[Tube Amp] A little bit off-topic, but I try...

[lazerphea]

Hello all!
I know tube amps are not the topic here, but I try anyway: I'm having some problems understanding how bridge rectifiers work, in particular I bought a Fagor B250 C1500/1000 silicon bridge rectifier (http://www.datasheetarchive.com/indexer.php?file=Scans-0031258.pdf&dir=Scans-001&keywords=fagor+B250&database=user-highscore) , and it outputs way more voltage I expected.
If I understand right,  in this document http://www.hammondmfg.com/pdf/5c007.pdf  it seems a full wave bridge rectifier should outputs a V.D.C. of 0.9*V.A.C sec., that is, if I input 380V~ I should measure 0.9*380 = 342V- at the output, right?
The problem is: I measure 480V- at the bridge output. 
Could please someone explain to me this mistery?

[bobindah]

Hi,

A guitar amp will typically have a capacitor input load, so I would expect a silicon rectifier to produce voltages around 1.414* VAC un-loaded so your measurement of 480V is pretty much spot on - especially if there is no load on the rectifier - this figure will be likely to go down when there is a load (e.g. a power amp circuitry) connected i'd expect about 450V under load. For more info on Tube Amp check out Merlin's excellent website http://www.valvewizard.co.uk/bridge.html (and his more excellent books on the subject!)

Hope this helps

cheers

Rob

[lazerphea]

Hi,

A guitar amp will typically have a capacitor input load, so I would expect a silicon rectifier to produce voltages around 1.414* VAC un-loaded so your measurement of 480V is pretty much spot on - especially if there is no load on the rectifier - this figure will be likely to go down when there is a load (e.g. a power amp circuitry) connected i'd expect about 450V under load. For more info on Tube Amp check out Merlin's excellent website http://www.valvewizard.co.uk/bridge.html (and his more excellent books on the subject!)

Hope this helps

cheers

Rob

Hi bobindah,
thanks a lot, you've been of great help! Now I understand the meaning of the numbers!
I don't want to abuse of your kindness, but I have a couple more questions...
I'm trying to build the AX84 SEL (schematic here), but I failed at ordering the transformer, so my secondary is 190-0-190 instead of 275-0-275. I'm new to the HT world, so someone suggested me to use a rectifier bridge to get a V.D.C. near to the desired one (366V on the schematic, out of the rectification diodes), but that was wrong (as we've seen from your numbers and my not working build ). Is there a configuration of diodes, maybe involving the transformer center tap, I could implement to make things right for what concerns AC rectification?
Thanks again!

[PRR]

> Is there a configuration of diodes, maybe involving the transformer center tap

No.

Build it with the 2-diode connection just like shown on the plan.

Your B+ will be near 260 V DC.

The amplifier *will* work with 260V.

Tube amplifiers are not fussy about the voltage you give them.

Power output will be less, but not a lot less.

If you built it that way, and it isn't working, something else is the problem.

The Debugging Page should be your next step. Half of tube-amp faults show in the DC voltages. Since you have the AX84 voltages, you can compare them yourself. Expect them all to be about 70% of "normal"; '209V' at first plate is likely to be near 150V. If it is zero or 50 or 250, that's a Clue.

[lazerphea]

Paul and bobindah, thank you very much!!!
Can you believe I tried to post the problem in an italian forum about tube amp diy, and after a 3 pages long thread, no one gave me useful advices? The last post was the one of this guy who told me: "you don't seem do know about electronics, and it is not possible to waste pages and pages of posts to answer to every trivial question. So read this and this, and find out why it doesn't work"...
Even thought he was right (I'm not an expert at all), I expected a trivial question to have a trivial answer, right? Nope: three pages of useless nonsense... :\
If I'd have to answer this way to every "trivial" question posed in computer science-related forum I used to follow, well... I'd surely have spared myself some headaches, but I don't think this is the point of a support community.
Anyway, you guys are always the best! Thanks again!