Hello, I am very interested in building an amp, and I used to be all in to like heavy metal and all that crap, but now my style is more bluesy, so I've decided to build an AC4. Also cuz they're freakin awesome!!! Any way, I kinda need advice. I've been doing kind of a lot of research, and I have a few schematics, which are thankfully almost the same. And I've done alot of research on how to be careful when building tube amps and stuff. But what I havent really looked up is what kind of components to use. I found this old tube power amp in an attic, (A 12at7 with two 6L6's per channel) and they used Sprague Atom caps with what look like carbon composition resistors? I also have heard something about carbon composition resistors, but I forgot what... I also don't know where to get Transformers, I don't even really know what the specs are that I need. I found the exact ones from Murcury Magnetics, but i'm not really stoked about paying $290 for transformers for a tiny amp.... I will if i really have to. I found this Link http://www.musicalpowersupplies.com with REALLY REALLY cheap transformers but again, I don't know the specs that I need so I don't know If i can get from them. Also I'd like to build a top boost into it, so i'm wondering how I would go about that.

Sorry about being so long winded. I'll just ask my questions below.

What is the diff. between Carbon Composition and Metal Film resistors (besides what they are made of) and does it matter?

What kind of transformer do I need, and do you have any suggestions on where to get them? does Hammond have them?

What exactly is a schematic for a top boost? (I kinda just took the bright channel off an AC30) will that work? and will I be able to do that with a typical AC4 Transformer?

As you can probably tell I'm new at building tube amps. I've been soldering since I was 10, and my dad is an Electrical Engineer, so I know about electronics and stuff, I just have NO idea about THIS stuff.
Thank you so much for reading this far! Any help you offer would be GREATLY appreciated. Thanks.
comp resistors have "mojo" meaning that they make your gear sound better than anyone else's with absolutely no scientific explanation. metal film resistors are supposedly quieter and can be made to high tolerances. according to an article by a reliable source, carbon comps have the unique ability to 'distort' when used in high voltage or high current applications (like as plate resistors). this "distrtion" makes the tone awesme or whatever.

Hammond has any transformer you'd ever need for a tube amp. just look up the specs and see which one will work. dont cheap out on transformers, especially the output transformer.

idk what you are asking in the 3rd question.

what other projects have you built? just because you can solder well doesnt mean you can build a tube amp.
I've really only built one other thing. Its basically a Zvex Super Duper 2-in-1 but instead of the volume knob and the master knob, there is just a toggle switch to turn the second boost on. There is also a 12 position Veritone in front of the boost.

and the Third Question is I'd like it to also have a top boost like the AC30 has. I guess its kinda 2 questions.

1. Can I just take the Bright Channel of the AC30 and use it for the top boost thingy?
2. If I do use the Bright Channel, will the transformer I need to use be different?

So I SHOULD just go with Hammond? Because the transformers on that website, are supposedly REALLY Good, they just don't cost a crap-load. Thats what I ment by Cheap
Last edited by Mr_Zap at Sep 12, 2010,
not the smartest idea, but not nearly as retarded an idea as that guy trying to build a 200W with no electronics experience at all a few threads down.

if you can read a schematic, excercise proper lead dress, troubleshoot, and are well aware of the dangers of tube circuits, then there's no reason you cant do this.

ive built hundreds of projects over the years and only recently was confident enough to build an amp. my first build is a transistor amp and my second is a simple tube amp (basically a champ with radio tubes) built inside an old tube table-top radio from '61. both are being debugged at the moment.
This should be a nice first project. However, I'd try to keep (close) to the original schematic first. The only thing I'd change would be from a tube rectifier to a solid state rectifier. In a single ended amp, you'd have no reason for tube rectification.

As for transformers, EdCor makes some reasonably priced ones that are also quality. The only problem is, you don't get multiple output taps on the single ended output transformers.

With the stock EL84, this transformer would work with an 8 ohm speaker:


This would be a good power transformer for you, as well. It won't burn up your EL84.

So the solid state rectifier would be Diodes? because on the AC30 I think it is.
Thanks for the links
Oh I totally spaced and forgot my other question.

What type of Capacitors should I use?
that is purely personal taste. it also depends on where the cap is in the circuit. if it was me i'd just keep it simple and use all poly-film and ceramic for any pF values.
On the power amp I have they are all rated at high voltages, what voltages should they be rated at for this one?
In the power supply, I recommend F&T electrolytics. They are the best electrolytic capacitor I've found. It says in that schematic that they should be rated at 350v.

The tube depot has a good selection of F&T:

As for coupling caps, I prefer mallory, but any poly type cap would do. I use silver mica for pf valued caps.

And yes, solid state rectification would be diodes. (I recommend you click on the link in my sig about amp building. It's still not complete, but I have covered quite a bit).
Last edited by end_citizen at Sep 12, 2010,
So ALL the caps would be 350v? and where does it say that? so I may learn.

And yeah, I have looked at your thread and read it like fifty times. Last time I saw it though, you were just beginning to cover power tubes, (in like May or something) It was actually the first time that I had actually seen ultimate guitar's forum ever, I found it while googling something.

Anyway... I'll check it out does it say exactly what diodes I will need? Do you know?

Thanks again!
that's a pretty complicated amp. lead dress is gonna be a bitch. honestly i get the vibe that you dont know nearly enough to be building this.

350v only for the filter caps. coupling caps as well, for safety. bypass caps can be typical 25v or higher.

the diodes have to be rated at the peak AC voltage that will be on them. you are rectifying a center-tapped 500v transformer, so multiply 250 by 1.41. that gives you about 352v. the diodes need to have a peak inverse voltage rating of 400v or higher for a good safety margin. any type of diode will do the job as long as it is rated 400v or more and can handle the current drawn by the amp.

1N4007 diodes are rated for 1 Amp and 1000v . they should work fine. i doubt that amp will pull 1A of juice out of those diodes.
For your first question, metal film resistors are more stable and typically have much tighter tolerances than carbon. Metal also do not vary that much in resistance value with temperature, carbon can drift quite a bit as they warm up. Metal film are also much less noisy than carbon.

Basically metal film are superior resistors in nearly every way, however some people feel that carbon ads more of a warm tone to a circuit. Really just get whatever is cheapest, it's not going to make a significant difference.

If you want a low watt AC30 top boost sound, why don't you build this?


I actually built an amp based on the original version of that amp, the chime 'n grind. It completely sounds like a Vox amp. Here's the schematic.


Here's the layout I made:

Note that the pots should be reversed, I goofed in the drawing , there's no switches (didn't draw them for whatever reason), and no heater wiring ,but that gives you an idea of the amp.
Last edited by earthwormjim at Sep 13, 2010,
Quote by Invader Jim
your diodes are also floating there...

fyi, its probably a good idea to mount transformers at right angles to each other. especially when they are close together.

Ya there's no switches in the drawings, the diodes go to the standby switch. I know I had a final drawing with everything correct, but I seem to have lost the file.

I was a little space constrained on where I could mount the transformers. It works, it's not noisy so meh.
Last edited by earthwormjim at Sep 13, 2010,
Why cant people make layouts like yours...? Are there anymore like this?
Schecter Omen Extreme FR w/ SD Blackouts AHB-2
Epi LP 100
Epi LP Special II

Randall RG100 Head
carvin x100b head
Carvin x100b 412
Fender FM412 Cab