using 2 different impaedance taps simultaneously

[Liam]

What you need to think about is the reflected impedance to the output tubes from the load. It goes with the square of the turns ratio. If you put parallel loads onto the output tranformer you are reducing the load impedance. Whilst that possible isn't as bad as increasing it, you're not doing the OT and tubes any favours.

[hasserl]

If you put parallel loads onto the output tranformer you are reducing the load impedance.

Are you? Or is each load reflecting a matched load back to the primary side indepenandtly of the other?

[Jag]

But would those impedances sum or paralell at the primary side?

[Liam]

Parallel, unless you connect one end of one to one end of the other.

[6267]

what about paralleling taps–or is this what those hammond diagrams are all about (All my opt's seem to only come with one tap anyway, so I've never really experimented). Although I do have one head with everything–4,8,16, 25v and 70v, I've always wondered what the impedance is with all paralleled or all series'd.

[kg]

ok, here's the deal:

4 and 8 ohm taps on tranny

4r load on 4r tap = nominal reflected primary z
4r load on 8r tap = half nominal z
8r load on 4r tap = double nominal z

basic stuff so far...

4r load on 4r tap, AND 8r load on 8r tap = half nominal z
8r load on 4r tap, AND 16r load on 8r tap = nominal z

if you want, think of it as if there were two discreet opts that had their primaries wired in parallel. once you really start mixing impedances you might have to resort to writing it out.

of course that analogy does not hold in terms of max core flux (saturation) and total current capacity, but it does work in terms of the reflected impedance.

if i were to plug a 16ohm cab into the 8ohm tap, and then an 8 into the 4ohm tap.

like you thought, that would give the "proper" primary load.

having said all of that, i would experiment. most reasonably constructed opts have no issues mismatching one "step" in either direction, and the tone might be preferable.

ken

[mooreamps]

Well, I'll tell you how I set up my stereo tube keyboard amp. I have an 8 ohm woofer tied to the 8 ohm tap, and a 4 ohm ribbon tweeter tied to the 4 ohm tap. The output power is split evenly between the two, and it appears to me the reflected load impedance back to the primary coil of the output transformer is unchanged.

-g

[Steve Conner]

Do you have a crossover for the tweeter? If you do, the speakers aren't really paralleled, because the crossover more or less disconnects the tweeter at low frequencies, while the woofer's voice coil inductance does likewise for it at high frequencies. So what you did would be correct.

With two full-range speakers and no crossover, I still think the original hypothesis was right.

[Liam]

ok, here's the deal:

Thanks Ken, now can you answer my Marshall presence control question? (Don't worry, hopefully I'll get the soldering iron out tomorrow.)

[mooreamps]

Do you have a crossover for the tweeter? If you do, the speakers aren't really paralleled, because the crossover more or less disconnects the tweeter at low frequencies, while the woofer's voice coil inductance does likewise for it at high frequencies. So what you did would be correct.

With two full-range speakers and no crossover, I still think the original hypothesis was right.

For the keyboard amp, it's a 4.7uF cap in series, because it is a ribbon tweeter. But when I do the combo Harp amp for Jenny, and will be using 4 10's for speakers, I'll have two speakers on one 8 ohm tap, and two speakers on one 4 ohm tap. This will be "full-range" speakers and no crossover, probably some flavor of Celestion or Emminence with the Neo Mag speakers to help keep the over-all combo wieght in check

-g

[Enzo]

For what it is worth, I would view them as parallel. A 16 ohm load on the 16 ohm tap reflects a certain impedance to the tubes. An 8 ohm load on the 8 ohm tap reflects the same impedance to the tubes. One of each reflects that impedance twice, so as far as the tubes are concerned it just sees two loads in parallel. All the tube ever sees is the reflected impedance, it has no idea which tap they are on.

[kg]

For what it is worth, I would view them as parallel. A 16 ohm load on the 16 ohm tap reflects a certain impedance to the tubes. An 8 ohm load on the 8 ohm tap reflects the same impedance to the tubes. One of each reflects that impedance twice, so as far as the tubes are concerned it just sees two loads in parallel. All the tube ever sees is the reflected impedance, it has no idea which tap they are on.

that is exactly correct.

8r load on 8r tap PLUS 4r load on 4r tap

is the same as

4r load on 8r tap.

[mooreamps]

that is exactly correct.

8r load on 8r tap PLUS 4r load on 4r tap

is the same as

4r load on 8r tap.

I just don't see it that way. I see it the same as a 4 ohm load on the 4 ohm tap, thus no change in the reflected input impedance back to the plate of the power tube(s).

-g

[Enzo]

Would you then say that if the amp had a 4 ohm load on the 4 ohm tap already and we add another speaker on another tap, NOTHING changes in the amp?

[kg]

I just don't see it that way. I see it the same as a 4 ohm load on the 4 ohm tap, thus no change in the reflected input impedance back to the plate of the power tube(s).

-g

gary, to be blunt, it doesn't matter whether or not you see it that way. i'm telling you the way it is.

bust out your LCR meter and your test resistors and find it out for yourself.