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In Reply to: Re: Interstages with very high inductance posted by mqracing on April 1, 2007 at 09:53:57:
Thank you Mike for the thorough explanations.Right now the design does use two 260H anode chokes; I'll test with a CCS soon. Coupling is teflon cap. Sounds good, but i prefer transformer coupling which seems more dynamic. Adding a transformer in the existing case is impossible, but i might try in my next design. One question i have, does the leakage C and inductances of anode chokes + interstage + mosfet tail CCS might induce HF resonances?
So far the specs i was thinking of are 50K:50K 600H CT primary, CT or dual winding secondary (2x 12K5 ?), allowance for maybe 5mA or less unbalanced current in the windings to account for less than perfect tube matching. The output stage is sitting on a CCS too, so there won't be no bias current in the secondary. Since you are quite the expert in the matter, i'd ask you if such a unit would be extremely costly? If you prefer answering this last point privately, my email is jorisdallaire AT yahoo DOT ca
Follow Ups:
Hi Joris:High impedance tranney designs are trickier to design and balance the requirements of having a good bottom end along with good extension on the top. But not impossible. In the archives, as just one example, is a line output trans designed by Peerless with a 30K nominal primary impedance--- whose bandwidth (if you evaluate success on this parameter) is listed as 5Hz to 65khz plus or minus 1 db. As a funny aside--- this particular trans was designed for and sold to the Fort Wayne Steel mill. Don't ask me for what--- I have no clue.
As a general note on the costs of these small signal specialty tranneys.... extremely costly is itself a relative term--- that said--- frequently the cost of housing the unit (in multiple mu metal cans, sometimes with copper shading rings btwn the cans) was a large contributor to the final end cost of the product. If you application does not require this sheilding the relative costs become more manageable--- though your still often looking at some of the most expensive lamination materials (high mu lams) available--- so that you can get a walloping amount of L from a relatively small core area with relatively few turns--- all of which helps in getting a well balanced design with good bottom end capabilities along with good extension on the top end (high freq response).
In advance, we are not currently offering any small signal tranneys with pri impedances much above the 15K range.
best of with your project,
MSL
Thank you Mike,I must admit the technical difficulties being pointed out cool my enthousiasm :O) It seems that they are surpassing the advantages i first saw to the setup. However, the CCS plate load + transformer coupling might be a promising avenue.
Joris:Hi. I wanted to add or mention another strategy that I think works better with tubes having high or relatively high r sub p's....
I think in these cases one of the best coupling methods is to LC couple to the next stage. If your going to try to use a conventional 2 winding IT--- you'll already have the L and the C in the signal path--- but perhaps not so much optimized.
So--- why not use a grid choke where you can get several kilos of L's and the high impedances produced by that L. Yeah... you'll need to use a blocking cap in series... but remember that the series fed IT will also likely have an even larger cap in the signal path (the last C in your power supply feed).... and if you are going to use a parafeed IT (an improvement over the series fed IT) you will still have the cap in the signal path... but even the parafeed IT will not be able to produce as efficiently the L that the grid choke delivers.
then for loading your anodes... you can resistively load them or use plate choke load the anodes. I am assuming that if it's an LTP--- that you would want to use a CCS in the tail... to make it very long and tie it down...
LC coupling gives you the speed, dynamics and punch of having some iron in your diet. With none of the relative drawbacks of a bifilar wound transformer and without the sub-optimalities of trying to design a conventional 2 winding IT that is capable of delivering the same magnitude and quality of load imepedance to the anode of your driving tube as well as the grid choke can.
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