Individual cathode resistors for an AC15 / AC30.

[Mike_J]

I have been working on a friend's AC30 clone.  When he brought it to me it squealed and had a loud hum.  That has been fixed and now it has a small hum.

He wants to use it in his recording studio so it should be a little bit quieter so I am making a few changes to the amp.  One of the changes I was contemplating was individual cathode resistors.  Does anyone think there is an advantage to giving each tube its own cathode resistor?

His amp has a 15/30 watt switch that allows for the use of two or four EL84 power tubes and switches between a 50R resistor for four tubes and a 100 R resistor for two tubes.  If I used individual cathode resistors I could use one of the 4PDT switches I purchased a while ago.  Think I figured out how to wire it but don't know if it is worth the effort.

Can anyone enlighten me regarding their experience in this area?

Thanks
Mike

[sluckey]

The only advantage I can think of is with separate cathode resistors you can quickly see how much current is flowing thru each individual tube. This gives you an indication of how closely the tubes are matched. I don't really see any operational advantages.

[shooter]

what sluckey said.  I use 1 R and 1C on each tube, even went so far as to tweak R values to get bias more balanced.  played with a quad build changing cap values on 1/2 the tubes, nothing really discernible to me.

[Mike_J]

The only advantage I can think of is with separate cathode resistors you can quickly see how much current is flowing thru each individual tube. This gives you an indication of how closely the tubes are matched. I don't really see any operational advantages.

Thanks Sluckey

Can mismatched current flowing from power tubes cause noise in a single ended amp similar to mismatched tubes in a PP amp?

Thanks
Mike

[Mike_J]

what sluckey said.  I use 1 R and 1C on each tube, even went so far as to tweak R values to get bias more balanced.  played with a quad build changing cap values on 1/2 the tubes, nothing really discernible to me.

Thanks Shooter

Between you and Sluckey you have saved me some time chasing something with what appears to have marginal value.

Thanks
Mike

[sluckey]

Can mismatched current flowing from power tubes cause noise in a single ended amp similar to mismatched tubes in a PP amp?

I wouldn't think so.

His amp has a 15/30 watt switch that allows for the use of two or four EL84 power tubes and switches between a 50R resistor for four tubes and a 100 R resistor for two tubes.  If I used individual cathode resistors I could use one of the 4PDT switches I purchased a while ago.  Think I figured out how to wire it but don't know if it is worth the effort.

If you use four individual resistors you can use a SPST switch to take out a pair for half power.

[Mike_J]

If you use four individual resistors you can use a SPST switch to take out a pair for half power.

That would simplify things considerably.  Might consider experimenting with this as a last resort.

Thanks
Mike

[tubeswell]

Individual cathode resistors won't make any improvement in hum reduction. The answer is in the grounding.

[Mike_J]

Individual cathode resistors won't make any improvement in hum reduction. The answer is in the grounding.

This amp must have 20 different grounding points.  I know there are many ways to build a quiet amp but I don't think this way is one of them.  I have created a board based closely on the Hoffman AC30 board that I am going to install in the amp.  I will reduce the grounding points to three.  I have had luck doing this in the past.  If that doesn't do enough to reduce the hum I will experiment with increasing the HT power cap value and see if that helps without significantly impacting the AC30 chime.

Thanks
Mike

[tubeswell]

Read the article by Merlin Blencowe. Galactic grounding is the way to do it. One connection point to the chassis (at the amp's hi-gain input jack). The ground return points for each component go to the same point on the ground buss as the ground return of the filter cap from whence that part of the circuit draws its power supply. The output stage ground returns (including the screen supply node ground return) connect to the same point as the reservoir cap ground return (along with the ground return of the heater reference/bleeder and the bias-supply ground return) and a single wire goes from that point to the end of the buss bar (that is the opposite end to the input jack). Similarly the PI node gets its own ground return that attaches to the buss bar between the output stage attachment point and the pre-amp node(s) attachment point(s). Don't spread the ground returns out - keep each set as close to the relevant cap ground return as possible.


Note that this doesn't work well in a cap can arrangement - you need separate caps for this to work.


A 100R wire-wound humdinger pot for the heater reference (instead of 2 x 100R resistors or a heater winding CT) is a superior solution for dialing in the hum-neutral point as well.

[Mike_J]

Read the article by Merlin Blencowe. Galactic grounding is the way to do it. One connection point to the chassis (at the amp's hi-gain input jack). The ground return points for each component go to the same point on the ground buss as the ground return of the filter cap from whence that part of the circuit draws its power supply. The output stage ground returns (including the screen supply node ground return) connect to the same point as the reservoir cap ground return (along with the ground return of the heater reference/bleeder and the bias-supply ground return) and a single wire goes from that point to the end of the buss bar (that is the opposite end to the input jack). Similarly the PI node gets its own ground return that attaches to the buss bar between the output stage attachment point and the pre-amp node(s) attachment point(s). Don't spread the ground returns out - keep each set as close to the relevant cap ground return as possible.


Note that this doesn't work well in a cap can arrangement - you need separate caps for this to work.


A 100R wire-wound humdinger pot for the heater reference (instead of 2 x 100R resistors or a heater winding CT) is a superior solution for dialing in the hum-neutral point as well.

My method is similar.  For example with this amp each preamp tube has its own power cap.  I run all the grounds for each tube to the ground end of the cap for the tube and then run a wire from the cap to the appropriate spot on the buss bar feeding that portion of the circuitry.  All of the preamp and PI caps are tied to the buss bar which is terminated at a point near the input jacks.

I usually just ground the OT and the speaker jacks to a spot near the speaker jacks.

The HT/CT, HT and screen supply, heater CT and grounds for the bias pot (for a PP amp, this one is single ended) are terminated between the PT and the rectifier tube.  It could be lifted and a single wire could be sent to the PI end of the buss wire if you think that would work better.

I would like to know where to get a 100 ohm wire-wound humdinger pot.  If they work as well as it sounds like they should I would install them in all my amps although some are already very quiet.

Thanks
Mike

[sluckey]

I would like to know where to get a 100 ohm wire-wound humdinger pot.

AES...

     https://www.tubesandmore.com/products/R-V5W-100L