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Guitar tube preamp + class D power amp project
I don't have anything technical to post yet and I'm fairly sure that there are only a handful of members on here that will dig this build thread. That's ok with me. I really enjoy working with vacuum tubes. They just sound and feel right. Figuring out the point to point wiring layout and experimenting with different component values, B+ values, and different tubes is quite fun.
My son and I have been getting into learning to play electric guitars and I'm presently looking for a decent used bass guitar (for me) on craigslist.
Right now we have two crappy little solid state practice amps. It would be fu to build a 100 Watt tube amp with a Marshall stack but the prices for the output stage parts has gone over the top IMHO. The power and output transformers plus the output tubes combined cost more than a decent playing/sounding guitar!
Luckily I have a bunch of small preamp tubes, small transformers, a few class D amp boards, and laptop bricks laying around. That's where this project is headed. First step is to get the preamp tube stages and class D board gain stages right. After I get that part right I'll work on getting a tone stack that I like. Luckily the schematics for most Fender, Marshall, HiWatt, Vox, etc amps are plublished. So I can mix and match the front end parts to get what I like.
The ouput stage is simple really. I've built enough class D amp modules with switching power supplies (laptop bricks) to know where to go with that.
Comments
The LM1875 is supposed to be a great chip to emulate preamp tone and is considered to be neutral of a distinct sound signature of its own. Take that with a huge grain of salt. It is a really simple IC to implement. 20watts and single supply means that wallwort is still an option.
Sounds like a great project! You are stepping into a different world of controlled distortion.
The first Line 6 modeling amps used two LM3886 chips as the power amps.
I still have one of these laying around and a laptop brick to power it:
https://www.parts-express.com/TDA7492-Digital-Audio-Amplifier-Board-2x50W-320-606
So that's what I'll start out with.
You may need to put a load on the unused channel. Or run 2 speakers.
Yeah, I'll short the unused channel input and place a load resistor on that output.
I found a Hammond 6K1VF plate/filament transformer in one of my junk boxes. That should work perfectly for a single 12AX7 circuit.
Single? Come on now.
That plate/filament transformer should easily handle 3 of the 12A*7 family of tubes in regards to total heater supply current as well as the plate currents. Baby Steps Rocker!
I'm going to actually prototype up the circuit(s) with some cheap china 6N2J tubes I grabbed for $2 each. Almost identical except for the filament wiring.
http://beavisaudio.com/projects/valvecaster/ I have built this one a few times. Pretty fun and a lot of tone for such a simple project.
Thanks for that link Ken. I've never explored low voltage tube circuits.
That's pretty bitchin👍🏼
I've started to build my prototype circuit for this tube guitar preamp so I can play around with different values of tube bias and such.
When prototyping low voltage stuff like passive speaker crossovers I'm not so anal and just let alligator clips run wild. However, when working with vacuum tube designs that sport B+ voltages of 300 Vdc or higher I step up my prototype game. High voltage can kill you!!!
No high voltage here yet but this is the start of my prototype build:
Pictured above is just the wiring to the filaments (tube heaters). It's supposed to be 6.3 Vac per tube specs:
As seen the voltage measured across the tube filaments is a bit high at 6.68 Vac. I'm not sweating that at all. Firstly because there is zero load on the transformer's high voltage windings at this point in the project. So the heater voltage might sag a little bit when there is addition load on the transformer. Secondly I'm not worried about 6.68 Vac on the filaments because the tube data sheets for almost every tube in this world states: +/- 10% for almost every parameter, almost always for filament voltage. (6.68V - 6.3V) ÷ 6.3V ×100 = 6.03%. Well below the spec. Tubes are tuff in my personal experience.
Last post for today... NFL playoffs starting soon. Here is a picture of the glorious glowing tubes
Playing with the bias on power tubes is a thing. For ‘kicks & giggles ’ I put a toggle adjusting bias on my pre/buffer tubes (running then through a solid state amp). Don’t get the same ‘play’ as with power tubes - but wt heck…
Bias points on preamp tubes is definitely a thing with guitar amps. Hifi, you pretty much bias for as clean as possible. Quite the opposite for instrument amplification as it's where the tone of the amp lives.
Nothing too exciting but I did get the full wave bridge wired up using UF4007 ultra fast diodes.
Heck, that's more than I've done so far tonight!
My goal with this prototype step was a B+ voltage of 225 to 250 Vdc. I'm only using a simple 1 watt resistor to simulate the queiscent load current of one tube (will need to change as I add two more gain stages). But I'm totally ok with a B+ of 220 Vdc. I can/will adjust the power supply resistor values to hit the final value.
Nice fluke. I looked at that one a lot but bought a BK precision 393. I like the 393 but it sucks down batteries.
So this project has run into an issue right off the bat
I wired up the straight forward DC bias circuits and it is very clear one of the cheap china tubes is junk. I don't really want to order more of those tubes and have to wait 3 - 4 weeks to get them. Not sure where I'm going to go at this point...
If your interested in continuing the project I might have a few china tubes. Shipping might cost more than a kit from Amazon though.
Thats a nice meter you bought. I have a Fluke 179 in my big drawer of meters. Love it, I nick-named it the "Lifesaver".
I also have a Fluke 87V and a Fluke 101.
And a Fluke 116 and Fluke 120B Scopemeter at work.
I can send a 12ax7 for the cause, let me know.
Oh wow! What a generous offer thanks man!!! I got this... Just need to rewire the heater lines and run away from the junk china tubes. I have a whole bunch of made in America NOS 12AT7's, a few 12AU7's and one 12AX7. Thinking I might not need all the gain of the 12AX7 on the final two stages since I'm not driving output tubes. In fact the 3rd stage feeding the tone stack will probably be just a cathode follower with ~0 dB gain. The 12AT7 might work better in that position. We'll see... this is just a fun "let's play with tubes during the cold winter months" type project.
I also have a few Russian made 5751 tubes which heaters are wired exactly the same as the US 12A...7 tubes. Looking at the tube data sheets they seem to be somewhere between the 12AX7 and 12AT7. About 10% more gain factor than the AT's.
Yup, they are, but it really depends on the brand and age. Numbers can vary wildly, I have a new JJ 12AT that's "hotter" than quite a few of the used 12AX tubes I have.
My pre sounds great with all the AX/AT/AY tubes I've tried. The one new JJ AU I have just doesn't sound good in either position, just not enough gain for the way I'm running things.
I rewired the heater lines to the tube sockets and have the 5751's installed now. Plate and cathode voltages are within 2% tube to tube