How to Design & Build a Vacuum Tube Amplifier 

Glad you enjoy it!

Glad it was helpful!

Glad it was helpful!

Glad it was helpful!

It is as long as you use it correctly ..............seen many use it the wrong way .

It is!!! I love using oddball tubes "off label" like this :)

I normally don't change from the default unless I have those numbers. It can change the results but I always consider tools like this a "ballpark" type of thing anyway. So far it's mostly been very close for me.

hehe good one! I like it.

No I haven't, but that could be done.

For large holes I'm using Greenlee knockout punches.

@@SkunkieDesignsElectronics Ah, so an annular cutter would not phase you 😉 It looks like a knockout punch would make a nice clean hole 👍

It does and you don't need a drill press :)

@@SkunkieDesignsElectronics Yeah, I saw that, and it looks pretty cool 👍 I also liked that you could use hydraulics if so inclined 😁

If you lower the voltage across the tube to maybe 225-250V and increase the current to get to the same wattage, then 3.5K might work.

@@SkunkieDesignsElectronics Many thanks.

I plan to use a single resistor.

I simply read them off the schematic Kegger posted. I assume he measured those off the built amplifier.

What output tube and is it fixed or cathode bias?

@@SkunkieDesignsElectronics Both my PP 300B and KT88 have fixed bias that has 100k ohm and 0.47uF

@@Warclock07 That's a different scenario than these cathode bias types. .47uf should be fine.

@@SkunkieDesignsElectronics Thank you, I can see it now. That was a little bit confused 👍

I would think you need to measure the DCR from the grid to ground on a fixed bias amp, I know it's not just that 100K resistor.

Sure please share the info!

@@SkunkieDesignsElectronics will do my best to explain in comments, see if it comes across as usable information as soon as I have time later.🙂

Step One. Box. This bit is pretty simple, ideally you want 120 litres for the Faital Pro 12PR320 to work in. The displacement of the 12PR320 is 1.8 litres and the Beyma TPL-150H roughly 2.3 litres from my calculations, so that's an extra 4.1 litres needed in volume. Then factor in the desired amount of bracing, for me I went with 127 litres total. Dimensions for internal I chose was 14W, 42H, 13D. I built them so they are effectively floor standers, due to volume required and also to negate the need for speaker stands. It's a large volume, but in this format it's not really overbearing, plus height of baffle gives that sound reinforcement and brings tweeter to my ear level at listening position. Obviously these will be higher, wider and deeper due to box construction and will depend on thickness of wood used. For construction I tend to use both MDF and Plywood with a silicone layer in between to bond them together and reduce any resonance's, a constraining layer construction. There is room to manoeuvre in regards to construction, however crossover may need to be altered for largely different variation of the baffle. The Beyma is 1 inch above the woofer, no baffle step correction is used, largely because most people's listening rooms, especially here in the UK are not overly large and I find putting in BSC for the most part in such scenarios just tilts the bass up too much. With the baffle length below woofer, it will largely not be required. This is a ported design, port placement is largely down to preference here, front, rear, down firing even if you raise the box to allow, do whatever is best for your listening environment.

Step Two. Crossover. Crossover is a LR 4th order, this seemed the best suited for these drivers and allowed the greatest level of adjustment and also easier phase alignment. For the start of the tweeter circuit we have a 12ohm resistor inline with a bypass cap of 2.2uf. Next is a 4th order featuring a 10uf cap with 0.3mh inductor to ground, followed by a 7.5uf cap with a 1.8mh inductor to ground. The resistor and cap at the beginning can be adjusted up and down in value if required to raise or lower tweeter level and also tilt up and down if desired. This circuit as per mentioned is for a maximally flat response with about a 1.5dB treble rise at the upper end, which to my old ears is nice, but younger listener's with better hearing up there may want to adjust cap and resistor at start, a lower cap will tilt down, and higher will tilt higher, you can then adjust the resistor to compensate if you adjust the tilt and find region higher or lower in amplitude. Doing so within the realm's of where most are going to adjust either way has a very minimal effect on the phase, so it's fine to play around with this. Obviously you may want to make crossover accessible or even have crossover outside of the box if like me you like to adjust and tweak things to certain genre's. After all, Hi-fi is largely about the interaction in my book, be it media or the system itself. The low pass is 4th order again, albeit before the first inductor there is an impedance correction to keep flat beyond the woofer resonance. This consists of a 0.16mh inductor joined to a 24uf, joined to a 16ohm resistor that goes to ground. To get 16 ohm's, use multiples in parallel, so say 2 32ohms to spread the heat dissipation. Given the sensitivity of this speaker, it's unlikely you are going to be feeding this a whole load of power, but check resistor loads in a suitable program if you are concerned or listening at concert levels. Next is 2.33mh inductor with a 13uf cap to ground, then a 0.75mh inductor with a 2uf cap to ground. I'm trying to write this so all can use, having a diagram of a 4th order 2 way next to you and writing these values in corresponding place should be helpful to those new to crossover design. Lowest impedance with this design is 7.3ohms, very briefly, and then 8ohms or above, a nice easy load for all tube amps.

General information. Driver's. Faital Pro 12PR320. May seem like an odd choice to many, but for high sensitivity goals this driver is about the best I can find with the trade-offs that come with higher sensitivity, after all, bass is tricky to pull off in such a scenario, especially so in a reasonable sized box. This driver will also be available pretty much anywhere and has great response and a well regarded tone. Driver has a 97dB sensitivity, but alas putting into a box it measures 95dB, hence this overall builds solid 95dB rating as opposed to some manafacturers claiming 98dB for a speaker when that's either an average, or worse at peak, which many deliberately leave peaks in the response to claim a stated sensitivity that looks more appealing. They aren't dirt cheap, but not insanely expensive either at £144, I mean buying equivalent high sensitivity speakers is going to cost a lot more than cost of this build in many countries. Any concern regarding the use of a pro driver is pretty negligible, the moving mass is only 51.4g, pretty impressive for the size of the driver and being a pro driver this thing will be under no stress at all in a Hi-fi setting. With a response down to 40hz, and then any room gain depending on placement and you will get usable range down to low to mid 30's. Subwoofer isn't really a necessity here, which is probably quite welcome with the tube crowd purists. The tweeter, Beyma TPL-150H is quite expensive, £400+ in UK. It is however highly regarded and really rightly so. This thing has a 102dB sensitivity so plenty of wriggle room for many high sensitivity designs. The sound, once the midrange hump between 2-10khz has been taken care of via crossover design is really something, this thing is fast and just an experience in my book. Porting for the design is 2 x 82mm ports, 70mm in length. This will have port velocity below 20 m/s up to 50 watts of input power at which point your ears will be toast, so no issues there. If anyone has any questions, fire away. Just sharing in the hope it helps those finding their ideal speaker for tube amp journey.👍🙂

Sorry I have been so slow updating my website, here is a link showing the final build schematics www.skunkiedesigns.com/6lu8

@@SkunkieDesignsElectronics Thanks Steph, I really did not want to pressure you, you are doing so much. I appreciate the schematics, now I can finalize my purchases. Roger

Lemme know how it turns out!

That is why I said "Whatever is a close standard value"...

If you want a specific value, which is not standard, just connect standard value ones in series and/or parallel, to get exactly what you want.

The 25W will work, but are much larger and there is a point where extra large transformers can negatively impact performance. Also, I wouldn't buy anything until I am finished and have tested this design!

​@@SkunkieDesignsElectronics Thank you. I'll wait. I don't want to mess up on my build.