Jolly Meow (tube preamp)

[Smrtokvitek]

This is my take on the infamous <you can guess the model> Two Rock preamp. The layout was heavily inspired by Vahlbruch Kaluna. The plate voltage SMPS is my own design and I am also using a buck converter for the heater. It is still a work in progress. I have learnt a lot, but it was also in the making since the last August.

Few notes:

• I really like MELF resistors in a 0207 size. They are large enough to solder easily and they can still save space.
• I had to make a custom 3D-printed LED light pipe because I put it in a place that is not the best in the end. The LED is the only fancy one I had, OSRAM warm white.
• I wanted to have the tube mounted the same way the Kaluna has but the connector approach needs more mechanical support and just soldering the pin header works ok for me. There should be still enough space to get the tube out.
• I have the faceplate ready to get manufactured but I will have to deal with it in the future. Not enough resources (time, money) on my side right now.
• I would love to have the enclosure in a nice powder coat but I don't know anyone who could do that for me.
• I hit the cap with the soldering iron because I was populating it in waves. Testing the functionality on the smallest circuit and continuing after to not waste components.
• The middle knob has to be replaced, I got 2 identical ones and the third one is from a different batch and without the set screw.

I will have the opportunity to try it today. Let's hope for the best.

 

[szukalski]

I really like the look of this. Did you get the enclosure done by Tayda?

 

[Smrtokvitek]

Thanks, I did it myself using a step drill for circular holes and I have access to a hobby-grade milling machine but it is hand-operated so it is a bit flimsy and time-consuming.

 

[JamieJ]

This looks awesome! Is the preamp of a certain Mr JM? I’ve always wanted to build that preamp but could never find a schematic (not that I have any clue about tube based layouts) or PCB. I’d be super interested to hear how it sounds!

 

[Smrtokvitek]

Thanks. Well yes, I think we are talking about the same guy. There is this guy and also this post in this section where you can find inspiration.
I will try it in about 2 hours hopefully in the rehearsal room.
I don't have an amp at home and my motivation was also to have this as a part of a "home silent rig". I will see how it will go.

 

[Guardians of the analog]

 

[Guardians of the analog]

Also @vigilante398 right now

 

[Brett]

Nice build! I'm currently in the beginning stages of working on a HVPS. Would you be willing to share more details on the power supply choices?

I'd be most interested in the Vin minimum requirements and the amount of power being generated/efficiency of the two SMPS circuits.

Thanks!

 

[dawson]

That's really impressive- let us know how it works!

 

[Smrtokvitek]

That's really impressive- let us know how it works!

So good news and bad news. It works quite well. I had it in front of blues junior and it can definitely add some new flavour to the sound. The bad news is I am an idiot sandwich and I swapped mid and bass pot pins so they go in the opposite direction. They are log pots so the behaviour is not great. It can be fixed by cutting legs and using jumper wires. But it won't be very elegant...
This is a note for my future self. This is the consequence of working on something for a long time and returning to it from time to time not checking the basics.

Nice build! I'm currently in the beginning stages of working on a HVPS. Would you be willing to share more details on the power supply choices?

I'd be most interested in the Vin minimum requirements and the amount of power being generated/efficiency of the two SMPS circuits.

Thanks!

Thanks, I started more than 2 years ago with exactly this step. I found the voltage boost schematic using UC3843AD it worked so from that point, I just stayed with this design.
At first, I was using a 7805 linear regulator with diodes to raise the voltage to around 6.2V for the heater. This works but it has obvious downsides. It is big bulky and hot. I have found the AP62150 buck converter that works, with high efficiency, and a good input voltage range. It has one problem, it automatically switches between PWM and PFM to optimize efficiency. It can introduce some possibly audible noise so I switched to AP62201WU-7 which is basically the same IC with just the PWM function (at the cost of lower efficiency).

I am not sure what level of detail you want but the minimum voltage is around 8.4V for UC3843AD to start up if I remember correctly. It can withstand up to 30V input but the AP62201WU can deal with only 18V so that is the upper limit. I use a 12V power supply as I don't have to draw that much current as if I used 9V.

I have it set to 250V and the total current draw of this preamp was about 270mA at 12V I think. But I am not sure right now. I would need to do some proper measurements. The only thing that is really hot is the tube so I guess this fulfils the requirements I had.

 

[Smrtokvitek]

Little update. I swapped pins of those 2 pots today. It works as expected now. Also slight correction about the voltage. It is 242V for B+ and ít draws 278mA at 12V from the power supply. The heater is set to 6V.

Switches offer mid-boost, deep and bright. All of them are pretty useful. I like all of them in contrast with the default setting. So now I have to incorporate it into my board. I can imagine having it as a boost colouring the tone or pushing the amp. It has pretty enough volume. There is obviously not that much overdrive as there is only one triode being pushed but together with the overdrive (I have used a Zendrive clone) in front it can sound really nice. Basically as a preamp.