240vac xformer and tube amp

I've been in electronics a long time but it's been years since I've messed with tube stuff but I thought I "may" build a small guitar (practice) amp.

First question is...

Has anyone explored the possibility of using a simple (and generally inexpensive) 120 to 240vac step up transformer with a solid state bridge rectifier to get high voltage for a tube?

Is there a design/schematic someone can refer me to on a particular tube that could operate on the voltage from this arrangement....I would imagine you could get close to plus 280-290 volts or so under load?

I see where Hammond makes a few 230vct transformers, one that will supply .35a for about 20 bucks....not an autotransformer.

Wonder if anyone has seen a design with these for a small practice amp with just a few watts?

That's an 80 VA transformer. Use a bridge rectifier on it and get 325 VDC out. You could get more than a few watts out of that with a class B output stage. Use the center tap as a 162 volt DC output for the earlier stages.

"Michael" wrote in news:g9adnVN87OBXhBbeRVn- snipped-for-privacy@comcast.com:

You need Duncan's very fine tools.

Go to:

And download PSU Designer. While you're there, you'll want to get the other tools too. TDSL is my fav. You'd be hard pressed finding better value on the net.

That's almost all I use! (Sorry, my schemo is a proprietary device!)

The peak voltage will be around 330 but will drop under full load back down to

250 or so - perfect for a 6BQ5 power tube. If you run a class A SE amp the voltage will be stable... don't go over 300...

Preamp tubes like the 12AX7A will run on anything over 150 volts with no problem...

Don't forget you need a 6 volt filament supply at a few amps...

Oh, I hope you have experience with wiring a high Z high gain wide bandwidth device - or it won't work... it'll either hum or oscillate or both, probably...

Then there's the matter of Tone...

Bob Amp builder

Not at load... I get 275 volts after a 150 ohm filter... about 10 less than the bridge output, for a SE EL84 5 watt amp. Higher loading would reduce it more...

You can't use the center tap if you use the full secondary with a bridge...

Bob Amp builder

Why not?

Bob wrote in news: snipped-for-privacy@4ax.com:

Hi Bob. You can use the center tap. It will be at exactly 1/2 the B+.

the

more...

Sorry - my bad - you can... I must have been pre-occupied!

You're right - I must have been thinking of getting the bias from there, which is negative...

Hey Bob! You're the one who told me to tie two secondary (82v + 125v) windings of that old Heathkit TV power tranny in series and it would give more than enough B+ for a Champ like thing. I haven't seen you around in a while. Thought I'd mention that it worked out just like you said it would. As I recall, B+ was in the neigborhood of 250VDC. I built it with a diode bridge, 12AT7, and 6AQ5. Works great! Amp building is addictive. I built two more since then (5E3-almost, and JTM45). Phil

Hi Phil - I remember you!

I don't come here too much because of the off topic political bullshit - some people can't understand this is a guitar amp group...

As for addictive amp building - I'm now working on number twenty something... I'm trying toroids this time!

Well you dropped in at a decent time. Between the filters and whatnot, things are down below a dull roar at the moment. I don't think it will stay that way. Anyhow, it was nice to see you drop by. Toroids, huh? What's the deal with them? Phil

Bob wrote in news: snipped-for-privacy@4ax.com:

Amp junkies can be itentified by the solder burns on their hands.

Do you know of a source for toroidal transformers with both a high voltage and filament windings or do you wind your own? I found Hammond have decent toroidals but don't have mixed windings. They also tend to be quite pricey. I find it had to justify the extra cost when recycled iron can be had so cheap.

Is there such a thing as a toroidal output transformer?

......... R.K. style......................................

Would the core end up being unrealistically large? What do you think?

-snip-

A toroid has the advantage of a more compact magnetic field, and I thought that would give an advantage to players who like to sit on their amps, and get hum broadcast into their single coils. I'm putting the toroids on the bottom of the amp chassis, and there should be no radiated field outside the amp.

I will test it with my Strat, a sort of $1000 magnetic field finder!

Toroids aren't all peaches and cream though, their wide frequency response can bring more line noise into the amp... I may put a ferrite choke on the power wires...

not to mention their price, about double a standard unit. I need 2, for filament and plate supply, cost me about $100.

-snip-

I'm trying a pair of Hammond's right now! I will do some tests between identical amps with and without torus transformers. I've never seen a tube type power toroid... you'd have to order it made.

I might also try a standard filament tranny with a toroid plate tranny... I bought a box of E-I filament trannys cheap - about $7 each, with slight damage cosmetically speaking... 12.6VCT @ 2A ... was a good deal so I want to use them.

I haven't seen one - don't know if there is an advantage to one...

I'm not a tranny expert by any means, although I have wound a lot of them! I couldn't tell you how a torus would do as an output... but for fun I may try a toroid filament tranny as an output... it would match an EL84 to 4 ohms! But it may saturate with the DC...