Grounding BF-style amps

[tubeswell]

I recently finished a Weber 6A14 kit (BFPR 'clone').

(Voltages are more or less normal albeit slightly on the high side (425 on the plates) and the 6V6s are biased at about 26-28mA, which I know is hot. The heaters are at 6.5VAC.

I initially more-or-less followed the Weber layout (which follows the fender layout) and had quite severe hum issues. Then I re-did the grounding as follows:

Separate wires from each of the pots' ground lugs to one point on the brass grounding plate.

The main board grounds each going separately to the same point on the brass grounding plate

The reverb recovery stage (220k) grid load resistor and the RT secondary ground return going each separately to the same point on the brass grounding plate

A separate pre-amp filter cap going separately to the same point on the brass grounding plate.


The reservoir filter (off the cap can) and the screen filter (off the cap can) and the unused (former pre-amp) filter now tied to the screen filter, going to the same grounding point as the PT's High-Voltage winding centre tap.

The heaters are DC-elevated to 37V, and the ground return from this goes separately to the same point as the HV CT/main filter ground. The bias supply ground return also goes separately to the same point as the HV CT/main filter ground.

The hum is reduced substantially, however it is still there but it is intermittently affected/eliminated/reduced/enhanced by the vol pot and tone pot settings. (With the vol at zero I get a bit of hum, but it goes away completely when the vol is at about 1-3, then comes back with the vol at 4-5. Similarly when teh tone controls are at zero there is hum (when the vol pot is at zero, but when the vol pot is at 1-3 'quiet', I can get hum back by adjusting the tone pots). I guess this is inevitable as some of the main board ground returns are tied together before the rest of the pre-amp grounds are.

When I have scratch-built my own amps, I have learned to keep the ground returns separately wired to a common star point and never have any grounding issues.

How did Fender get the grounding quiet in his BF amps?

[dai h.]

not sure what the problem is but maybe it's something to do with the grounding. Noise being affected when you turn the volume pot must be some clue (would have to think about this a bit). For an audio taper, (up) around 3 could be half the R which is midway between AC and DC ground (AC grounded plate on other side of coupling cap, DC from pot lug to ground).

[olddawg]

Have you tried different tubes. I have chased my tail on new builds with hummy tubes new from the box. If you terminate the input do you have the same symptoms with the volume control? Have you tried reversing the speaker phase?

[tubeswell]

yep I've changed the 6V6s and the rectifier. Still the same

I was just fiddling with it before, the hum is worst (although not too bad but not as quiet as it should be) when everything is zeroed. Then when you turn up the vol past 1, the hum goes away, and past about 4/5 it comes back, but if you turn the bass pot up to about 2, the hum goes away again, but comes back it you keep turning the bass pot up??. Turning up the treble had no effect on the hum - so it all seems to be controlled with the vol and bass pots. Must be grounding.

[Bruce / Mission Amps]

yep I've changed the 6V6s and the rectifier. Still the same

I was just fiddling with it before, the hum is worst (although not too bad but not as quiet as it should be) when everything is zeroed. Then when you turn up the vol past 1, the hum goes away, and past about 4/5 it comes back, but if you turn the bass pot up to about 2, the hum goes away again, but comes back it you keep turning the bass pot up??. Turning up the treble had no effect on the hum - so it all seems to be controlled with the vol and bass pots. Must be grounding.

The up and down reaction to hum with the volume control is indicative of a ground loop.
Did you follow the classic black face PR grounding scheme or buy into some NET guru's drivel about how to ground an amp?

[tubeswell]

The up and down reaction to hum with the volume control is indicative of a ground loop.
Did you follow the classic black face PR grounding scheme or buy into some NET guru's drivel about how to ground an amp?

Ah Bruce the only net Guru I pay and attention to is you M8

Initially I did something similar to the classic theme but I had difficulty getting the cap can to solder to the chassis even with my 80W iron (I suppose I need to take all the 'iron' off the chassis to get the chassis to heat up properly do I?) so I ran all the cap's ground wires into 1, and wired that to the same pint as the High-voltage CT and the other power amp ground there. The rest of it was pretty much a standard grounding (but I had the main board ground going by separate wires to one point on the brass grounding plate. The pots were all grounded on their respective rears. The hum was worse than what it is now.

[Bruce / Mission Amps]

Ah Bruce the only net Guru I pay and attention to is you M8

Initially I did something similar to the classic theme but I had difficulty getting the cap can to solder to the chassis even with my 80W iron (I suppose I need to take all the 'iron' off the chassis to get the chassis to heat up properly do I?) so I ran all the cap's ground wires into 1, and wired that to the same pint as the High-voltage CT and the other power amp ground there. The rest of it was pretty much a standard grounding (but I had the main board ground going by separate wires to one point on the brass grounding plate. The pots were all grounded on their respective rears. The hum was worse than what it is now.

OK ... ground loops suck and sometimes a different approach fixes it while the same fix doesn't work on a similar amp.
If you have a wire from the common CT ground up to the brass plate that should be fine. Make sure the first preamp stage cathode bypass caps are grounded right at the input jacks ground, the volume pot is grounded right at the lug on the back of the pot and subsequently, onto the brass plate with a zinc coated star washer behind the pot (not black phosphate, they are almost non-conductive)... and the cathode bypass caps of the other 12AX7 is grounded to the brass plate near the tone controls.
It sort of sounds like a bad pot too but I still think it could be a simple ground loop between the reverb recovery section and the 1st preamp stage.

[pdf64]

It could be a bad pre-amp tube causing the hum, try subbing out those also.
Can you measure the hum frequency, to determine whether it's at line freq (ie from heaters), or 2 x line freq (B+ smoothing or grounding prob)?
Are all the grounds on the doghouse cap board linked on the board? It's best to use a seperate ground return wire at least for the first reservoir cap, back to the B+ winding CT.

[tubeswell]

Grounding sketch

I have sketched out the ground set up I have currently implemented. It looks weird because it is based on the logic of the main board and then what I did with the wires after that.

As you can see, the pre-amp grounds all end up with wires soldered to the brass grounding plate near the input jacks. (Oops! I forgot to draw where the 100k ground lug of the trem speed pot ends up - it goes to the same vicinity on the brass grounding plate as all the other leads)

The bias ground(s), and the 6V6 rk, and the filter cap can ground(s), and the heater voltage divider, and the HT CT all go to a tag terminal strip which has a separate chassis termination bolt near the PT. The HT CT and the filter cap can grounds are grounded on the same lug on the terminal strip, which is also the same lug the connects the the chassis bolt - so these ground technically meet 'first')

I now have the pre-amp filtered with a separate 22uF 450V cap and the otherwise spare terminal on the cap can is now in parallel with the screen filter cap node (I initially had it in parallel with the reservoir cap, but the B+ was 425).

The heater CT is elevated to 37V and is supplied by a voltage divider/bleeder network which comes off the normally 'unused' filter cap in the cap can. There is a separate 100V decoupling/smoothing cap on the voltage divider.

Note that the "22k" bias resistor is actually 39k in series with a 50k pot.

With this setup and all the pots cut, there is a hum (It sounds like 100Hz to me). Then when I turn the vol up to 5.5 the hum goes almost entirely/completely away, but starts to comes back as the pot goes above 6 or below 5.

Whereas if I have the bass pot set to 3 the vol pot is quietest at 3 also, but as I turn the vol pot up to 5.5, I have to roll the bass pot back to zero progressively to keep the amp quiet. That is exactly how it is at the moment.

In either case, the hum comes on further and further as the reverb pot is turned up.

(I haven't tried different pre-amp tubes yet, but I have tried different 6V6s and a different rectifier. At the moment I have a NOS 5R4 (cool-looking old coke bottle) in there and tung-sol RI 6V6GTs. The bias is ~22mA on one tube and ~24mA on the other. By my reckoning the B+ voltage is about 410. I say this because it was 425 with a 5U4GB, but I didn't measure it with the 5R4).

I am almost certain this is a ground loop (and I realise the grounding is less than optimal and that I'm going to have to re-do the grounding again). But looking at the fender BF layout, I am surprised that the vintage amps don't hum like crazy anyway.

(Edit -I am certain there is also a lead dress issue, because there is a thumping bassy oscillation which develops when the amp is turned up to about 7 or 8 - but only happens when there is a signal present - not under 'quiescent' conditions)