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more readily available?
845 is cheaper, as mentioned, and has MUCH more variety out there. The fil voltage is lower so less chance of hum, and finding a good quiet filament supply is a lot easier. And the GM70 is FUGLY! damn it's a bad looking tube, physically. I just think it looks like ass... with the ho-hum shape, weird base, inelegant proportions and ugly stepped tubular bulb... ugh. bring me a 845 any day, way better looking IMNSHO.
I think any hype/comparison vs. chinese build quality is moot at best and if anything favors the chinese. They have another 20-30 years of technology, materials and skillsets to draw upon vs. when the GM70 was made and engineered. not to mention the 845 was designed as a class-A audio freq. amplifier, which counts for something too. the so-called military might of the USSR, while large, was not generally known for quality control. they threw a lot of money into it, but they didn't have any motivation for good QC as it was a state job, guaranteed and subsidized by the govt. if you want build quality, look to US military tube developments, esp. stuff designed and built by the likes of Eimac, Bendix, WE, etc. The russian stuff looks like the poor quality 2nd-rate ripoffs of western tech that it is. They (russkis) had a few interesting tubes, but with a few exceptions (6C33 comes to mind) almost anything they made can be had in a better version in the world of western tubes. not that it was ALL junk of course, and the GM70 has its place, but the 845 is clearly superior in several ways.
With those gorgeous copper plates lit up they are quite beautiful.....but as with women, if you only like blondes, you could easily miss that brunette fox. :)
You have to remember that the Chinese 845 is produced for industrial
and commercial radio service as replacement spare parts... Same with
the 211 and and 805 which is widely used in all kinds of sundry RF
industrial equipment. There's no incentive for the Chinese manufacturers to do 20-30 more years innovation beyond the Russians.
Why? Not exceptionally reliable is better because it creates a constant stream of demand. This is also known as "recurring revenue".
On the other hand, the Soviets were enticed to screening their materials and testing their production processes and ultimately inspecting the product. Why? Military reliability calls for it. Why? Because communications equipment is at least as important as ordnance in the field :) The military command, the field technicians and the factory management would not accept producing complete crap!
Further, the Soviets continued innovating in RF tubes far after the West vacated the field. This is evidenced by the large number of unique ceramic RF power tubes and microwave devices out there.
Not all Soviet hardware was crap. People mistakenly "make fun" of
their ICBMs as being foolishly dangerous and using liquid fuel. The
reality is the USSR *mastered* storable liquid propellants. The SS-18
and SS-19 is an amazing example of a highly reliable ICBM that uses
stored liquid propellants. These missiles were rolled out in the 1970s. They had the same combat response time as American solid-fueled
ICBMs ; they could be launched in under 5 minutes of prep time. Why did the Soviets wait on Solid Fuel? Liquid Fuel has other advantages solid fuel doesnt: You can throttle and turnoff the engine! ; something you cant do with solid fuel. The result? Better control of range and targetting options ; simplification of the software for guidance. With solid fuel, you can only apply the brakes. With
liquid fuel, you can hit the gas as well as hit the brakes. It's
Nationalistic jingoism to make fun of their storable liquid propellant ICBMs. From an engineering perspective, the work is admirable and the feature set it provides makes sense!
Soviet Military Stuff is very very good. Where they messed up was
spending all their money funding monkey business in Africa, the Middle East, Central America, Eastern Europe - as well as catering to giving "hand-outs" to countries like India, Cuba, Iran.
Not to mention the overpriced and the latest and greatest rifles that the US bought from its crony contractors jammed and just don't work in the field; whereas a Rusky rifles designed in 1947 just keep on working in all conditions. AK-47 is the name.
> > > the so-called military might of the USSR, while large, was not generally known for quality control. they threw a lot of money into it, but they didn't have any motivation for good QC as it was a state job, guaranteed and subsidized by the govt. if you want build quality, look to US military tube developments
this just sounds like more right-wing Republican bullshit. Russian teflon caps, capacitors, and certainly GM100 are excellent products. Just because you stamped "made in USA" on some "made in china" junks does not mean they are any good.
The GM70 was used in mobile (field) RF equipment by the Soviet
Red Army. You can bet your bottom baloney that the tubes were manufactured with far more care than anything that comes out of Chinese factories... For military parts it means that the materials that go into the devices have to qualify, then the manufacturing and inspection process is monitored. Last but not least, there is reliability testing. There's no way Chinese factories with the goal
of producing consumer parts needs to make things at this level of repeatability, nor could they afford to match the capabilities of the Russian plants.
USSR stuff is the ultimate boondoogle for us tube heads and ham
people... This isnt the first time its happened either.. As it turns
out, after the Cold War, many aircraft factories in the USSR began turning out their aircraft-grade aluminum and titanium tubing to feed the mountain bike industry.
That said, I'd expect the GM70 to have longer filament life, much
greater resistance to shock and shear than 845s.
They were only made at Ulyanovsk factory that has closed, AFAIK.
However, a lot of these tubes have been produced and stocked, so NOS supply is pretty good. The main use for GM70 was for wire translation service. Every house, apartment, and workplace in the USSR had a 30 V audio outlet feeding a speaker with a matching transformer. The network provided broadcast from 6 am to 1 am, and was powered by local audio amplifiers using GM70 and GM100 tubes. Both tube types were designed for years of continuous service and had very conservative ratings.
So, GM70 was not primarily designed for the military. However, because wire broadcasting was a very important element of Soviet propaganda, the quality and dependability of this service was an important concern for the government, which reflected on the quality of these tubes.
GM70 is reliable unlike Chinese 845.
GM70 is generally cheaper.
the GM70 has double the heater power nd comes in a copper plate version.
wut TK said.
it is also interesting to note that in the case of the GM70 the double cathode power comes from increasing the voltage which also doubles the voltage gradient across the cathode.
With the right plate voltage (1,500 V), potential difference across the cathode is less than 1.5%. For 45, it is almost the same (1%), yet nobody sees it as a problem.
but what about the ratio of the filament gradient to the bias voltage? 20V of a 100V bias is a lot different than the 2.5V of a 50V bias for a 45.
As ivan mentioned, the tube to test this with would be the 304TL which you can configure as a 5V 25A or a 10V 12.5A tube. It is interesting to note that when wired for 10V operation two of the plate will cheery up before the other two, and when wired for 5V operation they all cherry up at the same time.
I actually think it would be kinda cool if listening experience showed the 10V connection of the eimac to sound better :-)
Potential difference to greed is more important. But still GM70 isn't so bad:
DC filaments suck the life out of DHTs. I've never heard a DHT with DC that didn't sound better with AC. Ultrasonic filament supplies are the ultimate solution.
GM70 was mostly used with AC heating in PP amps where filament hum cancels out.
DC filaments sound wrong not because of principle, but because of implementation. Diode bridge plus capacitor, or voltage regulator are not the right way to do it. The right way is dedicated filament transformer, choke input filter, and CM choke to block line contamination by audio signal. It could be 40 pounds of iron per one GM70 filament, but it sounds good.
I just like to write something down. Are we looking at something like this:
The critical value for the input choke is about 6 mH, but it should be rated 9 A. The second choke has to be rated only 3 A, so its value can be higher than 10 mH. 50 mH 3 A choke with 1-2 Ohm DCR has a reasonable size. CMC is easy as it does not need a gap. Cap values are right, but they may be higher than 10,000 uF. A tuned snubber circuit for bridge rectifier would be also helpful to keep the line clean.
I researched further into the subject matter, and found an article by Lynn Olson. It appears that I should use 4 chokes and a CMC.
If you were really hard-core, there would be four chokes: a pair on the high and low-side between the bridge and first cap, and a second pair on the high and low-side between the first and second cap. The first pair, since it emits magnetic noise, would be physically isolated from the audio circuit (on the far side of the chassis, close to the dedicated filament transformer), and the second pair would be close to the DHT, since it is part of a LC filter circuit. Another improvement would be to insert an RF common-mode choke of the type seen in the input of computer power supplies. This would reduce the common-mode RF noise that can sneak through the large power-frequency chokes."
However, I'd rather have one choke of higher inductance than two smaller chokes. Or even better, a choke with two windings: one in the positive and one in the negative rail (diff. mode choke). This would allow to reduce the amount of iron without compromizing the performance. Too much weight is a problem of GM70 amps.
With GM70, power supply on a separate chassis is the way to go. Therefore, it is logical to keep CMC and the last filter capacitor on the amp chassis, and everything else on the PSU chassis.
I do not think that small computer supply RF CM choke with ferrite core is appropriate. Such small choke (microhenries) would do nothing to prevent audio signal from leaking into the power line. Proper CMC should have inductance of at least 100 mH. It can be wound on a silicon steel toroid. A great source for toroid cores are current transformers. They can be bought cheap from surplus dealers.
i have built with all of the above mentioned tubes and like the 45 and the copper plate GM70. I also love the 2A3. Boy it really sucks when you cannot find science to justify your opinion.
I'm not convinced that it is such a problem. I sure have a hard time faulting the few M-70 amps i have heard.
Hows about a 304TL wired with all four cathodes in series? ;-)
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