Re-tubing a tube amp, trim pot across heater pins?

Thread Starter

hrs

Joined Jun 13, 2014
362
Hi,

I'm going to re-tube a tube amp that I have, barely readable schematic attached. I've figured out what I need to do to set the bias current.

What I haven't figured out is how to adjust the second trim pot that I designated TR2, I think it reads 250R. Somewhere in a youtube comment someone claimed that it's a hum balance pot that balances the heaters. To me it doesn't look like it balances anything, instead it lets you put more or less load on the heaters. How do you set it correctly? Simply listen to the hum and make it as low as possible? How / why does the heater load influence the hum?

Thanks,
hrs

heater_trimpot.png
 

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KeithWalker

Joined Jul 10, 2017
2,409
Hum is caused in a tube amplifier by leakage of the filament voltage across the filament insulation to the cathode of an electron tube. It is usually very small and can be balanced out with a potentiometer connected across the heater AC supply, with the wiper connected to ground. It is adjusted by setting it for least hum on the speakers with no input and maximum gain.
 

MisterBill2

Joined Jan 23, 2018
12,439
In addition, the load on the heater supply is not affected by the adjustment. And in some amplifiers that pot is tied to a positive voltage, not common. Not sure if that is better or not.
 

Ian0

Joined Aug 7, 2020
5,550
In addition, the load on the heater supply is not affected by the adjustment. And in some amplifiers that pot is tied to a positive voltage, not common. Not sure if that is better or not.
That's probably where there are some cathode followers in the circuit. The heaters are put at a higher voltage with respect to the normal "ground" so that the maximum heater-cathode voltages are not exceeded.
 

MisterBill2

Joined Jan 23, 2018
12,439
I had not considered that! I presumed it was to reduce heater to cathode emission current. It does make sense, though. I only used a cathode follower in the split-load phase inverter stage. That would make sense in a cascode amplifier stage also, even more so.
 

MrAl

Joined Jun 17, 2014
9,186
Hello,

Here is a new drawing might be a little easier to read, and easier to see the potentiometer in question.

Yes the hum pot helps to reduce the hum you hear at the output of the amplifier.
There are two sources of hum. One is the filament current change due to being run on AC, and the other is just hum pickup from any wires at the input of the amplifier.

The hum pot injects a little AC voltage into the input signal, so it partially helps to cancel out the AC noise. Now since the filament changes temperature at a rate of twice the line frequency, the pot probably can not compensate for that so the pot is probably just adding an AC offset to the input signal to help reduce the true AC hum signal. The pot adjusts the phase of the offset signal so you can match it to the hum pickup in the lines.

I talk about the filament issue in this manner because if we look at the filament temperature change it goes up and down for each half cycle but the hum adjust signal goes positive for one half cycle and negative for the other half cycle so that can not compensate for the heater temperature.
However, if the Ac hum signal itself can get into the input signal path via the filament then the hum pot could help to reduce that hum.
To find out for sure which of these effects play the most significant role we would have to do some careful measurements.

Here is the image a little better but mainly for the pot in question...
 

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Ian0

Joined Aug 7, 2020
5,550
Yes the hum pot helps to reduce the hum you hear at the output of the amplifier.
There are two sources of hum. One is the filament current change due to being run on AC, and the other is just hum pickup from any wires at the input of the amplifier.
This is the explanation from Blencowe, which @ericgibbs posted seconds ahead of me.
"Hum and buzz is caused by leakage current between the heater and cathode, and by the electromagnetic field around the heater and associated wiring. The magnetic field modulates the electron stream inside the valve, leading to hum. "
There is a problem in some dual valves, where the heater in one half goes up-down-up inside the cathode and then links across the top of the valve to the other half, then goes down-up-down back to the base. When used with AC heater supplies these types tend to hum, and are best used with DC heaters.
 

MisterBill2

Joined Jan 23, 2018
12,439
Hum and buzz are entirely different! I fought with buzz caused by using silicon diodes i the HV supply instead of a rectifier tube it caused little spikes on the HV line, it was solved by adding tiny hv capacitors, i don't recall where. Those diodes did not switch on and off smoothly, but with a snap. Thus the buzz spikes.
 

MrAl

Joined Jun 17, 2014
9,186
This is the explanation from Blencowe, which @ericgibbs posted seconds ahead of me.
"Hum and buzz is caused by leakage current between the heater and cathode, and by the electromagnetic field around the heater and associated wiring. The magnetic field modulates the electron stream inside the valve, leading to hum. "
There is a problem in some dual valves, where the heater in one half goes up-down-up inside the cathode and then links across the top of the valve to the other half, then goes down-up-down back to the base. When used with AC heater supplies these types tend to hum, and are best used with DC heaters.
Hi there,

Oh that makes more sense because that leakage would be at the same frequency as the line so that would mean the hum would be the same frequency too. A simple measurement of the frequency at the output with no signal input would show this as well as any double frequency harmonic.
I would think the feedback would take care of that though, but apparently not enough.

The heater temperature will go up and down though, but at twice the frequency of the line.

The thing that bothers me about driving with pure DC is filament degradation over time. Maybe it's not that bad though. In regular bulbs it shows up.
 

MrAl

Joined Jun 17, 2014
9,186
Hum and buzz are entirely different! I fought with buzz caused by using silicon diodes i the HV supply instead of a rectifier tube it caused little spikes on the HV line, it was solved by adding tiny hv capacitors, i don't recall where. Those diodes did not switch on and off smoothly, but with a snap. Thus the buzz spikes.
Hi,

That's interesting too. Some of the older style wall warts use small capacitors across each diode. That's the kind with a transformer, rectifier, and filter capacitor. They are usually low voltage though 3v to maybe 40v or so with the typical around 12v.

The Si diodes have fast turn on, and unfortunately they also have storage time which has to be mitigated with what usually ends up being a high reverse current for a very short time to turn the diode off. Often referred to as "recovery time".
 

MisterBill2

Joined Jan 23, 2018
12,439
Mr Al is exactly right with the storage time but I did not think that anybody would accept that explanation. It is exactly what I discovered before I fixed the problem. Very small very sharp spikes on the B+ line.
 
think of it as a trimmable center tap on a transformer winding - in a real world, center tapped winding there is always a slight resistance difference between the two coil outputs. this will cause the two voltages to be a bit different and not able to be totally cancelled by wiring in a twisted pair. when you can adjust the two voltages to exactly equal (aka balanced) you end up with less EM noise because the two lines are (more) equal and opposite and cancel out. if you use DC heaters (my fave) none of this matters because you have no AC signals on the heater and they are dead quiet B^)>
 
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