ST-70 just finished - very little bass

I just finished my ST-70 kit. It is beautiful and whisper quiet. It is driving a set of Klipsch KG4's which have wonderful full range sound. The ST-70 is equipped with Genalex KT88's which are new, along with a JJ Electronics rectifier and Genalex 12au7's. It has the TDR, stepped attenuator and Russian PIO upgrade.

For some reason I'm getting very little bass, almost like a high pass filter in play. It's being fed by an iPod which was previously driving another tube amp to the KG4's and had full range sound. The ST-70 has several hours of use thus far. I'd be amazed if this was a break-in issue as the roll off is hardly subtle.

Any suggestions or thoughts? As I said, the rest of it sounds great!

Filippo

I'm assuming you didn't mean 12AX7's? That's not the right tube for that amp...

Well it would be helpful if I actually provided the correct information .  I have 12au7's not 12ax7. So we are okay there. These are all brand new, matched tubes, so nothing vintage or worn out.

I will try driving it with a CD first. If that's not the issue, I'll set it up for 4 ohm. Report back soon. Thanks for everyone's input!

Filippo

I've decided I'd like to go ahead about put a DPDT On/On switch in the back of the chassis for 4/8 ohm switching. Any issue with doing this? Obviously one wants to switch this only when the power is off ...

Filippo

fmorelli wrote:I've decided I'd like to go ahead about put a DPDT On/On switch in the back of the chassis for 4/8 ohm switching. Any issue with doing this? Obviously one wants to switch this only when the power is off ...

Filippo

Sorry bob, but i have the opposite opinion.
Do not install a switch to select output circuit. It is a unneeded source of failure, many switches
that stays in the same position and is subject to heating cycles may experience dust and
oxidation product on the unused surfaces. Thus some day it might be an open circuit and
subsequently risk your amp.

If you will have the opportunity to change output circuit then install proper connectors
( like the ones dynaco used , or maybe like the ones audio research uses)

Those "banana connectors" that seems to be popular at the moment suffers from no locking, any activity behind the amp may accidentally pull them out , thus risking your amp.

Real men uses screw connections :-)

Hi there,

I would suggest to use tripple posts. I bought the ones from tubes4hifi and never looked back.

Best

Michael

Thanks you all. I prefer the triple binding posts and ordered those up. I'll report back once I've installed them. In the meantime, I continue to burn in the amp. We'll see if we can figure out what's causing the mystery bass roll off.

Filippo

your order going out when the post office is open again on Tuesday.
Just make sure none of the connectors are touching the chassis.
You can use the extra nuts provided to space the terminals away from the chassis.
The PCB part should be on the outside of the chassis (same as Dyna original)

I did go ahead and order the triple posts, which makes sense to set up with this amp, especially given my speakers.

That said I discovered the issue, and pass it along should someone else search for this problem. Turns out the bias on the two front power tubes was off from original adjustment (I'm not sure what happened, but replaced multimeter batteries and went to manual setting rather than auto ranging). Anyway with bias set, it sounds exactly as it should.

Thanks for sending the triple posts - I'll look forward to installing them.

Filippo

I felt like I should add two things to this for future searchers...

Bass Issues - I thought I was having these as well, and all of them ended up being something I had done. The two main ones were related to me using multiple 12au7s that I assumed one of which had to be good, along with the same issue with power tubes. I had a set of Mullard XF3 EL84s, the best EL84s made, which are known for having a very long life. They worked and biased up okay, but sounded thin. I had also bought a lot of 12au7 tubes listed as untested on ebay, which I think were actually rejects. Don't be cheap. Buy at least some decent tubes so you have a frame of reference.

My other issue was in my Klipsch speakers. I damaged a tweeter when I was working on the finish, so I replaced them with some supposedly factory replacements from a third party vendor.

They were either more efficient, or just screechy and annoying, which made me think the st-70 was light on bass. My other amp rolled off the treble so much that the tweeter wasn't noticeable.


A correctly biased VTA ST70 is a fantastic sounding amp with plenty of bass (I listen to everything from Coltrane to the Beatles to Rage Against the Machine to Black Sabbath to George Jones). It is capable of shaking my house at 105db through my Klipsch Chorus II's without breaking a sweat.




When you put in new tubes and bias them, the bias will drift as the tube gets older. This is why you should buy tubes that are matched AFTER a burn in period.

When I set bias, I set it as soon as I turn the amp on so I dont blow anything up, then I play it for awhile (you can hear if there is an issue) and check it again after about 20 minutes, and then again after an hour.

I check my st70 about once a month and adjust the bias to around 450 mv on my DMM, which I think translates to 45ma, but I am not sure of that...