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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Allen model 75 with hum

Ok, I have two model 75's one is perfectly quiet and the other has some hum. I have one input on each amp that uses the 12AY7 and one input that goes to the Ef86 so that I can use the input I need.

One of the amps I have to turn the 12Ay7 input up just a little to null the noise. The other amp doesn't have this problem.

Any ideas?
 
schematic:
 

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Ah, I see it is 110volts. What frequency is the hum? 60Hz means mains hum, could be a leaky heater in one of the valves also. 120Hz is usually main smoothing issues.
Does it vary with the volume or bias settings? If volume then look at the stage directly before the volume control, if bias, check the output valves and associated bias network. There is a bias balance pot that can cause issues.
Don't change random components, keep a record of what you do to save going over the same work again.
 
The hum sounds like 60 cycle. The hum changes in level when turning the input control in the input stage. I have two inputs that used to go to the 12AY7 the input tube. I removed the one on the right and bypassed the 12AY7 and went to the EF86 still using the input control. Now the control on the right goes directly to the EF86 for lower gain.

When the left control the 12Ay7 input is turned all the way down and I'm in the Ef86 input or not terminated at all there is a hum. If I turn up the left control a bit the hum is nulled.
 
Yes, correct I now have a high level and low level input. Note the noise was present before I added the low level modification. Also, I have two of these amps. One amp is perfectly quiet and the other has the noise problem.

So far I've swapped tubes...no effect. tried new tubes...no effect.

I just tried removing the inputs and noise is still there. I made a shorting input plug and shorted the inputs and the noise is still there unless you turn the high gain input up a touch and then you can get to about 5mV of AC across the negative speaker terminal and the 8 ohm terminal. Remove the shorting plug with the input turned up slightly and no effect.

I don't know what I am missing here.
 
I don't think what I just did is accurate but I took my meter set it on HZ and measured the output 8 ohms and common and 4 ohms and the signal appears to be 120HZ on the 8 ohm tap.

Power supply? The multi cap? Parts A,B, C? with B being faulty maybe? Hard to read my schematic very blurred.
 
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baudouin0

Member
2019-03-22 5:41 pm
UK
I don't think what I just did is accurate but I took my meter set it on HZ and measured the output 8 ohms and common and 4 ohms and the signal appears to be 120HZ on the 8 ohm tap.

Power supply? The multi cap? Parts A,B, C? with B being faulty maybe?


Though is might be difficult to tell by ear. There may be nothing faulty, try removing the EF86 next.
 
With V3 pulled no noise.

Voltage across R29, R30 70VDC, 70.1VDC

Across R34 "B" bias test point bias set at 240mV
Across R33 "A" bias test point bias set at 240mV

Bias balance does vary slightly as amp warms up. A little adjustment needed on the balance to keep the balance A B as close to zero as possible.
 
Of course V3 is the splitter so if removed the amp is not working.

I rebuild many AO75 and I have 2 pairs myself, totally quiet.

I removed the 12AY7 and go straight to the EF86, as the amp has too much gain for Hifi.

Make sure the RCA ground touches the chassis.
 

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baudouin0

Member
2019-03-22 5:41 pm
UK
With V3 pulled no noise.

Voltage across R29, R30 70VDC, 70.1VDC

Across R34 "B" bias test point bias set at 240mV
Across R33 "A" bias test point bias set at 240mV

Bias balance does vary slightly as amp warms up. A little adjustment needed on the balance to keep the balance A B as close to zero as possible.

Pulling V3 will still leave the output stage biased up. So no V3 no noise. Output stage looks fine. So I think you have a small 120Hz between pin 1 and pin 4 of V3. The fact that the hum also changes when the volume pot is adjusted could point to a hum loop. Is the phono grounded on the chassis - try isolating it. Its hard to go any further without layout and photos of the mods. It could also be supply ripple entering via R16.

The thing is V2 is supplied from the main reservoir caps whose ground will be different to that of C6. I think I would have locally decoupled the rail to v2 rather than this.
 
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