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In Reply to: RE: 5755 and a ll1630 for preamp posted by amnesiac on February 22, 2025 at 17:21:26
Forget about it. With an Rp as high as these of the 12Axx you will get MASSIVE frequency response problems with an output transformer. You will need an ungodly inductance for an acceptable LF rolloff and that many windings will give you a hell lot of interwinding capacitance screwing your HF response. so au7 is out 6j5 types leftedit should I start drilling?: ^)ah 12b4 across parallel winding 12 has low rp low enough? Ilike 12b4 my epay name. Holding back on the step drill until somebody help me do the math i'm a fitter turner done 1st year radio and tv 20 years ago. I just bias and listen. Need to get a scope. learn more.
Ok found 6 6cm7. According to data I can run 200v 5ma on plate hows that?
Edits: 02/22/25 02/22/25 02/22/25Follow Ups:
I agree the 6CM7 looks good for this. It seems well suited for the 5 DCmA limit of the LL1630. Many tubes would work well here, including the 6SN7.
Edits: 02/23/25
Iam going with 6cm7 then. I have a dumb question TK I can't remember for sure the trans phase. Does the asterisk indicate positive phase? Its been ten years.
Edits: 02/24/25
Phase in this case isn't positive or negative, it's simply the direction of the primary and secondary windings relative to each other. The diagram of this transformer at Lundahl's site shows suggested hookups. Note that their use of 10K terminating resistors in the SE example means the tube must be suited to a 40K plate load (in parallel with whatever is reflected back from the load).
I just realized the 6CM7 is two dissimilar triodes. Is that what you intended, to use each section for a different purpose? If this is a straight line amp with a level control at the input, I think you'll be better served with a 6SN7 or similar.
I added 32 ohm carbon to the grid maybe a little more air but less dynamic. Iam going to try find a cadock or vishay pair for the grids. I dont know about the load still have 875 vishay across it still not sure if Iam ib phase with the astrix as ground. It could explain the poor sound.
Edits: 03/01/25
Tk Iam mixing them up they are twin n7 types and shockingly ;[ its driving my ab workshop amp. Iam biasing the tubes through a motor blue alps 5k I never think thats a good idea. Iam not sure if I have the phase right if the astrix is positive iam inverting phase.
Edits: 02/27/25
Little thing built on an old chassis every hole is crooked and covered with copper sheet its quiet!. I love using old chassis Rat preamp build. It's not even folded strait.
I love to copper out everything and use as a ground plane occasionally. 2.1 Earth bus on amps I use sakuma's method. I got lazy here and used only one bus not separate signal and ps. But I am still careful where I place bipases and signal. Not a fan of blue alps I usually use the cheaper black or a alps carbon. with the circuit board what I might do if I can't get the warmth Iam looking for. I will use a 6ak5 triode if I have too I have better 7 pins basses I trust also. I'll keep working on it trying different valves. Or start a new dht or p. I really think globe 47 triode would make a great preamp. It's a rat but quiet I enjoying it now. Tidy it up.
image
Edits: 02/28/25
Its built Iam testing it with my penon serial iem Sounds kind of sterile but super quiet but Iam sorry I stripped caps and trans out of my 26 project now it had the warmth this one does not. Since its so quiet I will start afresh on a dht preamp 26 71a or 47 triode and try to sweeten this one up. Maybe I need to put a cap in front of the triode rather than bias through the blue alps. It's just not sweet maybe the bases I never used them before. Too much untested parts. Iam not sure where to start more oil caps I used polly bipas on the power supply and an echoke. I might try the tube power supply. Maybe just break it in more. I compromised for a safe build and it's quiet and dynamic it's sure quiet max volume its silent. Iam sus on biasing through the alps maybe try a gridstop.
R =
V
I
=
7 volt
.05 ampere
= 140 ohm (Ω)
P = V × I
= 7 volt × .05 ampere
= 0.35 watt (W)
140 ohm cathode resistor seems off 5 milliamps 200 plate .35 watts. 6cm5 very tired but keen to wire in something tomorrow I have a plethora of 8 pin 6c6 and all the top cap pentodes and dual triodes and loctals but 9pins are easy and installed and 6 is enough I haven't tried them in decades because I didn't have nos but these look fine, wire them alternate triode two swap triodes left to right to get ballance.. 005 maybe 1400ohm more likely
Edits: 02/23/25
140ohm seems so low.
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