Single 12AT7 Circuit Design

[Jolly]

Hey Everyone!

I've been an intermittent lurker over the years and am finally getting around to building a few of the projects I've been planning. I'd like to thank all of the knowledgeable and helpful individuals here for the information I've gained from this forum.

I recently completed my first tube amp build. It uses a 12U7 and 12K5 to generate ~1/4 watt from a 12 volt cordless drill battery. The entire amp is housed on board a guitar. For anyone interested the build thread on that can be found here:

https://www.projectguitar.com/forums/topic/52765-jollys-tube-amp-in-a-tele-build/

It turned out great as a proof of concept so naturally I'm planning to build a second version. For various reasons I decided that the next one needed to use more "traditional" tubes and I settled on a single 12AT7. I threw together a schematic for what I have in mind but I'm pretty novice as far as circuit design goes so I'd love some feedback if anyone's willing to give it.

For size and widespread availability of parts I'm pretty set on using a fender reverb transformer.  I think my B+ needs to be ~220V, again for availability of components to bump up my 12V battery supply. The stereo jack between stages is ideally there so I can play around with adding a volume and tone stack but always switch it back to a bare bones amp with no controls.

Let me know if anyone sees any major issues or room for improvement!

[Stuff4bikes]

Looks interesting. I use a single at7 half pre and half pi to run two 6v6's Not super loud but I am running an eminence 10" cajun or heat if I recall, I make up a little with the speaker spl's.

[Keppy]

Looks cool! Seems like you might want a grid stopper on V1b.

[PRR]

Welcome!

With Sleeve drawn as shown, Tip and Ring are swopped-up.

As drawn, when a plug is inserted, it relies on the thing plugged in to provide grid-leak for the power stage. (Stopper may be wise also.)

There may be no reason to break the Send? Leave it live all the time.

I don't think the 1Meg at input is optional. Again, don't plan for your source to leak your grid.

The operating point of the power stage, 220V 100Ω, does not look happy.

[tubeswell]

1 x 12AT7 triode @220V with 20k reactive load and 11mA tube current (dissipating just under ~2.5W) requires -1.45V bias.

For max power output under these conditions, Rk = 1.45V/0.011A = 132R

130R would probably be safe

[AmberB]

I know that it's not the easiest tube to find these days, but a 12DW7 may be more useful for this application.  It's basically 1/2 a 12AX7 and 1/2 a 12AU7.  Higher gain from the first triode, a bit more power handling from the second triode.
Just a possibility...

[mresistor]

I know that it's not the easiest tube to find these days

https://www.tubedepot.com/products/jj-ecc832-12dw7-preamp-vacuum-tube

[Jolly]

Thanks for all the feedback everyone! I really appreciate the help!

I throw out a little more context and what I'm trying to accomplish this amp. Ideally I'd like to optimize it for clean headroom and power output, then add some solid state circuitry in front to create or coax out the distortion. The first generation prototype sounds great but its a little quite. While I was breadboarding it I had it pugged into a 12" speaker and it had much more usable volume. The onboard speaker is 4" and pretty much fixed, so my only way to get more volume is going to be through the amp.

The prototype also has a really simple clean boost circuit built in to the selector switch. With that activated it has tons of distortion and more reasonable volume, so I know I can get the volume I want through the current speaker, it just needs to be driven harder.

Any suggestions for a solid state circuit to put in front of the 12AT7 version? Maybe even a buffer to control the signal into the preamp a little more precisely? Ideally I'd like to use that to create a clean and an overdrive "channel".

I looked into the 12DW7 and decided against it for now. The onboard amp setup runs into feedback issues with high gain, so I figured I'd start low and work my way up if the 12AT7 doesn't have any issues.

I really appreciate all of the advice on specific component values and configuration as its the part of this I'm still working on learning. I'll put together an updated schematic with all the suggested changes to make sure I understood everything correctly. 

[1blueheron]

Jolly,

Neat little project you have there. I will be interested to hear how it turns out.

I played with a similar idea I was calling a micro-Mace but like many of my projects, never finished it up.

If you are familiar with the Peavey Mace or Duece you know they had a SS preamp with tube output but they were monsters.  Not very practical for in home or bedroom use.  A miniature version would be great.

I was planning to use a simple LM386 as the preamp/driver/distortion provider.  They actually are quite natural sounding and the harmonics of their distortion are not bad for a SS device.  Also conveniently run off a 12VDC source.   IIRC the output tubes I was looking at was a pair of 6SN7's. 

For the SS front end,  You might want to look at the little gem, ruby, etc. that employed the LM386 to get an idea for the range of sounds it can produce.

There are some 12BH7 tubes used as output tubes in a few amps. One example that comes to mind is the Blackstar HT5.  Might want to explore them as well.  Lastly, look at LM386 based pedals.

For good results on an LM386 you need a good stiff 12v supply.  Bad sounding LM386 is usually the result of not enough power.

How small of a package are looking to end up with?

[Jolly]

I started down the LM386 rabbit hole and it led me to the TL072. It similarly will run straight from a 12V supply and as a somewhat arbitrary perk has the letter T in it. I'm loosely basing this project around a Tele style guitar and with that most of the components and themes I've picked so far have had a tie to the letter T, so why stop now. Ideally I'll just use this as a clean signal boost and try and get all the distortion from the tube.

Leading into package size, it just needs to fit onboard a guitar. Thickness is my main limiting factor, I have ~1-1/4" to work with but would like to stay a little thinner for mounting/clearance reasons. Other than that I can spread components out thought the body as needed. Weight and power consumption are bigger concerns but most everything will likely be minor compared to the big hitters; output transformer and battery for weight, heaters for power consumption.

As far as I understand it a battery is about as stiff of a power supply as one could ask for? Is there anything else I should consider to protect or regulate it? This is what I'm thinking about using to get my high voltage, any thoughts on if it will work?

https://omnixie.com/products/nch6300hv-nixie-hv-power-module

[Jolly]

Here's an updated schematic with some of the recommended changes. I also think I'm going to split the project up a little bit and save the solid state section for another day. I also removed the optional signal split in the middle, going to stick with a passive tone and volume control for now. Let me know what ya think!

[AmberB]

What are you planning to use to convert the battery voltage to the 200 or so volts you need for the B+ for the plates of the tubes?

[Jolly]

I'm thinking about using this to bump the 12V supply up to 200+V:

https://omnixie.com/products/nch6300hv-nixie-hv-power-module

I'm a little worried it might be kind of noisy but it seems to be about the right power handling, was designed for tubes, and fits my packaging requirements.

[Jolly]

It lives! I've got some tweaking left to do but at least I have sound coming through.

Here are a few quick cell phone videos. It sounds better in person but at least to give you an idea. The distorted demo is using a simple clean boost.