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In Reply to: RE: Hammond Galore Part 2-GM70 posted by deafbykhorns on May 29, 2015 at 19:52:02
You are losing your bass because the 44hy's of inductance is not enough for your 12K-ish source impedance. This is partially being masked by the CL resonant bump which is at around 40hz.
dave
Follow Ups:
I need to crack open that RCA handbook again, probably not a good idea using IT with that tube....
It would probably take some serious custom iron. I'll play around with the pentode more but I like the looks of
driving the GM70 with the 300B. Just need to find a tube to drive the 300B with about 40volts or so
(prefer a little headroom plus a hint of A2).
Hey,
A2 isn't an easy proposition and it isn't going to happen with that coupling cap in place. (for me grid chokes and cathode followers need not apply) Going to a 1:1 will help a bit but the DCR of the windings is added to the source Z again limiting the A2 possibilities. The next step in the design logic places a IT with a slight stepdown to keep a reasonably low impedance drive but I find the price in bandwidth and gain do not help the sonic situation. Maybe the best option is to simply direct couple the 300B to the GM-70 and then use the 1:1 to add a 5687 or 6H30 in front of the 300B to finish things off. The 1:1 can also serve plate choke duties on the 300B if you parallel the windings. 44hy is plenty of inductance but the 30ma is a bit light on the current.
dave
I paralleled the 126B and used as a plate choke, then DC coupled the 300B to the GM.
Changed bias scheme slightly
yes.... kinda....
when you parallel the wires of a transformer you do not double the inductance or current carrying capability so you still end up with a 44hy 30ma device.
dave
but why wouldn't I end up with 60ma? Wouldnt each winding be rated at 30ma x 2?
Either way, that's pretty slick, never knew these could double as a plate choke. Thanks for the info!
Edits: 06/04/15
The Bdc (DC flux) in a transformer is directly related to the amp-turns of current in the windings. When you parallel two identical windings, you do not change this relationship since you still have the same amount of current going through the same number of turns of wire. The DCR does typically get cut in half but that is about it.
dave
is above my pay grade....
I've made several attempts with spice but cant seem to get the basics of DC.
Would you still do a fixed bias with DC?
I did a IT with stepdown but I'm thinking twice about it since I get the hint this has already been tried and didn't sound too good.
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