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Old 15th August 2011, 07:03 PM   #11
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Yes, use one cathode resistor per pair. I would start with 300 ohms to be safe. Measure the plate voltage and the combined screen and plate currents and ensure that the max dissipation is 14W max. Your plate voltage is almost 350V according to the schematic (this violates the max plate voltage!), so you want no more than 80mA through the cathode resistor for 24V at the cathode of the op tubes. If 300 ohms results in an acceptable dissipation, then you can try 250 ohms.

Once you get it running correctly, try connecting the input to a stereo 100k pot and the wipers to the grids of V4 to hear if you like it better than through the tone controls.

Last edited by Palustris; 15th August 2011 at 07:07 PM.
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Old 15th August 2011, 08:32 PM   #12
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thanks I'll break out the iron tomorrow -- let you know how I get on of course we are assuming that the amp is similar to the AP30 -- this one is an AP35 -- can't find the schem -- it has 2 12AX7's one 12AU7 and 2 6EU7's....

Last edited by bendix6384; 15th August 2011 at 08:43 PM.
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Old 15th August 2011, 08:40 PM   #13
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This was a common way to cost reduce an amp and get better phono preformance. you get a free DC heater supply and don't have a high wattage resistor to buy. Many amps used this. I have a H-K A300 done this way. It just depends on what you want to do.
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Old 15th August 2011, 08:44 PM   #14
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what happens when one of the 12AX7 filaments burns out ?
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Old 16th August 2011, 04:58 PM   #15
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Hi I measured with the 12AX7 bias -- dc V

Rect 364 --- plates 356 - screens 328 - cathodes 20

with separate 300 ohm unbypassed resistors I get rect354 - plate 348 - screen 326 - cathode 20v all good ? 100mf bypass caps ? thanks again -- am I in the safe poreating range for the 7408s ?
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Old 16th August 2011, 05:22 PM   #16
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That looks good: less than 12W plate dissipation. Yes, use a 100uF cap.
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Old 16th August 2011, 08:29 PM   #17
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thanks again it sounds good unbypassed -- I tried using bypass caps but I heard no audible difference (to me) -- keep it w/o them ?
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