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#1
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24LQ6 Substitute for 6LQ6; More Hum?
Hello All,
I plan on making an amp that originally used 8 6LQ6 tubes. I really don't feel like spending the $50 per tube that these cost. I see that there is an equivalent but with a 24 volt filament; the 24LQ6. My question is this: If I use these tubes with a 24 vct transformer and ground the center tap of the transformer, will there be more "hum" present at the speaker output? The output is "Unity Coupled" so there is no voltage gain in this stage. I don't know if this makes any difference or not. The 24LQ6 looks very attractive because of it's price. Thanks, Daniel |
#2
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Surely you are talking about the Mac MC3500/MI350. If you can wind the
OPT or get someone to wind it for you you are way ahead of this game. I would series two of the 24* heaters on each side with a ballast resistor and build a DC supply running 55 or 60 volts, or rather two of them, one for the pushes and one for the pulls. There is a lot of DC offset and AC differential heater-cathode voltage in the Mac circuit. See how they solved the problem in the MI200. A pair of Vicor brick switchers might be the ticket here if you are not anti-silicon. |
#3
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Yes, Thanks for your reply.
I was lucky enough to acquire some outputs from a pair of MI350's. I purchased them from a HAM who bought the amps as scrap. All he wanted was the B+ transformers to build a transmitter with. I consider myself extremely lucky. As for using 24vac for the output tube's heaters, I want to keep it as simple as possible. I know that with Directly Heated Triodes, the higher the AC heater voltage, the more "hum" present at the output. I am hoping that tubes with a unipotental cathode (like the 24LQ6) don't suffer this same drawback. As for heater - cathode voltages, I don't see them getting much higher than 150v in this circuit, maybe 200 volts max. Each winding in the output transformer is 150 ohms. These amps get there swinging lots of current! If anybody with experience using high heater voltage tubes can share with me their knowledge... Thanks! Daniel wrote: Surely you are talking about the Mac MC3500/MI350. If you can wind the OPT or get someone to wind it for you you are way ahead of this game. I would series two of the 24* heaters on each side with a ballast resistor and build a DC supply running 55 or 60 volts, or rather two of them, one for the pushes and one for the pulls. There is a lot of DC offset and AC differential heater-cathode voltage in the Mac circuit. See how they solved the problem in the MI200. A pair of Vicor brick switchers might be the ticket here if you are not anti-silicon. |
#4
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They are worth a _lot_ of money, so he would have been far better off
to sell them and buy new transformers. |
#6
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DanF wrote:
Hello All, I plan on making an amp that originally used 8 6LQ6 tubes. I really don't feel like spending the $50 per tube that these cost. I see that there is an equivalent but with a 24 volt filament; the 24LQ6. My question is this: If I use these tubes with a 24 vct transformer and ground the center tap of the transformer, will there be more "hum" present at the speaker output? The output is "Unity Coupled" so there is no voltage gain in this stage. I don't know if this makes any difference or not. Biasing the heater around 50VDC above the cathode will reduce any hum you might have had if the centertap were tied to ground. You don't need much current capability in this bias voltage, a voltage divider hung off the B+ using resistors around 100K and 27K would do. |
#7
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On 16 Apr 2005 14:26:01 -0700, "DanF" wrote:
Hello All, I plan on making an amp that originally used 8 6LQ6 tubes. I really don't feel like spending the $50 per tube that these cost. I see that there is an equivalent but with a 24 volt filament; the 24LQ6. My question is this: If I use these tubes with a 24 vct transformer and ground the center tap of the transformer, will there be more "hum" present at the speaker output? The output is "Unity Coupled" so there is no voltage gain in this stage. I don't know if this makes any difference or not. "Unity coupled" means a gain approaching two, but yeah, noise sensitivity is reduced. If you want to null any residual 60 Hz hum, check Steve Bench's pages for an elegant universal solution (injection at grids). Chris Hornbeck 6x9=42 April 29 |
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