? On using two taps on OT simulataneously

[Enzo]

I guess the question then, Gary, is this: If you have a 16 ohm speaker on the 16 ohm tap and the amp running at rated output. Does adding a second speaker - say an 8 ohm speaker on the 8 ohm tap - change anything on the primary side? If it does, then the two loads have added. If it doesn;t change anything, then we get free power to one of the speakers.


And we know that if we connect a 8 phm speaker to the 8 ohm tap of a 50w amp, we get 50w in the speaker, and if we instead connect two 16 ohm speakers in parallel to that tap, we still get 50w, but 25w in each speaker. SO, starting anew, if we connect a 16 ohm cab to the 16 ohm tap and get 50w, and then an 8 ohm cab is added to the 8 ohm tap, then either the added speaker gets 50 free watts and the 16 still gets 50 watts. Or the two speakers now split the 50w between them for the same 25w each. If that split happens, then by not touching the 16 ohm speaker, we can change the power to it by plugging the 8 ohm speaker into the 8 ohm tap or not. And the only way that can happen is by the load on the tubes changing.

[mooreamps]

Basically, yes. Is it the same as driving two 16 ohm speakers from the 8 ohm tap. and to answer an eariler question, yes there is a simple cross-over network in that stereo tube keyboard amp, but it was only a non-pol series cap in line with the ribbon twitter.

-g

[R.G.]

Even the most casual reading of how transformers work and what they do shows that loads on multiple taps/windings appear summed in parallel according to the impedance and turns ratios at the primary. It used to be that you had to go find a text book to find this. With the internet, it's the work of a few seconds to find the explanation, worked examples, whatever you need to learn. Assuming you want to learn.

Arguing against this, or even arguing that it's mysterious, is the same thing as admitting that you not only don't understand, but also that you either don't know how to find the information, or possibly don't want to learn.

I don't know exactly why, but this brought to mind the two rules of pigs.
1. Never try to teach a pig to sing. It wastes your time and annoys the pig.
2. Never mud-wrestle a pig. Not only can you not win, but after a while you realize the pig enjoys it.

[mooreamps]

Assuming you want to learn.

My business is not learning electrical theory. My business is inventing it.

-g

[Merlinb]

My business is not learning electrical theory. My business is inventing it.

Talk about shooting yourself in the foot...

[mooreamps]

Talk about shooting yourself in the foot...

no i'm not. . . . I've stated many times, all I want to do is measure it for myself....

[Wilder Amplification]

My business is not learning electrical theory. My business is inventing it.

-g

So basically what you're saying is that you're trying to re-invent the wheel?

OK super genius...since we're dealing with impedance here, at what frequency do you plan to take this measurement at? Please enlighten us.

[R.G.]

Somehow I think that post will get edited. I've saved it to my file of interesting things I've found on the internet for just such an eventuality.

ROFLMAO

[mooreamps]

So basically what you're saying is that you're trying to re-invent the wheel?

Well, who else uses fixed bias in a guitar pre-amp? I am personally not aware of any other builder who does this. or who else is using electronic volume control in a guitar amp ? or who else is using a screen rail power brake in a guitar amp ? Yes, maybe I am.

OK super genius...since we're dealing with impedance here, at what frequency do you plan to take this measurement at? Please enlighten us.

440 cycles.


-g

[R.G.]

Somehow, the first rule of holes just popped up in my head.

The First Rule of Holes:
When you find you're standing in the bottom of a hole, stop digging.

[Wilder Amplification]

The interesting thing is that I've described how to properly do it and what it does, everyone in the know who has visited this thread has been in agreement with it...except for Gary.

Gary, it's really simple. In order to ensure that a 16 and an 8 ohm cab cannot attempt to consume more than 1/2 the total output power each, you must drop the voltage to the loads by 70.7%. Relocating the 16 ohm cab to the 8 ohm tap while relocating the 8 ohm cab to the 4 ohm tap accomplishes this. Each cab sees 1/2 of the total power and since the total power consumed = maximum output power the impedance on the other side stays the same as it would be if you just had a single 16 ohm cab on the 16 ohm tap.

If you connect a 16 ohm cab to the 16 ohm tap while connecting an 8 ohm cab to the 8 ohm tap, you are applying full output voltage to each load and each load attempts to consume the max rated output power. Essentially the impedance has been halved at that point.

You HAVE to stop thinking in terms of impedance in order to grasp it. It's all about voltage across the load (which is determined by which secondary tap the load is connected to and its turns ratio with the primary) and simple Ohm's Law.

Well, who else uses fixed bias in a guitar pre-amp? I am personally not aware of any other builder who does this. or who else is using electronic volume control in a guitar amp ? or who else is using a screen rail power brake in a guitar amp ? Yes, maybe I am.

And this has WHAT to do with "inventing electrical theory"?

[mooreamps]

If you connect a 16 ohm cab to the 16 ohm tap while connecting an 8 ohm cab to the 8 ohm tap, you are applying full output voltage to each load and each load attempts to consume the max rated output power. Essentially the impedance has been halved at that point.

and it may even end up this way. But, I still want to measure it.

-g

[Wilder Amplification]

...FWIW, Randall Aiken has this question answered in one of his Technical Questions postings for those wishing to read up on *what & why*

Here it is...copied straight from Randall Aiken's FAQ. Best explanation of it I've seen yet -

Q: Attempting to simultaneously operate a 4-ohm and a 8-ohm speaker from the 4-ohm and 8-ohm secondary taps of an output transformer...not a good idea---but, why?
A: If you run a 4 ohm speaker on the 4 ohm tap, it will reflect back, say, 8K to the primary. Now, if you also connect an 8 ohm speaker to the 8 ohm tap, it will also reflect back as 8K to the primary, so the tubes now are seeing a 4K load instead of the 8K they require. That's why it is a bad idea. Now, if you connected an 8 ohm speaker to the 4 ohm tap, and a 16 ohm speaker to the 8 ohm tap, the reflected impedance would be correct.

[Enzo]

Look, he already stated he didn't believe the RDH and wanted to test their explanation of theory, so why should this be any different?

[teemuk]

Well, who else uses fixed bias in a guitar pre-amp?

Magnatone did, already in the mid 1950īs.

or who else is using electronic volume control in a guitar amp ?

I've seen one in plenty of rack preamps. Orange's OMEC Digital, Hughess&Kettner and TUSC might have been the first ones to do it already in the mid 1970's and early 1980's.

or who else is using a screen rail power brake in a guitar amp ?

Guy Claret (late 1970's and patented) and Blackstar amps, for instance.