ruffrecords
Well-known member
I am not sure the output stage bias circuit will work unless the transformer primary has some significant dcr. The grid needs to be negative wrt the cathode.
Cheers
Ian
Cheers
Ian
Cheers
Ian
Cheers
Ian
Valve mic preamp design incoherent rambling
- Thread starter GussyLoveridge
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Winston OBoogie
Well-known member
Yep, total primary dcr of that transformer, with all windings in series, is only 352R
With the E81L biased for 20mA that's only 7V on the cathode. Your 200H chokes, albeit you don't need 200H here, have a much higher dcr which puts the cathode at a nice 50V or so.
Edit: parafeed your cathode output?
Edit 2: never mind, you already dissed that idea...
With the E81L biased for 20mA that's only 7V on the cathode. Your 200H chokes, albeit you don't need 200H here, have a much higher dcr which puts the cathode at a nice 50V or so.
Edit: parafeed your cathode output?
Edit 2: never mind, you already dissed that idea...
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Winston OBoogie
Well-known member
P.S. One thing with the choke loaded anode of the 6SK17 is that it will help PSRR as well as act like a constant current source (an imperfect one but nevertheless...) so the cascode stage will be more linear.
It's also a high gain stage which "may" be more than you want. One way you could reduce gain (besides degenerating the J-Fet more) is by giving it a stiffer restive load to work into.
A 100K or whatever ... from anode to ground.
It's also a high gain stage which "may" be more than you want. One way you could reduce gain (besides degenerating the J-Fet more) is by giving it a stiffer restive load to work into.
A 100K or whatever ... from anode to ground.
I'm a little concerned with so much iron in the signal path, though.
Winston OBoogie
Well-known member
Solid state current sources or gyrators for the cascode and the follower then?I'm a little concerned with so much iron in the signal path, though.
No output transformer.
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Maybe ditch the tubes too? Use a BJT over the K170, and a MOSFET in lieu of the E81L.
Now you're talkin' !
Agreed. I've always considered parafeed an economically-challenged solution.
Both the choke and the xfmr must have very low parasitic capacitance, and the combination of the choke's biased magnetic point and the xfmr's non-linearities are very challenging. I'm not neglecting the advantages of parafeed, but I think a SE xfmr can be made for a lesser cost than a choke+xfmr combo for equivalent results.
Of course, I can be completely wrong, since I have ceased using valves and xfmrs long since...
Is it really an issue? The cathode will swing negative due to magnetic storage.
Indeed. 7V is probably not enough. Adding a cathode resistor (by-passed or not) may solve the issue. Or find a way to apply fixed negative bias.
Winston OBoogie
Well-known member
I'm not neglecting the advantages of parafeed, but I think a SE xfmr can be made for a lesser cost than a choke+xfmr combo for equivalent results.
Of course, I can be completely wrong, since I have ceased using valves and xfmrs long since...
No you're not wrong. A good choke is an expensive item. And I'm not convinced that the inexpensive bigger inductance, lower current power supply chokes from Hammond etc. that folks have used is a good solution.
If you're going parafeed, I think you need to commit and buy a proper anode choke.
Personally, I would prefer using a higher H.T. supply and going solid-state loading if I were doing parafeed.
2 X £1.50 Mosfet's and a few resistors replaces your choke, use a nice Hi-Nickel non-gapped OPT
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Winston OBoogie
Well-known member
If the anode of hs first stage is at about half H.T., then maybe just DC couple to the follower and use a resistor under the cathode for bias.
Cap into the OPT
I looked at the E81L datasheet. For 20mA at 200V, Rk is only 120ohms. For 15mA, 180 ohms.
With 7V, it is nearly at cut-off.
I believe with 350 ohms, it would idle at about 11mA.
Correct me if I'm wrong.
Chokes, even the audio ones, used RF winding techniques (honeycomb, fractioned...)
Is there anybody left doing them?
ruffrecords
Well-known member
Apart from a few MHz for FM, aircraft and DAB, almost everything below 0.5GHz has been given over to amateur radio. They probably make their own
Cheers
Ian
Winston OBoogie
Well-known member
TBH I didn't/haven't looked at the E81L data sheet, I didn't even factor in Rk, I just pulled numbers out of the air based on his biasing technique and the DCR of his transformer ie. he puts 7V on the top of the primary winding.
Sloppy advice but mainly to bolster Ian's point that the output transformer doesn't really have enough DCR to work into.
A DCR of 2K or so would be better.
The best I've found are the Lundahl. Specifically the 1668 with DCR of 680 for anode loading at various inductance values/current:
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Why? You don't need voltage across a xfmr for it to work. You need current. In what is it different than putting the same xfmr in the anode?
I agree you need some DCR for cathode-bias to work. The datasheet shows perfect adequacy.
That's my feeling too...Apart from a few MHz for FM, aircraft and DAB, almost everything below 0.5GHz has been given over to amateur radio. They probably make their own
Yep I neglected the dcr side of the thing. I will stick to the parafeed line stage and resistor loaded cascade. It is a big challange to have many magnetic component in one chassi.
I will start a new build tread for this I just need to buy a few small components at the local electronics store
This is the new schematics
I will start a new build tread for this I just need to buy a few small components at the local electronics store
This is the new schematics
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Replacing parafeed choke with SE Trafo. A resistor in series with the transfomer mimicks higher dcr. Right. If we look at the v74 it has a 45H choke, dcr ca. 1k in the line stage. If a add a 670ohm resistor i series with the LL1671 (total cdr 1k) it will drop 20v at 20ma. In technical sense your are making the trafos parameter worse, but there is no practical reason not to do it right? A resistor i cheap and can put the cathode voltage were you want it. As long as you have enough primary inductance...
