Debateing on wether to tackle this project, will need a bit of help

[svstee]

I just finished building the Firefly over at AX84, and I have officially been bit by the amp building bug. Problem is, I am dirt poor and absolutely broke at the moment. Economy will have to be the central focus of my next build. with that in mind, since I have a halfway decent supply of parts (mostly from building stompboxes) I figure my current budget is limited to no more than 100$ USD. I know that doesn't sound like much, but I figure all I'll need to buy are transformers, maybe a couple resistors and caps (should have most on hand) and eyelet board. I was browsing around at Mojo Musical Supply and ran into these

Mojo Surplus » Sundowner/Swamp Thing Power Transformer (for SE Application)
Mojo Surplus » Sundowner/Swamp Thing SE Output Transformer

Followed the link and ended up with this schematic
http://www.cs.cmu.edu/~ehn/ax84/sche...o-Broiler2.pdf

Does this seem doable for a second build? This amp looks good to me for a couple reason; I don't have a SE amp, it looks fairly easy to build, it looks like it is very tweakable as far as controls go, and I have most of the parts I need already.

There are also a few things about that schematic that worry me, like the lack of other documentation as well as the lack of some information I'm used to seeing on schematics, like voltages and voltage ratings for components. I'm hoping that this forum will fill in the gaps in my knowledge.

Thanks!

[Sir Cuitous]

It looks about as easy as a Champ clone, so yes it definitely is doable as a second amp or even a first. I would also say it could meet your budget depending on what you can build yourself. I would not worry as much about the voltages since Mojo has specified the trannys. You can most likely build and bias it. But how you orient the transformers and the tubes IS important. You are not given a complete layout here so you will want to learn a little about orienting the parts. You could follow the Champ layout (tubes and trannys) as an example.

[svstee]

Aside from this http://www.adamamps.com/PDF/mojo_transformers.pdf which I found via Google, I can't find any specs on these transformers, where are you looking?

As far as orientation, I'll look at some champs and read up on the subject a bit, but what about a variant of this?
http://www.cs.cmu.edu/~ehn/ax84/imag...r_top_1024.jpg
Big pic, so I just linked.

[olddawg]

If you have more time than you have money you might want to check out yard sales and thrift stores for a doner amp. A lot of old record players, tape recorders, radios, small PAs etc used open ended 6V6 or EL84 5 watt amps or 6V6 or EL84 push pull 15-20 watt amps. Many times you can pick them up for next to nothing and use the tubes and iron. Sometimes you can do a conversion and even use the chassis. Also many times people will give you an organ just to haul it away. Many have a usable tube amp inside. If you keep your eyes open you will be suprised.

[tubeswell]

Yep in terms of a SE 6V6 or EL84 type amp with one or two pre-amp tubes, for a PT you can get away with 50mA on the HT winding and 1.5A or so on the Heater winding, so that is just about any tube PT you can find that works (in any junk shop) with a high-enough secondary voltage.

Similarly for an OT you only need 5W or so min for a SE 6V6 or EL84 type output tube. That pretty much is any OT out of most kinds of old tube radios. (assuming the thing ain't fried)

Take care when you test old parts

[svstee]

If you have more time than you have money you might want to check out yard sales and thrift stores for a doner amp. A lot of old record players, tape recorders, radios, small PAs etc used open ended 6V6 or EL84 5 watt amps or 6V6 or EL84 push pull 15-20 watt amps. Many times you can pick them up for next to nothing and use the tubes and iron. Sometimes you can do a conversion and even use the chassis. Also many times people will give you an organ just to haul it away. Many have a usable
tube amp inside. If you keep your eyes open you will be suprised.

I have been, I got beat to some old tube stuff at my churches yard sale by about 5 minutes a couple months ago. I also have an old tube organ at my cousins house, but its like a three hour drive, so I'll wait until I'm going up there for something else.

[Sir Cuitous]

I can't find any specs on these transformers, where are you looking?

I'm looking at the name that Mojo gives the transformers in your link. "Sundowner/Swamp Thing Power Transformer". It doesn't get any more explicit than that. I'm wagering that Mojo would not name these transformers that way and insert a link to the swamp thing project unless they have been tried out in that project. Is that much of a leap to make?

You can email Mojo, but surprisingly they may not actually have the specs. The secondary wiring pictured looks correct for the swampthing project.

[svstee]

I'm not saying they won't work, I'm sure that the project they linked to is good, I'm just unsure, without transformer specifications, about things like power ratings for resistors.

[svstee]

The project and schematic I link to are solid, they just don't have all the info that a novice such as myself needs. I'm fully aware that the transformers are the same ones used in that project, put that project and included schematic just aren't specific enough for me to feel comfortable with, and I can't find specs for the transformers either, so a simple thing like figuring out voltage ratings becomes a major impediment to getting this build off the ground.

[critter01]

Hey there, brand new to this forum and to tube (or valve as we call them in NZ) amp building.the Firefly is also going to be my first valve amp build and if you have any advice or things to be aware of during the build, i will listen and learn. i have been researching this addiction and getting lots of info from the Valvewizard and AX84 sites and i am starting to understand alot of the basic principles in regard to Valve guitar amps. as i am also financially challenged i plan on trying to source componants firstly from old Valve Radios that are often on the online auctions here.and secondly from members of a Guitar forum i belong to here in New Zealand.a couple of guys who are members on the NZ Guitars forum are also members here(Tubeswell and Unparalled) both of these gentlemen share alot of there knowledge about amp building with us here. i am looking forward to seeking advice from them as well as everyone on this forum.cheers

[Sir Cuitous]

I'm sure you can get help with the power ratings on this forum. Don't be discouraged. It looks like a fun project. Nearly all of the resistors can be 1/2 watt with some exceptions. The cathode resistor for the 6V6 should be 5 watts. There is a 1k resistor going to the screen (pin4) of the 6V6 which probably should be 1 watt. Also, there are some resistors in your power section 180, 1k, 33k, 33k. 2 watts for the 180 and 1watt for the rest. If I am wrong, certainly someone will post a correction.

As for the transformers, here's another layout that might help:


Do some searches for "champ" "head" "amp". You might be able to get some additional photos of layouts.

[tubeswell]

Hey there, brand new to this forum and to tube (or valve as we call them in NZ) amp building.the Firefly is also going to be my first valve amp build and if you have any advice or things to be aware of during the build, i will listen and learn.

Hi there critter01

Welcome to the forum

AX84.com has great forums for discussion on their projects where the guys there have focussed particularly on those projects they sponsor. While you can find 2nd-hand trannies around, it might be a wee bit harder to find 2nd-hand ones with firefly specs (particularly the OT, which has a higher load resistance ratio than most commonly-available pentode driven trannies). So if you are intent on the firefly, you might find you end up having to get an OT made up.

(Also probably might pay to start a new thread about your project - where you'll be more likely to draw attention to your questions)