[Tyson]

Talking about bypass caps. With tweeter cap I have Miflex KPCU-01 0.022uF 600Vdc with cheap yellow cap of 3.9 uF.

3.9 / 0.022 = 177.27 > 100, it is OK

But with the woofer of my BIG 3-ways maybe it is better idea a bigger value. 68 uF -> 0.47 or 0.33 uF ??? Bigger size and more expensive too.

[IMG, link] http://maty.galeon.com/WP-imagenes/hum/cajas-grandotas-filtro-esquema-tres-vias.png

What do you think? Tyson, Danny and others.

Danny mentioned to me at anything over .1uf in a bypass cap will start to have phase shift issues.

[maty]

Then bypass with 0.022uF is the best choice. Cheaper and smaller too!

Thank you very much.

[choff]

I was kinda surprised to find the construction of the integral stand. Seems like this could be improved upon to help isolate speaker from floor. Also, there is no way to level with uneven floors with these cheap hollow glides. I ended up popping them off and filling all of them with Blu-tac while I was at it. They come off super easy. 

[choff]

Just spit balling here....but I think this lower section stand would be easily improved if the hollow portion was filled in with mdf or similar to create a solid base instead of this open design.  Also, trying to figure a way to get them on roller balls. That seriously improved the sound of my last speakers.  Pretty tricky given the weight of these speakers and the stock base design.

[Danny Richie]

I thought that they needed about a one foot tall stand myself.

[choff]

I thought that they needed about a one foot tall stand myself.

Interesting....I have read comments from quite a few folks saying the same thing about these speakers.

[maty]

Maybe something like Klipsch Heresy III



But Forte III has a big woofer in the back too -> more floor reflections with this angle.

[choff]

It had never occurred to me that one might run speakers that are 8ohms on a 4ohm amp tap as Tyson suggested doing for my Forte's to help tighten the bass.  Since I need to resolder the binding post to transformer connections to make this happen, I decided to do some research on the subject until I had the time to do that work.

Like a lot of things on the internet, there sure are a wide variety of opinions on this approach and whether it is in fact safe for a tube amp.  I'm sure Tyson wouldn't have suggested it if he didn't think it was safe or a smart thing to do, but I would love to know more on the subject and what people think about it here?

[FullRangeMan]

Yes but not the contrary.

[Rusty Jefferson]

Just because a transformer coupled tube amplifier has outputs marked 4, 8, or 16 ohms doesn't mean the output impedance is actually 4, 8, or 16 ohms. The reason the bass may be tighter on the 4 ohm tap is simply because output impedance is significantly lower on that tap than the 8 ohm.  Low output impedance is the reason many like solid state (voltage source, not current source) amplifiers.  They have very low output impedance, typically less than .5 ohms, and many as low as .001 ohms.  That low output impedance can be good if woofers aren't being controlled well by an amplifier with high output impedance resulting in woolly bass.  However, if speakers have high impedance, putting a low output impedance amp on them (or using the 4 ohm tap) may over damp the woofers and give the speakers a leaner sound.

I have a small PP el84 amp that has 4, 8, and 16 ohm taps whose actual output impedance measurements are 5.6 ohms, 11 ohms, and about 20 ohms respectively.  Those numbers (4,8,16 ohms) are a guideline.  Also, there is always an assumption with smaller SET and PP transformer coupled amps that you aren't going to put them on speakers with extremely difficult loads that could stress them.  Your Klipsch speakers are not a difficult load.

[FullRangeMan]

Just because a transformer coupled tube amplifier has outputs marked 4, 8, or 16 ohms doesn't mean the output impedance is actually 4, 8, or 16 ohms. The reason the bass may be tighter on the 4 ohm tap is simply because output impedance is significantly lower on that tap than the 8 ohm.  Low output impedance is the reason many like solid state (voltage source, not current source) amplifiers.  They have very low output impedance, typically less than .5 ohms, and many as low as .001 ohms.  That low output impedance can be good if woofers aren't being controlled well by an amplifier with high output impedance resulting in woolly bass.  However, if speakers have high impedance, putting a low output impedance amp on them (or using the 4 ohm tap) may over damp the woofers and give the speakers a leaner sound.

I have a small PP el84 amp that has 4, 8, and 16 ohm taps whose actual output impedance measurements are 5.6 ohms, 11 ohms, and about 20 ohms respectively.  Those numbers (4,8,16 ohms) are a guideline.  Also, there is always an assumption with smaller SET and PP transformer coupled amps that you aren't going to put them on speakers with extremely difficult loads that could stress them.  Your Klipsch speakers are not a difficult load.

The transformer output impedance 4, 8, 16 ohms are the speaker impedance recommended.

[Rusty Jefferson]

The transformer output impedance 4, 8, 16 ohms are the speaker impedance recommended.

Again, a guideline.  If the 4 ohm tap actually has 6 ohms of output impedance, it may work better with some labeled 8 ohm speakers. 

[FullRangeMan]

Again, a guideline.  If the 4 ohm tap actually has 6 ohms of output impedance, it may work better with some labeled 8 ohm speakers.

This is a prob, do you have seen a amp like this?

[Tyson]

The lower output impedance from the amp gives you more control in the bass.  Especially with a speaker like the Forte where it dips to almost 3 ohms in the mid bass. 

Using the lower output impedance definitely will not cause problems with your amp.

[Rusty Jefferson]

Yes, I just referenced one.  It is not common for the actual measured output impedance to be 4, 8, and 16 ohms.

....I have a small PP el84 amp that has 4, 8, and 16 ohm taps whose actual output impedance measurements are 5.6 ohms, 11 ohms, and about 20 ohms respectively.  Those numbers (4,8,16 ohms) are a guideline. .....

[FullRangeMan]

Interesting, please PM me the details.

[jmpsmash]

The output taps (4/8/16) in a tube amplifier with output transformer has very little to do with the output impedance.

The transformer is a impedance transforming device that transforms the low impedance of a speaker (4/8/16 Ohms) to the high load impedance needed to be a load of the output tube. Eg. in a 300B SET, one of the most popular load impedance is around 3500. the transformer taps (4/8/16) merely says if you put a 4/8/16 Ohm speaker at the output side of the transformer, it will transform it to give the 300B the optimal load of 3500 Ohm.

As for the actual output impedance, it is calculated by the plate resistance (Rp) of the tube itself, and then transformed back into the output side. so as an example for 300B with a 4 Ohm load, a 300B has a Rp of 720, and it is a 3.5k->4 transformer, the output impedance will be 720*4/3500=0.8 Ohm. Since that is connected to a 4 Ohm load, the damping factor is around 5. Which is nothing to write home about!

[choff]

Made the switch to 4 ohm tap. The change is actually much more subtle than I had anticipated, but I do like what I hear on initial listening. Definitely a more balanced sound with highs and mids out front and not getting overwhelmed by the bass.   No reduction in power either, which surprised me as I thought by stepping down to 4ohm on 8ohm speaker would equal less power?

One odd thing when I was making the binding post change is the 4 ohm lead from transformer was reading at .7 resistance and the 8ohm at .9 to 1.0.....is that normal?  Would have thought 4ohm would be .4 and 8ohm read .8?

[jmpsmash]

Made the switch to 4 ohm tap. The change is actually much more subtle than I had anticipated, but I do like what I hear on initial listening. Definitely a more balanced sound with highs and mids out front and not getting overwhelmed by the bass.   No reduction in power either, which surprised me as I thought by stepping down to 4ohm on 8ohm speaker would equal less power?

One odd thing when I was making the binding post change is the 4 ohm lead from transformer was reading at .7 resistance and the 8ohm at .9 to 1.0.....is that normal?  Would have thought 4ohm would be .4 and 8ohm read .8?

regarding resistance, using a simplified analysis, the winding ratio (and thus resistance) is by voltage, and since power should be the same whether 4 or 8 ohms, and power is voltage^2, 8 ohm resistance being ~1.4x of 4ohm is correct.

[poseidonsvoice]

The output taps (4/8/16) in a tube amplifier with output transformer has very little to do with the output impedance.

The transformer is a impedance transforming device that transforms the low impedance of a speaker (4/8/16 Ohms) to the high load impedance needed to be a load of the output tube. Eg. in a 300B SET, one of the most popular load impedance is around 3500. the transformer taps (4/8/16) merely says if you put a 4/8/16 Ohm speaker at the output side of the transformer, it will transform it to give the 300B the optimal load of 3500 Ohm.

As for the actual output impedance, it is calculated by the plate resistance (Rp) of the tube itself, and then transformed back into the output side. so as an example for 300B with a 4 Ohm load, a 300B has a Rp of 720, and it is a 3.5k->4 transformer, the output impedance will be 720*4/3500=0.8 Ohm. Since that is connected to a 4 Ohm load, the damping factor is around 5. Which is nothing to write home about!

Thank YOU and Rusty Jefferson for communicating this truth. I am shocked by how many tube owners don’t know this!

What the output impedance is at a respective tap has to do with the dynamic plate impedance (which is related to the BIAS) of the tube used, the turns ratio of the output transformer and whether it is a SE design or PP design. Not to mention the fact that the output impedance isn’t a static number. It varies by frequency. Most “estimations” are at 1khz, but most dynamic driver based loudspeakers have their impedance nadir at 100-300Hz!

So how does one know the actual output impedance of an amplifier? Measure it from 20-20khz.

Best,
Anand.