So I’ve been reviewing EL34/6CA7 data sheets preparing for an EL34 SE-UL optimization study like the two I’ve done before for the 6V6 and 6L6. However, I’ve run into a little snag.
Choosing what I think will be the best SE-UL load impedance is a combination of calculation, estimation, and intuitive feel. The DC bias points for UL optimization come off the triode strapped characteristics for a given tube. This is because at DC, UL topology is really just triode topology with a small resistance between the plate and suppressor. But the AC operation is a compromise between the triode and pentode operating characteristics. One way to estimate this is to plot a DC load line on the triode characteristics and then estimate the how much the plate characteristics will straighten and shift to the left under UL operating conditions.
The long and short of all this is that I have determined that the load impedance I want to use for the EL34 SE-UL optimization study is 2.5kΩ. And this is where the problems arose. When I went digging through my parts stash I discovered that I currently have only four output transformers available; all 5kΩ primaries. These are all Edcor GXSE10-5k 8Ω transformers.
The astute reader may point out that I could just terminate these transformers at 4Ω and then the effective primary impedance would be 2.5kΩ. And I did consider that option. However, whereas the impedance transformation would be correct in this case, the core magnetization characteristics and parasitics would not. Since the study itself requires a significant amount of testing, measurement, recording, and analysis I decided that I really didn’t want the added questions hanging over my head due to the suboptimal output transformer load. So until I can get Edcor to send me a pair of GXSE10-2.5k transformers, the EL34 SE-UL study will have to wait.
But wait… there may be another option in the short term.
The thought occurred to me that I’ve been doing these studies on big octal based tubes and it might be nice to perform an optimization study on a smaller 7 or 9 pin “miniature” tube. There are lots of candidates in this class. I can take the 6AQ5 off the table because it’s just a lower powered 6V6 and the data for that study can be used for the small tube (pursuant to the latter’s power dissipation limitations). But that leaves some other tubes I could use with a 5kΩ load.
One candidate is the 6AS5 beam power tube. This is a small power tube originally designed as the audio output amplifier for automotive radios. It’s a low power tube with a maximum plate dissipation of only 5.5 watts. Another candidate is the always ubiquitous 6BQ5/EL84 power pentode. This is a higher power tube more akin to the 6AQ5 with a maximum plate dissipation of 12 watts. And a third candidate that I found while search through my stash is the 6CL6 power pentode. This is a pentode designed primarily as a video output amplifier in televisions. It is a low distortion wide band amplifier tube. The 6CL6 has a maximum plate dissipation of 7.5 watts, between the two other candidates.
The question obviously is which one do I choose. All will optimize well with the 5kΩ primary output transformers I have on hand. The 6AS5 and 6CL6 are both readily available on resale sites and eBay for very little money. The EL84 is widely available but more expensive than the other tubes.
My first inclination was to choose the EL84. But two things gave me pause. First was that EL84 designs are numerous, widely available, and widely varied. In short, the EL84 has been “done to death” on the internet. Both as a basic audio amplifier and in guitar amps. The second was that the 6AQ5 and 6BQ5/EL84 are so close in specifications and performance that I am not sure an 6BQ5/EL84 optimization study will provide any great additional knowledge or insight verses the 6V6 family optimization data set.
The 6AS5 is admittedly a lower power tube, but I’ve always wanted to see what I could do with it. In the 6AS5 µ-Power UL Amp I built over ten years ago, I don’t think it ever produced more than a watt per channel and I know for a fact that the operation was not optimized in any way. It would be nice to revisit this tube and see if I could do it justice.
The 6CL6 is a different kind of beast. It is a true pentode but it was specially designed to be low distortion and to have very low parasitic capacitances so it could be wide band. With it’s higher plate dissipation limit I could probably get maybe up to 2 watts out of it. It might make a really nice example of a tonally neutral lower power amplifier for a small room or office.
The other benefit that both the 6AS5 and the 6CL6 have over the EL84 is the lower power supply current required. This limits the size and the expense of the power supply and case.
So does anyone have any opinions about the path I should take? Should I just wait for my EL-34 transformers from Edcor (probably March or April)? Or should I choose one of the small tubes? And if so, which one? I’d like to know what people think.
As always, questions and comments are welcome.