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For the amplifier part: http://sound.whsites.net/projects-0.htm#mus

For the tube preamp part though, there should be plenty of schematics of famous amps online. Odds are good you might end up needing more than just one 12AX7 though, but that totally depends on what you wanna get out the other end.

Later edit: http://sound.whsites.net/valves/analysis.html#s3
 
Blues junior is about 15 watts from two EL84's .
If you want to make something with a transistorised output stage and you want as much loudness as the B.J. I'd be thinking at least 30-50 watts output. Transistor output stages dont normally sound good when over driven ,but you could use the 12ax7 in the preamp  to soak up the peaks.
There are many fairly good sounding transistor guitar amps based on the 2n3055 in pairs ,there cheap ,very available ,and robust enough to handle overloads ,quite easy to make 15-50 watts from a pair  . An easier alternative would be to use an Ic poweramp ,many choices here ,TDA7293  might be a good one ,will do upto 75 watts .
Another option would be to do something like a single el84 with a 12ax7 pre/driver ,its only about 5 watts ,so not a gigging amp by any means ,but for recording at a moderate volume you can get to that nice place between clean and crunch without blowing out the neighbours , and even if you drive it full throttle its still probably below 100 db spl.
 
Brian May has an amp made out of a vintage transistor radio output section and a small speaker, apparently used on a lot of records, go to the five and dime store and pick one up for two bucks, use the 12ax7a for the preamp and your done, older radios will have Germanium transistors which has the funk.
 
An old buddy of mine used do small pubs gig for a while with a valve radio as his amp ,it had an 8'' speaker with Alnico magnets, no more than about 5watts single ended output .
The thing is many vintage tube radios came with an external input to high z ,so just installing a jack socket and you have yourself a ready made amp with a cool 50's rockabilly sound to it .
 
Well even if the amp isnt strong enough to carry the full sound front of house at a gig you could always mic it up and put some through the pa system , even baby single ended tube amps with some sound reinforcement can really rock the house , you can go from clean to mean to scream without stomp boxes ,right from the volume control on the guitar , you wont be giving the sound engineer a headf**k by cranking big amps full on either .
 
Yeah trying to drive a speaker with a 12ax7 a fraction of a watt is all you'll get .  There is however a few small amps that use a 12au7 in a push pull output stage to get a watt or two  ,one of these is made by blackstar .
To do it properly you'd probably need a 12 ax7 at the front end ,one stage for voltage gain maybe followed by tone controls , the second triode set up as a phase invertor (split load) ,then a 12au7 in push pull coupled through a transformer . A 12ax7 only draws a milliamp or two of anode current max and it expects a very high load impedence compared to what normal transformers are designed for, the 12au7 is much lower impedence ,so maybe a small edcor push pull something like 12-15kohm : 4,8 or 16 ohms depending on the speaker you have will work ok .

Have a look at what this guy did with a couple of ax7's and an au7 on the output, even though the volume isnt massive ,its still an impressive sound .

https://www.youtube.com/watch?v=bgue2NYCVgY

The 48 volt mains transformer you have wont be much good either ,you'll be wanting around 220-250 volts ht .Try flea markets and garage sales ,something like an old taperecorder chassis might have everything you need,mains ,output transformers etc.
 
Fender Champ, model 5F1?

champ_5f1-schematic.png
 
Matador said:
Fender Champ, model 5F1?

champ_5f1-schematic.png

In what will likely be an effort to show off my lack of tube knowledge, just for gits and shiggles, what would it sound like if one were to replace the second 12AX7 stage cathode 1.5K resistor with a pot, and move the feedback from the cathode to the wiper on the pot?

Would this be some sort of an "overdrive" control? Wiper to cathode, no change from original, wiper moved towards ground, less feedback and more gain?

I ask, only for my own education.

Gene
 
I did up a guitar amp before from an old radiogram  chassis 2 xEL84, It had feedback from the transformer secondary ,I arranged a pot to adjust the feedback level and a footswitch to engage or defeat feedback ,it works great ,you could adjust your levels so with feedback you had clean with an edge or even crunch ,then you kick off the feedback and the whole character of the thing changes into a completly different wild animal  of a thing , well worth fooling around with .
 
The earlier grid leak 6SJ7>6V6 5C1 champ circuit is my bacon amp.
Use on full with really old Gibson P90 pickups. Great direct response to your hands.
Run it into a celestion blue.

No feedback loop.

I built mine with a single ended Philips radio OT. More tar there than an Islay Whisky.

The Gibson Les Paul version is the same circuit, except they used a 10m grid leak resistor. I could only begin to tell the difference on a Tele. 5m is fine.
 

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> cathode 1.5K resistor with a pot

First problem is that the NFB in a cheap amp cuts the hum/buzz from the cheap power supply. Power upgrade wanted.

There are no 1.5K pots. 1.5K is a "magic number" with Fender. (Yeah, we could fake it.)

Many folks make the "22K" a 100K variable. You need a stopper to keep from killing the bias, and you can't reach "no NFB".

Perhaps the most rational hack is to make the 22K into 5K or 10K, to the wiper of a 10K pot across the speaker. One way is more hi-fi than stock. The other way goes to dead-zero NFB.

It may not be a "wow overdrive". There is not a ton of NFB stock, or with any reasonable NFB values. In fact there may be insignificant NFB with nominal load. It may be more about controlling the rise at speaker bass resonance.
 
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