The Hundred-Buck Amp Challenge

[pinkmouse]

Hunoz, you'll never get in trouble with the mod team for constructive posting regarding safety.

[flatheadmurre]

Hunoz, I think the challenge for most people in this thread is to take some $1 RF/TV tubes and torture them into sounding like some classic amp they played on.

It has inspired me to try but i have to confess that the thing has beaten me 5-0 and right now it´s sounding like something the cat dragged in.

But to me the journey is far more interesting than the non existing chance to beat the more skilled people in this contest.

[Printer2]

Originally Posted by hunoz

I was not suggesting that the challenge was bad or that it had no benefit. It has many indeed. Having said that an amp that is an interesting project and one that working on it many things can be learnt from is not necessarily one that is worthwhile to build copies of especially beginners to amp building.

While on that note: the forum members may forgive me for being an old timer and that I grew up with high standards regarding safety while working on a project with high voltages. As such it gave me cringes seeing some of the photos in the posted pics with of lots of loose wiring, loose temporary connections and other unnecessary items on the bench while building and testing amps. (I am not pointing the finger at you, I am talking generally!) I did not even see a safety isolation transformer used on any of the pics. An Isolation safety transformer should always be used when working on any tube amps or where high voltages running in exposed wiring. If there was one somewhere in the pics, I missed seeing it. I think readers who are not aware of the dangers of this beautiful hobby may interpret what they see in the pics as safe practices. Go forbid someone having an accidental shock or much worse!

Generally speaking, while not up to the standards of days gone by, it does allow me to try out circuits rather quickly. The loose wiring is to allow me the option of changing circuits and positions on my breadboard, if I want to add a stage I have the wire length to do it. one switch and two pots are not being used so the wires are bent out of the way from the live circuit. It would be nice to have shorter lead length on the parts in the board but since I do not have an unlimited supply of parts I do not clip the leads unless they actually go into a finished circuit.

The temporary conections such as to the output transformer are just that, temporary. Three transformers in the picture, one output, one for the heaters, one for the high voltage. I would assume they are isolated enough from ground even though I have not hipoted them. The messy parts laying around, mainly parts that I swap in and out to judge the effect on the circuit. It would be best to have a spot for them and to mark down the change that they cause, when I get time to build a proper workbench that may happen.

So for all you newbies, do play safe. I do not go changing parts on a live circuit and I always check to see if there is any voltage stored in the circuit before I do any changes. I have loose wires running out from the key points I want to measure that I clip onto rather than sticking my scope or voltage probe into the parts on my breadboard. With the long lead length of the parts in the breadboard it is too easy to accidentally nudge one over and touch another part of the circuit. Mind you, while the photo does not show it well, I do allow enough physical separation between parts and wires to limit the dangers and the interaction of the circuit.

[TheGimp]

Originally Posted by hunoz

...snip...
I am not here to make enemies, so I apologise if my concerns came across as me telling you how you should work in your garage. It was not my intention.

No offense, I'm too touchie this morning.

That said, I have never seen anyone here advocate building the old 'death-trap' amps with direct line strings of heaters and un-isolated B+.

Every time someone comes here with such a schematic there are several posts advising NOT building such circuits. People wishing to use old radios with such circuits are advised to purchase an isolation transformer to provide the necessary safety with such circuits.

Regretably, pushing isolation below bench out of the way leaves it out of sight giving the impression of a less safe environment.

[Printer2]

Originally Posted by TheGimp

No offense, I'm too touchie this morning.

That said, I have never seen anyone here advocate building the old 'death-trap' amps with direct line strings of heaters and un-isolated B+.

Every time someone comes here with such a schematic there are several posts advising NOT building such circuits. People wishing to use old radios with such circuits are advised to purchase an isolation transformer to provide the necessary safety with such circuits.

Regretably, pushing isolation below bench out of the way leaves it out of sight giving the impression of a less safe environment.

I picked up an old 'American 5' radio, thinking of using it for this build. I did modify it a bit and I am sure it will go through another change or two. Propped up a 8" speaker in position, might loose two of the knobs for switches though. Here is the parts of the radio I am keeping.

[TheGimp]

Nice cabinet, it looks like it would lend itself well to a guitar amp.

You could remove the feet/relocate them, and turn it with the knobs on top.

[Wavebourn]

Looks good! Plenty of knobs and a hole for jack.

[Printer2]

Originally Posted by TheGimp

Nice cabinet, it looks like it would lend itself well to a guitar amp.

You could remove the feet/relocate them, and turn it with the knobs on top.

Nah, then it's roots as a tabletop radio would all but disappear. I already cut the wood strip that held the knobs and tuning scale and repositioned it to the right side. I have a sort of vintage looking grill material for it, including some artificial 'weathering'. It would be easier if I were to go open back, since I want a sealed speaker I will have to be a little creative with the chassis.

[MrResistor]

I have been following this post with great interest. I think one point which may escape some people is that everyone wins from this thread. Its true that different styles of playing benefit from different styles of amps. That is exactly what you will have here. Regardless of the "winner", there will be many amps with many sound clips to choose from. How great is that? Build the amp that suits your style! Variety IS the spice of life!

[Wavebourn]

Originally Posted by MrResistor

Regardless of the "winner", there will be many amps with many sound clips to choose from. How great is that? Build the amp that suits your style! Variety IS the spice of life!

Right!

But it is hard to choose.

Toying yesterday with super-dirty overdrive of a single ended output I decided to "improve" it in this particular regard. Additions included EQ in preamp to attenuate fundamentals of mostly low frequency strings, and to add an overdrive tone control. Here is the schematic attached. It sounds less nasty when overdriven, but did I loose some bottom end on clean sound, as the result? I don't know yet, because instead of a speaker box I use a small baffle with 12" driver from GE console.