want to build tube amp from spare parts

[Ashurbanipal]

This ain't a pedal thread, sorry, but several of you here always have good advice, so I'm asking. And I promise to make sure I know what I'm doing before electrocuting myself.

I have a spare hammond organ amp, and it occurs to me I have all the parts to build a single ended amp. I've got a power transformer, a 5U4, a 12ax7, a 6v6, an output transformer, high quality tube sockets, and vintage resistors. I'll probably need new electrolytics and some different value caps.

It seems I could build something similar to a Fender Princeton. Any advice on where to start? Does the book The Ultimate Tone have enough info to tell me how to use these parts and build a good amp?

[bancika]

First measure transformer output. Most likely, you have center tapped HV secondary, 5V secondary for 5AU4 and one or two 6.3V for heaters.
Then you need to determine output transformer impedance with that tube (or even easier, measure speaker resistance, if you get around 3ohms than impedance is 4ohms, and about 7 it's 8ohms). As for resistors and caps, I suggest you trow them (or keep as souvenir). Get metal film resistors (less noise than vintage), new electros and nice caps (like mallory or orange drops), not expensive but must be better than old ones.
It's best to go to www.ax84.com

[Peter Snowberg]

I agree with bancika, new parts are the way to go (except for the tubes and transformers). Old tube sockets should be fine too, but I would toss the resistors and probably most or all of the caps. Any film or mica caps should still be fine, but cheap ceramic and electrolytics are best replaced. The power cord should be grounded and be sure to use a fuse too.

Read this: http://www.angela.com/catalog/how-to/Single.6V6.html

Think of http://www.ax84.com as the DIYstompboxes of the tube world.

Best wishes on your project!

[brett]

Hi.
Good luck with your project.  I'm sure it will sound good when you finish.
Some of those Hammonds had 2 x EL84 outputs, which offer great potential for Vox or Marshall tones.

Then you need to determine output transformer impedance with that tube (or even easier, measure speaker resistance, if you get around 3ohms than impedance is 4ohms, and about 7 it's 8ohms).

Hmmmm....  I'm no expert, and I've only ever measured the DC resistance of the output transformer (OT) in one of my amps, but the primary of my JTM60 is about 160 ohms, and the secondary only shows 8 ohms, but takes an 8 ohm load.  I believe that some Marshal OTs go as low as 10 ohms on the primary and maybe 2 or 3 DC ohms on an AC 16ohm secondary.  RG Keen mentions this in his article on biasing tube amps.  DC resistance depends on wire thickness and things somewhat unrelated to AC resistance, which depends on number of turns, etc.

The good news is that you only have to check the speaker impedance to find out what the OT secondary was made for.

cheers

[bancika]

I was talking about speaker. It's good if you're going to use same output tube (at same voltage). In that case it's enough to measure speaker resistance.