I have a tube amplifier that I built with a Hammond 270EX power transformer. I understood most of all of this 40 years ago during school. I want to make an amplifier for a friend to play guitar and my question is two-fold:
1. Is there a more economical transformer than the 270EX?
2. I'm assuming that I really need to understand the power "needs" first?
I'm here for a while, so my first thought is (since I didn't design my amp's schematic) to design new circuits around my 12AX7 (two-preamps) and EL34(power) tubes. I want to design an amp that I could legally sell on order from guitar friends. And, back to the Hammond, are their other (cheaper) alternatives that I could look at even if I have to design based on differing power requirements? I know nothing about power transformers other than the curly lines on schematics and in textbooks.
thanks
David
1. Is there a more economical transformer than the 270EX?
2. I'm assuming that I really need to understand the power "needs" first?
I'm here for a while, so my first thought is (since I didn't design my amp's schematic) to design new circuits around my 12AX7 (two-preamps) and EL34(power) tubes. I want to design an amp that I could legally sell on order from guitar friends. And, back to the Hammond, are their other (cheaper) alternatives that I could look at even if I have to design based on differing power requirements? I know nothing about power transformers other than the curly lines on schematics and in textbooks.
thanks
David
When you mentioned EL34 power tubes I am assuming a pair in push-pull Class A-B. Yes you would need to look at the power requirements before deciding on the power transformer and output transformer as well. Are you planning on an amp with about 40 watts of output, or perhaps something more modest like 15 to 20 watts?
Maybe go down to a pair of EL84. The output transformer will be less costly and you may be able to save a bit on the power transformer too. The power tubes are less expensive than the EL34, so overall it make more sense to build lower wattage amps, keeping the quality up but reducing the overall cost. I think anyone looking at buying a hand built amp would expect to pay more than a mass produced amp, and if they`re not..well there's not much point in not making any money off building them.
Maybe go down to a pair of EL84. The output transformer will be less costly and you may be able to save a bit on the power transformer too. The power tubes are less expensive than the EL34, so overall it make more sense to build lower wattage amps, keeping the quality up but reducing the overall cost. I think anyone looking at buying a hand built amp would expect to pay more than a mass produced amp, and if they`re not..well there's not much point in not making any money off building them.
First, thank you for replying. I'm basically trying to spark my thinking and design my own amplifier. The one I have is called an AX84P1 Extreme with 12AX7 two preamps and EL34 power tube (just one). I think it's 15W. I can't find all my chassis layouts, bill of materials, and schematic. It was open source. Here's the closest I can get to what I have http://93.48.145.201/schematics/ampfactory-master/non-free/AX84/P1X.pdf . It really sounds good but I can't sell the amps because of copyright I think. I'm not sure if I copy the schematic myself and change what 1, 2 ... parts? Not sure there. But ... I'm excited about returning to my guitar (nearing retirement and may teach and play) stuff and maybe design a similar amp but see how much I can cut the costs. My wife seems to remember me spending about $900 but that was tools, PC board punch, chassis, new soldering station, wood for the amp, speaker (Celestion green back). I don't remember but I do know that I like the hand-wired in the chassis thing. I've done etched PC boards but this was much more fun and I realized that I could do what Leo Fender did in the 50's. What would would be your tube choice and number for a 15-20watt amp that could be clean and also have breakup (overdriven). I hear that EL84 sounds like Vox. I really like the Fender Deluxe Reverb because it can be clean and also have breakup. My Fender Twin amp actually is too clean for my liking overall. I do LOVE the clean part. And, I have the EL34 amp so my goal would be to make a 15/20 watt tube amp clean and overdriven-able and with a reverb, probably put a spring assy in the head. I'm all open to opinions. Thanks again for the reply.
I wrote the book (sort of) on the P1-EX (https://www.yumpu.com/en/document/view/17478109/introduction-to-tube-amplifier-theory-ax84com). You can sell the built amp for whatever price you want; the copyrights were on the schematics and build documents and were pretty liberal respect to redistribution (think an open-source model). If you spent $900 on the amp including tools and cabinet and speaker, you did okay. The kits from Chris back in the day went for about $300-350 or so, but that was bare chassis. Cabinet, speaker, face plates, etc. were all extra. Throw in tools to build it and you could easily triple that kit price. I built one without a speaker cabinet for about $500 or so some 20 years ago, but I had all the tools on hand (https://www.geckoamps.com/gecko002).
Stph
Stph
Stephen, thank for you for the info. I'm having Sandy (in Indiana, Precision Design) send me a nameplate and then I'm making an attenuator. I play and record at home into Apple's Logic and I want to put a mic in front of my amp because it sounds so good at exact settings. I'm going to enjoy the PDF because I really want to understand these circuits around the tubes. I would like to make another amp and also determine if I could source the power tube with a modern preamp (op amps). Anyhow, I'm excited to dig into all this.
I don't think of the power tube selection as sounding like one brand or another. Yes if the preamp section /tone controls and speaker are similar to a Vox design, it may sound similar. A single ended class A output tube (such as the one you built) may make more of a difference sound wise than a Push-Pull type circuit.
I think it comes down to personal preference. With push pull you need some sort of signal inverter stage so that would add on component count and some cost.. but it would in theory run more efficiently than single ended.
Stephen, I forgot to ask. Is it possible to find my old documents for chassis with dimensions and bill of materials? I tried and found some archive but nothing was working correctly? I want to draft an entire set of my own documents for "a" tube amp when I'm done with my design review to give to a friend in Virginia, lol doing PT and wanting to make his own amp and maybe even play guitar again.
Attached are the last P1-EX options page & schematic (with BOM and chassis layout). The chassis doesn't show dimensions, but you might try printing it at 100% without scaling. As I recall, it was drawn to scale. You might need a large-size printer to make that work.
Stephen thanks! If I do another even with the same stuff and chassis, I'll draft the plans and instructions. If I remember correctly, the chassis was a one piece buy, and then I looked at something and drew out dimensions on a sheet of paper and started drilling on the press. I remember the tube holes to be difficult because I think I used hole saws really low pressure lol. I want to run down all the materials and source them. I'm trying to convince a school friend that he "wants" to do this to get back to playing. He was in a bad accident and he gets sad that I'm so into my guitars now. I'm retiring and want to find something fun to be doing instead of sitting on my ar$e. Thank you again for the help. I'm studying for a work certification I don't need but reading this theory document this weekend. P.S. This is exactly what I worked from. Thank you thank you thank you.
One more thing: I know not to be putting my fingers in the amp when it's on or if I'm checking something with a meter or oscilloscope. But, are there any capacitors in there that need grounding that could shock me? I know about those big can capacitors but here?
If you don't have a bleeder built into the circuit, then the power supply caps need to be bled, It never hurts to bleed them even if there is a built in bleeder. You don't want to be the path to ground for any charge held in the caps.