New tube advice ST70

jeffjmr wrote:Time to get serious about new tubes.

My TriodeUSA ST-70 has all JJ tubes; EF34Ls behind two EF806 and a ECC82 or sometimes I swap it out for a RCA 12AU7A, and a 5AR4.

It is important to note that I have a powered subwoofer system, so the ST70 is asked to only drive my extremely efficient Altec A5Xs from 80hz and up.

My major complaint is a lack of dynamic range. My other system is a solid state amp running a set of B&W DM605S2s. High dynamic range source knocks me over with that one. My tube setup sounds compressed across the range in comparison.

I am likely to replace all 8 tubes, but I’d like to get some advice as to whether a lack of punch is more likely to be determined by the driver tubes, phase splitter, or power tubes, or even the rectifier.

At this point I am leaning towards KT88s, hoping the reputed additional headroom will get me that punch I’m looking for but thought I’d run this by more experienced tubers before I take the dive.

Thanks for any advice.
Jeff

jeffjmr wrote:Thanks Peter. That was a good idea, though the results are inconclusive.

Subjectively of course, the sense of compression is not as readily apparent with the B&Ws. But the rooms are so different that an A/B comparison is impossible. Moving the B&Ws is plausible but not easy. Moving the Altecs is not an option.

The original questions, even if just for my education, remain.  Can I expect to experience a dynamic range change with tube replacement? The Tubes are of unknown age as I bought the amp used, and I have put the amp on my bench and see clear distortion beginning at 22W so I think new tubes are indicated anyway.

I was just curious how various gain tubes in the driver positions may affect dynamic range, and what changes I can expect by moving from EL34s to KT88s. And how will additional headroom, a supposed advantage to moving to the KT88s, affect dynamic range, if any?

Thanks,
Jeff

jeffjmr wrote:Thank you both for your feedback.

I am aware and capable of making the internal changes necessary to move to KT88s, bias circuit mostly, and a phase splitter change from 12AU7 to 12BH7 according to the manufacturer. Your points are well taken, given the EL34s are not only weak but one channel’s pair are not well matched either. My drivers are also not well matched, so I do believe they need to go as well.

Since you both seem to have a lot of experience on the subject, is there any consensus on brands? Any advantage to the EL34L? Remembering I don’t need low end, which the EL34s supposedly excel at, given my powered subwoofers.

Jeff

jeffjmr wrote:Apparently I failed to mention this is a TriodeUSA ST70, likely factory built as the wiring and soldering are very professional looking.

A 12AU7 is spec for the phase splitter and EF86 for the drivers. Mine came with a JJ ECC83S that was not particularly well balanced so I replaced it with a perfectly balanced (mhos, tested before I had plate current measuring capability) RCA cleartop 12AU7A. The manual calls for the 12AU7 not the 12AX7 (ECC83S), so I made the change but there was not a significant audible difference. The only other meaningful changes I made was the yellow paper diode mod to the rectifier, and an inrush current limiter.

Hope that makes it a bit clearer.

Jeff

jeffjmr wrote:

peterh wrote:I agree with 12AU7 as the correct tube here.

BTW it seems according to the schematics that g3 on the EF86 is left unconnected, something that
will increase dist. Check the board, it should be connected to cathode ( pin 8 to pin 3 )

There are some differences between the finished amp and that schematic, but the G3 on EF86s is not one of them. Mine are not connected, and I looked at the driver board assembly manual and the driver board PCB diagram shows pin 8 as having no connection.

That’s a big change counter to the manufacturer’s design so please pardon my caution. What are the potential risks to making this mod?

Jeff