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In Reply to: RE: Oh wow! So I could do that too with my ST-70? posted by JoshT on March 15, 2023 at 09:39:37
Airtime is a member over on AK where we all who built the VTA kit/s went to help or get help. There was a thread over there a while back may around 2015 and Skizo was the one who mentioned about putting a 6SN7 in the middle splitter spot I tried iy and I think Airtime tried after I did but I could be wrong. That is where the biggest difference in sound for me happened. I was using Northern Electric 6SN7's in that spot but they kept humming. Then I was on the Upscale Audio site and saw Kevin found a bunch of Russian 6H8C (Equivalent to 6SN7) and were cheap. I bought some and that was the ticket! Had the same sound as the NE 6SN7 but with better mids and bass. I have been running it ever since.Yes, you will need a 6SN7 to 12AU7 adapter to make this work. My PCB came with my kit but if I'm correct, Roy offers a 6SN7 Octal pcb for the VTA-120 and 70. Some will say you can't run that tube in the 12AU7 spots but I only used it in the middle splitter spot and has worked fine. I would check out the threads over there to see what we all did. My Moniker there is Drummerboy2.
Skizo, Airtime, and another person were all helpful and offered a lot of info and help to others. Just goto the Tube section and search for the VTA-70/120 kit builds threads around 2015-2017. Mine was ("Just Pulled the trigger on Bob Latino's VTA-120 tube power amp"). I think that's were most of the info is at for the VTA amp builds between those years. Also, these amps are nothing like the Dynaco amps other than they look like them. Totally different circuits and better heftier transformers.
btw, I got the Auto-Bias PCB for my VTA-120 a few months back, I still need to install it. I will do that after cleaning and tightening all the socket pins
Happy Reading! :)
Edits: 03/15/23 03/15/23 03/15/23Follow Ups:
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"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
After posting yesterday I contacted Roy and asked some question about the Octal PCB and the Auto-Bias PCB. I decided to pick up an Octal PCB and do some upgrades like I did with the original pcb and some that I didn't do that came with the kit. This should be a nice improvement in sound. I will post some pics of what I do over on Tube/Diy and on AK when I'm finished.
Airtime and I have long been curious how much improvement would be gained by 6SN7s used as the phase inverters, over the two 12AU7s.
I've thought about converting my phase inverters to 12AT7s (VTA's original configuration) since the 12AT7 is a slightly more linear tube than the 12AU7, but I suspect that your all octal upgrade is really the way to go.
I look forward to the updates and pictures.
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"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
I've had a VTA (or Bob Latino at the time) ST70 for over a decade. This amp came stock with the 12AT7 board. Fortunately at least in my case, I've been quite happy with the 12AT7. Another friend who has the ST120 recently had the 6SN7 board installed. This replaced his 12AU7 board. Although he wasn't unhappy with the 12AU7 board, he says the 6SN7 board and companion 6SN7's of course have been a noticeable improvement. He had bought a 6SN7 adapter, but opted instead to do the entire board. So using a 6SN7 adapter in the middle position of the 12AU board seems to be a good inexpensive option to try for either the ST70 or ST120. Some may later opt for the entire board or be perfectly happy with just the adapter. Regardless, I think the VTA amps are some of the best value tube amps out there. I never tire of listening to mine even though I've got another P-P tube amp that lists for 2.5X
the cost of the ST70.
According to Roy Mottram, on the VTA forum, about 90% of his customers who tried both the 12AU7 and 6SN7 board, prefer the 6SN7 version.I only wish I'd tried the 12AU7 socket to 6SN7 tube adapter in the gain section years sooner than I did. The improvement took my ST-70 with the VTA board from an amp that almost never left storage, to one I can now enjoy listening to.
Oh, and for the record, according to Bob Latino himself on the VTA forum, the amps aren't, and have never been "Bob Latino" amps, as shown above.
Edits: 03/16/23
Are you continuing to use single 12AU7 socket to 6SN7 tube adapter or was that just a stepping stone for swapping in a new board?
I've emailed Roy and he confirmed the single tube approach works, but I get the sense many people do that as an interim step to confirm its an improvement before replacing the entire board. I'd really rather not replace the entire board because it's a new amp and I am already enjoying it as is.
Thanks!
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"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
...but that's only because this ST-70 with the VTA driver board is one of my back-up amps, and only sees occasional use. If it were my main, best sounding amp, I'd opt for the octal board over the noval.
Regarding tube adapters in general, Jim McShane told me that they can sometimes cause somewhat diminished highs, depending on the particular tube and circuit. I didn't notice that in my system. but eliminating them with the proper octal socket(s) for the 6SN7(s) would really be the best way to go.
I bought one based on a link from Roy and received an email from the seller - screen shot below.
Is this just a generic warning about putting a 6 volt tube in a 12 volt tube socket? I assume it would be a concern if I were replacing all three tubes but isn't where I'm just replacing the center tube?
Thanks for your input on this.
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"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
All tube filaments in a Dynaco ST-70 (including the VTA version) use 6.3 VAC, other than the 5AR4 rectifier. So the 6.3 V adapter is the one you want, even if you were adapting all three driver tubes.
I'm going to assume that you have a continuity function on the meter you use to check tube bias. If so, above is a crosscheck I came up with from the data sheets of the two tubes, to be certain that the adapter is wired correctly before you use it in your amp. I wasn't willing to leave the adapter to "It's probably OK", and it only took a minute to test.
The column on the left are the 12AU7 pins, and on the right are the corresponding 6SN7 sockets.
Good luck with it, and I strongly suspect you'll soon be a member of the 90% Club (who prefer the 6SN7 over the 12AU7 in the VTA). Please keep us posted.
Now to order a tube, most likely from Jim, Brent or the Tube Depot. Or an Inmate, lol. I've had good luck in the past with all four sources. :-)
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"If you are the owner of a new stereophonic system, this record will play with even more brilliant true-to-life fidelity. In short, you can purchase this record with no fear of its becoming obsolete in the future."
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