Please help with a late 30's preamp
« on: June 13, 2010, 01:02:49 AM »
Amp: Northern Electric (Canadian Hammond) organ preamp circa 1937, no internal B+ supply. Newly rebuilt, original power and output transformers. Please refer to schematic:

Symptoms: somewhat quiet output and a prominent 60 cycle hum.

Here's what i've got so far after poking around with the scope and multimeter. Any ideas are appreciated!

- Power transformer measures 6.3v with centre tap to ground.

- Measured 6 to 8 volts 60hz ripple between chassis ground and either side of the output transformer secondary.

- No ripple on the primary.

- Only 0.1V ripple between two ends of secondary. Same 0.1V ripple is present on the B+ feeding this amp from the Leslie cabinet (320VDC).

- Checked for transformer leaks to ground, got infinity resistance between all transformer wires and ground. Tried disconnecting the centre tap ground, same results - I don't know the purpose of this ground.

- Checked output transformer with 12V AC adapter (actually reading 16V). Measured 1.25V on secondary, giving 12.8:1 ratio. Spec is 4000:200 which would be 20:1. Bad transformer?

Thank you,

Re: Please help with a late 30's preamp
« Reply #1 on: June 13, 2010, 02:01:40 AM »
Do all the plate and cathode voltages look reasonable?  Sounds like a wire may not have gotten connected during the rebuild.

 Have you replaced all electrolytics?

The step down ratio is ~ 4.5:1  so a 12VAC signal on the pri should give you ~3.5V on the sec.

Check all the existing circuit grounding and make sure it's tied correctly within the preamp and to PS ground. 

The CT ground is for providing a balanced reference to ground in the filament circuit - it's there to minimize hum.  Double check that that connection is solid.

Swap tubes if you have spares.

Have you tried interfacing it to 2 different destinations?  Same results?  If one hums and the other doesn't check to see how the input ground is tied in on the Leslie power amp.

One last thing - Are the leads on the OT pri connected as they were originally(red to plate, Brn to B+)?.  I've had a strange case of AC voltage where it shouldn't be in a single ended circuit that was solved by reversing the pri leads.


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