• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Which amp to build for a class

Hi,

The Electronics instructor where I teach wants to run a class to build a tube amp and is asking for my assistance. I may have planted the idea in his head, but cant be sure :rolleyes:. In any event, we've come up with three options for an AB1 amp; bargain, mid priced, and full tube Williamson circuit amp. In tallying up the cost of parts, just the expensive ones, prices are about $250, $330, or $400. At least I think that is what my notes say. Most of the cost difference is in the transformers.

My question is, for an introductory electronics class is one a better option than another? People taking this class may not have any experience with electronics, or may be fairly well versed. I don't know the cost of the class at this point, but we would be providing the parts and there is a very noticeable difference in the "kit" prices. We'd like a good turnout so the lower price is attractive. We also want something that sounds respectable and provides about 8 to 10 watts. Any suggestions for which way to go?

We talked about building the PS on it's own chassis, and the low and high priced alternatives on their own chassis for comparison purposes. That sounds like a good experiment, doesn't it?
 

Tjj226

Member
2012-12-02 2:49 am
To be clear, that's a headphone amp (not for speakers). It has a transistor output stage, and no output transformers.

Yes. Hence why I said "go in the opposite direction".

I do not know the particulars of this class. It sounds like you only want to build 1 tube amp for 400ish bucks and have the whole class work on this 1 amp. Now if I am wrong, please correct me.

The starving student can be made for close to 50 bucks. You still get to learn about tubes and stuff. But now each student might be able to build their own amp to take home with them.

Yeah you don't learn about transformers, and yes its for headphones, but you do get to learn about mosfets, and who can't use a nice desktop headphone amp :D

Im just giving you alternative ideas.
 
That's a complex undertaking for a class where no-one has specific skills in electronics. Without knowing the actual instruction environment, length of the course, student profile etc, I make the following suggestions and observations.

If your aim is 10 useful watts and tube, I'd be inclined to do a tube input in front of a chip amp (tube as a buffer) or a mofo type board (tube as a voltage amp).

This would give ample opportunity to cover: the basics of electronics - what are the components, what do they do; calculations for parts specification and design including Ohms Law, frequency corner values, gain requirements etc; point-to-point techniques; PCB manufacture techniques (or not); soldering and practical build skills; enclosure construction etc.

It keeps the voltages relatively low and so safety much more manageable.

It reduces the fragile parts count, and also reduces the expensive magnetics count. If you go "mofo", you could increase the experience by hand-winding the required choke.

The key here is to have a project that is not only interesting but more importantly achievable for everyone in the class without the instructor spending the entire last 1/3 of the course personally troubleshooting and building the project for those who struggle.

Been there, done that.
 
I immodestly suggest that "El Cheapo" be considered.

Some points in no particular order follow.

While the safety consideration previously mentioned is valid, B+ at 45 V. pretty well restricts the project to "flea power" 3S4 O/P tubes.

A complete set of power magnetics for "El Cheapo" can be acquired from Allied Electronics at modest cost. Buying in quantity, for a group, brings the unit cost per set down. Allied stock # 70218190 is the Triad N-77U main B+ trafo. Allied Stock #: 70218145 (Triad C-24X) is the B+ filter choke. Allied stock # 70218344 is a Triad VPS-24-1800 dual 12 VAC unit that provides heater power and B+ boost (1 winding each). Allied stock # 70009000 is an Allied/Hammond 6K27VF trafo that provides B- and 12AT7 heater power.

Ultra-linear (UL) mode O/P tubes can be employed for approx. "12" WPC, if Edcor's CXPP25-8K O/P trafo is selected. Restricting to "6" WPC triode mode "finals" allows Edcor's lower cost GXPP15-8K O/P trafo to be employed. FWIW, the "Gold Standard" in "El Cheapo" O/P "iron" is the Dynaclone Z565, but costs go up.

Any member of the extensive 6V6 tube family, including 6CM6 and 12AB5, works with the parts values shown, along with the 'AQ5 shown.

If a "standard" CDP is the signal source, no separate preamp is needed.

All around good guy Jim McShane carries kits of parts for the project.
 

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I immodestly suggest that "El Cheapo" be considered.

I second your suggestion. I think the OP needs to set a limit on parts count to 25 parts, ElCheapo qualifies for that. Personally I wouldnt be attracted to a class making a PC board amp, why, because its not old-school-fun, and the students should have to check and double-check wiring a point to point project. And even have their lab partner cross check things.

You'd probably want the chassis all pre-punched, after all its not metal shop, bread boarding it is probably too dangerous. (I guess those two points argue having a PC board that they just screw to a piece of pine). Making a few separate power supplies to go around would save a lot of time and add a lot of safety. As you decide who gets some juice and when. A variac with dim-bulb outlet would be handy too.

When they take the amp home they can make their own power supply at the end.

I took a 5 watt Champ guitar amp making class once, it was fun (but overpriced), the chassis was punched already. Each student applied initial power one at a time under supervision.
 
Headphone amp maybe?

Is a headphone amp under consideration? How about the Cavelli SOHA from the old Headwise site? It's a hybrid, 12AU7 gain stage and OPA2136? output. Will drive anything but planars and electrostats. Powered by a 15VA, 12volt transformer, B+ about 60 volts. Uses mosfets for CCSs. Can be built on a proto board and a cheap eBay chassis from the Orient would make it domestically acceptable. Probably all in could be built for less than 100 bucks.

Steve
 

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Thanks

Thanks everyone for the replies so far. Let me clarify things a little. I am making Frugel-Horn flat packs and hope to tie into this amplifier class with a speaker building class of my own. So we are looking at amplifiers that will easily drive an Alpair or Fostex driver in a FH enclosure. We hope to market the class as build your own "complete" stereo system. Or at least the heart of it. We can accommodate 5 or 6 participants per class.

Each participant will be taking the amp home with them. I believe we will be doing most of the mechanical work ahead of time; how much is there to learn by drilling or punching holes in metal or wood? The Fab Lab (where we work) is very successful with our Luthier program and I think this might be a natural fit for us. We also have a recording studio available for testing speakers, with an omnimic on the way.

I'll show the El Cheapo amp to Mike and see what he thinks. He is a retired engineer and has a lot of experience, but not necessarily with hi-fi stuff. Most things he talks about go right over my head.

My original thought was to build the Amp Camp amp and therefore was hoping to keep the cost of the parts in the $325-$350 range. If you want to check out the Fab Lab, here is a link: Fab Lab | Lifelearn
 
So we are looking at amplifiers that will easily drive an Alpair or Fostex driver in a FH enclosure.

FWIW, the approx. $150 Fostex FE206NV driver is claimed to be 96 dB. sensitive. Applying Paul Joppa's 102 dB. rule to that 96 dB. sensitivity tells us that 4 WPC will be sufficient in most listening spaces. Therefore, the approx. 6 WPC produced by a triode wired "El Cheapo" will be fine.

As magnetics are where most money is spent in building a tube amp, let's run a magnetics "tab" for a 6 WPC triode wired "El Cheapo". $94.26 for 2X Edcor GXPP15-8K O/P trafos. $32.43 for a Triad N-77U. $8.21 for a Triad C-24X. $16.81 for a Triad VPS24-1800. $16.83 for an Allied 6K27VF. $168.54 is the total magnetics cost. IMO, you're very much within budget limits.

Less sensitive speakers will require the 12 WPC UL mode "finals" deliver. O/P "iron" costs increase. Still, that $350 figure seems doable, unless Z565 O/P trafos are desired.

"El Cheapo" is not a toy. Built with Edcor O/P "iron" it's nice. Built with Z565s it's more than nice.
 
Building a chassis that looks presentable is always a tougher task than most beginners realize. I'd recommend starting with an existing kit, maybe something like this:

JBH 6N1 6P3P Tube amplifier HIFI EXQUIS Single Ended DIY SET or Finished 6L6 Lamp Amplifier JBH6N16P3P-in Amplifier from Consumer Electronics on AliExpress

It can be built as a single ended pentode with global feedback, single ended triode with no global feedback, or creatively strapped to be a monoblock. Parts are straightforward and can be upgraded (better caps and resistors, tube rolling, prettier power switch and knob) if desired. I bought one (not from this vendor, but it looks exactly the same) and it comes very well packed. The iron is OK, not great, but for a beginner, it's an easy path to success.
 
If you are going to have to try to explain how the amp works, a single ended amp will be far simpler to deal with. If you're going to have your students scratch build these amps, do as much prep work before class as you can. Mount all the hardware and get everything ready to solder before you start.
 
Thanks Eli, I was thinking of the FHMk3 and a 5" driver. Using your example above the FE1246En at 93 dB sensitivity would borderline or ok for the 6wpc El Cheapo. I know that 3dB requires a doubling of power to reach the same volume level. We don't yet have a FH cabinet for an 8" driver, but I believe that Dave et. al. are working on it.

Applying Joppa's Rule to a 93 dB. sensitivity yields an 8 WPC requirement. In a small room, 6 WPC could work out, but I wouldn't run the risk.

The "package deal" amp/speaker cost will be quite similar, if you pair FE1246En drivers with 12 UL WPC. What is saved in driver costs goes to an increased "iron" expenditure. TANSTAAFL!

If you go with Edcor's CXPP25-8K O/P trafo, consider adding UL/triode mode switches (1/channel) to the setup. Triode mode is more "elegant" and the reduced power O/P should be adequate for music like small Jazz combos or string quartets. OTOH, "head banger" Rock or 19th Century Romantic "potboilers" (think Berlioz, Liszt, or Suppe) will need all the "grunt" you can muster. Never, ever, change a UL/triode mode switch's position, while the amp is powered on.
 

6A3sUMMER

Member
2016-06-07 6:50 am
The difference between 6W and 8W (1.25dB) does not follow one of the old rules.
Given two amplifiers of the same quality, and you are considering moving up in power:
Purchase an amplifier that has at least 2X the power of what you have (+3dB), or keep the one you have.