2.65 ohms with a tube amp?

[okcrum]

We used to deal with reflected power in RF systems in the Air Force (Ground Radio Maintenance). Has anyone measured reflected power in a bass amp with a Bird inline watt meter? Is it the same as measuring reflected RF? If different, how? Thanks!

It's basically the same idea, save for iron core transformers instead of ferrite or air core with RF systems.

A transmission line hybrid (as is used in Bird wattmeters) is impractical to build at audio frequencies, so most actual measurements get done with a resistive bridge. Sad to say, that doesn't help you out much for full power testing, and that lack of measurement leads to the myths we see here. In reality the rules are about the same for audio or RF systems.

EDIT: Actually it'd be more specific to say "directional coupler" instead of "hybrid".

 

[Hawaii Islander]

It's basically the same idea, save for iron core transformers instead of ferrite or air core with RF systems.

A transmission line hybrid (as is used in Bird wattmeters) is impractical to build at audio frequencies, so most actual measurements get done with a resistive bridge. Sad to say, that doesn't help you out much for full power testing, and that lack of measurement leads to the myths we see here. In reality the rules are about the same for audio or RF systems.

Thanks for the clarification.

 

[Hawaii Islander]

Curious. What about generating a harmonic of the audio frequency to a radio frequency and measuring that with a Bird or similar power meter? The output level of the harmonic would need to correlate the audio signal generated. Some affordable harmonic generators may exist http://www.thegleam.com/ke5fx/cg.htm. I haven't worked with electronics in nearly 20 years, so I'm very rusty with theory.

 

[billfitzmaurice]

So if you were to run a 2.6 ohm load, what tap would you use , 2 or 4 ohms? I'd guess the 2 ohm tap...since that is the "minimum" load. If you use the 4-ohm tap, that is the minimum load for use with that tap.

Use the four ohm tap. You're still thinking along SS lines. Tubes are exactly the opposite.

 

[James Judson]

I would be very careful taking technical advise from a forum like this. No one brought up the fact that impedance matching is critical for power transfer. Output impedance (amp) matching input impedance (cab) gives you the most power transfer. When there is a mismatch where do you think those missing WATTS go. A big enough impedance mismatch causes your amp to work to hard soaking up those extra watts and components start to fail. I would consult owners manual.

YMMV

 

[Nephilymbass]

It says on Fenders Website they recommend to run this particular head at 2,4 or 8ohms. I've seen guys push two 4ohm ampeg 810s with a single SVT2 all tube head so what I recommend is running two 4ohm cabs and getting down to 2ohms. Which could be as simple as having a speaker cabinet modified. Back when I used to have 15s I had a local music store convert a couple 8ohm 115 cabs to 4 ohms and its not that big of a deal. But yeah at 2ohms I bet that amp would be awesome.

 

[Hawaii Islander]

I would be very careful taking technical advise from a forum like this. No one brought up the fact that impedance matching is critical for power transfer. Output impedance (amp) matching input impedance (cab) gives you the most power transfer. When there is a mismatch where do you think those missing WATTS go. A big enough impedance mismatch causes your amp to work to hard soaking up those extra watts and components start to fail. I would consult owners manual.

YMMV

okcrum mentioned reflected power in his post.

 

[billfitzmaurice]

I would be very careful taking technical advise from a forum like this. No one brought up the fact that impedance matching is critical for power transfer.

In truth, not really. The impedance of an SVT cab, for instance, varies from a low of 3.5 ohms to a peak of over 20 ohms. As there is no such thing as a constant impedance speaker there's also no such thing as a 'perfect' tap to speaker match. It's an approximation, and the rule is to use the tap impedance rating that's equal to or higher than the speaker impedance.

 

[kuys]

But what about the 3 10' epi's? That's why I'm confused why would cabinet makers put out cabs that can't be used by 99% of the market.

 

[billfitzmaurice]

But what about the 3 10' epi's?

What about them?

 

[kuys]

What about them?

They have a ohms rating of 5.3.

 

[okcrum]

In truth, not really. The impedance of an SVT cab, for instance, varies from a low of 3.5 ohms to a peak of over 20 ohms. As there is no such thing as a constant impedance speaker there's also no such thing as a 'perfect' tap to speaker match. It's an approximation, and the rule is to use the tap impedance rating that's equal to or higher than the speaker impedance.

Well, it'd really be better to take the nominal output impedance that is closest to the nominal speaker impedance. It's a grossly simplified method, as you point out. Given the wild variance of speaker impedance (since it is a type of motor after all) the question really becomes "which amp tap most closely matches the speaker at the frequencies where most of the power in the signal is?". If you look at speaker impedance curves, you see right away why Bill recommends the higher impedance tap. Even an 8 ohm tap is somewhat mismatched to a single 8 ohm (nominal) BP102 in a cabinet over most of its range.

The thing to remember is, compared to RF systems, most audio systems are pretty broadband, covering 10 octaves or so in a full range system. Also, there's very little isolation (return loss) gained from the speaker cable, compared to the transmission line to the antenna in RF systems.

So the reflected power is dissipated in the output transformer and output tubes, as well as the speaker (yes, some of it reflects back). That happens even when all the impedances are "matched".

There's a good description of what Bill is talking about in Link Removed.

 

[epic!]

To the OP...I did it no problem! Ran a Bassman300 into a 3x15 rated at 2.65 ohmns. Never had an issue.

 

[billfitzmaurice]

They have a ohms rating of 5.3.

So use them on the 8 ohm tap if you have one, the 4 ohm if you don't. Either way there's no worries.

 

[lem8r]

Tube and SS amps behaves equally when connected to lower than required impedance. Output transistors and output tubes start to conduct more current as B+ didn't change and impedance lowered. That extra current can overheat transistors or tube's plates.
Other things that are changed as bandwidth, output power etc aren't dangerous to amp. Only extra current.
So safer to connect 2.65 Ohm spkr to 2 Ohm tap(and get less than nominal current) rather than to 4 Ohm(and get more than nominal current).

 

[eastcoasteddie]

Use the four ohm tap. You're still thinking along SS lines. Tubes are exactly the opposite.

There seems to be a lot of contradiction on this subject...which impedence selection to use with 2.67 ohms. I've searched this forum extensively and, quite possibly, in every single thread regarding this subject, one guy says "use the 4-ohm tap" and the next guy says "use the 2-ohm tap".

However, I seem to be more inclined not to take Bill's response for granted, being that he has a LOT of credibility.

One of the most interesting threads I came up on, is that one gentleman actually emailed Ampeg with tis very question. Ampeg replied that one should use the 2-ohm tap with a 2.65 ohm load. Another member replied that most of the technical information in the Ampeg manual and what you get from Ampeg tech support....is wrong.

I think this topic specifically should be in the "Ohms Sticky".

 

[billfitzmaurice]

There seems to be a lot of contradiction on this subject...which impedence selection to use with 2.67 ohms. I've searched this forum extensively and, quite possibly, in every single thread regarding this subject, one guy says "use the 4-ohm tap" and the next guy says "use the 2-ohm tap".

In this instance it's splitting hairs, either tap will work. IME those who are the most adamant on perfect matching of taps and impedance also tend to be those who least understand the entire concept of impedance and how amplifiers work. As a quick test of your impedance chops, what is the impedance of the three cabs shown here?

The answer is 8 ohms, for all of them. And it's a bit of a trick question, because all three are loaded with the same driver.

 

[throbbinnut]

Just to clarify -

Solid state amps fry when they put out too much current - this happens when you put too small of an impedance on their output and then push them too hard.

>>>Pentode-based<<< tube amps fry when you put too high of an impedance on them because the output transformer inductance causes a voltage spike, which usually shorts out the output transformer winding due to the high voltage. That's why tube amps usually have a shorting jack on the first output, because they are happier driving a short-circuit than they are driving an open circuit (open circuit usually fries them).

If you want to know why this is, look at the curves of a pentode. Now draw an infinite impedance load line (horizontal line). What happens when you try and vary the grid voltage? Plate voltage goes to infinity. If you draw a 0-impedance load line (vertical line), if you swing the grid voltage the plate current goes insanely high and low, but the voltage doesn't change. So you would eventually kill your tubes, but not your output transformer.

Back to the original question - plug 2.65 ohms into the 4 ohm tap.

Chris

 

[barbe-et-hache]

That page contain a lot of usefull informations about speaker impedance : Loudspeaker Impedance :

This is the most important :

a loudspeakers impedance is everything but consistent across its frequency range

 

[lem8r]

>>>Pentode-based<<< tube amps fry when you put too high of an impedance on them because the output transformer inductance causes a voltage spike, which usually shorts out the output transformer winding due to the high voltage. That's why tube amps usually have a shorting jack on the first output, because they are happier driving a short-circuit than they are driving an open circuit (open circuit usually fries them).

When you connect cabinet to trans, output pentodes get loaded with reflected load(rather high load, kOms) everything works fine. Next you disconnect cabinet and now pentodes are loaded with impedance of primary windings(too small load, tens Ohm) and now tubes putting to much current. Red plates are not good thing to do with tubes.

HV spikes due to primary inductance are usually shunted with 1-3kV diodes.

Engineers of tube bass amp usually tend to squeeze as much power as they can. Thus loadline for 4 Ohm spkr connected to 4 Ohm tap lies near max plate dissipation curve. So when you connect 2 Ohm spkr to 4 Ohm tap new loadline can cross max plate dissipation curve or exceed max cathode current limit.