The Hundred-Buck Amp Challenge

[tubelab.com]

Now we are the old folks.

I hear that from the "kids" that I work with every day. I was working there before some of them were born.....no most of them..... since I have worked in the same place for 38 years.

[dovla]

Originally Posted by Wavebourn

You can try it with 6V6, but I will keep 6P30B-R in mine. Sub-miniature tubes are dirt cheap, don't require sockets, and are very reliable, being manufactured for space and military equipment.

Wavebourn, I'd like to replace the guts of a SS Fender practice amp with your 'micro-champ' but currently have no high-mu small-signal tubes. Can I get away with ECC81 in the same circuit while looking for the 'proper' tube (I believe 6n21 has mu of 70 and ECC81 is 60 so not that far away)? I can only find 6n16 on epay - do you have a source for 6n21? Also, did you ever post the latest schematic with the 'patent-pending two-pot trick'?

[costis_n]

Originally Posted by Printer2

Since it has been quiet.

Not the best thing that I done but it is getting there. Bit of work to do on the other side of the circuit board and then start wiring up to the tubes. I have jumper wires in place for the plate, cathode, and screen resistors for the preamp pentode, still have to play with the values and it beats swapping parts on the board. The couple of parts that are standing off the board are also temporary till I pick up the right ones.

Sorry, but I do not agree! If you tidy up the wires it will look very seksi. Chassis colour looks sooooo 70's!

[kevinkr]

Originally Posted by costis_n

Sorry, but I do not agree! If you tidy up the wires it will look very seksi. Chassis colour looks sooooo 70's!

I agree, looks quite good actually.. All that is required is a wire tidy..

[Printer2]

Still missing a lot of wires, I'll do what I can to keep it neat. The color was a tough one, I stood in front of the paint display for quite a while deciding on which paint can to pick. The color is off in the picture due to the light, it is much brighter, more like the tone in between the board and the over exposed bit.

[Printer2]

Played around with some 6AU6's in a 6V6 SE amp and think I found values I like. Lost the pentode/triode switch, it worked but you can roughly do the same thing with the volume control. Now I am thinking the versatility of the amp will be more with a bass and treble control rather than a Tweed type of tone control. The question is whether to put it before the 6AU6 or after it? Any suggestions?



Oops, cut off the left side of the circuit. Negative feedback from the output and a filter capacitor missing.

[ChrisA]

Originally Posted by Printer2

Played around with some 6AU6's in a 6V6 SE amp and think I found values I like. Lost the pentode/triode switch, it worked but you can roughly do the same thing with the volume control. Now I am thinking the versatility of the amp will be more with a bass and treble control rather than a Tweed type of tone control. The question is whether to put it before the 6AU6 or after it? Any suggestions?....

In that schematic, I think I see the tone controls inside your feedback loop. That is not supposed to work. If NFB is doing it's job it will "correct" out whatever the tone stack does. Same goes for putting a master volume control inside the loop.

So to answer your question about were to put the TS. Just before where the NFB loops back.

[Printer2]

Originally Posted by ChrisA

In that schematic, I think I see the tone controls inside your feedback loop. That is not supposed to work. If NFB is doing it's job it will "correct" out whatever the tone stack does. Same goes for putting a master volume control inside the loop.

So to answer your question about were to put the TS. Just before where the NFB loops back.

See what happens when you do a cut and paste. Good catch. Alright then, with no NFB (I was debating whether I really wanted it anyway) where do you see the tone stack?

[tubekit]

I agonized over where to put the tone stack for weeks. For one thing, I am working on a three-tube design (five gain stages in the bottles) that includes a spring reverb tank--a REAL tank, not an inventive arrangement of speaker parts.

And if there is no distortion control, then the amp is less than ideal, so . . .

Reverb has to go after distortion. Tone has to go after reverb, it seemed to me.

Most of my original ideas had one too few stages to pull it all off. That was before I decided to trust one stage to do two jobs (actually, three jobs, if driving the tone stack is one of them), and then proved the concept workable.

I considered all sorts of novel ideas but most didn't really appeal to me.

The most "out-there" of the potentially useful ideas was to control only the bass (and possibly mid) in the dry path around the reverb send-return and put the treble after the reverb recovery stage.

The reasoning was that the reverb send had to deliver current. So it could also drive a bass boost/cut circuit, and reverb is effective only at the mid and upper mid anyway, so perhaps putting the bass controls in the bypass around the reverb would be okay. But I later rejected this idea. All tone has to be in the same stage--after the reverb.

Well, long story short, and an announcement!:

I've got a design to submit. No circuit diagram yet, but the prototype is made and it sounds good!

Maybe it will make the price limit. Have yet to tally it all up, but it's close.

I'm using a 6EB8 (my one-tube amp tube) as a first stage (triode) and distortion driver, (with a Gain control between the triode and pentode). The 6EB8 pentode is the reverb driver and distortion producer. Out of the 6EB8 pentode with a low value plate load, the signal splits just past the Master Volume pot. Through a high-pass cap to the MOD reverb tank (4FB3D1B -- no tranny) on one side, and through a dropping resistor to tame the direct dry into the tone stack on the other.

The tank signal comes back to (and here's the beauty of this design, the tube!!!) a 6KE8!! This tube, which is an FM or TV mixer oscillator, has a pentode that can live on a fraction of a volt at the grid. (At less than one volt on the grid the plate current is already down in the cellar. Read: GAIN) VA is in the neighborhood of 220 when the reverb signal emerges on the output side where it mixes with the direct dry, and that mix is fed to the tone stack. A modified Fender three-way with some adjustment in the values gives a mid boost about as far as you could stand it--or drops it out completely in a nice phatt smile curve! The tonal possibilities of this amp are incredible! Especially since there is a treble bypass cap around the dry path attenuator that keeps the ching and clarity in there unless you dial it out with the treble control.

There's more reverb than I might want for some things, so there is also a pot that varies the amount of reverb return mixing back in from zero to "plenty". And it took some time and head-scratching frustration before I realized that with no dedicated reverb driver, the return was out of phase after going through the recovery pentode, which made everything oddly thin and trebly so that the reverb sounded very different than the dry. Solution: rewire the RCA out jack from the amp to put the ground in the center and the audio cable feeding the tank to put the center pin to ground at the amp end to reverse the connection at the tank end while keeping the shield grounded to the amp ground (whew!).

The medium-mu (40) triode of the 6KE8 takes the output from the tone stack to drive a 12A6 (to a 6K6 what a 6K6 is to a 6V6) single-ended into a 5W Edcor SE tranny with a super-linear screen tap.

This amp plays well with a perfectly clean acoustic guitar with a combo under-saddle piezo/soundhole condensor), when dialed in for clean, and goes far enough into fuzzy distortion to handle All Along the Watchtower, Jimmy version, or Stevie Ray's Pride and Joy. You want just enough overdrive for Johnny B. Goode? Okay. You got it.

You gotta hear this amp!

I'll have to make some recordings. Is an MP3 acceptable?

Later this week, the circuit diagram. The MP3s. The BOM.

Oh! Am I excited about having finally gotten the bugs out of this thing and realized the intent of the design!!! And just in time, it would seem.

[TheGimp]

I put my takeoff for reverb after the tonestack and second gain stage but before the third gain stage, and injected it right back between the second and third stage.

I've just about finished wiring all I can before adding the tagboard.

Progress has been slow as I stopped to make a cradle to make wiring easier.

The tube sockets aren't completely mounted, hence the odd angles.