Another 6l6 question

[jim1976]

I just built a 5e3 and I put 6l6's in it, just because it was what I had and it really works pretty well I think. But there seems to be a bit of discussion about the "right" output impedance for 6l6's vs 6v6's. So I checked out the tube manual for a pair of 6l6's running cathode bias and 360v and it says to run a 9000 ohm load vs 8800 ohm for 6v6's in a 5e3...
Why does everyone say you need 5000 ohms or less for 6l6's in a cathode bias? it even recommends the same 250 ohm cathode bias resistor....
The other thing is most people say is that there going to draw 120ma on the plates, the manual says running like this its only 100ma.
Just wondering really I'm happy with what I have.

[pdf64]

9k p-p for 6L6GB/C seems higher than I can remember seeing; usually in the range 6k6 - 3k8 for class A or AB1, eg http://www.mif.pg.gda.pl/homepages/f...7/6/6L6WGB.pdf
Whatever, if it sounds good then carry on; tubes can accommodate a wide range of load impedances which is just as well due to the significant variation of impedance with frequency exhibited by speakers, eg http://www.eminence.com/pdf/Legend_V128.pdf
I don't think that the operating conditions in a 5E3 push things hard enough for problems to emerge, unless the load impedance is raised to silly levels, eg above 20k, due to the saggy power supply.

Now you are going to advise that you are using a GZ34 and choke.
Pete

[jim1976]

Yes 6k6 -3k8 for fixed bias but not for cathode bias AB1 http://frank.pocnet.net/sheets/049/6/6L6GC.pdf
I did plug into the 4ohm tap with an 8ohm load oops was just cleaner with higher treble

I'm using a 5gc4 like a 5y3 no choke, has great bass nothing farty about it...

[pdf64]

Thanks for bringing that RCA info to my attention, the GE one is my usual reference.
My guess is that the high p-p load helps to keep it operating close to class A, thereby minimising the increase in cathode current at high signal levels, which otherwise would cause squish/crossover distortion.

'I did plug into the 4ohm tap with an 8ohm load oops was just cleaner with higher treble'

I think that would double the reflected primary p-p impedance, eg 8k8 rises to 17k6; the increase in treble you perceived may have been related to increasing harmonic distortion with load that the info sheets show on the chart on the last page.

You would need an 4 ohm load on the 8 ohm tap to get the primary load down to 4k4.
Pete

[jim1976]

I thought it might be interesting, thats why I thought I'd mention it, the rated distortion is high (4%) too.

I'm actually running a 10k primary which is probably even worse, I've noticed the same thing with other tube amps. If you look at transformer attenuators like say a Marshall power brake thats all they do except the other way, lowering the impedance.