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Old 30th Mar 2023, 5:10 pm   #41
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Default Re: Help with a HHSCOTT LK-72

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Old 30th Mar 2023, 5:17 pm   #42
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Default Re: Help with a HHSCOTT LK-72

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Going by the 1st schematic posted, the total resistance in series with the heaters is 87 Ohms with the original selenium rectifier, in the OP's amplifier the total resistance in series with the heaters is 123 Ohms according to the OP, the extra resistance (36 Ohms) is most probably there to compensate for the lower forward voltage drop due to the fitting of a silicon rectifier.

Lawrence.
Hi Lawrence yes this is what i also read through other forum's, usually because the rectifier has changed they used a 33ohm resistor to drop the voltage. 0I'm just trying to find out what the voltage should be? Should I measure the resistance of my 12ax7 heaters and adjust from there?

From the schematic and grahams input I need 43-45vdc.
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Old 30th Mar 2023, 5:24 pm   #43
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Default Re: Help with a HHSCOTT LK-72

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The original arrangement would have resulted in a voltage of 45 VDC across the heaters which are wired in series. What voltage are you seeing? You need to measure it between chassis and the end of the 55R triple resistor chain. That's more of less the South West tag on the smoothing capacitor.

If the reading isn't around 45 VDC you'll need to adjust the value of the 55R resistor(s). Unless of course whoever did the mods opted to change the heater voltage.
As I said.

I can't think why you'd want to measure the resistance of the heaters.
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Old 30th Mar 2023, 5:27 pm   #44
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Default Re: Help with a HHSCOTT LK-72

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The original arrangement would have resulted in a voltage of 45 VDC across the heaters which are wired in series. What voltage are you seeing? You need to measure it between chassis and the end of the 55R triple resistor chain. That's more of less the South West tag on the smoothing capacitor.

If the reading isn't around 45 VDC you'll need to adjust the value of the 55R resistor(s). Unless of course whoever did the mods opted to change the heater voltage.
As I said.

I can't think why you'd want to measure the resistance of the heaters.
You certainly did
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Old 30th Mar 2023, 5:42 pm   #45
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Default Re: Help with a HHSCOTT LK-72

I'm thinking as the tubes warm up they draw less current, that means the voltage will increase.

I'll leave them on to warm up and measure again.
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Old 30th Mar 2023, 9:32 pm   #46
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The tubes used for this amp are different brands but labelled v1 to v4, im wondering if the resistor choice was chosen based on the mismatched tubes, hence measuring the heaters going on what Lawrence said. Just thinking out loud, it may just be bull crap, im learning as i go along.
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Old 30th Mar 2023, 9:41 pm   #47
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Default Re: Help with a HHSCOTT LK-72

You're overthinking this. Matching has nothing to do with heaters, heater currents or volts across the heaters.
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Old 31st Mar 2023, 7:02 pm   #48
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Default Re: Help with a HHSCOTT LK-72

Agreed Graham I've ordered some different value 5W resistors to up the voltage.

I've also ordered some 7591A tubes to replace these unknown 6L6 tubes as I don't think they can handle the voltages.

I've also got some other issues, with the rectifying tube in I'm getting 450V on pin ones of the 12AX7's when it should be 105V.
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Old 31st Mar 2023, 7:37 pm   #49
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Default Re: Help with a HHSCOTT LK-72

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Agreed Graham I've ordered some different value 5W resistors to up the voltage.
What voltage are you actually reading across the heaters?

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I've also got some other issues, with the rectifying tube in I'm getting 450V on pin ones of the 12AX7's when it should be 105V.
Is that valves in or valves out? If it's valves out I'm not surprised as there'll be no volt drop across the chain of voltage dropping resistors leading from the cathode of V11 the rectifier. R202, R203, R204, R205 and R206.
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