Primary Impedance

[alexmoose]

Hello!

I am currently tweaking my EL34 SET amplifier, and I am trying to decied on what primary impedance to run it at. I tried it at 2.5k, and I changed it up to 5k, with little chance of changing back.

I recently started reading Tom Schlangen's website, specificly the part where he suggests a 10k load for his EL34 SET.

What does everybody sugest? and more importantly, what changes sonicaly when one changes the primary impedance?

[Giaime]

I prefer high loads with high voltage and low current. You'll get lower distortion, higher damping factor but at slightly lower power.

[Tubes4e4]

Hi Alex & Giaime,

Originally posted by Giaime
I prefer high loads with high voltage and low current. You'll get lower distortion, higher damping factor but at slightly lower power.

Also, for the very same OPT, you will get extended bass frequency range due to the lower iddle current, which means you get some iron saturation reserves (the iron will saturate at a lower frequency).

Acoustically, the added low end and damping factor easily outruns the slight loss of Po.

Tom

[rdf]

Originally posted by Tubes4e4

Also, for the very same OPT, you will get extended bass frequency range due to the lower iddle current, which means you get some iron saturation reserves (the iron will saturate at a lower frequency).

That never occured to me, great point! However, re: distortion I find consistently that across most of the useful operating range of near every tube as B+ rises and idle current falls, 2nd harmonic distortion rises and higher harmonics drop. That's my preference anyway (as well?) however a standard, single-numerical THD meter will see it as a higher distortion circuit than 'low and hot'.

[Tony]

ypu can try connecting your 8ohm speaker to the 4ohm tap if your amp has it, it has same effect as rasing primary impedance..

[alexmoose]

Originally posted by Tubes4e4

Also, for the very same OPT, you will get extended bass frequency range due to the lower iddle current, which means you get some iron saturation reserves (the iron will saturate at a lower frequency).

Acoustically, the added low end and damping factor easily outruns the slight loss of Po.

Tom [/B]

I'll have to get to work raising my B+ Voltage!

Originally posted by Tony
ypu can try connecting your 8ohm speaker to the 4ohm tap if your amp has it, it has same effect as rasing primary impedance..

Thats exactly what I was going to try, I have a hammond 125CSE, I am going to use this to find the optimal impedance, then buy an expensive transformer according to that configuraition

[AndreasS]

Originally posted by rdf


...2nd harmonic distortion rises and higher harmonics drop

Maybe the distortions comes from the driver?

regards Andreas

[rdf]

Good thought, but my tests use a CCS loaded driver. The operating point doesn't change with B+.

[AndreasS]

And the voltage swing was the same?

regards Andreas

[rdf]

Yep. The same behaviour manifiests itself with drivers BTW. The method I use is pretty simple, one variable power supply for B+ and a second for grid bias. For any given drive level the two are adjusted simultaneously to maintain a constant plate dissipation while monitoring the harmonics.

The results are surprising. For example with about 4 VDC on the cathode, equating to around 230 VDC plate, my samples of 6C45Pe will show no harmonic higher than second above ~ 125dB with a current around 5 ma CCS loaded. A more popular 20 ma lowers the (already low) second but pops higher harmonics well above the noise floor.