VTA ST70 Bass response

[rockinrob86]

EDIT AFTER ISSUE WAS FIXED. I was having issues with cheap replacement tweeters for my Klipsch speakers, and also some hum coming from stacking the ST70 on top of my preamp. I also was using old preamp and power tubes, that although they "worked" were not nearly up to spec. Kegger gave me some mods to do later on, but I hadn't fixed my other issues before I did them. At this point, with titanium tweeters, good tubes all around (Ampex 12au7s, telefunken 12ax7, a mullard 12au7 in the middle on the st70 and EH 6CA7 output tubes) it sounds FANTASTIC and beats my expectations. The bass is big and bad, but not overpowering and it handles anything from classical, jazz, etc to metal, and I am totally happy! Long story short, you cant skimp out at any stage, and new or verified NOS tubes are must haves.

I am running a tubes4hifi (aikidio with regulated power and phonostage) sp8 into a vta low gain w 12au7 modded st70 (New filtercaps, the works), and Klipsch Chorus II speakers.

The problem is there is not enough bass. When I switch my marantz (fully recapped, sounds great) back in, the bass comes back.

I know the marantz stuff is "warm" sounding, but this is different.

It sounded terrible at first( as in ouch that piercing treble really hurts), and I realized that was a bias issue caused by my interpretation of the manual(dont ask). I biased each tube to 45ma and it was better, but still light on the bass. The sound is very detailed, and really great, but just too little bass.

If I use my headphones on the marantz with the sp8 going into the tape input on the marantz it sounds awesome, and significantly better than if I was just using the marantz alone. IT still sounds detailed, and actually very similar to how it sounds with the speakers, only the bass is right were it should be.


My headphones are grado SR80s, and are far from overly bassy. They sound basically neutral.


When I am running the st70, there is bass, it just has no punch and feels that it is just out of balance with the mids and trebles. I bought parts to build a tone control to install in the preamp, but I am thinking this is not the problem, and I need to solve the real issue.


I am running JJ EL34 tubes that were in the st70 when I got it, and I dont know how many hours are on them. They bias up fine.

The power transformer in the st70 is original (I have the nickel plated earliest version).

My power at the wall is usually 122, and the PT is untouchable for more than a split second within 30 minutes or so.




I dont want to buy new tubes or a PT if I dont have to. I have other things I want to spend money on...

But, does it sound like one of these is responsible for my issue?

 

[awise1961]

I too was not totally happy with the sound of my ST-70 after building the low gain VTA board. However, I found that the sound improved greatly with the following tubes installed:

2- 12BH7A's as drivers.
1- 5814 as input.
4- Gold Lion Reissue KT-77s' (Ruby Tube EL34's sounded almost as good)


The bass and overall soundstage was wonderful after that.

Al. Wise

 

[bktheking]

That and make sure all your voltages on the output and driver tubes/phase splitter are spot on. The build manual tells you what they should be and what pin's to check. One wrong resistor value in the vta board can wreak havoc on sound/balance.

 

[Kegger]

Also coupling caps both type an size can matter, and power supply reserve can help.

Yes check the plate voltage of the center tube, pins 1 and 6, you'd like to see about
150v or better there, much lower and it won't sound right.

As BK found out you may need/want to lower R13 an 14 if the voltage is to low there.

 

[bktheking]

I tested mine today after the "fix", man what a difference 100 volts makes on that center tube, it woke up the amp.

 

[Kegger]

Yah I cringe when I see designs that intentionally have like 70v at the plate of a triode.
(100v is the low end cutoff for most triodes I find to still perform decent, prefer 150+)

There are very very very very few triodes that perform even halfway decent with a low
plate voltage like <90v, about the only "audio" tubes used today that will are the 6DJ8's.

 

[Br1ck]

I have the high gain VTA board and really think VTA is the way to go. I was really puzzled by your description of the lack of bass. Glad you got it worked out. Of course, a power tranny upgrade would help.

 

[GordonW]

Yah I cringe when I see designs that intentionally have like 70v at the plate of a triode.
(100v is the low end cutoff for most triodes I find to still perform decent, prefer 150+)

There are very very very very few triodes that perform even halfway decent with a low
plate voltage like <90v, about the only "audio" tubes used today that will are the 6DJ8's.

Truthfully, you're really not in the "linear part" of the plate curves of a 12AU7, until you get about 250v of supply voltage, and about 125v of plate voltage, or higher, on it.

I like to run 'em at about 280-300 supply volts (to the supply side of the plate resistor), and about half that on the plate (on the tube side of the same plate resistor). Seems to really make them "hammer". Clean, too.

12BH7s are a little more "forgiving", but they also like that voltage.

6CG7s can run almost anywhere- but I still "give 'em the juice", just so they can swing lots of clean voltage on the output...

But yeah- on low voltage, the 6DJ8, and the 6H30 are about the only tubes that work well under 180v on the supply/90v on the plate...

Regards,
Gordon.

 

[rockinrob86]

I have the high gain VTA board and really think VTA is the way to go. I was really puzzled by your description of the lack of bass. Glad you got it worked out. Of course, a power tranny upgrade would help.

I still havent fixed it, but I now have some things to try when I get back in town next week!


I haven't been able to find a definitive answer about the PT.

If a PT is running really hot, but still working and putting out the right voltages, would replacing it with a newer one that wont run as hot make a difference in bass response? Or is it more along the lines of as long as it can steadily supply the voltages needed, no difference?

I ordered my own parts for this, so maybe we are on to something there... I tripled checked everything (at least I think I did) when I was putting it together, and I always measure resistors when I am installing them.

I am using the brown xicon polyprop caps for the coupling caps. I heard they were good enough, and I wanted to get it all working before I put in better caps. I dont think these would make as big of a difference as what is going on.

Unfortunately, this amp came with 3 xf2 mullard el34 tubes, which all bias up fine. I havent done this, but I think I may put a pair of those on one side, and the JJs on the other and see if I can tell a difference from that. I may need to order a single xf2...

 

[bktheking]

The stock pa060 transformers run hot to begin with, there was never an issue with bass response and transformer heat. They only get hot after an hour of so of use. Think about it, you'd lose bass after a while, not after turning it on. I think it's good fortune that it came with 3 xf2 mullards, only having to buy one is a money saver. I'd run those over JJ's any day of the week. The issue with voltage referred to a properly built vta board AND proper voltage going to the EL34's , if you haven't got the proper voltage readings on the driver tubes or ouputs it will not sound right. Now keep in mind, these st-70's without pre with tone controls don't have alot of bass to begin with. My pas 2 has the bass/treble controls disconnected and is by no means a bass machine. As a test, use the marantz as a pre and add some bass using the marantz. Zero bass out of the st-70 is something entirely different, they do have some bass but maybe it's not what you were expecting running it without a proper pre with tone controls.

Maybe a picture of the wiring underneath may help. I know my transformers run around 135-140F, they are hot but I can touch it for more than a second.

 

[rockinrob86]

I have the tone controls disabled on the marantz. I am mostly referring to recordings I know like the back of my hand and have heard on tons of stereos and headphones of all types, and the st70 always feels like there is 20% less bass than I would expect, if that makes sense.

I think I just need to do some voltage measuring and get a clearer picture that way. I can take some pics when I get back.

I am not wanting a bass machine, but Led Zeppelin II, Abbey Road, and Neil Young Harvest don't sound quite right. Come Together is a great yardstick imo.

I put a meat thermometer on my PT and it got up to 160 (I think I remember accurately...) with the probe resting in one of the holes, this was after about an hour. I just measured my wall voltage at 123.5.

I need a variac!

 

[rockinrob86]

If I buy a single xf2 mullard, how can I know if it will sound good with the other 3? I dont have a tester...

The other tubes bias up in the st70 okay, since the bias is adjustable per tube is this not a problem and any xf2 I get will work just fine?

 

[dcgillespie]

It is important to understand the difference between bass response with regards to overall frequency response, and bass response with regards to power response.

If you are operating your ST-70 with no more than (say) 10 watts peak output per channel (pretty darn loud with my Cornwalls), then no amount of increase in power transformer (or OPT) reserve capability will really improve your bass performance, because operating at 20Hz and even lower at this power level is entirely within the capability of these components in the ST-70. Therefore, if you feel the amount of bass is rolled off in this scenario, then it is an issue of frequency response. To that end however, it could be the actual response of the circuit that is in error, or simply your own personal preference. It is therefore always helpful to keep your ears calibrated to what truly flat response across the audio spectrum sounds like, so you can then determine if what you're hearing is in fact a response error, or accurate response which you simply do not prefer. Without this knowledge, you have no idea where you are, or where to even start in creating a system that performs to your liking.

On the other hand, if bass is lacking only when significant power is required from the amplifier, then it is an issue of power response. It is this scenario where upgraded power transformers and power supply enhancements in general can have the most effect. No doubt that increasing the capacitance of the power supply can improve bass definition at any power level. But increasing overall supply capability beyond that will have the greatest effect at higher power levels, because it is under those conditions where the original power supply components were the most lacking.

I mention all of this in reference to your questioning if an improved power transformer would help in your scenario, which is no small investment. Using these guidelines will help you determine when and where the cost of such components can make a worthwhile improvement to your equipment.

Dave

 

[bktheking]

Listening to ring of fire by johnny cash right now, plenty of bass for my needs. Something else may be up. Have your tubes tested before spending money to see where they are living, they may be shot for all you know.

 

[131east23]

I have an ST70 with a new Low Gain VTA; after I finished gutting it and upgrading it I thought it was magical but lacking bass. Of course it amplifies what I feed it and the almost stock PAS3 was the problem. (The Cornwalls, recently upgraded, and the ST70 are now a very revealing pair that I don't think I will be parting with anytime soon.) A few weeks ago I bought an Audible Illusions Modulus 3 and it has no tone controls and the bass is back, even in the phonostage, where it was really missing. Right now I have Sade's "Lovers Rock" playing and even a room away I can feel the bass, even at just a few measly watts of power. It might not be your amp, but instead the source you feed it. JMHO.

 

[rockinrob86]

I thought that too, but the pre I am using sounds amazing fed into my marantz and using headphones, with the EQ set flat. I switched over to using the marantz as the amp, and it has more bass, but it doesnt sound as good overall as the st70, just more even across the frequencies

 

[PB_Audio]

I thought that too, but the pre I am using sounds amazing fed into my marantz and using headphones, with the EQ set flat. I switched over to using the marantz as the amp, and it has more bass, but it doesnt sound as good overall as the st70, just more even across the frequencies

What Marantz Amp are you using; AND has it been refurbished... new caps and such? Old Marantz solid state (early to mid-70s in particular) are known to greatly accent the Bass as caps dry out and go out of spec, but with a distinct loss of definition, ('Muddy' Bass).
...or, are you using a Model 8B or pair of Model 9s???
Big differences...

For the most part, my tube experience is with decades of tube guitar combo amps, like Fender Tweed and Blackface, and Trace Elliot, etc.
It sounds like tubes, primarily. At least, that's where I'd start... or inadequate voltage to the tubes. Check that first.

 

[rockinrob86]

I am thinking this may be a tube rectifier or PT problem... need help determining which it is.

At pin 8 off of the 5ar4, I am getting 455v, at the b+ input, it is down to 393, pin 1 of the center tube is 135, 6 is 125, 210, 199 for right side 1 and 6, and 205, 197 for left side.

I thought rectifier tubes either worked or they didnt, so this makes me think the PT isnt working as well as it should be...

 

[bktheking]

B+ at the driver board should be 380-420V , typically 430-450 is where you want B+. I had center running at 150V and the outers around 225V. I'd be pointing my finger at the cap can before the transformer. Did you wire the cap properly (orientation as well for the sections) , notice there is a jumper between 2 of the cap sections then a 2.2K resistor which increases capacitance feeding the B+, at least that's my understanding.

 

[bktheking]

PS- What rectifier are you running, i can't find your post on what it is? They do work or don't, the importance is which one as you can stick a few different models in and get different results.