4 ohm or 8 ohm tap?
- Thread starter markloft
- Start date
Register to hide this ad
Nyjblbob
Hooked on Horn's and Tubes. Go big or go home
Either one. whatever sounds better. No doubt the Bass would be better on 8 ohm.
linuxslate
Well-Known Member
Is this more scientific?:
Hook up a True RMS Watt meter and speakers to one tap.
Drive it pretty hard, but not into distortion. Note average or peak power output.
Leaving the input at the same exact level, Connect speakers and TRMS Meter to the other terminals.
The configuration that sends more power to the speakers is the correct one.
Hook up a True RMS Watt meter and speakers to one tap.
Drive it pretty hard, but not into distortion. Note average or peak power output.
Leaving the input at the same exact level, Connect speakers and TRMS Meter to the other terminals.
The configuration that sends more power to the speakers is the correct one.
Makes sense, sounds more scientific, but the “try each and see which sounds better” worked well enough for me.
Not sure that would work very well with a tube amp and the complex load a speaker presents.
Plus I don't think max power and max sound are always the same thing.
Try both is always sound advise.
Would that hole true for speakers rated at 4 ohms?
Every amplifier is different also.
What is the transformer spec (5k?, 3.5k?) vs power tube used?
What is the transformer spec (5k?, 3.5k?) vs power tube used?
Is your suggestion of better bass, assuming a nominal 8 ohm load? I have read reviews of a few speakers, with low average impedances, sounding better with a amp's 8 ohm tap. That seems to be the exception, however. As mentioned: try both.
Is it correct that, using proper winding techniques, an output transformer can provide equally good (and powerful) signals to either 4 or 8 ohm taps? Some high-end manufacturers provide only one output impedance on their tube amps, stating that better coupling is possible that way.
On a bass amp I have made to run 6L6GCs in a PP AB config with a 450VDC B+, the OPT with its 5500 ohm primary is a perfect match for the 5600 ohm plate resistance of the tubes. The 4, 8 and 16 ohm secondary taps, are 4, 8 and 16 ohms. I am using an 8 ohm speaker on the 8 ohm tap. Easy.
If using alternate tubes with a different plate resistance presented to the primary, you effect the secondary impendence. You can find the new secondary impedance with some simple math by using the plate resistance of the tubes in use and knowing the turns ratio of your transformers you can find the impendence ratio. I decided to use 6550s in that amp (a popular option for this amp) with 3500 ohm plate resistance. The ZR of secondary taps with the lower primary presented came out to 2.5 ohm, 5.2 ohm and 10 ohm down from 4, 8 and 16 ohm. I tried both the now 5.2 and 10 ohm taps with my 8 ohm speaker. The 10 ohm tap was by far the better choice. The amp was louder and really opened up.
All this to say, yeah which ever one sounds better...
If using alternate tubes with a different plate resistance presented to the primary, you effect the secondary impendence. You can find the new secondary impedance with some simple math by using the plate resistance of the tubes in use and knowing the turns ratio of your transformers you can find the impendence ratio. I decided to use 6550s in that amp (a popular option for this amp) with 3500 ohm plate resistance. The ZR of secondary taps with the lower primary presented came out to 2.5 ohm, 5.2 ohm and 10 ohm down from 4, 8 and 16 ohm. I tried both the now 5.2 and 10 ohm taps with my 8 ohm speaker. The 10 ohm tap was by far the better choice. The amp was louder and really opened up.
All this to say, yeah which ever one sounds better...
Like markloft, I wondered the same when I got my Rogue 100wpc tube amp some years ago. My large Paradigm 100.v2 towers dip below 4 ohms for much of the bass "power" region. John Atkinson tested and reviewed the Paras and recommended a 4 ohm tap for tube amps, however, I don't know if he actually tried both 4 & 8 ohm outputs on the amp he used.
Because the Rogue requires top plate removal to re-wire the outputs, I chose the 4 ohm tap. I hope that I guessed correctly because it's a PITA to switch between 4 & 8 ohm. The speakers have well-balanced sound with very good bass so perhaps, in my case, 8 ohms might not transfer more power to the speaker.
Because the Rogue requires top plate removal to re-wire the outputs, I chose the 4 ohm tap. I hope that I guessed correctly because it's a PITA to switch between 4 & 8 ohm. The speakers have well-balanced sound with very good bass so perhaps, in my case, 8 ohms might not transfer more power to the speaker.
Hi-Q
Active Member
We used to check source (output) impedance (mainly 50 Ohms) by applying a non-reactive variable load resistance until the measured open circuit voltage was exactly halved, the applied load would then be equivalent to the source impedance. So one must assume that a 50 Watt amplifier with say an 8 Ohm tap will do the same, delivering half the power (25W) into an 8 Ohm load with the other half lost in the amplifier. Assuming this to be the case, it can be proven with simple math that the 4 Ohm output tap will deliver more power into a 6 Ohm load than if it was connected to the 8 Ohm tap. So the 4 Ohm tap should give the sense of better Bass as more power (which it needs) is now available although not that much.
FWIW - McIntosh recommends the 4 ohm tap when nominal speaker impedance is below 6.3 ohm. Without checking the manuals I believe this applies both their tube and SS models. Keep in mind the speaker will likely dip to 3 ohms at certain frequencies. 4 ohm tap will be easier on the amplifier.
