extra $1000 due to some consideration, I would hesitate. In fact you can buy any number of used amplifiers for that much and less. I prefer to buy used gear for the greater value, particularly from private parties. Those are my best bargains. Sometimes there are problems but in the long run, my money goes a lot farther. Often you can get a good deal due to distressed situations and if you have to make repairs you get a bit of education in the process.Of course I am assuming your goal is to have QSOs rather than just a pretty station.So if you pick up an old SB-220 for a few hundred, you can get a lot of fun per dollar.

author: noreply
author_rank: Frequent Contributor
timestamp: December 09, 2020, 12:33:40 am
content: Yep, the LDMOS ACOM 1200S looks like a fine Amplifier - ticks ALL the boxes ... however there is a big BUTWhat's the BUT?I believe there is an inherent design flaw with this amplifier and from the numerous user reviews (see eHAM) this theory seems to correlate.What's the 'flaw' ?In simple terms - there is insufficient isolation from the control logic / CPU circuitry and the active RF circuitry.Have a look at the attached photos of the RF section (1200S)You can see clearly that PIC 1.0 - does not have any shielding on the RF coupling transformers of the LDMOS device - and there is a 'logic' board sitting 'piggy back' on top of the main RF board - only inches away from the 1KW+ of RF energy without adequate isolation.To prove this theory ACOM in a later revision made changes to the RF board - see PIC 2.0 - here you can clearly see they isolated the RF section with a metal shield plate (poor attempt - but better than nothing) - they also did a better 'job' at winding the coupling transformers. Finally they integrated the 'piggy back' board circuitry on the main RF board - so they could make sure the 'logic' circuitry is now properly coupled to the PCB ground. (Still a poor job).All of the above - correlates to the various reported failures of the CPU and logic circuitry of the 1200S - when ever there is a 'failure' - usually caused by a transient or a significant amount of RF going back as REVERSE power from an antenna mismatch (yes user error - but the so called 'protection' is not good enough). Does not require much energy in this environment to 'fry' a microcontroller!The fact that ALL of the ACOM resellers (Dealers) - have no idea - or at least do not want to admit that there is a design flaw - which at one stage has been attempted to be rectified by ACOM (by virtue that you can clearly see from photos that they did try to make significant changes to improve the RF from leaking into the Logic Circuitry) but not, IMHO, to an effective level.The complete design is 'flawed' in the significant engineering level required to make proper and effective RF isolation within this design.I think if Dave did a 'teardown' on this puppy - he would have a great field day at ridiculing their ad hock attempts in 'fixing' problems - somthing you dont do with RF design - it needs to be done from the ground-up and not as 'add-ons' (IMHO)Making 'retrofit' revisions - without admitting that there is a problem - is not giving me a level of confidence to 'buy' an amplifier which may or may not fail - because I might be unlucky and get an earlier revision of the HW - which does not have any of the 'attempted' fixes - that's assuming that these 'fixes' do indeed work - still unproven.On the other hand the 1000 and 1500 'tube' designs are over 10 years old - they are virtually bullitt proof and very well refined - they simply work - even with massive operator abuse.I hope the above is some insight as to why the ACOM 1200S (their flagship LDMOS amplifier) is a risky purchase despite the massive theoretical advantages of the solid state design and operation.Take Care

author: noreply
author_rank: Frequent Contributor
timestamp: December 09, 2020, 12:57:17 am
content: @bob91343I agree with your comments.The problem here in the UK - is that there is simply not enough good 'used' kit available.Sure with amplifiers you really need to know who 'owned it' before you - and you are correct sometimes you can indeed pick-up a great bargain.With solid state amplifiers - I simply would not trust to buy a used amplifier - simply because you will most likely need to replace the LDMOS devices - and they are not cheep if you can get the most likely now 'unobtanium' devices (these amps were designed years ago and use discontinued LDMOS devices).At least with the 'tubes' you can 'see' what you get and at worse - the most costly part will be the 'tube' - if of the older variety - these can be found relatively 'cheep'.Yep, there is also 'fun' in fixing.I looked into the cost for a new LDMOS amplifier using the current NXP MFR300 devices (2 of these will give you 600W)By the time you get all the parts , get ALL of the PCB's made and build the bandpass filters, protection circuitry, control circuitry, and get appropriate metalwork, heatsinks, fans, switches, display panels, and a 50V 40A PSU - just the cost of the components will top US$1000 - then you have the 'fun' to build and test the design.If you have time - this is and can be an enjoyable experience - but if you don't have time - its worth paying an extra 1K to get something that is working immediately and has a proven design with a warranty.Problem is that I simply don't trust any high power solid state amplifiers which have significant 'logic' circuitry within their design.Usually control logic / microcontroller designers and software engineers - are not experienced with RF design, on the other hand the RF engineers - don't have experience with logic and control systems / CPU and microcontroller designs - so there is a natural 'potential' problem in realizing a functional and bulletproof design, not impossible with the extra care taken to bridge the knowledge gaps, but unfortunately in my humble opinion nobody at present can clearly demonstrate that they have indeed succeeded in doing this to the satisfaction of their customers.Just saying ...Take Care

author: Sal Ammoniac
author_rank: Super Contributor
timestamp: December 09, 2020, 06:38:44 pm
content: Anyone buying a tube amp in 2020 needs to consider future tube/valve availability. From my sources, I've heard that the 4CX1000A is more likely to be available in the future than the 4CX800A. The 4CX1500 is also nearly a drop-in replacement for the 1000A, so that's a consideration too.Solid-state devices are not immune to future availability issues either.

author: bob91343
author_rank: Super Contributor
timestamp: December 09, 2020, 06:52:04 pm
content: You can solve the obsolescence problem by stocking a few spares. As it turns out, I have used all my spare 3-500Z tubes and it makes me more careful how I operate the amplifier. Sure, it involves more cost but definitely less than having to find replacements down the line. And if you never need those spares, it was still good insurance.

author: Sal Ammoniac
author_rank: Super Contributor
timestamp: December 09, 2020, 07:06:17 pm
content: Quote from: bob91343 on December 09, 2020, 06:52:04 pmGood point. I've got two spare 4CX1000A's in the closet.

author: noreply
author_rank: Frequent Contributor
timestamp: December 10, 2020, 01:52:57 pm
content: Is there a 'shelf life' for 'tubes'?The conventional glass tubes and the ceramic and metal radial beam tetrode type?I know that LDMOS devices can be stored for very long time - as long as there is no corrosion in the leads of the package material - best to keep in nitrogen filled bottle I guess?Can the same be said on the tubes?Can the the tube loose its 'vacuum' over a period of time?Just curios ...

author: Sal Ammoniac
author_rank: Super Contributor
timestamp: December 10, 2020, 05:57:53 pm
content: Quote from: noreply on December 10, 2020, 01:52:57 pmThe answer is yes for glass tubes, particularly big ones like the 3-500Z. The glass-to-metal interface of the pins is prone to leakage over long time periods (years). Tubes that have had this leakage can generally not be recovered. The other mechanism that affects these tubes in long term storage is outgassing from the anode (which is the "getter" in these tubes) and this can sometimes be mitigated by applying filament voltage and HV (but no drive) to the tube to get the anode cherry red for an hour or two to activate the "getter" and reabsorb the gas.I don't think ceramic/metal tubes have the same issue with leakage.

author: bob91343
author_rank: Super Contributor
timestamp: December 10, 2020, 06:45:44 pm
content: I don't think solid state devices are especially better than tubes in the long term. There is migration, which compromises performance of transistors. I don't recall ever seeing a tube die from sitting on the shelf.

author: noreply
author_rank: Frequent Contributor
timestamp: December 10, 2020, 06:52:36 pm
content: @Sal AmmoniacThanks for the informative feedbackI always wondered if there is some special storage technique(s) that could be used for storing tubes for very long time.Quote from: Sal Ammoniac on December 10, 2020, 05:57:53 pmWas wondering if there is a way to mitigate this?Perhaps submerging entire tube into mineral oil for long term storage - the permeability of 'oil' is less than a gas like 'air' - so perhaps the leakage can be minimized to zero?Just a thought - wondering if anyone has experimented?

author: Sal Ammoniac
author_rank: Super Contributor
timestamp: December 10, 2020, 07:19:07 pm
content: Quote from: noreply on December 10, 2020, 06:52:36 pmNot that I know of. One possibility is to store the tubes in a vacuum chamber so that the gas pressure at the pin/glass interface is as low as possible.Mineral oil is an interesting idea. I don't know how viable this is or if anyone has tried it. Hams are probably the only users of large glass transmitting tubes these days. The remaining commercial users of large transmitting tubes tend to use ceramic metal tubes or have transitioned to solid state.

author: noreply
author_rank: Frequent Contributor
timestamp: December 10, 2020, 08:20:03 pm
content: Yeah, I thought mineral oil might be worth trying - its used quite often to 'cool' motherboards with 'hot' components - basically you run your electronics in a fish tank filled with mineral oilI am curious about the manufacturing process of the ceramic metal tubes - the 4CX1000A for instance - they are extremely expensive for a 'tube' - not sure if this is because of the materials / manufacturing process or simply demand / supply - low numbers means high costs?The problem is that if you buy a some equipment - like an amplifier - which uses one of these $1000 tubes - there is an instant incentive for a dishonest reseller - to 'swap' tubes - he gets your new and you get a working used tube - none the wiser the swap took place ...Not saying this happens everywhere - but with everything where there IS such a single easily swapped item - there will always be a 'potential difference' (using electrical analogy) - for some shady activity to take place.I guess its not a good idea to have one easily swapped component worth as much as 30% of the cost of the equipment - inside itBut the good thing is that there ARE more HONEST people in this world than the dishonest - but we only hear about the dishonest ones unfortunately.Take Care

author: Sal Ammoniac
author_rank: Super Contributor
timestamp: December 10, 2020, 09:55:22 pm
content: Yeah, it would be difficult to tell the difference between a new ceramic metal tube and one with thousands of hours on it. I'm sure there are unscrupulous people out there that wouldn't hesitate to swap tubes.Lots of tubes in commercial service aren't used until they fail--they're pulled out of service and replaced before they fail. These "pulls" are often sold on the secondary market and can be a viable option for hams wanting to save money. This source is drying up, however, as commercial users (broadcasters, MRI machines, etc) switch to solid state.Some of the larger metal tubes can be rebuilt, which can be a big cost savings versus buying a new tube, but that's mainly tubes used for broadcast operations, not ham use.

author: noreply
author_rank: Frequent Contributor
timestamp: December 10, 2020, 11:20:08 pm
content: Interesting ...Quote from: Sal Ammoniac on December 10, 2020, 09:55:22 pmSurely there must be some 'telltale' signs - like the way the 'tube' behaves or its quiescent voltages / currents?My understanding is that new tubes exhibit slightly higher currents until they 'settle'If no signs that can be measured - this is indeed bad news for innocent consumer and good news for the unscrupulous people out there that wouldn't hesitate to swap tubes

author: Sal Ammoniac
author_rank: Super Contributor
timestamp: December 10, 2020, 11:56:58 pm
content: Quote from: noreply on December 10, 2020, 11:20:08 pmThere quite possibly are, but I'm not aware of them. I was referring to visible indications. The two 4CX1000A's in my amp both look identical to my two brand new spares, despite thousands of hours of use.

author: g0fvt
author_rank: Newbie
timestamp: December 21, 2020, 06:53:32 pm
content: I believe that many of the ceramic Russian valve/tube types are no longer manufactured and those on the market now are old stock. Ceramics are not as "leak tight" as some of the old glass types, they need special care when being put into service after lengthy periods of storage. The literature for many of the modern tube amplifiers rather misleadingly refers to the GU74B as a 4CX800. That is an Eimac style part number but made by Svetlana I don't think that Eimac ever made such a device. I don't know many users of Acom amplifiers but those that do use them seem to be very happy with them. I use an amplifier with a pair of 3-500ZGs, unless something goes badly wrong they ought to outlast me. If I was buying an HF amplifier now or spending a large amount building one I think I woud go the LDMOS route. A no-tune amplifier has a lot going for it with the likes of remote operation or quick band changes.

author: geggi1
author_rank: Frequent Contributor
timestamp: December 21, 2020, 09:59:11 pm
content: You should have a look at the SPE amplifiers. They have been around for years. Dont buy a new the 1k-FA because they are using the MRF150 and they are pushed beyond the limit.The newer 1,3k-FA is a better linear with LDMOS transistors.But if you only are running about 400W a used and nice priced 1k-FA might be a good option just keep it under 900W and you will not push the transistors.

author: noreply
author_rank: Frequent Contributor
timestamp: December 22, 2020, 05:51:34 pm
content: Totally agree that having ability for 'remote operation' is a must have feature - but is solid-state only option for remote operation is the real question?I have gone down the rabbit hole with ACOM and ALPHA.Its interesting to see that the ALPHA 9500 - very old design and still going strong - is a nice amp - with 2 x 4CX1000A's - if used at sub 1KW should last a lifetime, and its fully automatic suitable for remote operation.The interesting thing is that I have not seen anybody taking an ACOM 'manual amp' and fitting some 'EXTERNAL' servo motors and control systems - just to totally avoid opening the amp and being exposed to HV circuits - and 'jerry rig' a blot-on automator which will turn any 'manually tuned amp' into an autotuned amp.In theory this is quite feasible - simply replace your hands with servo motors which bolt-on onto the external knobs and control buttons.Anyone heard of someone doing precisely this or similar setup?

author: charliedelta
author_rank: Regular Contributor
timestamp: December 22, 2020, 08:09:41 pm
content: Choose the amplifier with the best 3rd order IMD figures.The 4cx1000 was not really designed as linear SSB tube it was designed more for class C service.The 0 watt grid dissipation is also a concern. I understand that its well protected but its not really well protected.4cx1000 and 4cx1500b tubes sell for silly prices and the Chinese made tubes are questionable.

author: mrf245
author_rank: Regular Contributor
timestamp: January 04, 2021, 07:59:42 am
content: In 2016, I helped my Italian friend to purchase some FU-728F 2kw tubes. Very good quality. But I can't find FU-728F in good quality now (you can find some FU-728F in the Chinese market but actually salers don't know if it is good to use)

author: Sal Ammoniac
author_rank: Super Contributor
timestamp: January 05, 2021, 11:50:08 pm
content: Quote from: noreply on December 22, 2020, 05:51:34 pmThe Alpha 9500 doesn't use two 4CX1000A's. It uses a single 3CX1500 (AKA 8877) tube.You might have been thinking of the Alpha 8410, which uses either two 4CX1000A's or two 4CX1500's.

author: gorillamotors
author_rank: Contributor
timestamp: March 30, 2021, 02:08:25 pm
content: Who currently manufactures the most reliable 4CX1500 and 4CX800A tubes?