Anyone with experience building valve amps?

[StarGeezers]

 AES has a great little 8 watt SE OT.... P-T31   @ 13USD    I used one in a VJr scratch build (P2P) and it has worked fine over at NOG for a Long time ... gets played Every day by musicians trying out the NOG guitars... Many offers to buy the amp... all refused......

[Brymus]

Actually if you are concerned about cost then I would recomend the 1.8 watt Marshall over at 18 watt.com
I am planning on building this soon just becuase of the cost.
The trannies for that are really cheap -you can get tha trannies and tubes and chassis for about 100$
Trannies 45$ for both tubes 30$ for all 4 and small hammond chassis 25$ all at AES.
You proly have everything else except the large 400V caps on your bench.
Other wise for ease of building and help from forums I would go the EVJ route.
But then your looking at least 200$ by the time your finished.
And the 1.8 watt is push pull and is half as loud as the 18watt lite its modeled after plenty for practice.
The EVJ will be working already and you can learn as you go so that may be something to consider.
I've built about a dozen tube amps and the 18 watt is still my favorite.
I started with the EVJ though -its like a 101 experiments for guitar amps kit,great for learning.

[brett]

Hi

the 1.8 watt Marshall over at 18 watt.com

That looks really great. 

WRT using a PT as a SE OT. 

This guy managed to do it

In general, you'd have to do something substantial to compensate for the DC in the transformer.  A large power transformer (e.g. 20VA for a 2W amp) might have enough iron in it to leave "headroom" (spare magnetic flux capacity) for the AC.  It would probably give some transformer distortion/clipping (which would be asymetrical and might sound good). I've been thinking of other solutions, and a simple one would be to hacksaw through the outer "loops" of a typical "two loop" transformer.  That is, cut through the laminations.  With a sharp hacksaw, and a small transformer (e.g. 5VA) this might not take too long.  Hmmmm.....  Also, curring through a toroidal transformer might work if you could avoid the wiring..

AES has a great little 8 watt SE OT.... P-T31   @ 13USD   

That is SO cheap.  An 8W SE Hammond costs me Oz$94 = US$75 !! Arghh.

Thanks

[deeleyer]

Interesting.
I'm currently designing a SE Amp using a PT as OT and I would like to know what could happen in the future if I continue with that topology. In my tests the OT doesn't get hot at all but reading your post makes me think if it could be dangerous...

[petemoore]

  Eddie said just throw the transformer in there and see...
  Which is fine as long as it works. When the ratios and current ratings are good'.
  PT's as OT's...why not...I have, as long as it 'hangs' it's 'good', might not have the same transforming characteristics and frequency response, that might not matter either. That's what I'm using to drive a reverb transducer, a power transformer...perhaps one day I'll find another...seems to work fine though.

[GREEN FUZ]

I`ve pretty much assembled all the parts I need for my amp project. I`ve developed a bit of a mental block about the power transformer though, despite all my attempts to research the subject.

What I need to know is, is there any reason why this transformer couldn`t be used for the champ circuit?

[Ripthorn]

What exactly is that transformer supposed to accomplish?  If it is for the power supply of the whole thing, no way.  That might barely produce enough power to heat one tube, but not give you any decent plate voltage or power for anything else.  What is your mains voltage supply?  Remember that a single 12ax7 needs 300mA at 6.3V or 150mA at 12.6V for the heaters.  Power tubes typically require even more current than that.  If you are trying to do the traditional champ circuit, I would look at a hammond replacement transformer or similar.  More money, yes, but guaranteed to do the job.  When choosing a power transformer, it is critical to have the right rating, as one too low can create arcs, core saturation, or other such mechanical and electrical mayhem.

[kurtlives]

If you used 24AC as the primary it might work. It would give you a B+ in the 330V DC range. That should work.

100mA is enough to supply the plates.

Then you have the issue of the heater supply voltage. You would need some other transformer for that. Just gets messy... I'd just get a transformer made for a Champ circuit. One that has the HT< heater and recto taps.

[GREEN FUZ]

Thanks guys.

I picked this up, along with 4 identical transformers, in a job lot of components. What would they be good for?

[Ripthorn]

If you have multiple ones, putting them back to back would be great for plates.  If your mains voltage is 120V, you will be able to rectify about 160V out of those, but if your mains voltage is 240, then you get the higher voltage Chris mentioned.  These could also be great for pedal board power supplies (one would work well for a bipolar pedal board with the center tap grounded).  Pretty useless for tube heaters, though.

[brett]

Hi
for some reason that is unclear to me, I was advised some time ago (in the forum by a guru) not to use back-to-back transformers for anything other than low power amps (e.g. 1W).  Maybe there's an issue with the high currents spraying mains hum.  In which case, wouldn't twisting wires help (like twisting heater wires?)

Anybody have the facts?
cheers

[Joe]

I drew this up a while back hoping to build it sometime:
http://www.diystompboxes.com/analogalchemy/sch/7pinamp.html

I thought this was good because a lot of old radios already have those tubes. (The power transformer wasn't in the original diagram but I figured on using an isolation transformer.) If you're into the zvex kind of contraptions, you could probably make this pretty small

[Ripthorn]

Hi Brett, I can't say authoritatively what the answer is, but over at ampgarage I have had a few people actually recommend back to back transformers, especially when looking for secondaries that are a little off the beaten path.  I have heard, however, that if the current draw is too much on the secondary side, core saturation can occur which can wreak havoc on an amp.  The way to work around that (if I remember this right) is to simply never let the secondary current draw from the first transformer be higher than the rated value.  So in this case where the transformer is a 10:1 ration, the high voltage side of the second transformer should never pull more than 10mA, which for the plates in a champ type circuit should be no problem.  My little 2W that I finished a couple months ago uses back to back PT's and I haven't had a problem yet.  Then again, I haven't been able to play it more than a few hours.

[Shorty]

Just a quick note, here are two articles which outline how PA line transformers can be used as output transformers.

http://home.alphalink.com.au/~cambie/6AN8amp/M1115.htm

http://ozvalveamps.elands.com/optrans.htm

[Joe]

A while back I tried to make a class-A transistor amp using one of those. Sometimes you can get those pretty cheap too.

[snap]

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