I am thinking about putting my filament transformers (4 in total per channel) under the top plate of the PS chassis, as these are open framed units, but I am concerned about stray fields under the chassis. Am I worrying about something that I shouldn't be, by placing them there? I have never mounted any transformers/chokes below decks like this before for fear of inducing hum in the wiring. Its not a matter of space on the top plate, just going on over everything one last time before I start drilling holes. What would you do in this case?
Filament transformers do create strong fields around them. But these are magnetic (B) fields (rather than E fields), and unless the chassis is THICK steel, it will make little difference which side they're on.
Best plan is to pay attention to wiring: be sure that sensitive circuits (grid input-cathode return) are not in a large loop-area. keep these very short and closely parallel, twsit if more than an inch or so. keep input circuits furthest from the filament trafos. Avoid using the chassis for power/heat/signal current anywhere.
The orientation also matters. If at all possible, test the amp before drilling the fixing holes. try the trafos in different 90 degree orientations for lowest hum.
Best plan is to pay attention to wiring: be sure that sensitive circuits (grid input-cathode return) are not in a large loop-area. keep these very short and closely parallel, twsit if more than an inch or so. keep input circuits furthest from the filament trafos. Avoid using the chassis for power/heat/signal current anywhere.
The orientation also matters. If at all possible, test the amp before drilling the fixing holes. try the trafos in different 90 degree orientations for lowest hum.
I understand the 90 degree orientation of the transformers to each other, but I was concerned with putting any trannies/chokes under the chassis. I understand now that the magnetic fields will be there regardless now. Proper wiring layout will be a big hurdle.
I have attached a link to my proposed layout with all transformers on top of the power supply chassis. If you all would not mind reviewing this layout, I would appreciate it. I am about to breadboard the circuit as drawn to make sure that thr transformers will work in this orientation, but I have never embarked on a circuit this complex before, so I could use some sage advise.
http://themastins.com/PS_Chassis_layout.gif
T1: Main Power transformer
T2 and T3: Filament trhansformers for the two mercury vapor rectifier tubes (866A)
T4: Filament transformers for input/driver stage (on amplifier chassis)
T5: Voltage supply for bias circuitry (V6 (6V4), V7 (6U8A))
T6: Filament supply for bias circuitry V9 (6L6GC), V10 (12AX7))
The two squares in the lower right of the chassis are PS chokes.
I have attached a link to my proposed layout with all transformers on top of the power supply chassis. If you all would not mind reviewing this layout, I would appreciate it. I am about to breadboard the circuit as drawn to make sure that thr transformers will work in this orientation, but I have never embarked on a circuit this complex before, so I could use some sage advise.
http://themastins.com/PS_Chassis_layout.gif
T1: Main Power transformer
T2 and T3: Filament trhansformers for the two mercury vapor rectifier tubes (866A)
T4: Filament transformers for input/driver stage (on amplifier chassis)
T5: Voltage supply for bias circuitry (V6 (6V4), V7 (6U8A))
T6: Filament supply for bias circuitry V9 (6L6GC), V10 (12AX7))
The two squares in the lower right of the chassis are PS chokes.
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Depending on what function the 12AX7 has in the circuit, it may be upset by fields from the 5AR4 rectifier - the current through these peaks much higher than the dc, and has an unpleasant waveform. Placing the AX7 the far side of C6 would be an improvement - if it's a sensitve circuit anyway...
Thanks Rod. It is used in my +440V bias supply circuit. It will not be a problem at all moving it farther away from the rectifier tube. Thank you for taking the time to review this for me. Here is a link to the actual bias supply circuit.
http://themastins.com/440V_bias_supply.gif
Here is an updated version with the 12AX7 moved farther away from the rectifier.
http://themastins,com/PS_Chassis_layout2.gif
http://themastins,com/PS_Chassis_layout2.gif
The 300B amp I am building will have two transformers under the hood, and Kevin Kennedy, the designer, says the amp is very quiet, as long as care is taken to separate signal and AC power wiring.
Here is Kevin's prototype: http://www.kta-hifi.net/projects/amp_page/300b_amp/300bpp.html
Alex
Here is Kevin's prototype: http://www.kta-hifi.net/projects/amp_page/300b_amp/300bpp.html
Alex
transfomers are traditionally put on top not only for the magnetic field but also for heat lost
the fields are also meant to affect the caps as well as tubes (as far as i understand they do)
i would put plate metal between the transformers and any thing else and make sure you have good heat lost.
the fields are also meant to affect the caps as well as tubes (as far as i understand they do)
i would put plate metal between the transformers and any thing else and make sure you have good heat lost.
Must have been a long night. This circuit is for the regulation of the input and driver stages. The bias supplies are similar, but with negative voltages
No this is a pair on mono KT88's in PP. Separate PS chassis for each channel. I have always wanted to build a pair with regulated stages and bias circuits. Big undertaking, but part of my bucket list of audio projects. I have always heard so many negative aspects to regulated designs, but I have to try it for myself. I guess its the Missouri mule in me. Need to investigate shunt regulation a bit more before I actually start building.
Thanks Rod for your thoughts. The PS chassis will be approx. 3" tall, so I will have some room for a quiet fan if required to be installed in the bottom plate. I will have plenty of ventilation holes in the top plate. I guess my primary concern with a fan would be noise, but if required I can see how it would work.
Alex,
I just wrote a message to Kevin asking him asking about shunt regulation for my amplifier, instead of the series regulation that I had planned to use for these amps. The shunt circuits are more complicated and have more energy waste, but they seem to add less to the final sound, so I want to find out what is involved circuitry wise, before I start my final build.
I thought of using shunt regulation (which I already use in my preamp and active crossover) in this amp, but decided against it - partly because Kevin's design seems very well conceived already, and partly because of the huge energy inefficiency.
Alex
And I might not do it either, but I would like to see an implementation so that I can judge if the extra circuitry would be worthwhile or not. I know it is huge waste of energy, as a Class A stage would be too. It dissipates max energy either into the load or into the circuit. Just want to make sure that I leave no stones unturned. Good luck with your project. I look forward to hearing your thoughts on its performance once completed.
Will do. I would be interested to hear what Kevin thinks of using shunt regulation for the 300B amp!
Alex
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