DIY bucking transformer for vintage amps

[Heemis]

RG, thanks for the quick reply!  It's hooked up exactly as indicated on your article here: http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

Re: an indicator, I think you're right, after playing around with it the last couple days, I think I might install some kind of indication and also maybe a mains switch so I can leave it plugged in and not suck power.

So are you saying that under a heavier load the voltage should drop less? 

Regardless the -5 (-8) setting is giving me 115-117 Vac which is EXACTLY what I wanted.  Thanks again for the help!

[R.G.]

OK, makes sense. The primary of the small transformer always sees the AC power line, which doesn't vary (much...) with loading. The secondary is deliberately designed with its no-load voltage 10-15% high (and the smaller the transformer power rating the more over-nominal-voltage it is) so that under full load, the voltage on the secondary sags down a little to its full-load rating.

In this connection, the secondary subtracts the most voltage from the AC line when there is no load. As load goes up, the secondary end-to-end voltage sags some from resistive losses, and so subtracts less from the AC power line, so the output voltage goes UP, seemingly paradoxically.

There is an alternate hookup. In this one, you connect the secondary in series-aiding with the transformer primary, with the primary dot-end to the secondary non-dot-end; then the AC mains are put across the entire secondary plus primary, and the output taken from the junction of the transformer primary and secondary, or the secondary center tap.

In this connection, the (for example) 120V mains are put across the would-be 120Vac primary plus the 10V secondary. So the small transformer primary gets 120/130 of the mains voltage. This gets you about 111V at nominal from the connection. This is an autotransformer connection as opposed to a bucking transformer connection. Slightly different voltages.

But what you have works. Good work.

[Heemis]

Ah, I see what you're saying... Yes it is a little odd to imagine the voltage increasing as the load increases but it makes sense in this instance.

I'm tempted to re-wire it to the autotransformer config, but I think i'll just quite while I'm ahead!

My amps thank you!

[R.G.]

You're very welcome. And if it's not currently broken, don't fix it!   

[mac]

My house AC voltage is 125, which my vintage princeton hates.  I want to build a voltage adapter a la this article at geo: http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

Cool, I need higher voltage.
I bought my Laney in the UK, the toroidal PS is intended for 240V 100w, but in my country line voltage is 220v, so heaters run at 5.6v, (220v/240v*6.3v=5.77v)
I thought I needed a costly +100w variac, but I guess a cheaper 220v:18v-24v iron would do it.

Despite the voltage drop, tone is surprisingly good, and that's ok for the output quartet (BTW, a pita to match)
Nevertheless, I always wondered how it would sound at the proper voltage.
My little Valve Jr, initially set up for 240v, improved a bit when I used the 220v connection of the built-in PS.

mac

[R.G.]

Despite the voltage drop, tone is surprisingly good, and that's ok for the output quartet (BTW, a pita to match)

Tube matching. As usual, I have opinions on that.   

The most important thing to match on tubes is the idle bias current. This is what most tube matching places measure. If you set the amp up so there is a bias adjustment per tube, you can use unmatched tubes, and "match" them by adjusting the bias.

There is a long litany of issues with matched output tubes, including the issue that amps may sound better with slightly mismatched tubes for reasons you can, as an effects DIYer, guess. But individual biasing on the outputs is a handy, handy thing to have.

Some parts of the tube amp world think that preamp tubes sound better at 10% lower heater voltage. They will almost certainly last longer. But power tubes need full heater voltage to produce their much higher currents.

[mac]

But individual biasing on the outputs is a handy, handy thing to have.

Unfortunately my Laney has a 56 ohm resistor shared by 4 x EL84.
No individual bias resistor or Blumlein garter circuit 

But power tubes need full heater voltage to produce their much higher currents.

Let's say you have a 4 matched set running cold, 5.5v or so. What would you expect? Crossover issues, less output power?
In other words, is 5.5v that bad?

Tube matching. As usual, I have opinions on that.    icon_lol

I'd be surprised if you haven't! 

...for reasons you can, as an effects DIYer, guess

Don't bet your pay check on me 

mac