? On using two taps on OT simulataneously

[mooreamps]

Magnatone did, already in the mid 1950´s.

Then I would ask you to list a specific model. I looked at all the Magnatone prints and all I see is resistor bias.

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.

I looked at all the H&K prints. They carry the signal to the front panel, and then back onto the PCB. In my control, the signal stays on the board, and passes through a variable gain vacuum tube.

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

yes, they did. I was not aware Blackstar amps were still in production. I believe they use a FET based circuit for their power scaling. My circuit does not use a FET.


-g

[mooreamps]

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?

It first appeared to me the secondary impedances were in series, not parallel. That was over a week ago. You make it sound like I'm not looking at this at ALL.........

-g

[Wilder Amplification]

It first appeared to me the secondary impedances were in series, not parallel. That was over a week ago. You make it sound like I'm not looking at this at ALL.........

-g

And I still question how you even came to that conclusion as adding a speaker to a different tap would cause current draw to increase just like in a parallel circuit.

[mooreamps]

And I still question how you even came to that conclusion as adding a speaker to a different tap would cause current draw to increase just like in a parallel circuit.

What difference does that make? I'm still going to measure it, and find out.

-respectfully
-g

[R.G.]

What difference does that make? I'm still going to measure it, and find out.

That's a good way to approach it.

But do us a favor, will you? Report back whatever you find, OK?

It is always possible that, theoretical considerations and over a century of exploration of how transformers work aside, you'll find out that they really work differently than all the poor benighted people who tried but failed to understand before you.

When that happens, I'd like to be the first one to congratulate you appropriately for your discoveries.

Peace on earth and purity of essence.

[mooreamps]

Absolutely I will. If they end up being in parallel, I will report back as such.

-g

[Enzo]

Not sure just exactly what we are looking for prior examples of, but Seymour Duncan amps, in several models used diodes for tube cathode bias. and they also used triodes as a variable resistor to control signal level going into the power amp.

Page 2 of this file for example. http://www.schematicheaven.com/newam...ertibe_100.pdf

You make it sound like I'm not looking at this at ALL.........

Not my intent. I don't think you are not looking at things. The impression I get, whether fair or not, is that when the text book says something that doesn't fit your notions, your first reaction is that the text must be wrong.

[mooreamps]

Not sure just exactly what we are looking for prior examples of, but Seymour Duncan amps, in several models used diodes for tube cathode bias.

No. They use a current sink because it's phase splitter.

and they also used triodes as a variable resistor to control signal level going into the power amp.

That's an interesting set-up. I've never seen that before, but no, not the way I do it.

Not my intent. I don't think you are not looking at things. The impression I get, whether fair or not, is that when the text book says something that doesn't fit your notions, your first reaction is that the text must be wrong.

I never saw that in the "text" until it was pointed out and no it's not my first reaction. But, I am still going to measure it none the less.

[jasonguitar]

This was an entertaining and informative thread. So what ever happened to your measurement mooramps?

I have a friend who just brought me his Duncan Convertible which blew the mains fuse after running both the 4 and 8 ohm taps simultaneously cranked up! So the fuse blew and he puts in a 15 AMP and turns the thing on and says it just made a loud hum and he smelled something burning!! Geez... I really wish he would have called me and asked about this stuff. Ouch. About to pop the hood.

No. They use a current sink because it's phase splitter.


That's an interesting set-up. I've never seen that before, but no, not the way I do it.

I never saw that in the "text" until it was pointed out and no it's not my first reaction. But, I am still going to measure it none the less.

[tboy]

I have a friend who just brought me his Duncan Convertible which blew the mains fuse after running both the 4 and 8 ohm taps simultaneously cranked up!

What sort of load(s) did he connect them to?

[jasonguitar]

What sort of load(s) did he connect them to?

Internal 8ohm speaker on the 8ohm tap and a 4ohm ext cab on the 4ohm tap. Turns out he killed the rectifier and shorted one power tube. Not sure what he was smelling cause there was nothing burned out unless it was just the tubes. The tubes were several years old in the amp. Here's some interesting info on this whole subject: According to Duncan, you can run any load on any or all of the speaker outputs at the same time!!! Here's what the manual for this amp says:

"4 OHM, 8 OHM SPEAKER JACKS: You can use these jacks to match the amp impedance to the input of the speakers you use. Proper connection to these jacks will give you maximum power and maximum damping.
If you choose to run an 8 ohm speaker in the 4 ohm jack, no damage will result in the amp or with the speaker, it will just sound different from being played through the 8 ohm jack. The same situation is possible with a 4 ohm speaker in the 8 ohm jack.
You can also use all three-speaker jacks at the same time with different speakers. The Convertible amp's strong transformer can handle almost any load with no damage."

No damage to the transformer but what about the tubes?

I replaced the fuse, checked for shorts, checked amp voltages without tubes, with a new 5U4 rectifier - standby off, installed a fresh matched quartet of EH EL34s and the amp bias was sitting around 60% static dissipation wattage and sounds great. Luckily he didn't do any other damage. The transformers in this amp are monstrous so I understand Duncan's confidence in their transformer with mismatched loads but again I ask, what about the tubes?

[R.G.]

The actual details of what happens when you overload a power stage with multiple loads on multiple secondaries depend somewhat on how the transformer is wound inside. The secondaries may be separate windings, or stacked, somewhat like an autotransformer, or even series-paralleled. But the OT isn't what blew on this one.

A power amp cannot put out more power than it takes from the DC power supply. In fact, it cannot even put out that much, as it wastes some of the power as heat to run itself. So when you overload a power amp that does not have some kind of limiting in it, the weakest link in the chain of AC power -> DC power -> output devices -> OT will fail first. In this case, the first to fail was the AC power line fuse. That is a happy coincidence of its main job of preventing the amp from bursting into flames on a failure in the power stages.

Putting a 15A fuse in there was what we used to call an IQ test back at TIC. Of course, every amp tech has had one of these walk in the door, either as a 15A fuse or some aluminum foil or whatever.

With the fuse "repaired" and not able to fail any more, the next weakest link failed. Good that it was the tubes! That's *much* cheaper to repair than either transformer.

[J M Fahey]

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

-g

Problem is, electrons follow certain rules on their own, *we* have to learn them in order to successfully work on electronics and such.
You may invent as much as you wish, electrons couldn't care less about you and will continue doing their thing, their way.
Meaning anything you design or analyze using *your* invented rules , just won't work.
Or work very poorly and just by chance.

[Dave Mac Farlane]

Brilliant, wish I had been around when this was posting.
Dave.

[Dave H]

Putting a 15A fuse in there was what we used to call an IQ test back at TIC. Of course, every amp tech has had one of these walk in the door, either as a 15A fuse or some aluminum foil or whatever.

I once found a 1.25” 4ba tapped spacer in a fuse holder. It was OK. The transformer had protected it