New tricks

[leadtower]

Hi All; I know i posted earlier this question but I just would like to try once more. My ST 70 is sounding really nice. I would be interested to know of any other circuit modifications that might be beneficial.
Does anyone think that running the EL 34's in triode would be a good mod? Is the same method of triode wiring used if you have Shannons board installed?
Is there any other tube that people have tried that they like; such as 6L6's?

Does anyone have a particular opinion regarding this possibility; I have the Eiclone boards; considering that transformers and hardware would run about 200$ not including passive parts; does any one think the trade from ST 70 with all the mods to a pair of Eiclones is cost effective. I use Altec Valencias and Loth full-range speakers. I will eventually build the Ekies; but would put that off for a while and continue perfecting the Dyna if the concensus was that there is some difference but not a world of difference. I have to ask this dumb question because I have no way to hear the amps so I must rely on advice from people who know.

Last question. Anyone tried Bi-Amping between the ST 70/ST 35/Eikes?
Thanks Guys; One last request. Is it a good idea to get involved in a O-Scope for troubleshooting on these amps? And would the possibility of Shannon or one of you guru's walking someone through the procedure of utilizing the scope to check and calibrate the amplifier circuits be in the cards?
Thanks Much.

[WA4SWJ]

Hi Leadtower,

Sorry for the slow response to your question.

I have an ST-70 and 3 eiclones, all with Shannon's boards. I use the ST-70 for music and the eiclones for home theater. I have to say that the ST-70 sounds just a bit better than the eiclones when I try them with music on the same set of speakers. But I also have to say the difference is small. I still have not made Shannon's recommended mods in the eiclone area of the forum. So, I still have to do that yet. That might make them sound a little better - only time will tell. When I get time to make the mods.

I have used my eiclones for over a year as home theater amps. I hear things in movies now that I do not hear with some Yamaha amps I have - DSP-A3090's. Those are pretty decent amps but the eiclones sound better to me. I also have a Yamaha DSP-A2070 and the tube amps are definitely better than that amp to my ears. The transistor amps sound muddy to me if that's an appropriate term to describe sound. They don't sound bad, just that there is something missing. Clarity maybe is the word.

So, as for tweaks, I'm not sure what all you've done to your ST-70 but to me (and this is subjective) amps with solid state rectifiers and larger value electrolytic filter capacitor values just seem to have more ability to handle transients to me than amps with tube rectifiers. I have built some Mark III clones too that I am now using for the home theater. They have Sovtek 5AR4 tube rectifiers in them. They seem to lack just a little punch. Again, not that they sound bad, just a little less something. I am going to modify them eventually and put in some solid state rectifiers to try them out. Another project for when I have time.

Someone in the Poseidon list here talked about using KT-77's as final tubes. The new JJ versions are only about $18 apiece. I bought a set but have not tried them in my ST-70 yet. The point is that tube rolling might be another tweak that could be tried. Perhaps someone more knowledgeable than me about tube geometry might be able to point to some differences.

I haven't tried the triode connection yet for my ST-70. Not sure I want to. But, I'm open to other opinions and maybe some measurement facts.

Eric talks quite frequently about JJ KT-88's. If you build the eiclones those seem to be the tubes to get. Shannon seems to like Sovtek KT-88's. I don't have either of those tubes but I'll get some eventually. Again, when I have time. Work gets in the way of hobbies!!

Now, I'll have to say that my ST-70 is using a 5751 voltage amp tube and 12BH7's as the phase inverters. Shannon did some work with his distortion analyzer measuring this setup and his results show this is a pretty good arrangement. However, make sure you have enough filament current available since the 12BH7's require more filament current than 12AU7's. I have the Triode Electronics power transformer and it has plenty of capability there. If I remember correctly, Eric likes 12BH7's too.

I am quite happy with my ST-70. Right now I am not sure what else I would do to it except try the KT-77's. Maybe play with bias settings and use my analyzer to see if it makes a measured difference and then listen to see if it makes a real difference.

Well, enough rambling. Good luck with your tweaks and let us know how you wind up.

Happy 4th of July!! Telarc's 1812 Overture is a good one for this time of year. all the cannon shots would require something like 30,000 watts to not clip at actual sound levels I've heard. Maybe Tom and his power triodes could do it!!

Regards,

[leadtower]

Thanks Much WA4SWJ. The reason I was so persistant is this amp has turned out to be such a great sounding amp that it continues to amaze me. I like the idea of 12bh7's. I have the Handmade trans so that might work but I am not aware of the 5751's. Any circuit changes required to implement that tube?
I see a lot of 6L6 talk as power tubes. I was not sure exactly how to use them; what changes to make.
You know I have a couple pretty good amps but this ST 70 is my favorite; with the changes it became a very detailed and wide-range amp but still has that tube smoothness. So it was my intention to put this amp into a really nice enclosure; maybe the stock boxes you can get on Welbournes site. Before I finalised that I wanted to make sure I got that last tiny improvement possible.
Thanks Much for your help.

[WA4SWJ]

My knowledge of tube histories is limited. The 5751's I have are GE NOS. My understanding is that they are ruggedized tubes with additional internal element bracing that helps eliminate noise and microphonics due to vibration. They also have slightly lower gain. I made no circuit changes at all - just plugged them in. They work fine.

Like you I have found that my ST-70 is a fantastic sounding amp and is definitely the best of all the ones I have. Mine has the new Triode Electronics transformers. I now have obtained an old ST-70 with original transformers that I am going to convert using Shannon's board. I'm anxious to see if there is any difference between the two - Triode new transformers and the one using the old original Dynaco transformers.

Good luck with your project. I'll bet it will look nice. I have a Welbourne Labs chassis that I am using for an 832 amp that has been in progress for months now. It is definitely a nice looking chassis. But I like the look of the old Dynaco design chassis though with the cage off so I'm going to keep mine in that arrangement.

As for 6L6's, I had some in a guitar amp when I was a teenager but I have not used them in a stereo amp yet. I read lots of good comments about them. I do have some 807's that are similar. I'll try them someday too.

Regards,

[erichayes]

Hi All,

The 5751 is often described as a "special" 12AX7--even by GE, who should know better. It is, in fact, two triode sections of a 6AT6 in one bottle. It's very close, but not identical, to a 6SL7/5691. It has a slightly higher heater current (350 mA @ 6.3 V) than a 12AX7, but otherwise can be dropped into just about any non-oscillator 12AX7 application.

Because it's an instrumentation tube, its spec.'s are a lot tighter than "regular" tubes. I've tested several NOS brands on the Tek 570, and all the curves are remarkably similar. The new Sovtek 5751 doesn't adhere to the original spec. but is an extremely good sounding tube. Don't expect long life from it, though. I've had one go feeble after only about 100 hours. I'm keeping an eye on the other 3 I'm running.

[Pyre]

I have a St-70-C3 and run either three 5751’s or 12BZ7’s for the first stage. The 5751’s are great low noise very defined tubes the 12BZ7’s are very bold and warm. I have tried a few different 5751’s and found any of the black plate triple mica tubes to be great sounding.

Don’t bother with the double mica tubes you may as well stick with 12ax7s. The 12BZ7’s have a higher gain and are just about indestructible, I have not had a set fail yet. I have noticed that the 5751’s sound great out of the box but the 12BZ7’s take about 20 hours to break in before sounding right. NOS 5751’s are getting expensive these days and if you can get a pair of triple mica black plates for under 50 bucks your doing well. 12BZ7’s are cheap still I have bought black plates for 7 bucks each NOS. If you want to read a fairly biased but useful review of 5751’s here is a link (read down to part 3):

http://www.audioasylum.com/scripts/d.pl?audio/faq/joes-tubes.html#12AX7

The prices are a bit out of wack

[erichayes]

Hi All,

I mentioned this factoid elsewhere earlier, but since the subject's come up again . . .

A 12BZ7 is two 12AX7s wired in parallel. Its µ is the same, its plate resistance is ½ that of the 12AX7, and its Gm is double (as is its heater current). If you're having noise problems in a high gain 12AX7 stage, or if you have an amp that uses a 12AX7 as a direct driving phase inverter, try a 12BZ7 instead. Great tube.