Typical output impedance for tube amps

[Hylle]

I would like to take the tupe amp output impedance into acount when I design my loudspeakers. can anyone tell me in what range, the typical output impedance of a push-pull and a SET amp is?

[jameslover]

every day is market goes up and down so , i think that you should find nearest hardware cheap market for this project.

[Jeb-D.]

I'd suggest designing with a 2 ohm source impedance in mind.

[DougL]

I think 2 ohms is a good number for a non-feedback triode PP or SE amp.
For a Classic UL or pentode amp with feedback, the DF was usually around 8, so 1 ohm would be realistic for a ST70 etc.

Obviously gross generalizations.

Doug

[wakibaki]

Amplifier Output Impedance

w

[Hylle]

Thank you all, for the good imputs. I am suppriced to see that you belive there is not much difference in output impedance between PP and SE amps. I will probably simulate with a output impedance value between 1 and 2 ohm. Then cablesize and series inducteres can be used to trim the right bas performance, I think.

[kevinkr]

I would broaden the range just a bit, and say 1 - 4 ohms to be safe because I have several 300B PP 0 fdbk pushpull and SE amps with output impedances of > 3 ohms or so, and this would be typical..

Don't forget you also have to include the DCR of the transformer primary and secondary in the source impedance calculation, and a DCR of 1 ohm on the 8 ohm tap is not uncommon.

Note that the following example should be considered a gross simplification but is anecdotal of the sorts of impedances you can expect with an SE amp.

My 300B SE amplifier has a 3K:8 OPT, the primary DCR is about 100 ohms, and the secondary DCR close enough to 1 ohm for the purposes of this example..

The output tube rp is about 700 ohms.

Primary DCR is 100 ohms

The impedance transformation ratio is 375:1 primary to secondary.

Secondary DCR is 1 ohm

So figuring the source impedance on the primary is approximately 800 ohms then (800/375) + 1 = 3.13 ohms at the secondary for the 8 ohm tap.

Leakage inductance may increase the effective source impedance at higher frequencies, but I ignore it since I am primarily interested in the effect on the woofer.

The amplifier source impedance will be roughly 3 ohms in this example, and is in fact close enough for most purposes.

It is not at all unusual for a 0 fdbk PP amplifier optimized for output power as opposed to source impedance to have a significantly higher source impedance than an SE amplifier using the same tube type.

Note also that class A and class A/B PP amplifiers will have different source impedances even with the same OPT and in the case of the A/B amplifier that source impedance changes significantly as a function of power output beyond the class A region of operation.

[Jeb-D.]

Originally Posted by kevinkr

I would broaden the range just a bit, and say 1 - 4 ohms to be safe

I totally agree. Aside from your example, a lot of the common budget PP Pentode amps also have a Zout around 4ohms.

That was the range I had in mind when suggesting to design with a 2 ohm source impedance in mind (for an unknown tube amp). It's 2 x 1ohm, but 1/2 x 4ohms. It would compromise the extremes equally and be closer to ideal for amps in the middle range.

[Hylle]

ok Thanks, it sounds like I should go a little higher on the expected output impedance.

[tubelab.com]

Don't forget you also have to include the DCR of the transformer primary and secondary in the source impedance calculation, and a DCR of 1 ohm on the 8 ohm tap is not uncommon.

The DC resistance of the speaker cables is a factor too, but I don't even want to consider another "cables" debate.

There are many texts and articles on controlling speaker cone motion by lowering the output impedance of the amplifier and associated wiring. It is important to remember that the speaker generates a voltage when it moves. The "cone control" is provided by forcing that voltage generator to operate into a short circuit, or as low an impedance that is practical. What is often overlooked is that the speaker voice coil has a DC resistance too, and it is often several ohms.

Many speaker design programs and texts do not consider this since it is beyond our control.