Merlin Grounding Design

[scstill]

In Merlins Book "Designing Valve Preamps for Guitar and Bass" he describes an additional dropping resistor on the ground side after the reservoir cap be added to retard the ripple current from entering the audio ground.

I do not recall schematics with this ground approach. Should we be building our smoothing filters with this additional ground dropping resistor?

On page 267, Merlin says "...it is beneficial to split the usual dropping resistor into two parts (usually but not necessarily equal) and so deliberately create a real balanced filter." Does split mean halve the value planned on the HV side or match the value planned on the HV side?

[pdf64]

You’re missing the conditional clause. Very few amps have an additional stage of filtering.

If implementing the idea, the planned value should be split into 2 parts. So 100ohms might be comprised of 47 ohms in both the HT and 0V sides, or 68 and 33 or whatever etc.

[scstill]

You’re missing the conditional clause. Very few amps have an additional stage of filtering.

I thought the "additional stage of filtering" was describing what comes after reservoir for the output plate, output screen, and Preamp. All amps have this additional filtering.

[sluckey]

No. The "additional stage of filtering" is the cap he labeled as reservoir. If you remove that cap and the two resistors connected to it, THEN you will have the topology that most push/pull amps use.

[PRR]

> what comes after reservoir for the output plate

No. Look where he has the bubble "Power Amp".

I did this once, on an over-complicated "studio" build. The ripple after C-2R-C filtering was very low, and there were no rectifier spikes in the audio ground.

That was actually more a 2C-2R-2R-2C-2R filter: the Cs were doubled-up to support 550V. The project got overwhelming and my life changed about then so I never explored it in depth.

[d95err]

I can see this as useful on a single-ended amp. There, you really want to reduce the anode voltage ripple, and sag is not really an issue.

[Williamblake]

You could also replace the two resistors at the first filter cap with a common mode choke. I did this once in a single ended amp and it didn't hum but i could wonder time an again what the correct orientation of the coils is. Cant rember if they should add or cancel, if i remember correctly HT- and ground-current should travel in the same direction for this to work? Would love an educated answer to that so i don't have to wonder when i start working on that amp again.

/ Looking back at this i think the currents should travel in opposite directions but i actually don't know.

[tubeswell]

… with a common mode choke… i could wonder time an again what the correct orientation of the coils is. Cant rember if they should add or cancel, if i remember correctly HT- and ground-current should travel in the same direction for this to work? …

/ Looking back at this i think the currents should travel in opposite directions but i actually don't know.

Google is your friend


https://www.coilcraft.com/en-us/edu/series/a-guide-to-understanding-common-mode-chokes/

[Fresh_Start]

Keep in mind that Merlin has been a major contributor at AX84.com where the focus was single-ended, DIY amps for a long time. In that context, adding a reservoir capacitor with parallel resistors is both less expensive and easier to spec parts than a filter choke for the screen grid node.

Chip