Low Voltage Tube for Learning?

[AndrewD]

I'm getting into tube amp building and have a couple of books, but I learn best by actually doing! I've been researching low-voltage tubes to do basic experiments with. I figure that should keep me a bit safer, use a cheaper power supply, and not blow up the oscilloscope I'm learning to use at the same time!

I'm basically looking for the lowest power, simplest amplifier that still resembles how a "regular" guitar amp works. Don't care how loud it gets or fidelity. Just a platform to test changing component values and see if I'm calculating things correctly based on the schematic. 

Something like an ECL82 looks pretty cool as it has a triode and pentode, so I could experiment with both. That said, no idea if I'm about to run into a brick wall with that...

Does anyone have recommendations for tubes and power supplies? What's the smallest plate voltage I can get realistically away with? (Wild card option and wrong forum, but are there are solid-state circuits that would also give me a good feel for how these amps work?)

P.S. I know even low voltages can be lethal, so I'll still be careful: I won't lick it to see if it's working! I'm not an electronics whiz, but should know enough to keep myself safe.

Thanks all!

[kagliostro]

Ciao AndrewD
In the old times there were vacuum tubes planned to be used with a low B+ voltage (on car radios)

today to find such tubes may be not so easy, however is possible to use also usual tubes at low voltage (performance aren't at the best, however, if you want to experiment ...)

Here some link that can be of your interest

https://www.nutsvolts.com/magazine/article/beginning-to-build-with-tubes

http://www.junkbox.com/electronics/lowvoltagetubes.shtml

http://beavisaudio.com/projects/valvecaster/

https://hackaday.com/tag/vacuum-tube/
 
https://audioxpress.com/article/a-portable-tube-preamp

https://hackaday.io/project/188788-diy-simple-pcl82-low-voltage-12v-tube-amplifier

https://www.qsl.net/kl7h/12v.htm

Here a project to feed at "High" voltage tube projects with low voltage trnsformers

http://solorb.com/elect/hamcirc/tubesupply1/

Franco

[bmccowan]

but I learn best by actually doing!

I do the same, so thought I'd add some opinions.
I would suggest a somewhat different direction. Because working with something unusual you will find it harder to get great help like you can get on this Forum and elsewhere. Many will say start with a kit, but I am not so sure that is the way either if you are seeking to learn.
Circuits - can/should be learned with no voltage. Whether you draw by hand or use software like ExpressSCH (recommended) understanding circuits can be done by drawing and modifying schematics. Rob Robinette's site has great explanations of schematics.
Taking your schematics and drawing a layout - DIYLayout is a great tool for this, as is VISIO because you can get Sluckey's help.
Physical circuit building and soldering - again done with 0 voltage. I would suggest buying a small turret board from Doug, some buss wire, a few components, and go at it.
Building - ECL82 is a great tube - you are right. But I would start with simpler tubes. 6V6 would be a good choice. Used in more amps than just about any other tube. Still made today. And they sound great. I would choose a well known circuit based on those. No reverb, no trem, to start. One of many singe ended amps - the term "Champ" is the "Jell-O" of the amp world, but there are many others. When ready you can download a bill of materials and buy what you need from good suppliers like Doug. I'm not suggesting a kit, because they can be kind of paint-by-numbers. You end up with an amp, but maybe not the same degree of learnin'.
Oscilloscope can be learned post your first build IMO.
Solid State? A different beast.
Most of the new builders here who end up with a noisy mess, started building before they learned the basics. You are starting with reading - that's great. What books do you have? Folks here have good recommendations, no doubt.

[thetragichero]

this is certainly a good place to start the book learnin`: https://www.hnsa.org/manuals-documents/2575-2/
this breaks down 'circuit blocks' you'll see in a lot of amps: https://www.valvewizard.co.uk/

i found i did my best learning with both the book stuff and modifying existing stuff (what happens when i embiggen this cap? sorta thing)

[acheld]

I'd echo what bmcowan is saying, with a twist.

Amp building is really an art of the whole.   Most decisions you make have an effect on something else in the amp, and it's the ability to see ahead in the build that makes the difference.

So, when bmcowan (softly) advises against a kit -- if the goal is to learn -- he is giving great advice.

There is a middle path with Hoffman's "kits."   These are not the same thing as fully provisioned kits like you would purchase elsewhere.    You will have source some parts separately, but Doug's BOM provides great guidance.    You will have to make decisions.   

For someone new to this sport, I do suggest using Doug's online store for parts, as they are known to work in the project.   Things like lead length, size of the cap, etc, are good.   And, not expensive.

Not to mention that this forum knows these projects inside and out, and will give great advice. 

The point I'm trying to make in this long post is that the "art of the whole" comes from making decisions that you just don't make when building a fully configured kit.   Oh yes, there will be mistakes!  But that's ok, or even a good thing if you learn from them.

[Lectroid]

Hey Andrew,

You can certainly trust all of the above advice. All of the responders have helped me learn over the last few years with tons of good advice and patience with my endless questions.   

[AndrewD]

Whoa, thanks all for the great replies. The links are greatly appreciated, and I'm going to spend some time with them. A good chunk I've already visited, so good to know I'm on the right track!

This all started basically as a way to make a cheap Deluxe Reverb (oh, how naive!), but it's now more about understanding the theory; not just tubes, but electronics in general. I've dabbled a lot over the years (arduino, raspberry pi, etc.), but there are some big gaps in my knowledge of the fundamentals I'd like to fill. And when I say "theory," I don't mean becoming an expert at calculating things from a schematic, but for getting an intuition on how the circuits work so I can mod them.

I've actually had a bit of a journey deciding what to build first. As mentioned, my end goal is to build a Deluxe Reverb. I wanted to start with something simpler, so I ordered all the parts for a 5e3 build (via, the store on this site, mouser, etc.). I got as far as making the turret board, but then saw the Champ Micro on Rob Robinette's site and the EF80 version is exactly what I was looking for in a first build. Relatively simple, low power, etc. I'm mostly a bedroom player at this point anyway.

So I'm scrapping the 5e3 idea... I did some research and it looks like I can use the transformers I already ordered in a DR so the parts aren't wasted. At the rate I was going, I was going to have like 10 amps which seems a bit excessive, haha. I'm sure others here may disagree though!

My current plan is to build the Champ Micro and then do a big jump to the DR. I figure between learning from mistakes on the Champ Micro and spending extra time on the fundamentals, I'll have a good chance at getting the DR done with reasonable quality. I think the plan is a bit ambitious but doable if I take it slow. If the Champ Micro doesn't go well, I'll re-asses =)

I also plan to make my own chassis and cabinet. I've built a cabinet before and the chassis is just for learning/to say I've done it.

Factoring in all the great advice on this thread, I think I'll do my testing with Champ Micro. There's no reason I couldn't wire up the pre-amp section and try to run the 12AU7 with a low B+ and see what happens. Or even swap in a diff tube that will take lower plate voltages that has the same pinouts.

As for books, I have Richard Kuehnel's Basic Theory book and Robert Megantz's Design and Construction of Tube Guitar Amplifiers. I've also spent a lot of time on Rob Robinette's site and Uncle Doug's channel on youtube.

It's seriously amazing to me how many great resources there are out there and the passion for sharing and helping in the community. Well done! Thanks again everyone.

[bmccowan]

Good plan I think. I have a Micro EF80 amp on my thought list too, likely a push-pull with a Gibson or Valco preamp.
Good books too. I also like the Merlin B preamp book, but honestly I have to read sections slowly, think for a bit, and then read it a few more times.
There are some other good YouTube channels - some less annoying than others. Fazio is entertaining and helpful. And they get progressively annoying as you sift through Mr. Carlson, D-Lab, and the Misogynist, oh sorry I meant the Guitologist.

[sluckey]

Low voltage tubes are not what you need for learning how tube circuits work. Low complexity circuits is what's needed. The champ, even though it uses voltages around 400V, is a fine example. Low power, low complexity. Besides, getting stung by the champ is a learning experience that carries a valuable lesson that will stick with you much longer than Ohm's Law.   

[PRR]

Most amplifier confusion is not about the tubes/transistors, but BASIC OHMS LAW.

If I ran an amp boot-camp you'd do 200 Ohms Law problems before breakfast.

You say you "have" Richard's Basic Theory. Have it, read it, digest it.

[AmberB]

Also, there are sever amps that you can find schematics for that use the EL-84 output tubes, which have a lower plate voltage than a 6V6, so if you're looking to build an amp with a lower voltage, that might be something to consider.  250 to 300 volts on the plate of the output tube, or tubes.

[kagliostro]

6aq5 are low voltage 6V6, 250v B+

Franco

[HotBluePlates]

Most amplifier confusion is not about the tubes/transistors, but BASIC OHMS LAW.

If I ran an amp boot-camp you'd do 200 Ohms Law problems before breakfast.

When I went through the Navy's electronics training program, one of the first things we did was write a narrative version of Ohm's Law as Current = Volts / Resistance 100 times as homework:

   "The Current in a circuit is directly proportional to the Voltage across the circuit, and indirectly proportional to the Resistance of the circuit."

Then they had us do a ba-jillion Ohm's Law puzzles as found in NEETS Module 1.

... EL-84 output tubes, which have a lower plate voltage than a 6V6, so if you're looking to build an amp with a lower voltage ...

6aq5 are low voltage 6V6, 250v B+

You're both correct that these other tube type have lower maximum plate voltage ratings.

But we can always run the 6V6 with 100v screen and plate, as shown on Page 4 of the 6V6 data sheet.  The consequence is much-reduced power output (but people today are using a dizzying array of attenuators, load boxes or re-ampers to reduce the loudness of amps even as small as. a Champ).