I'm not familiar with the Triode mod. But if you haven't already tried the "SED" EL34's,those have a nice warm characteristic.Other people may be more familiar with the Triode mod to see if it has that characteristic.
7 responses Add your response
The ST70 is not the last word in either power or low-end. That being said, even in this age of multikilobuck products being de riguer, I'm hard pressed to find a more satisfying amplifier.
A few questions:
1) Are you sure the ST70 is working properly? Have you measured the bias on the tubes, as well as the typical voltages inside the amp that Dynaco spec?
This is ALWAYS the first thing to look at. You may well have a weak pair of output tubes, drivers, or rectifier. A tired rectifier tube will take all of the gusto out of your ST70
2) Have you tried the Dynaco with the speakers before you upgraded to the Triode Electric board? Of course, you may not have had the amp without the mod
3) Loudspeakers prior to what I call the Tube Renaissance beginning in the late 1990s are often far more difficult to drive than those of today. Solid state amplification was the far more common topology, and loudspeakers featured difficult impedance curve and featured more complex crossover networs which serve as "speed bumps" for an amplifier.
Further along those lines, What is the woofer size and alignment of your loudspeakers? A lot of products of that era are sealed aligment, which is a difficult task for a tube amplifier.
In my experience, the ST70 works beautifully with a couple of my loudspeakers, and not at all with others. In the end, presuming your amp is OK, you may just have a poor match
As the Les McCann and Eddie Harris song goes, "Compared to What?" What amp gave you better bass response and what preamp are you using? The Synthesis speakers were designed on tubes, so this should not be an issue. Oh, you said that you are using the Triode boards, which brand of coupleing caps are you using in them?
I think Trelja pretty much hits all of the points. Especially the power issues and the impedence matching ones as well. If your amp is biased properly (and this is very important) and you still have poor bass response, you may have to just consider getting either more suitible speakers or a different amp. I've had more success in the bass department as well as FR linearity, using amps utilizing the 6550/KT88/KT90 family of tubes.
Atmasphere (Ralph) raises a good point. Certainly worth checking this out. Years ago, I had a pair of speakers (Rectilinear III's) that came from the factory wired out of phase! In checking the phase, if it seems correct, try reversing polarity of only one of the speakers just for the heck of it and see what happens (just in case the speakers are incorrectly internally wired).