Transformer Questions

[tablebeast]

I was asked this question the other day and I wasn't sure about the answer so I am bringing it to you guys to decipher. OK, so if I have a tube amp that has multiple taps on its output transformer can I hook up to more than one tap at a time? If I do hook up to say an 8 ohm tap and a 16 ohm tap at the same time how do the two connections affect each other as far as load impedance and/or power output?

Jesse

 

[JohnRoberts]

All the taps look to the amp like they're connected in parallel.

Assuming the amp is fully loaded when you put one X ohm load on the X ohm tap, putting multiple X ohm loads on multiple X ohm taps will overload the amp.

To connect multiple speakers bump them down to lower ohm taps. Say one 8 ohm speaker on the 4 ohm tap and one 4 ohm on the 2 ohm tap should equal one normal load, or two 8 ohm speakers on the 4 ohm tap.

JR

 

[tablebeast]

To connect multiple speakers bump them down to lower ohm taps. Say one 8 ohm speaker on the 4 ohm tap and one 4 ohm on the 2 ohm tap should equal one normal load, or two 8 ohm speakers on the 4 ohm tap.

I knew it was something like that. I always thought you had to pick one tap and balance the load from there.

So, here is another question, this time about power transformers. Why do some tube amps have ceramic caps (usually .047 1000v) installed from the AC primary to ground? What does this accomplish?

 

[shabtek]

caps on primary can reduce noise (i.e. shunt transient rubbish) from the power lines and prevent it from getting to the secondary and your dc rails

this months 'nuts and volts' has a decent article on analogue power supply design

 

[tablebeast]

caps on primary can reduce noise (i.e. shunt transient rubbish) from the power lines and prevent it from getting to the secondary and your dc rails

I figured it was along those lines. AC only has to pass 60hz right? So its basically shunting everything above that to ground? IF that is the case shouldn't every tube circuit have this then? I was confused about this because so many older amps have a polarity switch shunting only one side to ground. What is the point of the switch? Why not shunt both sides all the time?

 

[PRR]

> Why not shunt both sides all the time?

Then the chassis is always 60VAC away from that radiator the musician is sitting on. Keeps 'em jumping.

 

[tablebeast]

[quote author="PRR"]> Why not shunt both sides all the time?

Then the chassis is always 60VAC away from that radiator the musician is sitting on. Keeps 'em jumping.[/quote]

Umm, what? That is an abstractly impressive reply. Are you saying this practice of shunting is dangerous? Only dangerous on one leg? Only SAFE on one leg?

Isn't there danger with the caps shorting AC to ground causing shocks? I'm just curious as to the specific function of this type of circuit. If it is so useful, why doesn't every circuit have it? Should I add this to all the circuits that don't have it or pull it from stuff I redo that has it installed?

Jesse

 

[mr coffee]

Jesse,

Cap limits the current. Dangerous only when the cap shorts. "Unpleasant tickle" might be more the right description.

You can also put a cap across the primary to reduce emi and rfi entering via powerline connections.