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Leslie 145 amplifier distortion
Leslie 145 amplifier distortion
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Old 16th September 2019, 01:36 PM   #11
Kay Pirinha is offline Kay Pirinha  Germany
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You mustn't change the value of any of those four 56k resistors. As plate and feedback resistors, respectively, they're critical, and altering their values also will alter the amplifier's sound. Try my suggestion, I think the DC voltage will be close enough to 420 V.
Does the stock rectifier feature 8 diodes, as per schematics, or just four, as I've seen in a 122 amplifier that I owned until January 2013? Howsoever, as I don't know the stock diodes' specs, get four 1N4007's for just a few cents, and they will do, even without voltage sharing resistors.
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Old 17th September 2019, 01:47 AM   #12
PRR is offline PRR  United States
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> convert the 145/147 amp's PSU for full wave

WHY?

He's got distortion "badly". He's within 10% on B+ and on cathode current. While that will give lower numbers on banch-test, "badly" to me implies a deeper problem-- at 10% low it should play fine up to a quite loud level.

Still waiting on pin voltage readings and a proper resistor in line with that gas tube.

I don't know why the factory 56k resistors were changed without checking voltages *first*.
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Old 17th September 2019, 08:20 AM   #13
Daniel777 is offline Daniel777
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So...an update folks

I did indeed convert to full wave rectifier, just to use all of the transformer winding. Slight improvement, not dramatic. DC voltage is 405 where should be 430 on B+

I can play in treble register softly without much distortion, but adding a bass note makes it fuzz up like a guitar amp.

Max volume, distorted as it is, is around half what a normal Leslie 145 would do.

So I'm feeling there must be more. I've replaced all valves... Checked all resistors and replaced a couple. I think it's in the output push-pull pair of 6550 valves, because I have modified the resistors on the 12au7 to give correct 115v (easily changed back, and it did make a 30% improvement to problem distortion)

Other parts are as per schematic now...

I don't need the full gain, 20W would be fine for my home use. Hence, I don't mind the lower B+ - if I can just lose the distortion!

Any more thoughts guys?
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Old 17th September 2019, 09:54 AM   #14
Kay Pirinha is offline Kay Pirinha  Germany
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Is the Leslie's 15" LF driver ok? Has it previously been replaced by a 8 ohms one, which isn't a good idea?
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Old 17th September 2019, 03:34 PM   #15
Daniel777 is offline Daniel777
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The 15" driver tests as 9.6 ohms DCR so I suspect it is a correct one....
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