DIY Audio Projects Forum

Just spent some time reading up on grid leak resistors and grid stoppers. I should have done that earlier. Once it sunk in, everything made sense.

You want 1M ohm to reduce load on input capacitor and improve sound quality, smaller input caps normally mean less cap distortion. The second reason is to reduce load on your music source.

By having the other of capacitor connected to 100k but otherside at tube grid with 1M to ground will yeild about the same noise input to the amp.
Compared to that with larger cap and grid directly to ground with 100k.

A capacitor is like short in ac so having 100k to ground on either end yeilds about the same noise input to amp.
Lower input resistance means lower noise and less sensitive to noise pickup.

Not all grids like 1M to ground, bit too big for some tubes like KT88, it has specified maximum resistance of 220k denpending on bias method. Meaning you need to use 220k or smaller grid resistor to ground.

Let's run some numbers...

First the high frequency response:
The miller capacitance of the 6EM7 driver is 162pf. The output impedance of the 12AU7 gain stage is ≈11.5kΩ. So, with the 250kΩ to ground, the f-hi is ≈89.3kHz. Replacing the 250kΩ with 1MΩ gives an f-hi of ≈86.4kHz. This is essentially no difference.

Now the low frequency response:
With the 0.22µf coupling cap and the 250kΩ resistor, the low frequency f-lo is ≈2.8Hz and τr≈57mS. Replacing the 250kΩ with the 1MΩ gives an f-lo of ≈0.71Hz and τr≈222mS.

So the 1MΩ has little effect at the high end, but at the low end it increases the bias excursion recovery time to an unacceptable duration.

As for noise, the overall noise figure for the chain is largely set by the input cathode follower in front of the tone stack. Coupling impedances this far back in the chain will have very little impact on overall noise.

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Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

Matt, I've been reading up on coupling capacitors and grid resistors, in an attempt to understand your explanation. I've read the following.

"As the voltage at the input side of the capacitor increases further, the voltage at the output side is dragged down by the grid, which refuses to rise above the cathode voltage by any appreciable amount. The resulting current that develops through RGS quickly charges the capacitor, increasing the voltage across it. This is a process known as clamping. When the input signal amplitude is finally reduced, the excess voltage across the capacitor bleeds off slowly through the grid resistor RG in a process known as bias recovery. Bias excursion and recovery are important characteristics of a guitar amplifier's transition to and from an overdriven state. Reducing the size of the coupling capacitor causes the grid bias to recover more quickly, but it also reduces bass response by introducing more reactance into the circuit at low frequencies".

Does this mean that the grid resistor and coupling capacitor affect the lower frequencies much more than the higher frequencies?
I would like to know how to calculate the output impedance of the gain stage. I've found how to calculate everything else. I know you've done all the calculations and I don't need to understand, to build the amp. Just trying to learn.

Also what would be the audible effect of a too slow recovery time?

In general, the coupling capacitor has no effect on high frequency response. High frequency response is driven by the values of the coupling resistances and the value of Miller Capacitance (Cm) of the following stage.

The description you quoted is discussing bias excursion and recovery. This is an anomalous operation mode when the tube is overdriven and the grid goes into conduction mode (i.e. begins drawing current). The size of the coupling capacitor and grid resistors affect both this phenomenon and the low frequency response. Here is the passage from my design notebook concerning low frequency response and coupling capacitance.
In designing my coupling circuits I am very careful about bias recovery time. Generally I design the signal chain so one stage will transition to grid conduction first. Then I manage that stage to limit bias excursion recovery time. Here is the passage from my design notebook concerning bias excursion.
Read these two passages carefully and it should become more clear what I am saying.

_________________
Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

I've just finished building Matts' 6EM7 as a pair of mono blocks. Each one has a line stage tone stack pre-amp also designed by Matt. I fired them up yesterday and to my delight they sound beautiful.... BUT, between tracks there is audible hum, enough to be annoying. Turning the volume controls up or down doesn't affect the volume of the hum. Identical on both amps. I've checked earthing. I've moved signal cables. I've tried different inputs and no inputs. Can't get it to go louder or quieter. I also have a lacewood V2 fitted with the tone stack..... no hum. Any ideas?

This means it is systemic and therefore most probably based around a design decision. The most important thing to do first is determine the frequency of the hum. Is it at mains frequency or twice mains frequency? The answer will help determine the next steps.

A tip: if you don't have an oscilloscope or frequency counter you can produce a mains frequency reference for comparison fairly easily. Just take a 6.3v filament transformer and put a ≈1kΩ resistor in series with a small speaker on the output. This results in about a 6mA current through the speaker for about 0.3mW output. This is loud enough to be a useful frequency reference.

If the hum is at mains frequency, then it's likely about field coupling; if at twice mains, it's likely a B+ filtering issue.

_________________
Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

You can also try tone generator https://www.szynalski.com/tone-generator/ to listen to 100hz mains sound.

Because the hum doesn’t change with volume I would say it is PS based and not earthing. First and simple test is get a big high voltage cap e.g. 450V 100uf and use alligator wires to clip it on the last filter cap. Don’t do it live. If the hum drops audibly then your PS is under filtered. If not I would look at where the earth or B- connects.

If I have B- connected to the RCA earth and I have a hum I move it to the pot and the hum goes. And (hard to believe) it also works the other way. If you have mains earth connected to B- try lifting it. If the top plate is not at B- try connecting it to B-.

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Projects: "Precision" - 300W Class AB Holton power amp | "retro2308" - chip based headphone amp | ”Calibre 834” - tube phono MM preamp |

[/quote]If the hum is at mains frequency, then it's likely about field coupling; if at twice mains, it's likely a B+ filtering issue.[/quote]

Thanks for quick replies. I've only just obtained an oscilloscope and have been getting used to it on a 12 volt headphone amp, so please excuse me if I make basic mistakes.
First of all I want to clarify for you what I'm hearing. With nothing connected to the input of the amp, and volume control turned fully anticlockwise, I get hum that is just audible from 2-3 meters away. I can increase this hum by turning the volume control clockwise.
With an input connected, (and I've tried various) and the music paused, the hum is again just audible from 2-3 meters away but doesn't get quieter or louder when I adjust volume.
On the oscilloscope I'm seeing a very distorted 50Hz trace.

Period = 4cm x 5ms = 20ms... Frequency = 1/0.02s = 50Hz
I'm pretty sure I've done that right?
Please excuse photo quality.