Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Roukedou on August 22, 2017, 02:17:34 pm

Title: 6G15 Hum-loop block network
Post by: Roukedou on August 22, 2017, 02:17:34 pm: I'm in the process of finishing a 6G15 reverb unit design. As the unit will be with a proper 3 prongs power cord, I want to implement a hum-loop block network, to make sure I don't get ground loops between the reverb unit and the amp. I would be using the same circuit as the RI unit.

http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-63-Reverb-Schematic.pdf (http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-63-Reverb-Schematic.pdf)

As I understand it, I need two ground connections to the chassis, one for the earth wire of the power cord, and one (and only one) for the circuit ground. What is left for me to determine, is where that circuit ground should be located in the chassis (near the input jack, or near the PT bolt), and how to route all the grounds to this point. I was thinking of having a ground bus that collects all the grounds and bring them to this grounding point. Would it be advisable to split this bus as to separate the power grounds from the pre-amp grounds?

Thank you.
Title: Re: 6G15 Hum-loop block network
Post by: shooter on August 22, 2017, 08:32:38 pm: Quote
it be advisable to split this bus

here's a link to dougs Reverb unit, it may help answer some questions

http://el34world.com/Hoffman/files/Hoffman_6G15.pdf (http://el34world.com/Hoffman/files/Hoffman_6G15.pdf)

the biggest problem I found n others were ground loops caused through the AC line because you are running cables between 2 "powered" sources.
common buss strip for gear, isolation buss are a couple ideas
Title: Re: 6G15 Hum-loop block network
Post by: tubeswell on August 22, 2017, 08:41:41 pm: I've used the hum loop blocker that is described in Merlins Grounding article http://www.valvewizard.co.uk/Grounding.pdf (http://www.valvewizard.co.uk/Grounding.pdf) in 6G15s. It works well.

Schematic I did from memory. The schematic doesn't quite depict the layout. The blocker was hooked to the 'input end' of the ground buss. The output end and the output (recovery) and buffer stages and all the preamp grounds went to this same spot. The 6V6 ground and reservoir and smoothing cap grounds in the CLC filter together with the HT centre tap, all went to the other end of the buss bar. The heater CT was elevated to the 6V6 cathode.

All the jack sleeves have to be ground-isolated for this to work properly.

The loop blocker has 2 x 6A diodes, 1 x 0.1uF 630V film cap and 1 x wire-wound 15R 5W resistor.
Title: Re: 6G15 Hum-loop block network
Post by: warioblast on August 23, 2017, 07:54:08 am: I'm about to add the hum-loop block network in my 6G15; just waiting for my parts to arrive.

Here are 2 ideas I found:
TAD 6G15 layout http://www.tubeampdoctor.com/images/artikel/Full_Documentation%206G15_21.07.14.pdf (http://www.tubeampdoctor.com/images/artikel/Full_Documentation%206G15_21.07.14.pdf)

Jeff Gehring https://www.thegearpage.net/board/index.php?threads/6g15-fender-reverb-unit-build-questions.1690482/#post-21700467 (https://www.thegearpage.net/board/index.php?threads/6g15-fender-reverb-unit-build-questions.1690482/#post-21700467) see post #6
Title: Re: 6G15 Hum-loop block network
Post by: prongster on October 31, 2021, 12:41:15 pm: I know this is an old thread, but I just built a Hoffman 6G15 which hummed noticeably (but not horribly) when connected to an amp. I had an Ebtech Hum X laying around so I plugged the unit into it and the hum is greatly reduced. I'm guessing the Hum X is based on the same principle as the Hum-loop block network...

..is there any advantage of having it attached to the actual grounding scheme discussed here? (Other than the obvious ones of cost and convenience of having it built into the unit.)

Thanks
Title: Re: 6G15 Hum-loop block network
Post by: PRR on October 31, 2021, 01:06:32 pm: https://soundforums.net/community/threads/ebtech-humx.1375/
Title: Re: 6G15 Hum-loop block network
Post by: tubeswell on November 01, 2021, 08:11:21 am: Quote from: prongster on October 31, 2021, 12:41:15 pm
I know this is an old thread, but I just built a Hoffman 6G15 which hummed noticeably (but not horribly) when connected to an amp. I had an Ebtech Hum X laying around so I plugged the unit into it and the hum is greatly reduced. I'm guessing the Hum X is based on the same principle as the Hum-loop block network...

..is there any advantage of having it attached to the actual grounding scheme discussed here? (Other than the obvious ones of cost and convenience of having it built into the unit.)

Thanks

The HumX isn’t ‘based on the same principle’ because it eliminates the reverb tank’s chassis safety (Earth) ground, and (then) you’re relying on instrument cable shield between the reverb tank and the guitar amp to be the safety ground connection.

Better to keep the reverb tank chassis safety ground and elevate the reverb tank’s signal ground with the hum loop blocker as per Merlin’s suggested circuit
Title: Re: 6G15 Hum-loop block network
Post by: PRR on November 01, 2021, 11:34:10 am: I believe the HumX limits ground-fault to about 1 Volt maximum.

If you read the link I gave, it is reported that UL labs did not refuse the idea on sight, but proposed their usual extensive (expensive) testing to get UL Approval.
Title: Re: 6G15 Hum-loop block network
Post by: prongster on November 02, 2021, 01:05:42 pm: Oh wow, OK. Thanks for explaining all that guys!