Leslie 145 distorting badly...

[Daniel777]

Hi folks

Leslie 145 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)

Any thoughts guys? Appreciated

[David Anderson]

Did it ever work correctly?

That you thought shorting the 470 Ohm resistor would fix this problem . . . well, the best thing I can say about that is that it's troubling. The 470 Ohm resistor is there to dampen surges when the 0C3 kicks on. Other than that, it doesn't really do anything other than sometimes act as a fuse if the screens short.

Leslie 147s don't have a particular problem with power supply hum, so all that massive power supply capacitance does is to strain your power transformer every time you turn it on. A little knowledge is a dangerous thing.

It does matter if you're using non-standard transformers.

[Daniel777]

Hello David

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 ideas?

[David Anderson]

I'm not sure the power transformer for an L100 is suited for this application, being designed for a very different power supply. The 6.3V winding is adequate, and you do get some extra headroom by not having the 5U4's heater demands, but the specs for the stock Leslie 147 PT B+ winding are 335VAC @ 190mA. The L100's amp is only drawing 100mA at idle, while 147s are usually drawing ~180mA at idle, with greater demands at full power. While the 147 schematic shows 420V off the bridge at idle, in the real world, it's usually 450-460V.

So you're trying to make the L100 power transformer's B+ winding operate at 80% over spec. While you can play with this by deleting the load on the 5V winding, you can only push it so far. Considering the inrush current demands of the large power supply reservoir, I wouldn't be surprised if you eventually blew the B+ winding. In a full-wave power supply like the L100, the two halves of the B+ winding alternate. In a bridge rectified circuit, the single winding is working on every rectification pulse, and the peak charging current is made even higher by your oversized capacitors.

I would bet your B+ voltage drops even lower once you get the 6550s into Class AB1 operation range.

Here in the U.S., the Classic Tone PT would be the obvious choice, but they don't make a 230VAC model. You can get a 230VAC Leslie 147 PT, but it is more expensive, which I predict will be the complaint as it so often is with DIY projects where people are trying to make do with whatever spare parts are on hand. Unfortunately, there's nothing I can do about that.

[Jyvoipabo]

Hello,

Check ~110V DC Anodes pin 1 and 6 of the 12AU7 and =0V DC G1 grids pin 5 of each 6550 output tubes.

JP