I'm not against parafeed - I am listening to a small parafeed spud right now - but I think for a GM-70 amp it's not a good idea.
You shouldn't count on PSRR to get it silent, just make a good PSU (mine is noise free on 95dB speakers). The insulation requirements are relaxed on the OPTs, because they are high on the anode chokes...
A GM70 amp is a complicated, dangerous and expensive project and you shouldn't save money on the output transformers (money that you spend anyway on a pair of special chokes and good PF capacitors).
You achieved better bass response with a choke loaded parafeed - that is normal, the LC resonance artificially boosts bass. More detailed stage? Yes, parafeed has an extended response in the high frequencies, when compared to a SE that uses an average OPT. But it also has an extra (big) capacitor in the signal path, which is not good for sound quality. This kind of amp deserves good OPTs, with good high frequency response.
Parafeed is not true SE. In SE, the output transformer has current permanently flowing and thus a non zero magnetic flux.
In parafeed, the signal has to magnetize the transformer, and very small signals are lost due to nonlinearities (the transformer experts could tell us more about this).
Sorry for not dusting them off.
Filament transfomer for a input (6C45Pi), driver (300B) and output tube (GM70).
Power / rectifier filament / bias transformer for output GM70
Output parafeed transformer for GM70
Filament transfomer for a input (6C45Pi), driver (300B) and output tube (GM70).
An externally hosted image should be here but it was not working when we last tested it.
Power / rectifier filament / bias transformer for output GM70
Output parafeed transformer for GM70
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I just meant as an anode material. I wasn't talking sonically. As conductor properties go.
Given the surface area and operating current I would not expect the poor conductivity of steel to be an issue. I'll probably never see any steel plates - they don't come on eBay very frequently if at all.
The quartet I bought are now more than 3 weeks overdue, coming from the Ukraine I suspect that they are stuck in a customs warehouse due to the shutdown. I figure at this point that there is a good chance I will never get them, and I have just 8 days left to file with paypal. The seller is blameless in this case.
The quartet I bought are now more than 3 weeks overdue, coming from the Ukraine I suspect that they are stuck in a customs warehouse due to the shutdown. I figure at this point that there is a good chance I will never get them, and I have just 8 days left to file with paypal. The seller is blameless in this case.
Well, as Kevin stated (and I've heard), the steel plates are not inferior, although the quality of sound I heard may have as much to do with their vintage as their composition; (as you point out, graphite should be superior – and in some respects, it is).
I am indebted to the man who let me use his steel plates, however briefly, and would love to own a pair, (if for no other reason than to have a different "flavor" – I use copper plates when bi-amping because I love their mids, but use the graphites, when running full-range, for the power and deeper bottom).
However,
"Large increases in cost with questionable increases in performance can only be tolerated in race horses and women." – William Thomson ("Lord Kelvin")
So, I doubt I shall ever own a pair of steel plate GM-70's (at least at my own expense – if someone wants to give me a pair...)
Besides, I think we all know, that – particularly in high-end audio – price has less to do with quality than availability or the "unobtainium" factor.
Having something – anything – that someone else can't have makes it all the more desirable.
When I first started hearing about the GM-70, there was barely a difference in price between the copper plates and the graphites. In fact, the few people who knew about them were saying, "yeah, the coppers are good, but the graphites handle more power".
As time went by, and copper plates became scarcer, the party line seemed to shift to "Oh you must own the copper plates – they're simply superior in every way).
Truth is, I've heard them all, and I've measured them all. The difference between any of them is not as great as you might think. The copper plate does have a slightly more pronounced midrange, but the difference becomes less significant at higher standing currents. (i.e. if you run 1250V @ 80mA, the copper's mids shine and the graphite's bass extension isn't really that much better) and vice versa (run 925 @ 135mA and the graphite really "packs a punch" down bottom that the copper just can't match).
The chief benefit of the steel plates, AFAICT, is that they seem to respond equally well across the board, and don't display any "E vs. I" preference.
It's all about perceived value and modeling expectations.
We all believe what we're told to believe.
An ol' schematic
In your design where you return the parafeed cap to the cathode rather than ground - you don't need the cathode bypass cap. The cathode AC signal is canceled by the out of phase current coming from the parafeed cap which gives zero AC signal (apart from a small error signal) across the cathode resistor. Effectively the cathode is at virtual ground, and the parafeed cap is all but eliminated from the signal path. Clever really.
I disagree with Vincents comment about magnetization current. Its hysteresis at the zero crossing point which causes signal bluring. Transformers designed for zero DC also have minimal hysteresis and so minimal bluring. My opinion is that a well designed parafeed amp should better a SE amp especially with such demanding operating conditions as your amp will entail.
However a 10H plate choke and a 2uf parafeed cap seem very marginal to me and are likely to introduce a resonance in the bass region - which my experience says isn't good. I suppose these are the limitations of building a practical parafeed GM70 amp.
Shoog
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Those aren't, by chance, coming from Boris (Miroshnikov)? If so, you will get them. I had some 6Ц4П's get stuck at UKRPoshta for eight weeks(!) and he tracked 'em down (eventually) and got 'em to me. I only mention Boris becuase his local post is one of the worst in the Ukraine. But he and Denis Moisyeyev are the two guys in the Ukraine that you can pretty much count on. I can't speak for all the Ukrainian sellers – (actually, a few I can, but won't!) – but those guys are both reliable.
Unfortunately I suspect the problem is actually here, I've heard from a few sources that international mail parcels are not getting through. (U.S. Customs is badly affected by the shutdown.)
I've never had a problem with any seller from Eastern Europe - reliable as a Swiss watch. (I buy mostly less known Russian tubes for current projects.)
Not to sound too repetitive, but a 10H choke is very marginal and you may be forced to peak the bass response by playing with the series cap - unfortunately this also probably means amplifier source impedance at low frequencies will also be higher than you might suspect or desire. (poor damping factor)
I thought that 10H was actually 40H so was not initially concerned.
What is the the inductance of the parafeed output transformer?
I thought that 10H was actually 40H so was not initially concerned.
What is the the inductance of the parafeed output transformer?
I agree with both of you. I just attached the copy of the schematic I had drawn in 2004 - see the date in the left bootom corner. The same year I joined diyaudio.com community. Since then I learnt a lot. Anywsy, thanks for your comments.
PS. Actually - three of you.
As to primary inductance of PF transformer - I lost the email with specs from the manufacturer. I have measure it. Ro = 6 ohm, Ra=7kohm
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I'm using a conventional SE transformer, 7K:8/150mA/40W - having done the load lines I thought the tradeoff of power for slightly less distortion and better damping factor a good one.. I considered a range of 5K - 7K, since I have a real bona-fide somewhat difficult load (3 way horn/Onken based system) I am sure the load impedance reflected is all over the place. The mids and highs because of xo design and the horns' ridiculous efficiency look resistive. (Lots of padding)
The completed GM70 amps massively outperformed the 300B amp that I was using up to the time, not just in grip, bass control and dynamics as you might expect, but in terms of detail, imaging and a lot of other things. Properly employed the GM70 is extremely capable.
The completed GM70 amps massively outperformed the 300B amp that I was using up to the time, not just in grip, bass control and dynamics as you might expect, but in terms of detail, imaging and a lot of other things. Properly employed the GM70 is extremely capable.
Reassuring comment Kevin, thanks . I'm just getting parts for a GM70/GK-71 amp - Ordered a pair of 7k transformers from Bud Purvine. I've been using KR300B's mostly for a few years, also some Sylvania & KenRad 6B4G's , but I heard a really nice GM70 amp a few months ago, only running from 500V supply and it was a real eye-opener . Before anyone asks - yes it was running on efficient speakers, only making about 3wpc . I bought some GM70 coppers after that !
I actualley started with a 7.5K primary and got a significant improvement in IMD when I moved to 10K – I also went from a 100mA to a 120mA core which could account for some (most?) of it. If you've got the voltage, (and the GM-70 will take lots – a good deal more than the spec sheets would indicate), you can get really good excellent damping without using NFB. I seem to recall we've discussed this before; IIRC, you use the copper plate exclusively, (which really "sings" at higher voltages).
I couldn't agree more. I think the GM-70 is one of the best audio tubes ever created. They're tremendously linear, exceedingly consistent, can dissipate >100W, and (at least the graphites are) in good supply, and they can be had cheaply (compared to other high-powered triodes at least). Plus, the GM-70 is the highest-powered triode that was actually designed for audio.
Seems to me the obvious choice for SET, yet you often see designs adapting transmitting tubes or voltage regulators.
Still, there are probably twenty times as many amps employing the 6C33C, which (vs. Gm-70): isn't directly heated, has about half the plate dissipation, burns even hotter, is a lot less stable, and isn't anywhere near as linear. I don't get it. (I guess it does have an attractively low plate impedance though).
These days I am using graphite which is surprising since I have more copper plates in my stash. I actually bought more graphites but had a few duds (glass bubbles actually) and gave a bunch away - big mistake since they have doubled in cost since I bought them..
I think fear of high voltage keeps many people away which imho is not a terrible thing if they are not quite ready for it, but I have heard a few GM70 amps running at well under 1kV and there the performance seems more typical of vintage dhts. Unrealized potential due to the lower plate voltage.
I'm running 1kV and 120mA, no tube failures so far even with copper.
I'm with you on the 6C33, must be the low plate voltage and rp that makes it appealing, although I will say it sounded good in my ill-fated circlotron OTL design.
I think fear of high voltage keeps many people away which imho is not a terrible thing if they are not quite ready for it, but I have heard a few GM70 amps running at well under 1kV and there the performance seems more typical of vintage dhts. Unrealized potential due to the lower plate voltage.
I'm running 1kV and 120mA, no tube failures so far even with copper.
I'm with you on the 6C33, must be the low plate voltage and rp that makes it appealing, although I will say it sounded good in my ill-fated circlotron OTL design.
In addition to high voltage with a CCS, the plate voltage swing is limited to no more than half that achievable with the plate choke. (For comparable output power twice the plate voltage is required, and added dissipation in the output stage will be much higher, about double due to the dissipation in the CCS.)
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