Need to upgrade my practice amp

[SolderFodder]

Currently I am using a Ruby build I threw together last fall as a practice amp.  I put my Marshall ShredMaster in front of it and it does great but now I need an upgrade (tone controls and a few more watts would be nice).
I am putting together parts for a AX84 P1, but it'll be a few months before I have more than $50 bucks to dedicate to practice and recording amps, so I am considering an intermediate step.

I am considering a Ruby Tuby build http://www.sophtamps.ca/schematics/ruby_tuby_1.jpeg

I have a TDA2005 so I was going to use the stock schematic for it for the power amp section (replacing the LM386).
I am also going to remove the opamp front end, as it looks like it'll just get in the way of anything I want to push that tube with (or make it an optional input).

So on to the questions:
Not sure if Stephane is even still around, but I am wondering why he used the 12AX7 instead of 12U7?  Do 12U7 sound better at 12v?  That's what they were designed for so I always pictured the clean sound being a less beefy 12AX7, but better than 12AX7s in starved mode (if this isn't true why the hell do they make 12U7?)
Does a starved tube sound better than a opamp for a clean amp?  And I know this is subjective, so I'm fishing for informed opinions.

[PRR]

> why he used the 12AX7 instead of 12AU7?  Do 12AU7 sound better at 12v?  That's what they were designed for ...

12AU7 is a 300V tube.
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/1/12AU7A.pdf

You are maybe confused with 12U7, a very different tube.
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/1/12U7.pdf

Any "300V" tube can work with 12V. Performance may be "bad". But in guitar effects, "bad" can be "good".

> why the hell do they make 12AU7?

An interesting question. Why would they put two souped-up 56/76 triodes in one bottle and waste decades building millions of them?

[SolderFodder]

An interesting question. Why would they put two souped-up 56/76 triodes in one bottle and waste decades building millions of them?

No, you're right PRR.  I went back and corrected myself.  I meant 12U7, the space charged 12v version.  I was not in my infinite wisdom questioning the need for tubes in general 
Just looking for an opinion really on opamp vs starved tube vs space charge tube opinion.

[PRR]

At a $50 budget, I think you have enough to do just getting a good 12V 1A power source and the tone controls.

I'm not a fan of cheap DIY tube stages. I think it takes very considerable experienced tweaking (and many prototypes, and ample supply Voltage) to hit a for-sure generally-good plan.

I'd be thinking JFET, volume, JFET, Fender tonestack, TDA2003.

[phector2004]

Yup, JFETs are the way to go if you're on a budget. They cost a few cents and never need to be replaced. They also run in the 30ish volts range, and don't sound like "crap" until you get down to just a few volts.

Have you considered looking at some chipamps? Taylor's got a nice one almost ready to go, but you can find kits on ebay or on some online electronics places.

If it's just a practice amp, doesn't NEED to have tubes...

[SolderFodder]

I was afraid of that.  I didn't want to kid myself into thinking this was going to sound good just because I was using a tube.
I read through the Taylor thread.  I liked the idea, but the amp he proposed at the end does not suit my needs as a practice amp.  No tone stack for instance.

JFET (esp MPF102 based) sounds appealing.  With the volume that early on, wouldn't it be more of a gain control?

[SolderFodder]

Yup, JFETs are the way to go if you're on a budget. They cost a few cents and never need to be replaced. They also run in the 30ish volts range, and don't sound like "crap" until you get down to just a few volts.

Have you considered looking at some chipamps? Taylor's got a nice one almost ready to go, but you can find kits on ebay or on some online electronics places.

If it's just a practice amp, doesn't NEED to have tubes...

I was considering a ToneMender + TDA200x.  I don't really mind opamps that much, but you they don't exactly make my Strat bark when I push them.

[PRR]

This should work with TDA2003:



It "must" use JFET J201; higher threshold JFETs are IMHO too tricky to get and keep biased with 12V supply. Aron will sell you a lifetime supply of J201 at an excellent price, though I agree that's more than you need today.

The FET Drains should stand at +4V to +8V. J201 should do that 9 times in 10 (why you want a few extra). If not in that range (and the next J201 from the bag isn't better) then change the 1.5K source resistor to 680 or 3.3K.

It won't easily crap-up like some starvin-tube plans do; it won't be soulless like some opamp plans are. It has basic volume plus Fender Bass and Treb.

If you must find bark, dirt, grit.... I think pedals do that better. And with a mere 4W-16W you can just slam the TDA2003 without hasty deafness. But you can try a second volume control after the tone-stack. Or you can change-up the 2.2K to 10K or 33K to starve the JFETs into more brown-out tone.

[SolderFodder]

Interesting, I was centering in on something similar with a Uno from ROG in front.
This is actually what I was looking for as long as it stays fairly clean. Is this your own design?

[PRR]

I drew it.

I didn't even put my mark on it; there's no more "creation" here than in a grocery list.

"Design"? The basic amp stage has been around for a hundred years, fifty years with the non-vacuum device. The part values are generally determined by the available devices, supply, and what the box has to do. The tone-stack is generally credited to Fender, though I did scale the impedances down to suit the lower voltage.

One of the sweetest musical instrument amps I ever met was a Yamaha e-piano head which was a lot like this, only more production-friendly. More parts so you didn't have to meter every FET for ballpark voltages.

[SolderFodder]

Right.  So it's two vanilla gain stages with a tone stack.  That what I was looking for.  Thanks again.  Going to try to get the preamp section done this weekend.

[SolderFodder]

It "must" use JFET J201; higher threshold JFETs are IMHO too tricky to get and keep biased with 12V supply. Aron will sell you a lifetime supply of J201 at an excellent price, though I agree that's more than you need today.

The FET Drains should stand at +4V to +8V. J201 should do that 9 times in 10 (why you want a few extra). If not in that range (and the next J201 from the bag isn't better) then change the 1.5K source resistor to 680 or 3.3K.

Ok, I just finished this.  I measured the voltage the transistors are seeing and it's 14v (the wall wart puts out a bit much as usual).  The drains are at 9v for the first stage and 10v for the second.  This may be ok, it doesn't sound bad at half volume but it distorts at full volume.  The distortion doesn't sound all that bad but if getting the voltages down will make it sounds better that would be cool.  Should a put a trim pot on the transistors power line maybe?

[PRR]

> 14v ... drains are at 9v

Reduce the 1.5K source resistors to 1K, 820, whatever, so the drains are at about half supply, 7V or 8V.

> but it distorts at full volume

I thought you wanted a guitar amp?

A guitar amp that won't go into distortion is no darn good.

On days you don't want distortion, don't turn all the way up.

Are you sure the preamp is distorting? There's only so much you can get out of a TDA2003. With 14V supply and 8 ohm speaker, 3 Watts. Less than a healthy Champ.

What gain are you running in the TDA2003?

[PRR]

I just noticed you are saying TDA2005.

What exact circuit and gain?

[askwho69]

mmm thinking!!!

[SolderFodder]

I just noticed you are saying TDA2005.

What exact circuit and gain?

No, I used a TDA2003 based on your recommendation.  I used the poweramp section presented in this thread.
http://www.diystompboxes.com/smfforum/index.php?topic=81129.0
Looking closer at that it does advertise 5W, I was thinking 10W but definitly wasn't hearing 10 last night.  That would explain it.

Now understand, the amp is very usable as is.  In fact it worked without having to debug it the very first time which is always a plus (and usually an indication of low part count  ).  It can get very boomy when you turn up the bass or volume, so I was worried the higher voltage may be changing the frequency response, but then again I'm running through a junker speaker that I've been playing my Ruby through.

[SolderFodder]

Actually the more I play with the amp, the more I like it.  Thanks for your help again, PRR.  I get some really nice jazz tones out of it.  It's a little hard to work a shredmaster in front of, but other than that no complaints.

[PRR]

The first stage may overload easier than a classic guitar amp.

Find the 2-jack input wiring from any tube Fender and use the low-gain input.