General DIY Questions/Answers

Wntrmute2

Not So Mediocre Member
I am hoping to create a thread for general questions related to DIYing tube gear and associated bits. Topics such as I'm going to post for instance so they don't get buried in someone's thread. Maybe a sticky if it merits.

Question # 1) Could someone explain the differences between the different schemes for cathode biasing?
Question #1a) Same question for grid bias?

TIA
 

paulbottlehead

Active Member
Question # 1) Could someone explain the differences between the different schemes for cathode biasing?
You'll have to explain what you mean. Typically when we say "cathode bias", this means there is a resistor between cathode and ground that controls the current drawn by a tube.

Above you have 40V across a cathode resistor providing cathode bias on the left, then a grounded cathode and -40V applied to the grid (fixed bias) on the right. We say "fixed" bias because the bias voltage is what it is until you adjust it, where the cathode biased circuit will self adjust. Note that fixed bias does not require a negative voltage to be applied to the grid. If you use an LED between cathode and ground with the grid at DC ground, that is still fixed bias.
Question #1a) Same question for grid bias?
Do you mean grid leak bias/contact bias?

If you put a huge resistance from grid to ground, you can create a negative bias voltage from grid leak current, and this can be used to bias a tube. The Mullard 5-10 is an example of using this biasing method. It's super uncommon to see these days and it only works under very specific conditions with very specific tubes. You wouldn't want to do this with a tube not loaded by a CCS or a resistor, for example.
 

Wntrmute2

Not So Mediocre Member
I think you understood my questions well enough to answer. I need help even formulating the questions 🤦‍♂️
Your picture didn't post after the contact bias?

I was going to ask next about LEDs instead of a cathode resistor and the use of a bypass capacitor in cathode biasing next.
 

MrEd

Senior Nobody
Cool, I get to learn more.
I may have a few questions.
I did not realise that placing a diode between cathode and ground was fixed bias.
 

paulbottlehead

Active Member
If you have an unbypassed cathode resistor, degenerative feedback will lower the gain of the stage. You can consider the cathode follower as an example where degenerative feedback eats up all the gain! With no cap, the degenerative feedback isn't particularly frequency dependent, so it's easy to calculate.

On the other end of the spectrum, if you have a really well bypassed cathode resistor, you will get more gain out of the same stage still without the variation with frequency.

As the cathode bypass cap shrinks, the transition from high gain to low gain will move up in frequency, to the point where you can even create a treble boost by using a really tiny cap across the cathode resistor.

Here is a calculator that will let you play with values:
Cathode Bypass Capacitor Calculator
 
I have an input selector box and a TDK volume pot that I've used as a stand-in or test preamp for years. I've been thinking about adding in a simple tube gain section and started looking around and found this.

But my question is why is this guy put all the wires in white plastic tubes?




100_1826.jpg
 
Last edited:
Looks like PTFE tubing over bare silver wire (or silver plated wire). Can't say what the reason was. Maybe it's cheaper to buy bare OCC silver wire and add PTFE dialectric - or maybe it's to get the advantage (assuming, purely hypothetically, that there is one) of an "air dialectric" between the bare wire and the tubing. Just guessing. But I've seen a lot of build pics with bare wire in PTFE tubes.

cheers, Derek
 
Thanks Derek. Yep, I've seen a lot of pix with that wire config before too. I'll stick to using color wire. Less chance for confusion in my addled mind.

That is what my wife said yesterday when I brought home a new to me chunk of wood.

20200903_124836.jpg20200903_152410.jpg
 

Wntrmute2

Not So Mediocre Member
Mine would have said "Whatcha need another piece of wood for. The 6 over there lonely?"
When I built my phonostage, I used clear tubing over the bare silver wire but I only used that in the signal carrying wiring. The rest was as color coded as I could manage.
 

Wntrmute2

Not So Mediocre Member
Next beginner question. I bought some of those ceramic sockets for breadboarding but when I look at them they seem really well built and grasp the tube pins really tightly. Any reason not to use them for the actual build? Normally, I'd use something like a Belton but these seem nice.

Oh, another bias question. What is filament bias? Does it function as cathode bias?
 

paulbottlehead

Active Member
I have an input selector box and a TDK volume pot that I've used as a stand-in or test preamp for years. I've been thinking about adding in a simple tube gain section and started looking around and found this.

But my question is why is this guy put all the wires in white plastic tubes?
I would strongly suggest skipping that circuit. The output impedance will be many tens of thousands of ohms and completely incapable of driving cables or any amplifier with even moderate input impedance, let alone something like a powered subwoofer!

As a general rule of thumb, if you see a linestage preamp with really tiny output coupling caps, like 0.1uF, 0.2uF, or even 0.47uF, you should be very skeptical of the rest of the design! (and the tubing)
 

MrEd

Senior Nobody
This may be one of the better explanations of filement bias. It is generally used with DHT tubes that have a lower ampeage draw due to the required resistors disipation requirements
 
This may be one of the better explanations of filement bias. It is generally used with DHT tubes that have a lower ampeage draw due to the required resistors disipation requirements
Waiting for Part 2
 
I would strongly suggest skipping that circuit. The output impedance will be many tens of thousands of ohms and completely incapable of driving cables or any amplifier with even moderate input impedance, let alone something like a powered subwoofer!

As a general rule of thumb, if you see a linestage preamp with really tiny output coupling caps, like 0.1uF, 0.2uF, or even 0.47uF, you should be very skeptical of the rest of the design! (and the tubing)

Thanks Paul. It was a late night thought anyway. That combo has served me well and why mess with a good thing.
 
If a power supply calls for a 20uf 440v cap and I have a small one a large one and a big can cap. Does size really matter if the are all the same values?
 
Ok, I thought there was another measure of a cap. Ripple current....I've never seen that on a cap. Thanks Paul.
 

paulbottlehead

Active Member
There are several dimensions of electrolytic cap datasheets that have to do with reliable operation in a given circuit. Ripple current, leakage current (functionally applied DC voltage), ambient temperature, and ripple frequency all kinda work together to kill caps.
 
Top