6EU7 filament pins

[Mr Boochie]

you can use the coax, grounded at both ends for that situation instead of a wire. Ideally the shield would then ground to the same point that the cathode resistor grounds to, and if the grid resistor is at the tube, both grid and cathode resistor would tie to that same spot. If you use a wire, the wire would ground to that spot instead. However you slice it no multiple paths to ground is the key. Tying the ground to the same spot as the cathode resistor prevents any noise difference between the two from being injected at the grid to be amplified.

Well, in my experience, the shielded portion of a coax cable can generate a ground loop — if you ground the woven-shielded outer portion at both ends of the coax cable. This is especially true in low-level inputs, like phono or mics.

I just posted this to be sure that the OP didn't double-ground the outer portion of that input cable.

 

[gadget73]

if there is a second ground path, yeah it can absolutely form a loop if both ends are connected. It won't do that if there is only a single path. However you choose to do it, just make sure there is a single path or its a whole lot more likely to have hum problems.

 

[Mr Boochie]

if there is a second ground path, yeah it can absolutely form a loop if both ends are connected. It won't do that if there is only a single path. However you choose to do it, just make sure there is a single path or its a whole lot more likely to have hum problems.

I think it's because the coax cable most manufacturers use (even in hi-end audio) has measurable resistance in the outer shield, over even a few inches. This resistance can lead to a voltage differential and a potential ground loop. A loop may become an audible problem in low-signal inputs of a high-gain section.

 

[egellings]

After ignoring it for about six years, I decided to try again, assessing why the Heathkit AA-32 integrated amplifier I built in 1968 then re-built in 2013 has a lot of hum on the MAG Phono input. All other inputs are quiet so my assumption has been that the hum is originating in the area concerning V1 which is the MAG Phono pre-amplifier.

View attachment 3831668

My initial attemps to find and eliminate said hum included rewiring it a 2nd time, installing 4 different 6EU7 tubes, reverting to the original capacitors connected to V1 and installing a tube shroud. Nothing helped. A vintage tube radio tech had a go at it and he couldn't diagnose the problem either.

All capacitors, resistors, tube sockets, power cord and switch plus the null hum pot are brand new as of 2013. The transformers, control pots and input selector are original. As mentioned, the amplifier works well with all inputs save for MAG Phono.

Last year I came into possesion of a variable DC power supply. To satisfy my curiosity I would like to disconnect V1 from it's 6.3 volt AC source and power the filaments from the DC power supply to see if that eliminates the hum. My question then concerns the 6EU7 in the V1 - phono pre-amp position.

I understand that the fillament pins are pin 1 and pin 2. Does polarity matter?

Polarity, if AC, should not matter. If one side of the AC source is grounded, then there could be a difference in hum level based on filament connection to the source.

 

[sddave]

After ignoring it for about six years, I decided to try again, assessing why the Heathkit AA-32 integrated amplifier I built in 1968 then re-built in 2013 has a lot of hum on the MAG Phono input. All other inputs are quiet so my assumption has been that the hum is originating in the area concerning V1 which is the MAG Phono pre-amplifier.

View attachment 3831668

My initial attemps to find and eliminate said hum included rewiring it a 2nd time, installing 4 different 6EU7 tubes, reverting to the original capacitors connected to V1 and installing a tube shroud. Nothing helped. A vintage tube radio tech had a go at it and he couldn't diagnose the problem either.

All capacitors, resistors, tube sockets, power cord and switch plus the null hum pot are brand new as of 2013. The transformers, control pots and input selector are original. As mentioned, the amplifier works well with all inputs save for MAG Phono.

Last year I came into possesion of a variable DC power supply. To satisfy my curiosity I would like to disconnect V1 from it's 6.3 volt AC source and power the filaments from the DC power supply to see if that eliminates the hum. My question then concerns the 6EU7 in the V1 - phono pre-amp position.

I understand that the fillament pins are pin 1 and pin 2. Does polarity matter?

Take a clip lead and ground the + signal at the jack. Does it still hum? Then repeat by attaching the signal lead to pin 5 (the grid of the tube) Does it still hum when grounding the input signal? If so most likely c8 will need to be changed.

A picture would be helpful to further troubleshoot this because small construction mistakes can cause a hum in a high gain circuit like this.,

 

[62vauxhall]

I have not been able to act on any suggestons since my last post 8 days. My only accomplishment was to re-install two Orange Drop capacitors that were connected to V1 when I did the re-build in 2012/2013. In chasing down a hum remedy, I removed them and put back into place the two caps which were original to the amplifer. My thinking then was that perhaps the more modern Orange Drops exacerbated the hum and the older, more primitive capacitors were less offensive in that regard.

I cannot claim with 100% certainty that the hum is now less than it was when the original capacitors were in place but it seems so. For now, I am leaving well enough alone as there is a "barn find" transitor amp I am keen on trying to resurect.

Many thanks to all who took the time to share their knowledge and experience. FWIW, this AA32 always did have some hum on MAG Phono but was only noticable between tracks when the volume control was about 2 o'clock. It appears now that that is again the case so I guess that the amp is as good as it is going to get.