Bush VHF61 Hum - Advice Please

[cathoderay57]

Hi folks, my latest project is a Bush VHF61. I have replaced all of the Hunts caps and the electrolytics all reformed OK. It works well on all 3 bands, but on all 3 bands including also gram there is a fairly obtrusive background hum at minimum volume that I am trying to silence. The hum doesn't change as the volume is turned up. I have tried tacking in replacement HT caps on all 3 sections but this has no effect whatsoever. If I pull the EABC80 the hum disappears. I've tried a brand new replacement EABC80 but the hum remains the same. The key seems to be the output transformer connections. The original had an o/c primary, and no hum-bucking winding. The replacement that I have fitted has 2 primary tappings providing either 45:1 or 43:1 turns ratio. For a 3 Ohm speaker I calculated that the EL84 wants a 42:1 turns ratio and so I have used the 43:1 tapping. Aha, I thought, I can use the short few extra turns to the longest tapping as a hum-bucking winding so I did. As per circuit the secondary has negative feedback going to the bottom of the volume control via a 0.1uF cap in series with a 4.7k resistor. I replaced the original 0.1uF paper cap with a ceramic 100nF which I had to hand. Here are the results of some prodding about:
1. Shorting out the hum-bucking winding (i.e. reverting to as per circuit) reduces the hum by about 50% by ear.
2. Disconnecting the negative feedback (with the hum-bucking still connected) reduces hum by approx 25%.
3. Doing 1 and 2 makes the hum almost inaudible.
4. In previous sets and amplifiers that I have worked on, getting the negative feedback phase the wrong way round causes howling i.e positive feedback. On this set reversing the connections at the speaker secondary does not do this, presumably because the closed loop gain of the feedback loop isn't sufficiently high.
The questions are: what is the best solution? dispense with the hum-bucking winding presumably, since it is yielding worse hum with than without. I would still like to retain negative feedback but not sure how best to connect it. Advice much appreciated! Cheers, Jerry

[cathoderay57]

I'll plug an AF sig gen into the gram socket and compare the phases of the input with that at the speaker using my double beam scope. That will enable me to ensure the feedback is connected as negative, not positive. Still not sure why using the additional few turns on the output transformer as a hum-bucking coil made the hum worse? I would have thought the additional inductance would have helped the smoothing. Cheers, Jerry

[ms660]

If it's any use, the attached scan gives a description of how a hum cancelling output transformer circuit works.

A hum cancelling output transformer should have a turns ratio for the two primary sections that equals the ratio of the valves internal resistance (ra) to the HT filter resistance (R1)

Lawrence.

[cathoderay57]

Hi Lawrence, thanks for that. By my calcs, and using ra for Mullard EL84 as 38k, Na/Nb=24 whereas Ra/r1=38 so clearly some way off. I could have experimented with a 1.5k filter resistor (R1), which would make things about right, but I've reverted the circuit back to standard by disconnecting the hum-bucking winding, made sure the neg feedback is the right way around, and shifted the speaker ground back to the output transformer mounting screw where it was originally. The hum is now very low, not quite zero but about what I would expect for a set like this. I think that'll do. Nice thing with this hobby is you never stop learning. Cheers, Jerry