Guitar Tube Amp


Step 20: OH, Man...I wish I had...

No project is without lessons learned. In this case, I wish I had:

--Used a heaver, possibly stock, metal chassis.

--Substituted a 10 inch speaker.

--Used a turret board, instead of point-to-point. The more I modified the project, the more a tangled web it became. After this project, layout for turret board makes perfect sense--tubes on the back, controls on the front and the component board between....

So, additionally, the chassis layout sucks, too. Initially my main concern was to keep the preamp tube far away from the power transformer. But the lead-dress is awful. The "star-ground" is in the wrong place, too.

It must be said: despite the "rat's nest," the amp is quiet.

--The filament tap is not separated from the HV secondary. Too bad I didn't just use a separate 3A 6.3v transformer for the filaments, and added the 6-7V from the main transformer to the HV tap (about 149V, vs 142V.) After all, I had to add the wallwart for the preamp anyway....

--Used tube sockets with some sort of retaining clips--the chassis is upside-down. It hasn't been a problem yet, but eventually... All new ceramic sockets might be better, too.

Oh, and I did add a negative feedback loop. Although it did lessen the punch and (audio) feedback, and tone down the bass, it added a bit of "fartyness," too. That flabby, farty tone was present whether the nfb was switched in or not.

So I ripped it out right away.

Sometimes this is just a solder-joint issue, sometimes it's a bad cap. But it might be a routing problem, which at this point is beyond fixing without a complete rebuild.
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Hi I have difficulties of figuring out some of the components. All that trafo talk made my head ache :D i live in Europe so my wallwart is 220V and why the current has to be 147 couldn't it be 150 or 250V. Sorry i may ask a stupid question but in my head it seems normal :). And do you have list for those components? Because i have trouble of understanding of some. Is that speaker trafo some special one? Because i havent seen those in no where.
Alright, thanks for all your help!
gmoon (author)  katatonicdean3 years ago
Antec--that was the company that makes a 150V toroidal transformer with filament windings too, at a reasonable price. It's the AN-1T150 model.

Find their stuff here. I've never used it, but that's the closest I've seen to the transformer I used, and it's not very expensive.
First off, I was just wondering if this can be converted into a bass amp with an added bass knob.
Secondly, do you know of a place that I can get a similar choke, power transformer, and output transformer?
And thirdly, what pins were used for the tubes? It would be really helpful to know. (I notices that you used the 12ax7 twice, but on the power supply you only had one of the filament pins powered?)

Thanks for all your help!
gmoon (author)  katatonicdean3 years ago
Converting to a bass amp? Probably by using somewhat lower value coupling capacitors. Where this amp uses 0.02 uF caps, you could go with 0.033 or 0.047 uF coupling caps.

That would be more "bassy". If you need a separate bass knob, then use a different tone stack in place of the existing one. This website will be helpful.

Edcor makes the output transformer. You can find similar voltage power transformers, but that's a bit harder. I had a nice link to a 150V toroidal PT, but I lost it when my computer died. If I find it I'll post it later. Chokes are easier, Hammond makes those.

You'll have to look up the pin outs yourself, they are on the datasheet I linked (and you can find these online)--in fact, if you've never built a tube amp before, I'd recommend you do a lot more research. You'll figure it out...

The 12ax7 is listed twice, but it's a dual triode, so two triodes are enclosed in the same tube. Hence, only one filament is needed (note: 12ax7s can be wired as either 12.6V or 6.3V filaments).
Oh, and can i use a higher ohm speaker. I was thinking an 8 ohm or 16 ohm speaker. What would I need to do?
gmoon (author)  katatonicdean3 years ago
Choose an output transformer with whatever impedance you need.

I.E., if you have a 16 ohm cabinet, get a transformer with a 16 ohm output impedance. I'm sure Edcor makes this trannie in both 8 and 16 ohm versions.

Some OTs have multiple coil taps. You'd need to special order that from Edcor...

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