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Leslie 145 amplifier distortion
Leslie 145 amplifier distortion
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Old 15th September 2019, 07:48 AM   #1
Daniel777 is offline Daniel777
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Default Leslie 145 amplifier distortion

Hi folks I'm new to the excitement and knowledge of this superb forum! I'm a Hammond organ enthusiast, with a lifetime of interest in audio.... I have a organ speaker Leslie 145 -6550s in push-pull, fed by 12au7 - which has distortion badly. I have 398v on the rectifier output (shows 430v on diagram). This gives 23.4v on speaker output to ground rather than 25v. Tried shorting the 470 ohm resistor from the OC3 to the 6550s but no difference. I'm running a non-standard O/P trans and power transformer, but not sure if this matters Valves are all new, with new power capacitors too (very large values - all 150mF for extra smoothing) I only shorted the 470 because it had been missing previously - wired out. It's not worked since I had it - it came to me with no valves, missing o/p trans, filthy. - and no relay, potentiometer, etc.

The output transformer is correct spec - mostly supplied for kt88 style guitar amps like Vox, but right windings and impedance. The power supply transformer which was in it buzzed and immediately got very hot and smelly, so is almost certainly failing internally. I've replaced it with one from a Hammond L100 amp chassis, which has a 315-0-315 secondary, as well as 5v and 6.3v windings.

I've used one half of the 315 side, and the 6.3 for heaters. Heater voltage is fine, secondary measures at 319 for the HT, and then almost 400v after bridge rectifier. I haven't changed any resistors - the ones i checked measure ok. The coupling capacitors are ceramics and orange drops, which i haven't changed. Any thoughts guys? Appreciated
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Old 15th September 2019, 08:11 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Check the anode voltages on the ECC82 (12AU7).
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Old 15th September 2019, 08:24 AM   #3
Daniel777 is offline Daniel777
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Thanks Jon...will do

It does sound like distortion on the output though....
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Old 15th September 2019, 08:55 AM   #4
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Daniel777 View Post
Hi folks I'm new to the excitement and knowledge of this superb forum! I'm a Hammond organ enthusiast, with a lifetime of interest in audio.... I have a organ speaker Leslie 145 -6550s in push-pull, fed by 12au7 - which has distortion badly.
What do you mean by that?
Does it distort *always*, at all power levels, or just when you push it hard?

Quote:
I have 398v on the rectifier output (shows 430v on diagram). This gives 23.4v on speaker output to ground rather than 25v.
What do you mean by that?
Are you trying to get 25V on speaker but it starts distorting whenn reaching 23.4V?
Quote:
Tried shorting the 470 ohm resistor from the OC3 to the 6550s but no difference. I'm running a non-standard O/P trans and power transformer, but not sure if this matters
Brand does not matter much but specs should be what circuit demands.
Quote:
The output transformer is correct spec - mostly supplied for kt88 style guitar amps like Vox, but right windings and impedance.
Ok.

Quote:
The power supply transformer which was in it buzzed and immediately got very hot and smelly, so is almost certainly failing internally. I've replaced it with one from a Hammond L100 amp chassis, which has a 315-0-315 secondary, as well as 5v and 6.3v windings.
Does it match what circuit demands?

Quote:
I've used one half of the 315 side, and the 6.3 for heaters. Heater voltage is fine, secondary measures at 319 for the HT, and then almost 400v after bridge rectifier.
So you are using a (presumably) full wave rectifier winding transformer from another amplifier for a different bridge rectified supply ... and of course onlyb half of it.
Then youb are severely misusing and under rating it, because that half winding both does NOT expect 100% duty but 50% one, and to boot it hs twice the resistance, so twice the resistive loss.
That alone will pull voltage under load more than expected and also overheat that winding.

IF your Hammond uses a bridge rectifier, get the proper transformer designed for that.
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Old 15th September 2019, 10:42 AM   #5
Daniel777 is offline Daniel777
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Hello

The transformer is a power transformer - the Leslie amp only has a single coil for power and one for heaters. Using a centre tap half of the secondary won't do any harm, so long as it's not overloaded.

It's a transformer from a completely different amp - which has a reverb section etc, so is used to providing plenty of current.

The distortion is on everything above a whisper. The 23v I speak of is a figure quoted on the circuit diagram - no at speaker level

It's a 40w amp, but I only need to achieve half that for home use...
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Old 15th September 2019, 04:11 PM   #6
PRR is offline PRR  United States
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Give link to schematics. Measure voltages all around the driver and compare to schematic values or report here.
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Old 15th September 2019, 04:24 PM   #7
PRR is offline PRR  United States
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Leslie organs and amp numbers overlap. I found a 147 which may be similar to 145.

The "25V" is actually the self-bias on the 6550s and 23.4V is all the same for now.

The "470" is critical. Without it there is no power to G2 or driver. With a short in place of 470 the gas-tube may oscillate bad.

The OT p-p load should be around 5k, not real critical. BUT the secondary winding on the Hammond is NOT a common 4-8-16 winding. It seems to be 16r CENTER-tap. The centering is vital (a 4 tap may or may not be close enough). The phasing is critical or it becomes Positive FB which is bad.

There is a 8 ohm dummy load on the input which sure would distress any normal preamp but it can be switched-off.
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Old 15th September 2019, 06:14 PM   #8
Daniel777 is offline Daniel777
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Hello

Yes, the amp is a 147 type. The common schematic online is correct.

I've improved matters a little by reversing the windings feedback on the primary. Still distortion though.

I've replaced the 0.1 coupling caps now, and reduced the smoothing caps down to 100mf (I did trial with the original can cap too, which made it worse)
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Old 16th September 2019, 01:04 PM   #9
Kay Pirinha is online now Kay Pirinha  Germany
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As the L-100 uses a 5U4GB full wave rectifier or something like that, I'd just convert the 145/147 amp's PSU for full wave. Get two 1N4007's in series from each plate winding end to the first filter capacitor, connect the CT to gnd and you're done.
To keep the final stage's design with your OT substitute, connect it's 4 ohms tap to the cathode bias combination, the 0 or Com end to one cathode and the 16 ohms end to the other one. If the amp oscillates, you need to swap both connections.
Best regards!
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Old 16th September 2019, 01:16 PM   #10
Daniel777 is offline Daniel777
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Hi Kay

Thanks....most helpful

So essentially a full wave rectifier? Looks like I would hAve 315V going into the diodes, giving about 445V DC no Loa, which would then be perhaps 400V on load

This is nearly 10% below design voltage - do you think this would cause distortion?

I've already improved matters by dropping the 56k resistors feeding the preamp valve to 47k, which lost some distortion. I also tried temporarily shorting the L1 choke, which is about 10 ohms DC resistance, and this did reduce the distortion further a little, but I don't think I can eliminate it - amp might become unstable?

With regards diodes, the ones installed currently I can reconfigure in series pairs - do I need bleed resistors on them too?

Any more ideas appreciated....
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