BTW if you disconnected HV slow start -delay- circuit (SS parts next to first cap), the resistors between rectifiers and first cap also unnecessary.
See polish site 300B version:
https://najlepszeodtwarzacze.pl/appluaseaudio-300b-6f3-western-electric.php
See polish site 300B version:
https://najlepszeodtwarzacze.pl/appluaseaudio-300b-6f3-western-electric.php
I concur with Tom Bevis advice.
I believe it might beneficial to know where and how hum finds its way into the circuit.
Install shorting plugs at the inputs. Does hum level very with volume setting? If the 6F3 tube is removed, does hum reduce dramatically? To an acceptable level? If hum becomes acceptable in absence of the 6F3 stages, attention should focus on hum ingress at the input stages.
I think it would be helpful to measure hum amplitude to quantify progress as experimental changes are introduced. If available, an audio voltmeter on the output would be great. If you don't have a audio VM, I suggest a series cap in series with a battery-operated multi-meter. Your bench DVM can likely measure AC voltages of a few mV at 60 or 120 Hz. In interest of best sensitivity, measure across the output transformer primary, i.e. B+ and plate terminals. (Be safe!) Because the transformer is driven by a relatively high impedance source from tube's plate, I predict you'll see less hum across primary winding than you would observe between plate and ground. I recommend a dummy load on outputs in place of speakers.
If you're nervous about ammeter floating at B+ potentials, you can relocate it to near ground. I can coach re the details you wish.
Good luck!
I believe it might beneficial to know where and how hum finds its way into the circuit.
Install shorting plugs at the inputs. Does hum level very with volume setting? If the 6F3 tube is removed, does hum reduce dramatically? To an acceptable level? If hum becomes acceptable in absence of the 6F3 stages, attention should focus on hum ingress at the input stages.
I think it would be helpful to measure hum amplitude to quantify progress as experimental changes are introduced. If available, an audio voltmeter on the output would be great. If you don't have a audio VM, I suggest a series cap in series with a battery-operated multi-meter. Your bench DVM can likely measure AC voltages of a few mV at 60 or 120 Hz. In interest of best sensitivity, measure across the output transformer primary, i.e. B+ and plate terminals. (Be safe!) Because the transformer is driven by a relatively high impedance source from tube's plate, I predict you'll see less hum across primary winding than you would observe between plate and ground. I recommend a dummy load on outputs in place of speakers.
If you're nervous about ammeter floating at B+ potentials, you can relocate it to near ground. I can coach re the details you wish.
Good luck!
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I recently build a preamp that had excessive hum on the output. When I disconnected the transformer and rectifiers and just shunted in 12V from a car battery (the design uses boost converters for the HV supply for tubes), there was no hum. When I connected another transformer 6 inches from the chassis, there was no hum. It turned out to be the capacitors (6 in parallel for the "first cap" were physically too close to the power transformer and the hum was magnetically coupling into the circuit.
Just yet another thing to consider.
Just yet another thing to consider.
Yep! And if the hum is coming in through the filaments it'd be 60 Hz as well.
If you have a sound card and some software (REW for example) it's pretty easy to determine the dominant frequency of the hum.
Tom
You can get 120Hz charge pulses to couple magnetically through the transformer and caps though - my hum was 120Hz in my previous example. Admittedly, the transformer was undersized and might have hitting saturation on the peaks.
I don't have an audio VM but I do have an oscilloscope on the way. Already have the isolation transformer. I don't know if that will work. As far as the hum, it doesn't change with volume. The hum is exactly the same with the 6F3's pulled.
Yeah I played a 60hz tone on my PC and compared it and it sounds exactly the same. 120hz is higher pitched. Actually it's a pretty clean sounding 60hz as well.
That's insightful.
I wish the schematic had reference designations. As an experiment, I suggest disconnecting the left-hand lead of the 220nF cap connected to plate, screen, 10K resistor of the second stage. Note any change in hum amplitude with lead open. Next connect this cap lead to junction of 150uF cap, 750 ohm, two 22 ohm resistors in cathode network of 2A3. Any change in hum amplitude?
I think I'll rectify the heaters first and if that doesn't work it'll be easy to do that while I'm in there. I did see that on the 300b amp this company makes they used DC for the heaters. Must be a reason for that.
I put a KBU2510 rectifier along with a cap on one of the heaters with alligator clips and I'm only getting half the voltage to the heater but the hum on that side sounds like 120hz now. In fact when I play a 120hz tone from the other room and get close to the speakers hooked to the amp they cancel each other out. I hear nothing from the rectified side and 60hz from the other. the other speaker must have 120 and 60 on it and I just can't hear the 120 over the 60. I must be getting hum from the B+ and the heaters.
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In theory, you just made your 60Hz AC into 120Hz pulsating DC... Either way, it seems like your noise is coming through the heater supply?
Thanks for this, Tom. This was the cause of some 120Hz hum in a preamp I built. Shortening the loops solved 99% of it.