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In Reply to: RE: Pushpull 46 amplifier posted by used-hifi on August 14, 2017 at 19:08:48
Funny you should mention that. I have, as a retirement project, a PP 46 project very slowly gathering steam. My last day on the job was 31 July. :>D The next order of business is acquiring the O/P "iron".
The data sheet shows a no NFB trafo coupled design, which I will not use. I am building a variation on the "El Cheapo" theme, that uses MOSFET followers DC coupled to the #46 grids.
BTW, I have RCA ST bottle #46s on hand. Talk about industrial art. The proverbial brick outhouse seems flimsy compared to that which RCA built.
Eli D.
Follow Ups:
Eli:
Congrats on hitting retirement - please keep us posted on your Class B 46 project.
Jim
Jim, thanks for the good wishes.
I want/need suggestions from knowledgeable people, like you, about chassis design.
The signal complement in each channel is a 12AT7, a Pete Millett MOSFET driver PCB plus heatsink , and 2X #46s.
The B+ PSU is a 5 Schottky diode 2 rails from 1 CT construct. The "tall" rail is low current and choke I/P filtered, which allows for hefty load resistances on the LTP plates that yield good net gain. An AnTek AS-2T230 toroid does lots of the energizing duties. Along with the toroid, I already have, on hand, Heyboer wound filament trafo and 20 H. choke I/P filter inductor. A 20 Kohm bleeder resistor is not especially wasteful of power. ;> )
Space for the PSU has to allow for 2 Octal relay sockets (Amperite and "standard") and a Loctal socket for a 14Y4. The 14Y4 delays LTP B- rail rise.
BTW, I plan on dragging the #46 grids sufficiently positive, at "idle", to dispose of the notching/crossover distortion issue. JMO, we should be stating near Class "B2" operating conditions. I'll gladly sacrifice a little O/P power for linearity's sake.
Eli D.
"BTW, I plan on dragging the #46 grids sufficiently positive, at "idle", to dispose of the notching/crossover distortion issue. JMO, we should be stating near Class "B2" operating conditions."
I'm confused by this.
How do you bias the tubes at or near cutoff when the grids are positive?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Look at the #46 data sheet . With the 2 grids tied together and 0 VDC on them, the tube is nearly cut off. In order to get a small, but appreciable, "idle" current, a small, positive, bias is needed.
The #46 is a "natural" for Class "B2" operation.
Eli D.
Sorry.
I was thinking triode connected.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Eli, would there be an issue wiring both 46's heaters in series?
I have 5vct transformers on hand.
IMO and what others say about class B amplifiers that notch distortion is much more prevalent in SS because the devices are almost cutoff, with tube that V or notch is much less and as we shall see less audible.
also can you please when you have a min answer the question above about parallel feeding.
Thank you
Lawrence
IMO, you should save the 5 VCT "iron" for 300Bs or 5 V. DH rectifiers.
I ordered a custom filament trafo from Heyboer that contains 4X 2.5 VCT windings, at very reasonable cost. Getting both "idle" currents and filament phasing right is easy. Remember, you want AC heating hum cancellation in the O/P "iron".
BTW, another winding on the custom filament trafo takes care of 'T7 heater power.
Eli D.
Ok, I am a cheap lad esp when it comes to experimental designs and if it pans out i like it i can go full Monty!
I don't design I am more of Tech/repair/thinker with lots of common sense, Thats why i am asking because I don't know
the data sheet does not specify separate heater winding's, so i thought why not use what i have to make it function if there is not objection?
Now I am going to need all the power these tubes can utilize min 20watts other wise this wont work for me so its class B all the way.
Eli if your not familiar with para-feed or don't know the answer to my question its all good, I can experiment and report back if no one cares to comment.
There are plenty of PP amplifiers out there class A or AB AB1 but nothing like what we plan to do class B is a totally different animal.
from what I am gathering class B amplifiers need excellent output iron meaning very low shunt capacitance and leakage inductance! Torrodial comes to mind.
What OPI are you thinking??
Lawrence
I'm thinking of "the usual suspects", for the O/P "iron" and it will not be toroids. The #46 data sheet calls for a 3.5 Kohm end to end primary. Plitron does not offer a suitable "off the shelf" model and it's very long odds both of us would be bankrupted by the cost of a combined Q4 custom order, let alone separate custom orders. :> (
In a little while, I'm going to request quotes from (in no particular order) MagneQuest, Onetics, Heyboer, and ElectraPrint. As my design uses global NFB, I need "iron" with at least a 35 W. power handling capability. A no NFB design needs less magnetic headroom.
Eli D.
Been Thinking and playing with a single channel 6hv5-IT-pp46 and plays ok at lower volumes but cannot get power out esp at lower frequencyIt maybe the tubes as I dont have tester for 46's
your power drive idea or mr millet fet followers can drive the grids quite positive :)
I am not a designer as stated above would a modest IT for phase splitting duty work followed by the fet followers?
tI also wanted to say I could NOT see crossover distortion and the amp has THE most accurate sound IMO , there is not the typical class A fuzziness :)
the sound is very very clear all of the musical information seems to be right there unlike other typical pp class A or AB amps i have played with its dynamic sweet and enjoyable :)
Edits: 08/27/17
Looking at the data again 20 watts continuous, I would bet much higher peak power can be obtained! Esp if you have plenty of drive current , but I would like to try interstage first then maybe move onto a power drive mosfet driver
Have to see if I can get some parts cheap to play
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