After a short hiatus thinking about the topology of the 7189 amp, I wonder if I would be just as well off using a CT plate load inductor at the input stage. One thing the output stage requires is good grid bias balance, and a CT plate load inductor would help ensure that. True, the amp wouldn't technically be 'DC coupled', but when I considered what an active solid state bias balancing circuit would have to do, the effect wouldn't be that much different than a high inductance balanced plate choke (I don't want to rely on the input stage plate voltage not drifting over time with completely separate bias networks to each half of the output stage), plus the CT choke would still allow me to drive into positive grid current at the 7189s. Now, I'm trying to see what real world options are out there. I'm thinking at a minimum of 1KH per side (maybe as much as 5KH) here since I still want to be able to employ a reasonable amount of local and global feedback without instability issues. Due to the DC cancellation of such a choke (similar to a PP output transformer), a moderate size part seems like a possibility here.
I notice at the Electra-Print web site that they advocate M6 silicon steel for output transformer use over amorphous or permalloy, on the bases respectively that the permeability of M6 is more uniform across the audio band and that permalloy loses LF permeability (before it saturates?). Any comments on the above?
thoriated,
I cannot comment on your choke idea, except to ask about the 1 kilo henry to 5 kilo henry specs. HUH? That is a pretty big device, or it has a very high DCR and uses amorphous core too. Nothing essentially wrong with either value, but do you really need that much inductance?
As for the core type issues, Jack has laid out the relative graces of the three materials. The M series, including M3, are more linear, but they are also more useless as frequency rises, being essentially out of the picture above 400 Hz. Used properly this can be a very good characteristic as it is core that causes distortion.
Nickle core does become phase anomalous as it enters the sub 40 Hz region and the closer you get to DC the worse it becomes, but this is really only an issue if you are running an AC signal on top of a DC current and the AC is driving the core above 8 kilogauss with AC rims signals for 48% nickel and 6 kilogauss for 80% nickle. These AC excursions do upset the mag field linearity in the window and phase problems are the result and they occur across the frequency bandwidth, not just at low frequencies where the disturbance arises.
Amorphous core does have a mid frequency sag, but it is still part of the power transformation out to and beyond 20k Hz. Means it is adding distortion all the way out there, but the distortion is significantly lower than that of M series core in their common bandwidth to 400 Hz.
In a general sense you add capacitive coupling to an M series core to obtain power transform support for the antenna event and throw it away for nickle and especially for amorphous core materials. How you go about dealing with that capacitive coupling that is used, goes a long way towards determining how musical, transparent and coherent the transformer sounds.
Bud
I cannot comment on your choke idea, except to ask about the 1 kilo henry to 5 kilo henry specs. HUH? That is a pretty big device, or it has a very high DCR and uses amorphous core too. Nothing essentially wrong with either value, but do you really need that much inductance?
As for the core type issues, Jack has laid out the relative graces of the three materials. The M series, including M3, are more linear, but they are also more useless as frequency rises, being essentially out of the picture above 400 Hz. Used properly this can be a very good characteristic as it is core that causes distortion.
Nickle core does become phase anomalous as it enters the sub 40 Hz region and the closer you get to DC the worse it becomes, but this is really only an issue if you are running an AC signal on top of a DC current and the AC is driving the core above 8 kilogauss with AC rims signals for 48% nickel and 6 kilogauss for 80% nickle. These AC excursions do upset the mag field linearity in the window and phase problems are the result and they occur across the frequency bandwidth, not just at low frequencies where the disturbance arises.
Amorphous core does have a mid frequency sag, but it is still part of the power transformation out to and beyond 20k Hz. Means it is adding distortion all the way out there, but the distortion is significantly lower than that of M series core in their common bandwidth to 400 Hz.
In a general sense you add capacitive coupling to an M series core to obtain power transform support for the antenna event and throw it away for nickle and especially for amorphous core materials. How you go about dealing with that capacitive coupling that is used, goes a long way towards determining how musical, transparent and coherent the transformer sounds.
Bud
You might want to read this thread. Bud explained in all detail the differences between silicon steel and amorphous cores.
Hi, Bud P. & Herr Grau -
You are definitely giving me some material to digest, so I may not be able to respond in full immediately, but the very large value I was thinking of would be to keep the input stage gain as high as possible at low frequencies to minimize phase shift because I wanted to have both a local FB loop from primary or UL taps on the output transformer and also the option of global feedback, so ideally, the rolloff due to the input stage plate load choke would be at a frequency lower than that due to the output transformer primary winding. I could do some tricks to get by with considerably less plate load inductance, say a few hundred H on each side of the CT, but that would force me to waste gain and perhaps do some FB frequency shaping also that would probably be avoidable with a higher plate choke inductance. IAC, I was curious how much would be practical with a 2000 ohm DCR in each half of the CT plate load choke for the input stage. Since the DC current would be fed from the center tap, I expect a maximum DC imbalance of 2mA in practice, or perhaps even 1mA would suffice. Ideally, I would want to keep the overall size of the CT choke small enough so that the smallest dimension would be less than 2".
I'll continue to read over the thread that Herr Grau linked to.
You are definitely giving me some material to digest, so I may not be able to respond in full immediately, but the very large value I was thinking of would be to keep the input stage gain as high as possible at low frequencies to minimize phase shift because I wanted to have both a local FB loop from primary or UL taps on the output transformer and also the option of global feedback, so ideally, the rolloff due to the input stage plate load choke would be at a frequency lower than that due to the output transformer primary winding. I could do some tricks to get by with considerably less plate load inductance, say a few hundred H on each side of the CT, but that would force me to waste gain and perhaps do some FB frequency shaping also that would probably be avoidable with a higher plate choke inductance. IAC, I was curious how much would be practical with a 2000 ohm DCR in each half of the CT plate load choke for the input stage. Since the DC current would be fed from the center tap, I expect a maximum DC imbalance of 2mA in practice, or perhaps even 1mA would suffice. Ideally, I would want to keep the overall size of the CT choke small enough so that the smallest dimension would be less than 2".
I'll continue to read over the thread that Herr Grau linked to.
The reason that I ask the above is that I might try winding my own CT plate chokes. I have located a source for M3 tape wound toroidal cores and also bifilar magnet wire that doesn't appear to be too outrageously expensive. A few evenings of thrilling toroid winding, and - Voila! CT plate chokes (I hope). What do others here think of this?
Along with the 33 gauge thickness the M3 material has an average perm equivalent to the max perm of M6. Since you are going to wind a toroid, make sure you cover the core in tape, if it does not come with an epoxy coating, to keep from scraping the magnet wire. You will need a hook to pull the wire through the hole with, a large plastic crochet hook can be modified for this and a small hammer to force the wire to go where you want it to. This should be either an amber plastic mallet or a "hide" mallet, no iron, no sharp edges and nothing that will bruise the dielectric coating on the surface. At the voltages you are working with corona will attack any dielectric discontinuity with vigor.
Once finished I would recommend you purchase a 1 gallon can of Dolph AC41 varnish from EIS inc (eis-inc.com). this is an air dry chemical and is impervious to mold and chemicals that will dissolve epoxy. You can also use it as a conformal coating for circuit boards after population, just a superior product all round. Plan on having it cure in a ventilated space OUTSIDE your home!!!!!!! This will provide a considerable boost in your fight against corona death.
Bud
Once finished I would recommend you purchase a 1 gallon can of Dolph AC41 varnish from EIS inc (eis-inc.com). this is an air dry chemical and is impervious to mold and chemicals that will dissolve epoxy. You can also use it as a conformal coating for circuit boards after population, just a superior product all round. Plan on having it cure in a ventilated space OUTSIDE your home!!!!!!! This will provide a considerable boost in your fight against corona death.
Bud
you did not mention you core area, however you can use this formula: 1750/A where A is your core area...
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