Tubes 101 - Intro to Tube Preamp Design

[szukalski]

Thanks a lot for this, and the other, thread, as well as the other tube based threads. I’ve learned (and learning) a lot.

The only downside, apart from tube prices, is that you’ve already made an optimal tube mounting PCB and I can’t improve on it !

 

[mnemonic]

great primer on how tube circuits work. With this info you can pretty much analyse how all tube amps work since they’re all just variations on this theme. Just add more or less stages, more or less interstage filtering, different component values, etc.

Now we can look at the JCM800 preamp and see why it’s so bright, look at all the treble peaking on every divider, and the small bypass cap on the first stage. Wouldn’t take much to make it way higher gain too, could change the second stage to a more traditional gain stage (or just copy the first one) and bypass cap on the final stage.

Also I clearly don’t browse enough here I didn’t know you made kits for the space heater, it’s such an exploitable circuit also, easily could be modded to whatever type of gain stage you want for a ‘V1’ in front of an amp to make it high gain. And I see you’re sending PCB’s to musicding, that’s good for me!

 

[MBFX]

How do you fiddle with and prototype tube circuits? I know I CAN hook things up to my breadboard, but that also seems like it could be a bad idea.

 

[vigilante398]

How do you fiddle with and prototype tube circuits? I know I CAN hook things up to my breadboard, but that also seems like it could be a bad idea.

There's nothing wrong with breadboarding a preamp circuit; they're low enough current that a breadboard will be able to handle it, you just need to be extra careful about the distance between things. I used to do it that way, I designed a breakout board PCB with a tube socket and 9 header pins so I could plug it into a breadboard, and even now the high-voltage SMPS module I sell PCBs for has the pins spaced so that it can be plugged into a breadboard. That's how I used to prototype tube circuits.

These days though my prototypes are a little more unconventional because I generally go straight for a PCB layout. I've built enough preamps that I tend to know what the topology should look like without breadboarding it, then when the PCB comes I can tweak component values as needed before finalizing the design, and when it's verified I have something a little more tangible and repeatable than a breadboard.

 

[rossbalch]

I'm just getting into tube preamp designs, as in I did the ADHD thing of buying a bunch Russian Sub-mini tubes, whilst still reading about the basics ha ha. I'm hoping to get some insight into the power / voltage flow.

In your (@vigilante398) Black Eye design, I see that C9 and C10 are high voltage caps, which makes sense given they need to filter high voltage DC from the next stage. But what I'm wondering is why C6 and C5 don't also need to be high voltage caps as they are directly connected to the 250dc rail too?

 

[SamuelG]

I suspect it is because they only need to manage the voltage drop across the resistors which is much less than the 250v plate voltage to ground.

 

[owlexifry]

I suspect it is because they only need to manage the voltage drop across the resistors which is much less than the 250v plate voltage to ground.

+1

e.g. C6 isn't seeing 250V. it's only seeing the relative voltage difference between each side of R11.
(the 250V value is with respect to ground, i.e. the difference between the high voltage/B+ rail and ground is 250V, but the difference between B+ rail and V2B plate is a lot less than 250V).

so if you wanted to know exactly what that voltage difference is, you could hook up your multimeter leads to each side of R11 and measure the DC voltage. (exercise caution here)

 

[vigilante398]

Yup, these guys nailed it^

Obviously there is no reason you can't use high voltage caps in those spots if you have them around and have room for them, but you're not dropping the whole B+ across the plate resistors, so it's not necessary.