The Hundred-Buck Amp Challenge

[TheGimp]

I'm still fighting my cab. For the past week I've been trying to figure out how to mount the chassis now that I've assembled the cabinet.

I should have mounted it before gluing the cabinet!

So, "Aftert-Thought Engineering" was called for help.

I finally figured a 12" long drill bit would be required. I drilled the four corner holes in the chassis, and a small wooden block that fit the corner. Clamp the wood guide in place, slide drill through it and the chassis to get a perpendicular hole. Four holes drilled.

Next I have to drill a 3/4" countersink and glue in "T" nuts. Now I can start thinking about the chassis layout.

[jonnyeye]

Originally Posted by Printer2

Curb my enthusiasm? It is good to see that others have gone down the same roads with mixed results. It makes it interesting. It is having to strip my kitchen floor that curbs my enthusiasm.

Haha, great!

Originally Posted by tubelab.com

I have thought I discovered some new tube circuit only to have someone send me a copy of my nifty idea printed in a very old book. I thought I had invented a really cool cathode follower idea until I heard from the holder of the original patent issued in 1957!

I have used this PI / output circuit too, in fact it is in one of my AMP1.0 designs. I have found that it will oscillate if you use a small cathode bypass cap (10 to 25 uF) like typically found in guitar amps AND a crummy OPT (like the Parts Express unit) that has a low frequency roll off and associated phase shift. I didn't come up with that idea either, I found it in a schematic on the web.

I have been told that all possible tube circuits have already been done. I was beginning to believe that until I built a class H design with the plate supply varrying in real time controlled by a DSP. They didn't have DSP's in the vacuum tube glory days.

So you're prestealing my ideas too? I don't have any goal of designing anything particularly new, at least not yet in my solder-slinging career - especially since I'm about 50 years too late to the tube party, and I'm working mainly on guitar stuff at the moment.

I checked and it turns out that I did not install a cap across the output tube pair's cathode resistor - so that lowers my chance of oscillation from low frequency phase shift, but also shifts the operating point about. Sounded fine as-is, but I'll try it later with one in to see how it changes the tone (I have some 220uf axial caps around that should do the trick) - thanks for the warning!

scitizen17, I listened to your amp clip this evening - it's pretty tasty, great cleans. Good job!

Also tonight bought chassis and cabinet materials, and found out that 4-40 hardware can be tough to find sometimes...

[TheGimp]

4-40 and 2-56 I go to Ebay for.

Here is the counter-sink for the "T" nuts and the cab with the chassis positioned.

[djgibson51]

Ok, cards on the table time.

Thanks for sharing. The variety of approaches in this thread has really expanded my design thinking. I've tried to reciprocate by putting my schematics up as they become viable. (New one coming soon)

[ChrisA]

Originally Posted by TheGimp

I'm still fighting my cab. For the past week I've been trying to figure out how to mount the chassis now that I've assembled the cabinet....

I like to have the chassis slide into the cab like a drawer. I make a kind of rail out of wooden strips. Then I have a wood panel that screws on and keeps the chassis form sliding back out. No mechanical fasteners. The chassis is held on all six sides.

[costis_n]

OK, so question time.

I wired it so G2 is fed through a 56K resistor from the second supply capacitor. BUT the power output on continuous sine wave was too low, like 5 watts. I saw on the scope an inverse input signal on G2l, which means that i got negative feedback. So far so good.
I then connected a 47K dropping resistor from the 3rd supply capacitor, which is after a 10K resistor. Less feedback, since G2 supply is now stiffer, power is 10.6 Watts.

The question is: I want g2 to have compression that is progressively heavier, like only on power output peaks, not linear like it is now. What to do?

PS We are talking about a GU50 single ended, with 480V HT and running at 34 W dissipation

[TheGimp]

Originally Posted by ChrisA

I like to have the chassis slide into the cab like a drawer. I make a kind of rail out of wooden strips. Then I have a wood panel that screws on and keeps the chassis form sliding back out. No mechanical fasteners. The chassis is held on all six sides.

That is an interesting idea ChrisA. Unfortunately with the controls up it would require the channel to be vertical. Sliding the chassis down would impinge upon the speaker frame, so the speaker would need to be removed before removing the chassis.

That would probably be a good technique for slope mount controls so I may try that on my next build if I can figure out the clearances for everything.

Mounting vertical with controls up allows me to mount all the tubes vertical down and transformers back. Three sides used for mounting should make layout easier. The downside is there will be a back panel on the top about 8" high. I'll need vent holes, and a mounting plate for the power cord and a fuse holder, etc. I could mount them on the chassis but access would be restricted.

[Cassiel]

Originally Posted by jonnyeye

Haha, great!

scitizen17, I listened to your amp clip this evening - it's pretty tasty, great cleans. Good job!

I'm going to do some playing and recording on Friday BUT I shiver with dread at the possibility of getting positive reviews. I'd rather not post my clips if that's going to be the case. Positive reviews are BORING. OTOH the destructive force of extremely negative reviews could get me down and impair my musical career. LOL. To be or not to be....she loves me, she loves me not.

[tubelab.com]

So you're prestealing my ideas too?

Yeah, I stole it, but.....I put it back! I have abandoned the connection from the plate of one output tube to the grid of the second. I am now using a self split arangement with a 150 ohm shared cathode resistor and no bypass cap.

I experimented with "your circuit" because the ultra simple design (about $40 total amp parts cost) didn't have enough gain. The plate to grid connection did bring up the gain but I didn't like the sound, and there still wasn't enough gain for overdriven distortion. I have decided to add another tube to really boost the gain. So now my $40 amp is a $45 amp. That design is being built now and I will post the schematic if it works.

Remember how this thread got started. An Ebay seller of a $69 tube guitar amp kit posted info about his kit in a few threads asking for ideas and circuits for low cost guitar amps. He stated:

The lowest cost guitar amp on the planet.....Only 69.95 COMPLETE! (Except for the speaker and the chassis)...... You can't build a guitar amp for less than this. Not no way. Not no how.

I replied:

Wanna bet? Sounds like a challenge to me. Anyone else? What kind of guitar amp can you build for $69.95?

The OP of one of the threads started this challenge with his $100 criteria which brought some criticizm from the Ebay seller. Most of the posters here have been working towards that goal. I have been working on the best amp that I can make for $99.999999 too.

I decided that since I was the person to say the 2 words that started this, I would also work on an amp that would fit into the $69.95 criteria. The OP declined to publish any info on his design, but we know it uses a 6EB8. I wired a 6EB8 up as a triode gain stage, pentode output stage using a typical guitar amp circuit and played with it. It can be built rather cheaply but there just isn't enough gain.

My $40 three tube circuit works much like the 6EB8 circuit and fulfils my "so simple that even a drummer can build it" criteria. Both designs however do not have enough gain for super overdriven distortion without an external pedal. The 4 tube design has not yet been tested, so we will see......

[Printer2]

A little farther from the $100 limit again, replied to another discussion and did a copy and paste of a couple schematics coming up with a composite of two designs. Circuit needs to be cleaned up but just wanted to show the general idea. Should be familiar from my previous schematics but a quick rundown.

Switch for tone stack, three position double pole. BF Bass/Treble with 22uF first stage bypassed, less mid cut no bypass, tone stack disconnected.

Three pole two position switch, puts tone stack between first and second triodes (clean Fender) or after second triode (Bassman style, sorry no cathode follower but might go Mosfet at some point).

Preamp B+ voltage switch. Lower voltages as earlier amps or higher for more cleans.

Four pole three position switch, Changes the two triodes from cathodyne, long-tail pair, and paraphase inverter.

Switch for feedback.

Switch shown for increasing output tube cathode capacitance but could quite easily go to fixed bias instead.

I'll find out how viable some of these are on the current builds I am doing, eventually want to add reverb also.