Valve amplifier for stompboxes

[luap77]

I did go to AX84.com only to find the core projects page under construction. I was also put off by some of the numerous qualifications etc I'm very sure that it is a great resource and that I should investigate it in more detail. Certainly a website that could occupy many hours, days, months etc!

I am just hoping for a simple well-sorted build that somebody has actually made and verified and that sounds good. I'm not considering a kit as i've come across a both of NOS valves (about 4 each of 10 different types of common ones) and also have a few slavaged transformers and a suitable chassis that i'd like to try and use.

So, were I'm at is:

1) Still looking for a low wattage tube amp build (A P1 sounds OK, but I'd like a verified version if possible), or anything below 50W

2) Looking for good kits available to Australia. I had a look at evatco and some others, but the use of low-end Chinese tubes put me off

Whatever I do, I'm going to need good documentation. Step-by-step is not necessary, but a error-free schematic and layout is. Any additional notes on bias values etc specific to the build would be a bonus.

Thanks again!

Paul E

[geertjacobs]

At ax84.com you need to go to the "Legacy Projects" and look there.
The P1 is verified by hundreds (thousands?) of builders (including me) so it's very safe to say it's a verified design.

The P1 theory document is one of the best (the best?) introduction you will get on tube amps, so read it at least three times before looking anywhere else...

Your post isn't very clear on whether you want a kit or not, but kits are available from http://dobermanamps.com/

[alextheian-alex]

At ax84.com you need to go to the "Legacy Projects" and look there.
The P1 is verified by hundreds (thousands?) of builders (including me) so it's very safe to say it's a verified design.

The P1 theory document is one of the best (the best?) introduction you will get on tube amps, so read it at least three times before looking anywhere else...

Yes, it is under the Legacy projects.   Core projects are up... power amp and preamp, but the amp+preamp page is not up yet... the P1 will go there--it is still under Legacy.  The BBS there is  great resource... just search and you'll find the answer to just about any tube related question.  Ii have personally built almost every amp design at AX84.  The main site is undergoing a formatt change, so there are a few links that have not made the transition yet.  The P1 is the best way to get your feet wet in tube amps audio... just 2 tubes, one preamp tube and one power amp tube.


here is the theory document:  http://ax84.com/?pg=associateprojects&project=p1&file=/media/ax84_m35.pdf

Build Guide: http://195.178.227.103/ax84/media/ax84_m60.pdf

and reference schematic for the thoery document: http://195.178.227.103/ax84/media/ax84_m60.pdf

[frickecello]

You should read some basic electric circuits theory about valves, power/output transformers, its limitations, its dangers, etc etc... and understand WHAT is every component in an amp design doing, if you not understand it first, it will be the same as walking blind, Im not saying it cant be done, but it is easier if you see the road... High voltage, (as well as low voltage) can kill you, or injure you...

And I think that you should think about the sound you want to get or first know the sound that you will get from an all valve amp, some people think that valves can achieve modern distortion by themselves, for example think of a jcm900, lots of guys think that the metal distortion it makes come from an "all tube preamp" and this is false, it makes use of clipping diodes in a bridge rectifier arrangement at the preamp section, so think it first... and there will be no "step by step guide to make a perfect electric guitar tube amp", you have to read and understand others designs and grab ideas to then build what suits your style and needs.

Regards

[luap77]

Thank you to all who replied. In particular, thanks to alextheian-alex for the links. I will certainly be heeding the advice to study the literature and know what i'm doing before i mess with lethal voltages etc

Much appreciated.

Paul E

[alextheian-alex]

Thank you to all who replied. In particular, thanks to alextheian-alex for the links. I will certainly be heeding the advice to study the literature and know what i'm doing before i mess with lethal voltages etc

Much appreciated.

Paul E

Sure.

Consequently, not all tubes have to be run at lethal voltages.  There are 'space charge' tubes that can run down to a dozen volts, as well as a handfull of other tubes that were originally developed for use in battery powered radios and cars... yes, cars had tubes in them at one point in time (i have a very nice pair of triodes from a caddilac).  There are also subminis and hearing aid tubesthat run with filament supplies down around 1v @ 20mA, and plate currents in the microamps!  You'll have to buy NOS (new old stock) versions since they are not made any more, but they are usually cheaper than a Radio Shack opamp!  There are also a few currently manufactured tubes that work well at low-ish plate voltages.

you can experiment with "starved plate" operation for the tube.... which is just another name for running them at low plate voltages.  You can't look on the data sheets and just copy down a circuit for starved operation, but you can mess around until you get some interesting sounds.  Not the way you are supposed to run them, but hey.. I'm sure that a 4558 opamp was never *meant* to be driven into distortion either!

[d95err]

Tube amp voltages are nowhere near as dangerous as website disclaimers make it appear (those are for legal, not practical, purposes). Only small children or people with a heart condition would be in any real danger. Typically, accidentally touching the B+ (high voltage) would get you a bad sting and perhaps a small burn. Changing a lightbulb also requires working with exposed high voltage leads. Don't be afraid, just have respect for it.

If you go for a kit with good instructions, you don't need to know all about tube electronics theory beforehand. You'll learn as you go along. If you try to learn all the theory first, chances are you'll never get started.

[zjokka]

Sorry to budge in and go slightly off topic (but only slightly) ...

I only started rewiring my guitar to prepare for active circuits (stompboxes),
but see that as a stepping stone to building a small wattage tube amp.

Now I build some boxes that went ok, I feel the next step approaching, but was thinking:

Wouldn't the cheapest tube amp project not be taking a tube radio and converting it to a basic guitar amp? portability is not an issue at this point, just to play at home and as tube exercise. Do have the P1 docs as tube introduction bible.

If I'm not very mistaken Keith Richards very first amp was a converted radio, and I suppose he was far from the only one? Tried to google but didn't find any 'convert tube radio to tube amp' sites.

Or would this be more dangerous?

thanks
j

[jaytee]

I just did this with an old PYE radio. The radio cost me £3. All I added was a 12ax7 and a few resistors and capacitors. It already had an output stage with an el84 and a ez81 rectifier. I already had the 12ax7 so it probably cost me about £7 including buying a schematic for the radio. Sooo cheap lol. It sounds really nice, articulate and musical and crunches nicely when it's full up. I also have the capacity to add more tubes if I want.

[alextheian-alex]

Yes, you can convert an old tube radio, but many of them are AC-DC radios which take the high voltage right off the wall and rectify it, which provides no safety ground and the possibility of high voltage on the chassis->into the ground bus->into the guitar cable->into the guitarist!!  Best bet is to convert a small HI-FI power amp, but by now they all need new capacitors, and they are usually free-wired, which is a mess to work on... i think you'll be debugging hum more than you will be playing.

Simplest circuit would be something in the single ended, class A vein, like an Epiphone Valve Jr/Gibson Ga-5/Garnet Gnome/Fender Champ or 5F2 Princeton.   But you won't get more support than Building a Doberman kit, and troubleshooting your problems on the AX84.com BBS.  Another option as was mentioned before, is to buy an epiphone Valve Jr, and rip the board out and throw it away, and just reuse the chassis, head enclosure, tubes, transformers, etc, and build up a custom circuit.

And as was also mentioned before... the voltages CAn be lethal, but are not as dangerous as many people make them out to be.  If you don't work on a peoject when it is "on" and let the capacitors discharge before poking around, and use good electrical practices--like keeping one hand at your side when probing with a meter-- then you will be fine.  I have personally takes hundreds of volts on several occasions and I am still kicking, and even 9v can be lethal if the voltage manages to bridge across your heart.